A Continuously Variable Power-Split Transmission in A

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A Continuously Variable Power-Split Transmission in a

Hybrid-Electric Sport Utility Vehicle

Miguel M. Gomez

Thesis submitted to the College of

Engineering and Mineral Resources

at West Virginia University

in partial fulfillment of the requirements for

the degree of

Master of Science

in

Mechanical Engineering

Victor H. Mucino, Ph.D., Chair

James Smith, Ph.D.

Nigel Clark, Ph.D.

Department of Mechanical and Aerospace Engineering

Morgantown, West Virginia

2003

Keywords: CVT, Power Split, Transmission, Hybrid, SUVCopyright 2003 Miguel M. Gomez


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ABSTRACT

A Continuously Variable Power-Split Transmission in

a Hybrid-Electric Sport Utility Vehicle

Miguel M. Gomez

Continuously variable transmissions (CVTs) have an infinitely variable ratio, which

allows the engine to operate more time in the optimum range given an appropriate controlof the engine valve throttle opening (VTO) and transmission ratio. In contrast, traditional

automatic and manual transmissions have several fixed transmission ratios forcing the

engine to operate outside the optimum range. Usually CVTs are used in small vehiclesdue to power limitations of the variable elements. Continuously variable power split

transmissions (CVPST) were developed in order to reduce the fraction of power passingthrough the variable elements. This configuration includes a planetary gear train (PGT),

which allows the power to be split and therefore increase the power envelope of thesystem. The PGT also provides a branch that can be used in a hybrid electric vehicle

(HEV) operation through an electric motor.

HEVs are receiving special attention nowadays because they require a smaller internal

combustion (IC) engine than they do in normal vehicles and consequently represent

savings in fuel. HEVs generally employ conventional automotive transmissions.

In this thesis a conceptual design for a CVPST for a HEV is presented, with increased

power envelope in the system for a sport utility vehicle (SUV) application. A

LabVIEW program has been developed, which according to the vehicle characteristics,

driving resistance coefficients and power source data, provides engine-vehicle velocity

relationships, CVT ratio, variable pulley radii, belt force and power trace following the

ideal fuel consumption curve. This thesis aims at extending CVT applications in the

automotive industry to bigger vehicles (and engines) such as pickup trucks and perhaps

small buses.


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ACKNOWLEDGMENTS

First of all I would like to thank God for giving me the capability to finish my

thesis work. I would like to extend my deepest gratitude to my research advisor and

committee chairman, Dr. Victor H. Mucino for his continuous guidance, encouragement,

support and patience through all this work. I would also like to express my appreciation

to my thesis committee members, Dr. James Smith and Dr. Nigel Clark for their helpful

comments and suggestions regarding my work.

I would also like to express a special thank you to the Mexican National Council

of Science and Technology (CONACYT) for their financial support, without which, this

work would not have been possible.

Thank you very much to all the people at the Council of Science and Technology

in the State of Queretaro (CONCYTEQ) for your help and support during the pre-process

of my studies, especially to Dr. Alejandro Lozano Guzman and Mr. Juan Sanchez

Ramirez.

I would like to show my gratitude to Dr. Jacky Prucz for his support and help

through my last year of my studies. I also would like to thank my parents and all my

family members who were a great source of inspiration to me.

Last but not the least I would like to thank all my friends at West Virginia

University who helped me in my work, especially to Rohit Paramatmuni.


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iv

TABLE OF CONTENTS

Abstractii

Acknowledgments..iii

Table of contents.iv

Keyword list....vi

Nomenclature.vii

List of figures...x

List of tables.....xiii

Chapter 1 Introduction1

1.1 Overview...1

1.2 Objective...5

Chapter 2 Relevant literature review ....6

2.1 Continuously variable transmissions....6

2.1.1 Types of CVT..7

2.2 Hybrid-electric vehicles..12

2.3 Power-split technology...18

2.4 Power-split hybrid electric vehicles....20

2.5 Continuously variable transmission controller...24

Chapter 3 PS-CVT concepts and kinematic models...26

3.1 CVTs and power-split concepts...26

3.2 Power flow configurations..31

Power-split case..31

Power recirculation case I...32

Power recirculation case II......34


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Chapter 4 Hybrid-electric power-split CVT conceptual design....36

4.1 Design proposed...36

4.2 Operational modes...41

4.3 Velocity relationship equations49

4.4 Force equations....51

Chapter 5 Example conceptual design.....54

5.1 Vehicle considerations.54

5.1 LabVIEW plots.....60

Chapter 6Control simulation...71

6.1 Control method proposed.71

6.2 Controllers needed in the design..73

6.3 LabVIEW simulation....76

Chapter 7 Simulation tests and results....83

7.1 Case I: Low speed83

7.2 Case II: Medium speed86

7.3 Case III: High speed.89

Chapter 8 Final Comments...92

8.1 Conclusions..92

8.2 Future work..94

Appendix: LabVIEW diagram codes......96

References102

Vita...107


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vi

KEYWORD LIST

Acronyms:

ACS = Advanced Control System

CVT = Continuously Variable Transmission

CVPST = Continuously Variable Power-Split Transmission

GM = General Motors

HEV = Hybrid-Electric Vehicle

HMMWV = Hybrid Electric Powered High Mobility Multipurpose Wheeled Vehicle

HP = Horse-Power

IC = Internal Combustion

MTU = Michigan Technological University

MVB = Metal pushing V-belt

OEM = Original Equipment Manufacture

PC = Personal Computer

PGT = Power Gear Train

PNGV = Partnership for a New Generation of Vehicles

SAE = Society of Automotive Engineers

SUV = Sport Utility Vehicle

VTO = Valve Throttle Opening

WVU = West Virginia University

ZF = Zahnradfabrik Friedrichshafen AG


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NOMENCLATUREa = CVT input pulley

xa = Acceleration in the forward direction

A = Frontal area of the vehicle

b = CVT belt

c = Output pulley CVT

C= Center distance between CVT pulleys

CD = Drag coefficient

d= Countershaft gear

DA = Air resistance

e = Idler gear

f= Control gear

fr= Tire and ground factor effect

Fx = Tractive force on the ground

Fb = CVT belt force

Fctr/i = Fcs/i = Force from the idle gear to the control (or countershaft) gear

Fs/p = Force from the planet gear to the sun gear

Fp = Force on the planet gear

Fr/p = Force from the planet gear to the ring gear

g = gravitational acceleration

cvt= CVT ratio

dI= Rotational inertia of the drive shaft

eI = Engine rotational inertia


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tI = Rotational inertia of the transmission

wI = Rotational inertial of the wheels and axles shafts

L = Length of the CVT belt

M= Mass of the vehicle

Mr= Equivalent mass of the rotating components

fN = Numerical ratio of the final drive

tfN = Combined ratio of transmission and final drive

Nr= Ring gear number of teeth

Np = Planet gear number of teeth

p = Planet gear

r = Radius of the vehicle wheel

rg = Ring gear

ro = Countershaft gear radius

rp = Planet gear radius

rs = Sun gear radius

rri = Ring gear radius

rro = Control gear radius

psr = Driving variable pulley radius

por = Driven variable pulley radius

P = Engine horsepower capacity

Rx = Rolling resistance

Rxf= Frontal tires rolling resistance

Rxr= Rear tires rolling resistance


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s = Sun gear

eT = Engine torque at a given speed (from dynamometer data)

Tin = IC engine input torque

Tem = Electric motor input torque

Tout= Total output torque

V= Vehicle velocity

W= Weight of the vehicle

= Air density

tf = Combined efficiency of transmission and final drive

in = a = Input angular velocity

c = Input pulley angular velocity

d= cs = Countershaft gear angular velocity

f= Control gear angular velocity

out= p = Planet gear angular velocity

r= Ring gear angular velocity

c = Ratio between the driven variable pulley and the driving variable pulley radii

g = Ratio between the sun gear and the ring gear radii

gc = Ratio between the counter-shaft gear and the control gear radii

= Angle between the x-axes and the line formed between the driving pulley center and

belt initial contact point

= Road slope angle (in radians)

)( = CVT pulley radii function in terms of the angle


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LIST OF FIGURES

Figure 2.1 Toroidal CVT...9

Figure 2.2 Metal push belt CVT....9

Figure 2.3 Variable diameter belt CVT...10

Figure 2.4 Example of a commercial flat-belt CVT....11

Figure 2.5 Electric car diagram...14

Figure 2.6 Parallel hybrid diagram block14

Figure 2.7 Series hybrid diagram block..15

Figure 2.7 Series hybrid diagram block..16

Figure 2.8 Power-split mode...20

Figure 2.9 Differential transmission and CVT system for motorcycles..22

Figure 2.10 Transmission configuration diagram proposed by John Anderson23

Figure 2.11 CVT belt type.25

Figure 2.12 CVT control schematic..25

Figure 3.1 Improved fuel economy plot; showing torque vs. engine speed of a

100kW IC engine with CVT ideal line..27

Figure 3.2 Improved vehicle performance plot; tractive effort vs. vehicle speed

for a CVT and a five-speed manual gearbox.27

Figure 3.3 Power-split diagram...28

Figure 3.4 Continuously variable power-split transmission....29

Figure 3.5 Pulley system diagram...30

Figure 3.6 Force analysis diagram for the power split mode......31

Figure 3.7 Force analysis diagram for the power recirculation case-I....33

Figure 3.8 Force analysis diagram for the power recirculation case-II...34


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Figure 4.1 Hybrid-electric CVPST diagram....36

Figure 4.2 Force analysis diagram for the proposed hybrid-electric CVPST.38

Figure 4.3 Power analysis diagram.39

Figure 4.4 Electric mode.43

Figure 4.5 Reverse mode.43

Figure 4.6 Engine start (P-split) / Battery recharge mode...44

Figure 4.7 Engine power-split mode...45

Figure 4.8 Hybrid-electric/ Maximum power mode...46

Figure 4.9 Idle-neutral (Case I) / Recharge battery mode...48

Figure 4.10 Idle-neutral mode (Case II)....48

Figure 4.11 CVPST sketch with radii, gears and pulleys notation....49

Figure 5.1 Loads acting on a vehicle...54

Figure 5.2 (a) The LX5 DOHC V6 by General Motors;

(b) Power and torque plot for this engine..58

Figure 5.3 The Unique Mobility SR286..59

Figure 5.4 Engine speed vs. vehicle velocity plot...61

Figure 5.5 Variable pulley radii (CVT) vs. angle .......62

Figure 5.6 Angle for high and low speeds used for several plots...62

Figure 5.7 CVT ratio vs. angle....63

Figure 5.8 (a) Belt force vs. CVT ratio; (b) Belt force vs. angle....64

Figure 5.9 Driving resistance curves Torque vs. rpms...65

Figure 5.10 (a) Torque vs. CVT ratio; (b) Torque vs. angle .....66

Figure 5.11 (a) Power vs. CVT ratio; (b) Torque vs. angle ..68


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Figure 5.12 Acceleration vs. vehicle velocity...69

Figure 6.1 Basic block diagram...72

Figure 6.2 Control computer system configuration for the hybrid Toyota Prius

(a)Starting and traveling at low speed; (b) Full acceleration.73

Figure 6.3 Controller diagram proposed.75

Figure 6.4 Specific-fuel consumption map for a V-8 engine 300 in3..77

Figure 6.5 VTO vs. rpms ideal curve (line with dots) compared with

parabolic equation developed (black line).78

Figure 6.6 LabVIEW environment for the hybrid-electric CVPST

simulator for the SUV82

Figure 7.1 Hybrid-electric CVPST LabVIEW simulation for case I...85

Figure 7.2 Hybrid-electric CVPST LabVIEW simulation for case II......88

Figure 7.3 Hybrid-electric CVPST LabVIEW simulation for case III....90

Figure A.1 LabV IEW diagram code for the engine speed vs.

vehicle velocity plot...97

Figure A.2 LabVIEW diagram code for the driving resistance curves.98

Figure A.3 LabVIEW diagram code for the pulley radii, CVT ratio,

belt force, torque and power plots..99

Figure A.4 LabVIEW diagram code for the acceleration vs.

vehicle velocity plot.....100

Figure A.5 LabVIEW diagram code for the hybrid-electric

CVPST simulator.101


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LIST OF TABLES

Table 4.1 Operational modes for the hybrid-electric CVPST....41


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Chapter 1 Introduction

1.1 Overview

A car must meet certain minimum requirements, in order to be useful and reliable for the

customer. In general, a vehicle should be able to:

- Drive at least 300 miles between re-fueling

- Be refueled quickly and easily

- Be able to reach a velocity to keep up with the rest of the traffic on the road

Gasoline cars usually meet these requirements however some of them produce a

considerable amount of pollution and also gas mileage is nowadays an issue of concern.

Vehicle pollution is an important issue particularly in large cities. An electric car, on the

other hand, produces almost no pollution, but it can only go 50 to 100 miles between

charges. And the problem has been that it is very slow and inconvenient to recharge.

Usually drivers desire for quick acceleration causes cars to be much less efficient than

they could be. It can be noticed that a car with a less powerful engine gets better gas

mileage than an identical car with a more powerful engine.

This is because most of the cars only use a small percentage of their horsepower for

typical operations. In the scenario of driving along a flat road on the freeway at 60 mph,

the car engine has to provide the power for three things:

- Overcome the aerodynamic drag caused by pushing the car through the air.

- Overcome all of the friction in the car's components like the tires, transmission, axles

and brakes.

- Provide power for accessories such as air conditioning, power steering and headlights.


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For most cars, it takes less than 20 horsepower to provide for the aforementioned items.

So, the purpose of having the extra power is for what its called "floor it", which is the

only time were all the power is being used. The rest of the time, it uses considerably less

power than what is available.

Most cars require a relatively big engine to produce enough power to accelerate the car

quickly. In a small engine, however, the efficiency can be improved by using smaller,

lighter parts, by reducing the number of cylinders and by operating the engine closer to

its maximum load.

There are several reasons why smaller engines are more efficient than big ones:

- The big engine is heavier than the small engine, so the car uses extra energy every time

it accelerates or drives up a hill.

- The pistons and other internal components are heavier, requiring more energy each time

they go up and down in the cylinder.

- The displacement of the cylinders is larger; so more fuel is required to move them.

- Bigger engines usually have more cylinders and each cylinder uses fuel every time the

engine fires, even if the car is not moving.

This explains why two of the same model cars with different engines can get different

mileage. If both cars are driving along the freeway at the same speed, the one with the

smaller engine uses less energy. Both engines have to output the same amount of power

to drive the car, but the small engine uses less power to drive itself.

Therefore, using small engines near their maximum power limit will result in less fuel

consumption, and in addition, if a continuously variable transmission (CVT) is used, this


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can improve efficiency since there are not gearshifts, preventing the operation in the least

efficient regimes of the engine.

A CVT offers the potential of allowing the engine to operate near the optimum efficiency

curve throughout a continuous range of velocity ratios without disturbing the driver with

discrete shifts. A disadvantage faced nowadays using CVTs, is the fact that shows power

limitations since a belt is used instead of gears. This power limitation is the reason why

CVTs are only used in small cars.

The concept of a continuously variable power-split transmissions (CVPST) brought by

Mucino, V. et al. (1997) for automotive applications features the allowance of increasing

engine power while reducing the power losses associated with power transmission,

specially at low-speed-high torque modes, providing CVT ratio capability. The main

reason on the use of a CVPST with an electric-hybrid vehicle (HEV) is to increase the

power envelope capability for a given CVT element in the vehicle, which enables the

CVT concept for light duty applications such as pick up trucks and small buses.

Internal combustion (IC) engines are nearing perfection, engineers continue to explore

the outer limits of IC efficiency and performance, but advancements in fuel economy and

emissions have effectively stalled. While many IC vehicles meet low emissions

standards, these will give way to new stricter government regulations in big polluted

cities in the very near future. As additional options for improvement, automobile

manufacturers have begun development of alternative power vehicles. The use of CVTs

in the automotive industry is found mostly (99 %) in Japan and Europe, and just 1 % in

the U.S. including the Honda Civic and the Subaru Justy. The reason for low popularity is


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that the torque capacity has been restricted to the production of cars with a small

displacement engine, as it was stated before.

Currently, there are no CVT applications in light duty vehicles. A new conceptual idea

for a hybrid-electric CVPST is presented in this work, which includes analysis and

development of velocity and force equations. Also a LabVIEW simulation program and

a transmission control system was designed.


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1.2 Objective

Develop a new conceptual configuration for a hybrid-electric CVPST with special focus

in a sport utility vehicle (SUV). This design would be generalized for most of light duty

vehicle applications.

Another special issue is the replacement of the driver who is in charge of the gas pedal or

valve throttle opening (VTO) for a controller. This controller has the purpose of matching

the gas required (VTO) with the torque needed in the output and the velocity that the

operator (driver) requests.

One more objective is to show changes in the behavior of the system in an engine-

transmission model simulated in LabVIEW for different scenarios that a vehicle

normally faces.

Once all these objectives are accomplished, the design that is being proposed is expected

to deliver more power than needed for a normal light duty vehicle, even though a smaller

engine is to be used. Also improved efficiency is expected due to the fact that there is a

power-split; therefore less power flows through the variable transmission as compared to

normal CVT vehicles where all the power passes through it. At the same time, it is going

to improve gas consumption and subsequent reduction of emissions may potentially be

expected as the idea of the HEV is taken into account as previously described, with a

smaller engine for the application.


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Chapter 2 Relevant literature review

2.1 Continuously variable transmissions

A continuously variable transmission (CVT) as mentioned previously, provides a

continuously variable ratio between the power source (engine) and the output shaft

(wheels). CVTs offer the potential to take advantage of engine power and reach peak

efficiency without using shifting gears.

A century ago rubber v-belt transmissions were used in Benz and Daimler gasoline-

powered vehicles. One important development with this type of transmissions was the

Variomatic in 1965 (Hendrix et. al., 1988) and since 1975 the Volvo model 66 cars have

been equipped with this type of transmission (Volvo Co. website). As mentioned before,

a limitation in CVT applications is power capacity, that is why different kinds of CVTs

have been developed in the past two decades. The metal pushing block v-belt (MVB) was

one of the most important developments. It had received very good compactness, power

density values and had higher efficiency than other types of traction, hydraulic or electric

CVTs (Hendriks et al., 1988). This type of CVT has been used in more than one million

of Japanese and European automobiles since 1994. The problem faced with this shaft-to-

shaft CVT is that it does not offer enough torque capacity. This limitation in power,

compared with gear transmissions, is another reason why CVTs are not widely used.

In order to take advantage of the CVT and overcome its disadvantages, the belt CVT was

combined with a gear train (Lemmens, 1972-1974, Takayama et al. 1991). This

development is known as the continuously variable power split transmission (CVPST),

which combines a planetary gear train with a CVT v-belt (Cowan, 1992, Mucino et al.,


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1997). The most important idea of this configuration is that the power flowing through

the belt can be less than 50 % at low speeds and around 90 % at high speeds;

additionally, the power split allows the system to improve its efficiency by around 10 %.

This power-split capability expands CVT applications thus giving the opportunity for

CVTs to be used in bigger vehicles.

The metal pushing belt is currently used in CVTs for small engine vehicles. It is difficult

to manufacture and consequently is very expensive. One of the ideas of CVPST is to use

rubber belts in order to reduce the cost of the transmission.

Reviewing the paper work presented by Mucino et al. (1997), there are several important

issues involved with the design of a CVPST. In this process, it is appropriate to choose

parameters to allow the input power split, one part through the belt and the other one

through the planetary gear set.

Since the inception of CVTs in automotive industry, it had undergone several changes in

the past twenty years; the developments include gear, hydraulic, toroidal-traction, push

belt, variable diameter belt and flat/v-belt.

2.1.1 Types of CVTs

CVT systems can be categorized into six main types, according to its functionality and

way of operation.

o GEAR CVT

This kind of CVT provides the continuously variable speed ratio using gear trains.

Gives a wide range of speed outputs, as well as high torque at low speed and vice

versa. The gear CVT was created by Cook in 1975 and it was different than


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automatic transmissions because it required a very complex fluid logic control.

The power source, such as the engine, is connected through a clutch to the

transmission input shaft and this in conjunction with the intermediate shaft, drove

the output shaft. The highest gear ratio is reached at low speed and when the

output torque reduces to a certain point, all the power is given by the planetary

gear train (PGT). A different gear CVT was developed by Won (1989) where gear

ratio changed by a combination of floating and differential gearing. A fully geared

CVT was created by Epilogic Inc. (Fitz; Pires, 1991), which was used in electric

vehicles. A servo driven actuator controlled the ratio adjustment, which varied

linearly with the displacement of the control actuator.

o HYDRAULIC CVT

Since it uses a hydraulic motor, it can provide a continuously variable speed ratio

with its adjustment, which includes the amount of hydraulic fluid in the closed

circuit. In this kind of CVT, either the hydraulic pump or motor are of variable

displacement (Kawahara et al., 1990). This type of CVT is not used in automotive

applications because its space requirements: Moreover it is noisy, low efficient

and costly.

o TOROIDAL TRACTION CVT

These transmissions use the high shear strength of viscous fluids to transmit

torque between an input torus and an output torus. As the movable torus slides

linearly, the angle of a roller changes relative to shaft position, as seen in figure

2.1. This results in a change in the gear ratio.


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Figure 2.1: Toroidal CVT (Machida; Murakami, 2000)

o PUSH BELT CVT

This most common type of CVT uses segmented steel blocks stacked on a steel

ribbon, as shown in figure 2.2. This belt transmits power between two conical

pulleys or sheaves, one fixed and one movable. In essence a sensor reads the

engine output and then electronically increases or decreases the distance between

pulleys and thus the tension of the drive belt. The continuously changing distance

between the pulleys (their ratio to one another) is analogous to shifting gears.

Push-belt CVTs were first developed decades ago, but new advances in belt

design have recently drawn the attention of automakers worldwide.

Figure 2.2: Metal push belt CVT (Fujii et al., 1992,1993).

Input

Block

MoveableFixed

Ring

Block


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The standard pushing belt is applied in engine ranges from 550 cubic centimeter

to 1.2 liters (Hendriks et. al. 1988) usually used in small cars, however, is very

expensive due to the difficulty of manufacturing.

o VARIABLE DIAMETER FLAT/V BELT CVT

This type of CVT is represented in figure 2.3. The flat belt CVT uses two rotary

disk assemblies, one of them is driven with an input shaft and the other one drives

to an output shaft. All the power flows through the belt and the continuously

variable speed ratio is produced by the variable diameter with respect to the center

of each disk, which have a flat cross section like the belt. In the case of the v-belt

CVT, the two pulleys have a V cross-section and each pulley connects to a

conical pulley.

Figure 2.3: Variable diameter belt CVT (Vibrate Software web-page, 2003)

This transmission has the same fundamental operation as the flat and pushing belt

where one pulley remains fixed and the other one movable. However the pulleys

separate at high gear ratios and can lead to the problems with creep and slip that


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have plagued CVTs for years. This inherent defect has directed research and

development toward push belt CVTs.

Figure 2.4: Example of a commercial flat-belt CVT (Honda Multimatic, 1995).

The Honda Multimatic CVT shown in figure 2.4 consists of an oil-pressure

variable input and output pulley, and a metal belt that connects the two. With an

oil-system clutch on the "driven" side, the Multimatic acts as an automatic

transmission. The two pulley widths adjusted by oil pressure react to the position

of throttle, speed and other conditions. For instance, when the accelerator is

depressed the driving pulley width increases. At the same time the driven pulley

width decreases - the two combining for a "lower gear" effect. In addition, the

metal belt is highly flexible and accommodates the ever-changing width of the

pulleys and transfers power efficiently without any slippage.


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2.2 Hybrid-electric vehicles

An American engineer, H. Piper, filed for a patent on his hybrid vehicles in November

23rd

, 1905 (Wouk, Victor; 1997). His vehicle could accelerate to 25 mph in 10 seconds, a

full 20 seconds faster than the average half-minute of its contemporaries. Piper achieved

this by combining a gasoline engine with an electric motor, today recognized as a

standard hybrid configuration. Three and a half years later, Piper finally received his

patent but by this time engines had become powerful enough to achieve Pipers

performance on their own. Engine developments, along with equipment that allowed

them to be started without a crank, led to the decline of electric and hybrid vehicles

between 1910 and 1920. Fifty years went by before the oil crisis of the 1970s led to the

construction of several experimental hybrid vehicles. But it wasnt until the 1990s that

major work commenced on hybrid technologies. This was helped in the US by the

formation of the partnership for a new generation of vehicles (PNGV) consortium. It was

comprised of the "big three" car manufacturers along with about 350 smaller companies

and the aim being to develop a car capable of giving 80 miles per gallon of gasoline. This

efficiency has to be achieved without sacrificing performance or safety while emitting

around one eighth of the pollutants of conventional vehicles and not costing significantly

more. The PNGV has not specified the type of vehicle powertrain that is to meet their

requirements but the IC engine/electric motor hybrid is the most likely ahead of fuel cell

and flywheel alternatives.

The two main objectives of this kind of vehicles are fuel consumption improvement and

emissions reduction. Currently Honda and Toyota have this technology and both


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Figure 2.5: Electric car diagram.

A gasoline-powered car uses an engine as the power source of the automobile and needs

fuel in order to be functional. An electric car uses a motor as the power source and

batteries as the energy storage supply, which usually provides for a range between 50 and

100 miles.

The two power sources found in a hybrid car can be combined in different ways. One

way known as a parallel hybrid has a fuel tank, which supplies gasoline to the engine, but

it also has a set of batteries that supplies power to an electric motor. Both the engine and

the electric motor can turn the transmission at the same time and finally the transmission

turns the wheels.

Figure 2.6: Parallel hybrid diagram block.


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Figure 2.6 shows a typical parallel hybrid diagram block. You can notice that the electric

motor and gas engine connect to the transmission. As a result in a parallel hybrid, both

the electric motor and the gas engine can provide propulsion power.

By contrast, in a series hybrid configuration (figure 2.7) the gasoline engine turns an

alternator and the generator can either charge the batteries or power an electric motor that

drives the transmission. Thus, the gasoline engine never directly powers the vehicle.

In the diagram shown in figure 2.7 of the series hybrid, it can be seen that all of the

components form a line that eventually connect with the transmission.

Figure 2.7: Series hybrid diagram block.

Finally, having a power-split box these two arrangements can be combined and have a

configuration that could work in either series or parallel. This configuration is known as

the series-parallel configuration and it has as stated previously a power-split box, which

allows the engine to either supply full power to the alternator or the transmission. This

configuration is shown on figure 2.8.


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Figure 2.8: Series-parallel hybrid diagram block.

Hybrid-electric cars contain the following parts:

Gasoline engine: The hybrid car has a gasoline engine just like the ones we can

find on most cars. However the engine on a hybrid-vehicle will be smaller and

will use advanced technologies to reduce emissions and increase efficiency.

Fuel tank: The fuel tank in a hybrid is the energy storage device for the gasoline

engine. Gasoline has much higher energy than batteries. For example, it takes

about 1 000 pounds of batteries to store as much energy as 1 gallon of gasoline.

Electric motor: The electric motor on a hybrid car is very sophisticated. Some

advanced electronics allow it to act as a motor as well as a generator. For

example, when it needs to, it can draw energy from the batteries to accelerate the

car. But acting as a generator it can slow the car down and return energy to the

batteries.

Generator: The generator produces electrical power for the batteries.


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Batteries: The batteries in a hybrid car are the energy storage device for the

electric motor. It can provide energy into the batteries as well as draw energy

from them.

Transmission: The transmission on a hybrid car performs the same basic

function as the transmission on a conventional car. Currently automatic and

manual transmissions are being used.


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2.3 Power-split technology

Power-split transmissions use differentials or PGTs in combination with variable

elements, such as a CVT. The power is split and a fraction of the power flows through the

CVT element to the differential (or PGT) while the remaining power circulates directly

through the differential. Power-split transmissions have the advantage of transferring

power that is greater than the capacity of the variable element, which is the limiting

reason in CVT applications. This kind of technology has been developed for automotive

applications using metal push belts and hydrostatic drives for the variable element. It has

also been used in farm tractors.

In 1972 Lemmens described the combination of a PGT with a v-belt CVT in a

transmission. The input shaft rotates the v-belt and the chain drive; this last one transmits

rotation to the planetary carrier while the v-belt transmits power to the sun gear. The

output shaft is connected to the ring gear. Most of the power flows through the PGT

while the CVT it is used to control the speed of the sun gear and at the same time to get a

variable range of speeds.

Lemmens improved his invention in 1974 in order to provide an automatic CVT where

the only setting required was neutral, forward and reverse. This arrangement can have a

non-desirable configuration known as power recirculation, which is not a real power-split

system. Takayama et al. in 1989 presented a power-split transmission, which consisted of

a CVT with a v-belt and a two-way differential clutch. Most of the power is transmitted

to the belt while the rest goes through the two-way differential clutch. When the CVT

reaches the maximum gear ratio, the output speed in the two-way differential clutch

becomes slower than the output gear where as the output pulley shaft gains the power


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2.4 Power-split hybrid-electric vehicles

A power-split HEV has been already developed in many different configurations. Recent

work has been done on the future truck competition for the Society of Automotive

Engineers (SAE) with the purpose of creating a SUV to be green and efficient with

performance, utility and affordability that customers expect. The vehicle presented by

Michigan Technological University (MTU) has come first in this competition as they

converted a production SUV to a HEV. The HEV drive system utilized a planetary

power-split transmission, which had the ability to couple the advantages of a parallel-

hybrid with the advantages of a series hybrid. The drive system consists of a planetary

gear set coupled to an alternator, motor and an IC engine and performs the power-split

operation without the need of belt drives or clutching devices.

Figure 2.8: Power-split mode (Beard, John et al.; MTU Future Truck; 2001)


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The system operates in four operational states and its configuration is shown in figure

2.8. When the engine is off and the electric motor alone propels the vehicle and

regenerates braking power it is called the electric mode because the engine is left off line.

The second mode is the engine start mode where the transition from electric to a hybrid

vehicle takes place. In order to bring the engine up to operating speed, the alternator

applies a torque in the forward direction of rotation. When the speed of the planet carrier

becomes faster than the ring gear, the alternator stops providing power and begins to

extract some of the engine power and at this point reaches the power-split mode. As the

vehicle speed increases, the amount of power being transferred through the ring gear and

chain drive increases with the speed of the ring gear. At highways where speed and load

is constant, the engine power required decreases allowing the alternator speed to decrease

and when it reaches zero we get to the final operational type called parallel mode. In this

mode all the power is transferred via the ring gear, as opposed to using the recirculation

loop made up of the alternator and motor, which has increased losses in energy

conversion. These operational states met fuel efficiency and low emissions that SUV

consumers expected and also maintained its manufacture feasible.

This thesis has the goal of increasing power so that it can be used in bigger vehicles such

as small buses and pick up trucks, and instead of using a chain drive it will be used a v-

belt CVT.

Kuen-Bao and Shen-Tarng (1999) have done some work on automatic hybrid

transmissions for motorcycles using CVTs where they presented a systematic approach in

designing an automatic transmission including a conceptual and kinematic design,

efficiency analysis, engine and transmission matching. Their basic concept is to combine


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a stepped and a stepless design into a hybrid transmission system. Four hybrid

transmission systems consisting of two degrees of freedom PGTs and rubber v-belt drive

units were synthesized. They also performed kinematic analysis and a kinematic design

of the hybrid transmission to obtain the range of the relative speed ratio of the differential

gear and provided the relationship between the major dimensions. In their results, the

transmission efficiency is not better than that of the existing CVT since the parts of the

prototype were roughly fabricated; nevertheless the overall results showed that their

design was theoretically correct and practically feasible.

Figure 2.9 shows the differential transmission with a CVT system used in their work.

Figure 2.9: Differential transmission and CVT system for motorcycles

(Kuen-Bao; Shen-Tarng; 1999).


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John Andersons thesis work (1999) also proposes a CVT transmission combined with a

HEV for the design and model of a torque and speed control transmission.

His study was conducted to determine the feasibility of creating a multiple input/output

transmission. This transmission, which was the torque and speed control transmission, is

combination of a PGT and a CVT. He studied six different operating modes for these

transmission technologies. These six modes are conventional, electric vehicle, series

HEV, parallel HEV, variate 1, parallel HEV, variate 2, and geared neutral. Each of these

modes has specific efficiency benefits during vehicle operation. This type of operation

allows for a transmission to be significantly more flexible than current automotive

transmissions. The engine can send power to the drive wheels directly through the PGT

and the CVT simultaneously. The results of his study showed a transmission that is

capable of efficient operation under a wide variety of circumstances.

Figure 2.10: CVT configuration diagram proposed by John Anderson (1999).


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From figure 2.10 it can be observed that this configuration requires all engine power from

the engine to be transmitted through the CVT. Variation of the CVT ratio allows the

overall range of gear ratios to match the engine output power required at the wheels. The

motor speed is prescribed by the engine speed and the CVT ratio total power output is a

function of engine speed, engine torque and motor torque.

2.5 Continuously variable transmission controller

The first electronic control system appeared on the market in the early seventies. These

were simple transistor control units for actuating in and/or out valves and were capable

only of determining shift points. At the beginning of the eighties first microcomputer

controls were presented by Y. Taga. The first transmission with fully electronic

microprocessor control of all major functions appeared on the market in 1983. This was

the ZFs 4 HP-22, a 4 speed automatic transmission with hydrodynamic torque converter

and lock-up clutch for vehicles with rear wheel drive. The electronic control function was

performed by a microcomputer enabling substantial functional improvements to be

achieved.

Of the numerous CVT concepts mechanical belt/chain type pulley-drive CVTs are the

most advanced and have a small scale of production. The offset input and output shafts in

their design makes them very compact and therefore ideal for front-wheel drive vehicles

with limited installation space, although their overall power ratio is less than a planetary

transmission.


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Chapter 3 CVPST Concepts and kinematic models

3.1 CVTs and power-split concepts

In recent years it has become important to decrease fuel consumption in motor vehicles

and get the best vehicle performance possible in a given engine. Both of these issues can

be improved by using a CVT as stated in previous chapters. To take full advantage of

such a transmission, two requirements must be fulfilled:

a) The efficiency of the CVT must be fairly high.

b) Its speed ratio range must be large enough to allow the engine to operate as

efficiently as possible.

Most CVT designs do not fulfill both of these requirements; however this can be

improved by a split-power CVT.

Figure 3.1 shows the typical characteristic of torque vs. speed of an IC engine. Level

lines for equal specific fuel consumption are included as well as hyperbolas for equal

engine power. The dashed line marks the most efficient combinations of torque and speed

that produce the required power determined by the speed and acceleration of the vehicle.

Using a CVT the ideal line can be followed by always choosing a proper speed ratio.

Thus the engine can operate at the most favorable speed for a certain power, which is

independent from vehicles speed. The continuous change between different speed ratios

provides a smooth jerk-free driving.


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Figure 3.1: Improved fuel economy plot; showing torque vs. engine speed

of a 100kW IC engine with CVT ideal line (Hedman, 1992).

Figure 3.2: Improved vehicle performance plot; tractive effort vs. vehicle speed for a

CVT and a five-speed manual gearbox (Mattsson, 1993).


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Figure 3.2 shows theoretically how the maximum engine power can be delivered to the

driving wheels at all vehicle speeds; this gives improved vehicle performance as

compared with using a manual gearbox with a finite number of discrete speed ratios. In

practice, the performance improvement obtained from the maximum engine torque is

smaller than in the loss-free case because a CVT generally has larger losses than a

manual transmission. In some situations it is necessary to limit the engine power due to

CVT restrictions.

Combining a PGT with a CVT the power can be split into two branches as shown on

figure 3.3. This kind of transmission is referred as the continuously variable power-split

transmission (CVPST), where the power through the variator is aimed to be less than the

input power.

Figure 3.3: Power-split diagram (Mattsson, 1993).

The CVPST for automotive applications presented by Mucino (1997) consists of a pulley

set (variator) coupling two of the three rotating elements of a PGT (sun and ring gears).

The power-split appears when the input shaft delivers power to the sun gear as well as to

the driving pulley, which in turn drives the ring gear of the PGT through the variator.

Variator

Planetary gear arrangement


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Since the input shaft delivers power in two directions, the variator carries only a fraction

of the total power flowing through the input shaft and the planetary gear set collects the

power flowing from the ring and the sun gear delivering the total output power. Figure

3.4 shows the diagram of the CVPST for automotive applications proposed by Mucino et

al. (1997).

Figure 3.4: Continuously variable power-split transmission (Mucino et al., 1997).

Some important gear ratio relationships that should be consider from the previous figure

are:

ri

s

gr

r= ,

ro

o

gcr

r= ,

ps

po

cr

r= (3. 1)


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Figure 3.6: Force analysis diagram for the power-split mode

Where

F1 = Belt force from pulley 1 to 2


F3 = Force from the counter shaft gear to the idler gear

F4 = Force from the control gear to the idler gear

F5 = Force from the idler gear to the counter shaft gear

F6 = Force from the idler gear to the control gear

F7 = Force from the planet gear to the sun gear

F8 = Force from the planet gear to the ring gear

F9 = Force from the sun gear to the planer gear

F10 = Force from the ring gear to the planet gear

F11 = Force from the output shaft to the planet gear

F12 = Force from the planet gear to the output shaft


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Power recirculation case-I:

This is a non-desirable case known as the power recirculating transmission (first mode).

Compared to the power-split case there is no idler gear, which changes the direction in

the control gear. The input power comes from the engine (HP) and again splits, a

percentage goes through the sun gear and the other part goes to the CVT and again the

part that goes through the belt is going to lose some efficiency. That power goes directly

to the counter shaft passing through the control gear. This power is added to the one

coming from the sun gear flowing through the planet gear, which transmits the total

power to the output shaft. As shown in figure 3.7 since there is no idler gear in this

system the direction in the control gear can either rotate in the opposite direction or give

no output power.

Figure 3.7: Force analysis diagram for the power recirculation case-I.

Where




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F3 = Force from the counter shaft gear to the control gear

F4 = Force from the control gear to the counter shaft gear







Power recirculation case-II:

The third and final possible case in a CVPST is where the power is recirculating through

the system and the power flowing through the CVT can be higher than the input power.

Since the power is recirculating through the variator and the PGT, the input power

instead of splitting power through the CVT is adding power to the recirculating power.

Also the direction of the output shaft is reversed, which is not desirable. Figure 3.8 shows

a force diagram for this case.

Figure 3.8: Force analysis diagram for the power recirculation case-II.


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Where



F3 = Force from the counter shaft gear to the control gear

F4 = Force from the control gear to the counter shaft gear








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Chapter 4 Hybrid-electric power-split CVT

conceptual design

4.1 Design proposed

This chapters objective is to show a conceptual design for a new hybrid-electric CVPST

configuration, power analysis and the development of velocity and force equations. This

design will allow the possibility to use a small IC engine which in addition to an electric

motor will give a higher or at least the same amount of output torque in a more efficient

way compared to nowadays light duty vehicles.

The benefit of this design is the opportunity to show a configuration in which a CVPST

can be used for light duty applications and at the same time improve efficiency, fuel

consumption and help reduce pollution.

Figure 4.1: Hybrid-electric CVPST diagram.


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Figure 4.1 shows the proposed system where the electric motor/generator is connected to

the countershaft and can either give variable torque or not. The engine has a constant

input torque and it is controlled by a clutch for idle modes. This torque given by the

engine is split, an amount goes through the belt and the other amount goes directly to the

sun gear. The amount of power flowing through the belt connects to a pulley rotating the

countershaft. Depending on the circumstances, the electric motor (where its input shaft is

connected to the countershaft) can either add power or not. The power flowing through

the countershaft passes to an idler gear used in order to have the control gear rotate in the

same direction as the input power shafts. This control gear is directly connected to the

ring gear of the PGT. This power in the ring gear is added to the one from the sun gear

and gives the output power to the planet gear, which connects to the differential shaft.

Force analysis:

The following force analysis diagram (figure 4.2) shows that the system is always going

to be in a power-split mode since the output velocity direction of the ring gear and the

sun ring are always in the same direction. This way it can be assured that the non-

desirable modes (power recirculation, high power circulation through the belt, etc.) are

not going to be present. On the other hand many clutches and brakes have to be placed

into the system in order to accomplish common vehicle performances, i.e. driving up hills

(high torque required), highways (constant velocity, low torque required), highway pass

(high torque reached in minimum amount of time), in the city (variable velocity, many

stops, variable torque needed), etc. In order to achieve all these states of operation, a

clutch is needed to decouple the IC engine another one for the electric motor, one more


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for the output shaft and finally one for the countershaft gear that connects to the idler

gear. These clutches are shown with more detail in figure 4.3.

Figure 4.2: Force analysis diagram for the proposed hybrid-electric CVPST

Where

F

1= Belt force from pulley 1 to 2


F3 = Force from the counter shaft gear to the idler gear

F4 = Force from the control gear to the idler gear

F5 = Force from the idler gear to the counter shaft gear

F6 = Force from the idler gear to the control gear






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Figure 4.4: Electric mode

STAGE 2: REVERSE MODE

The second stage is the reverse mode, which operates just as the electric mode but with

the difference that the electric motor operates in the reverse direction giving an opposite

output direction, as shown in figure 4.5.

Figure 4.5: Reverse mode

In this case, as the previous one, the engine is left off-line and all the power is supplied

by the electric motor flowing through the countershaft to the ring gear, planet gear and


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shaft (differential). In this mode the maximum amount of power is provided because the

two power sources are running.

Figure 4.8 shows that all the clutches are engaged and both brakes are released since the

maximum amount of power is provided and both input powers sources are running.

Either gear 1 or gear 2 can be activated depending on the torque required by the vehicle.

The amount of power given by the electric motor is limited by the energy that the battery

pack can provide, when it runs out of energy the system can either go back to stage 3 for

recharging batteries or stage 4 letting the engine run by itself. The stage chosen depends

on the amount of power required and can be switched eventually.

STAGE 6: IDLE-NEUTRAL / RECHARGE BATTERY MODE

The final stage is the idle or neutral mode, which can operate in two different ways. The

first possibility is recharging batteries by having the engine operating by itself. So while

the vehicle remains stopped the engine provides power to the electric motor-generator for

recharging batteries. In this case, as shown in figure 4.9, all the power provided by the IC

engine flows through the variator and supplies energy to the electric motor-generator.

Clutches A, B and C remain engaged since all the power goes to the generator; and

clutches D and E are disengaged giving a result of zero output power and velocity. Break

2 is activated stopping the ring gear, while break 1 is not activated and allows the ring

gear to spin free.

The second possibility for this idle-neutral mode is when the electric motor is running.

This case may be necessary at some point while the vehicle operates in stage 1 (electric

mode). The diagram for this second case is shown in figure 4.10.


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Figure 4.9: Idle-neutral (case I) / Recharge battery mode.

Figure 4.10: Idle-neutral mode (case II).

Here the electric motor spins freely without providing power. Clutches A, C, D and E are

disengaged letting the output shaft without any velocity. Brakes 1 and 2 are activated

stopping the ring and sun gear. Therefore just clutch B is engaged and prepared to switch

to an electric mode if clutch D and E are connected.

Comparing these two possibilities of idle-neutral mode, the first one is the more desirable

since the battery pack is being recharged.


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The characteristic parameters, which define the geometric configuration, involve the

overall radii of four gears and the pitch radius of the variable pulleys. Two clutches are

shown in the previous figure that illustrates the connection between the transmission and

the power sources (engine and electric motor). In this arrangement there are five gears

(sun, planet, ring, control and countershaft) whose radii can be used to express the input-

output velocity relationship. Consequently, the ratios are established by the following

(previously defined by Mucino et al.)

in

c

a

c

c

==

d

f

gc

=

r

p

g

= (4. 1)

rearranging this relationships can be written as

cincac == ; gcdf = (4. 2)

since dc = (4. 3)

then gccingccf == (4. 4)

for the angular velocity on the ring and sun gear

gccinfr == and ins = (4. 5)

using the general equation for planetary gear trains

g

p

r

sr

sp

N

N

==

(4. 6)

where rN and sN are the number of teeth for the ring and sun gears respectively

rearranging

)( rsgsp = (4. 7)


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and substituting ins = and gccinr = can be written as

)( gccininginp = (4. 8)

then it is possible to find a velocity relationship between the input and the output shafts

)1(1 gccgin

out

+= (4. 9)

and one between the output and countershaft using gcdr = on the previous equations

resulting in

)1

(1

gc

c

g

cd

out

cs

out

+== (4. 10)

The variable pulley relationships were previously shown in chapter 3.1.

4.4 Force equations

Referring to figure 4.2 with the force analysis diagram for the hybrid-electric CVPST a

useful relationship can be derived for the belt force based on the ratios described on the

previous section.

First the force equilibrium equations are developed

spspsbinrFrFT /+= (4. 11)

pprpsppout rFFrFT )( // +== (4. 12)

Where psF/ is the force on the sun gear by the planet gear and prF/ is the force on the

ring gear by the planet gear.

Deriving equation (4.12) it can be found that

ps

spsin

br

rFTF

/= (4. 13)

where bF is the force by the belt.


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From equation (4.12)

pr

p

out

ps Fr

TF // = (4. 14)

and for prF/

i

ro

ics

i

ro

ictrprr

rF

r

rFF /// == (4. 15)

since icsictr FF // = .

Where ictrF / is the force on the control gear by the idler gear and icsF / is the force on the

countershaft gear by the idler.

From equilibrium forces on the countershaft the following equation can be developed

o

pobem

icsr

rFTF

+=/ (4. 16)

Here emT is the torque generated by the electric motor.

Combining equations (4.15) and (4.16)

i

ro

o

pobem

prr

r

r

rFTF

)(/

+= (4. 17)


+=

i

ro

o

pobem

p

out

psr

r

r

rFT

r

TF

)(/ (4. 18)


s

i

ro

o

pobem

p

out

inpsb rr

r

r

rFT

r

TTrF

+=

)((4. 19)


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Chapter 5 Example conceptual design

5.1 Vehicle considerations

Many considerations have to be taken into account for the analysis of the system

designed. A vehicle consists of many components distributed within its exterior envelope.

For braking, acceleration and its most turning analysis the whole vehicle is often treated

as a lumped mass, excepting the wheels. Only the forward and longitudinal motions are

sufficient to be considered for driving test purposes and for the vehicle used for this

study.

DYNAMIC LOADS

The dynamic loads acting on a vehicle are traction forces, which push the vehicle to

move through the x direction and the rest of the forces acting in that direction are the

resistance forces. These forces are shown in figure 5.1.

Figure 5.1: Loads acting on a vehicle (Gillespie, 1992).

The traction force is the one coming from the engine and its transferred to the wheels

through the power train. The output torque given by the engine varies accordingly to the


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direction. From all of them, air resistance is the most considerable and important

aerodynamic force. This air resistance coefficient varies depending on the shape of the

vehicle and it is defined as (Gillespie, 1992):

ACVD DA2

2

1= (5. 2)

Where is the air density, A the frontal area of the vehicle and DC the drag coefficient.

Another important dynamic load is the rolling resistance, which is the total resistance

force from the wheels. In calculations the dynamic weight of the vehicle including the

effects of acceleration, trailer towing forces and the vertical component of air resistance

should be considered. When calculating rolling resistance a coefficient must be defined.

This coefficient rf is a factor that reflects the effects of the complicated and

interdependent physical properties faced by the tire and the ground, which are the

temperature, pressure load, vehicle velocity, tire material, slip, etc. At lower speeds the

coefficient increases almost linearly with speed. This coefficient is defined as (Gillespie,

1992):

)/160

1(01.0hkm

Vfr += (5. 3)

Where V is in km/h and fr is dimensionless and the rolling resistance is defined as

(Gillespie, 1992):

WfRRR rxrxfx =+= (5. 4)

where W andRx are in kg.m/s2

or Newtons.


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POWER AND ACCELERATION EQUATIONS

In the case of the acceleration of a vehicle two limitations can be present, engine power

and traction on the drive wheels, which may depend on vehicle speed. For low speed tire

traction may be a limiting factor, while at high speeds engine power may be considered

as the limiting factor. Limitations in power involve examination of the engine

characteristics and the power train.

Engines can be characterized by the torque and power curves as a function of speed.

Power and torque are related by speed. That is:

Power (HP) = Torque (lb-ft) x speed (rpms) (5. 5)

The ratio between engine power and vehicle weight is an important consideration for

acceleration performance. Taking into account the velocity, gravitational speed, engine

horsepower and the vehicles weight, an acceleration equation can be developed from

second Newtons law.

W

P

V

g

M

Fa xx 550== )sec/(

2ft (5. 6)

For a more exact performance it requires the modeling of mechanical systems in which

engine transmits power to the ground. The actual torque delivered to the drivetrain is

reduced by the amount required to accelerate inertia of the rotating components.

Considering the mass of the vehicle (M), the rolling resistance (Rx), the tractive force at

the ground (Fx), the aerodynamic drag forces (DA) and the hitch (towing) forces (Rhx), an

equation can be developed.

sin)( WRDRr

NTa

g

WWaMM hxAx

tftferr =

+=+ (5. 7)


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Where rM stands for the equivalent mass of the rotating components, tfN the ratio of

transmission and final drive, tf the combined efficiency of transmission and final drive

and is the road slope angle (radians).

VEHICLE CONSIDERED

As an example application for this thesis the Michigan Technological University (MTU)

HEV used for the future truck competition was considered. Its dimensions were used for

the calculations on the program created in LabVIEW. This SUV has the body of a

Chevrolet Suburban model 2000-2001 and it was considered because it is an example of a

light duty application and is one of the most popular vehicles nowadays in America. An

IC engine LX5 DOHC V6 by General Motors was selected and is shown in figure 5.2.

(a) (b)

Figure 5.2: (a) The LX5 DOHC V6 by General Motors; (b) Power and

torque plot for this engine (GMs website, 2003)


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This engine uses gasoline as fuel; it has a capacity of 215 HP (160 kW) and weighs 375

lbs. (168 kg). Based on the well-known Cadillac NorthStar V8, the GM LX5 was

introduced in 1999 Oldsmobile Intrigue and remained the OEM (Original Equipment

Manufacture) engine for the Intrigue in 2000 and 2001. The under-square bore-to-square

ratio exhibited by the LX5 provides high thermal efficiency and strong low-end torque,

while four valves per cylinder provide good top-end aspiration. It has a 9.3:1

compression ratio and is near optimum for regular octane fuel and when combined with

cam profile and intake tuning it enables the engine to produce an amazingly flat torque

curve with 90 % of maximum torque available from 1600 to 5600 rpm. The resulting

power curve is nearly linear and makes the engine particularly predictable and easy to

control in a power-split configuration.

The electric motor considered (figure 5.3) was a pair of Unique Mobility SR286, which

also works as a generator. It weighs 102 kg (225 lbs.), is 336 mm. (13.2 in) long and its

water-cooled. Its power capacity is 130 kW (175 hp).

Figure 5.3: The Unique Mobility SR286 (HMMWV PEIs website, 2003)


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5.2 LabVIEW plots

LabVIEW version 6i software is a powerful instrumentation and analysis programming

language for PCs. It uses icons instead of lines of text to create applications. In contrast to

text-based programming languages where instructions determine program execution,

LabVIEW uses dataflow programming where data determine execution. LabVIEW

can generate charts, graphs and customized used-defined graphics in a friendly way.

First a used interface is built by using a set of tools and objects. The user interface is

known as the front panel. Code is added using graphical representations of functions to

control the front panel objects. The block diagram contains this code. If organized

properly, the block diagram resembles a flowchart.

For this thesis purpose, a LabVIEW program was created, which can be used to design

the PGT and CVT as input values can be changed. It can also be used to see plot results

on different data such as velocity, pulley variable radii, belt force behavior, torque and

power.

1. Engine speed vs. vehicle velocity:

Figure 5.4 shows the CVT gear ratio range in an engine speed (rpms) with respect to

vehicle velocity (km/h) plot. The inputs needed are the high and low CVT ratios, the

differential ratio and the wheel radius. The area between both lines represents the

variable ratios that the CVT follows. The highest ratio slope is shown with a black

line and the lowest ratio with the red one.

The LabVIEW code that generates the previous plot can be found in the appendix at

the end of the thesis.


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Figure 5.4: Engine speed vs. vehicle velocity plot.

According to the input data for the vehicle considered (SUV) another program was

developed for the plots of the variable pulley radii, belt force, torque and power data.

This program shows the following useful plots.

2. Variable pulley radii vs. driving pulley contact angle:

On the left side of figure 5.5 the input data boxes are shown, which depend on the

capacity of the IC engine, electric motor, gear ratios, distance between pulleys and the

length of the belt.

The plot shows the values for the variable pulley radii with respect to angle , which

is the angle between the x-axis of the driving pulley and the line from the center to

the contact point between the pulley and the belt, as shown in figure 5.6. This angle is

changing depending on the CVT ratio.


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Figure 5.5: Variable pulley radii (CVT) vs. angle .

In figure 5.5 Rps= rps represents the driving pulley radius (black line), while Rpo= rpo

represents the driven pulley radius (red line). It can be seen that the crossover takes

place at 90 where the radii are equal. These radii are given in inches.

Figure 5.6: Angle for high and low speeds used for several plots.

=

=

High speed Low speed


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4. Belt force vs. CVT ratio and driving pulley contact angle:

The following plots in figure 5.8 show the reaction force by the belt, this is useful in

terms of design in assuring that the belt used for the CVT is going to handle the

maximum amount of force. The first plot shows the relationship between the belt

force (lb) and the CVT ratio, while the second one is between the belt force and the

driving pulley contact angle .

It can be seen in both plots that the maximum amount of force is when the CVT ratio

is high and the angle is low, this means that the vehicles speed is either zero or in

low speed. While the vehicle gains speed the belt force decreases and less amount of

power flows through the belt, which makes the system even more efficient.

(a) (b)

Figure 5.8: (a) Belt force vs. CVT ratio; (b) Belt force vs. angle.

5. Driving resistance curves Torque vs. rpms:

It has to be taken into account the driving resistance loads as described at the

beginning of the chapter. A small program was developed which shows the resistance


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(a) (b)

Figure 5.11: (a) Power vs. CVT ratio; (b) Torque vs. angle .

8. Acceleration vs. vehicle velocity:

The plot for the acceleration range for the CVT ratio vs. vehicles velocity is shown

in figure 5.12. The black line represents the highest CVT ratio and the red line is for

the lowest ratio. There are many vehicle-input considerations that have to be taken

into account such as the wheel radii, the vehicles weight, frontal area, specific weight

and the environment temperature. All this inputs depend on the vehicle considered

and are easy to determine. As it is shown in figure 5.12 the acceleration decreases as

the vehicle gains speed. When constant velocity is reached the acceleration is almost

zero and that is what happens in a vehicle. Mostly, acceleration is needed in order to

gain speed. This can vary according to driving conditions. If suddenly acceleration is

needed, for instance when passing a car in an interstate highway, this plot would look


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different. If the value for a particular CVT ratio wants to be known, the values for

highest of lowest ratio can be changed and the correspondent line would show the

plot for that particular ratio.

Figure 5.12: Acceleration vs. vehicle velocity.

This program can be connected to the CVT ratio controller and a single line would

show how the vehicle acceleration behaves. This plot shows optimal conditions for

the SUV considered.


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All LabVIEW codes and programming icons for these plots are shown in the appendix

at the end of this thesis. There are four code diagrams the first one is for the engine vs.

speed plot, the second one for the driving resistance curves, the third one includes the

pulley radii, CVT ratio, belt force, torque and power plots and the last diagram code is for

the acceleration vs. velocity plot. The inputs can be changed for any case giving the

opportunity to be a useful tool for design.


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Chapter 6 Control simulation

From the past demands concerning gearshift comfort, drivability and the need for

interaction between transmission and other vehicle systems provide the reason for

introducing electronic control systems for transmissions. The standard functions of such

systems have proven their worth and contributed towards satisfying these demands. For

this reason, despite the additional cost, most CVTs require to be computer-electronically

controlled.

The use of computer-electronically controlled systems in the driveline has increased

rapidly. Apart from engine management and brake systems, electronic transmission

control systems are the subject of intensive development work. This is not only with the

objective of improving comfort and drivability but also for reducing fuel consumption

and at the same time increasing efficiency.

LabVIEW software version 6i (as in chapter five for the plots) was used for the

development of a vehicle-transmission simulation program, which shows how the control

system proposed would work.

6.1 Control method proposed

The aim of this control system besides improving comfort and drivability is the

replacement of the driver who is in charge of the gas pedal (VTO) for a controller. This

controller has the purpose of following as close as possible the ideal operating schedule

of the transmission in the fuel consumption map. Obviously this will reduce fuel

consumption and at the same time will increase efficiency.


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is operating in the vehicle. Two controllers are going to be needed and it is explained

with more detail in the next subchapter.

6.2 Controllers needed in the design

The control system for a hybrid-vehicle is complicated. Implementation of the control

hardware is important for reliable execution of the control strategy proposed previously,

and for the normal operation of the vehicle. Two different types of controllers are needed

for the conceptual hybrid-electric CVPST configuration proposed in this work.

Hybrid-electric modes of operation controller:

This control system has been developed already by many of the HEV automotive

companies in the market, just to mention a few the Toyota Prius and the Honda Civic.

(a) (b)

Figure 6.2: Control computer system configuration for the hybrid Toyota Prius

(a)Starting and traveling at low speed (b) Full acceleration

(Toyota Motor Sales Inc., USA; Romans, Brent; 2000).

In the case of the Toyota Prius it has an Advanced Control System (ACS) that

monitors and controls the engine, generator, electric motor and battery pack.


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The ACS acts as a sophisticated control center that makes important decisions like

whether to have the engine on or off, whether to have the generator charge the battery

or whether to have the motor drive the wheels or store energy during braking.

Figure 6.2 shows two operational cases of the control system for the Toyota Prius. A

controller like this one is going to be necessary, which as it was just explained, will

be a smart machine and will decide when each component will be operating and will

allow to have the different modes of operation shown in chapter 4. Since many of this

type of controllers have already been developed and it is not the objective of this

work, a control system for this purpose was not proposed in this thesis. However it is

important to notice that is needed. This controller would give as an output data the

electric motor and engine powers for the second controller.

Transmission and optimum VTO controller:

The second controller needed for the hybrid-electric CVPST is proposed in this work.

This new concept is for the control of the VTO and the CVT ratio, which will solve

the problem addressed at the beginning of this thesis regarding the inefficient way of

drivers operating their vehicles.

The diagram shown in figure 6.3 explains graphically how this controller would

operate. It will have two inputs; the torque in the output shaft coming from a sensor

and the speed set by the driver, which will come from the accelerator pedal. The

controller will determine the driving resistance torque from the sensor in the output

shaft and according to the speed wanted it will determine how much torque is

required to reach that speed in the least amount of time in the most efficient way.


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Figure 6.3: Controller diagram proposed

Accordingly to the torque needed the controller changes the CVT ratio smoothly as it

gains speed following the torque-CVT ratio curve. The CVT ratio and the torque

supplied by the two sources are changed so the best efficient path in the fuel

consumption map can be followed. Finally this controller will allow the VTO to have

the smallest opening and will give an output signal to the IC engines throttle. The

second output signal would be the CVT ratio given directly to a hydraulic control

valve, which will be changing the diameters of the pulleys dependently.

The way of operation of this control system will be shown in a LabVIEW simulator

in the next subchapter and chapter 7. It will show how the variable pulleys of the

CVT, driving resistance torque, belt force, velocity relationships, VTO and output

power are changing.


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6.3 LabVIEW simulation

In a typical application the driver controls the VTO and the transmission ratio (with the

shift stick). The vehicle responds with acceleration, which depends on the excess

torque availability (from the engine) and the driving resistance from the road. Typically

in an acceleration maneuver the gas pedal is maintained at a high constant VTO level

(say 85 %) except when clutching and the operator controls when to shift gears.

Anecdotal training makes drivers shift when no further speed gain (acceleration) is

possible. In the case of a CVT operated vehicle the controller will control both the

VTO and the transmission ratio. A controller that matches torque and VTO with the best

efficient path would improve the overall efficiency considerably, leaving the driver with

the only responsibility of controlling the vehicles speed.

A software controller simulation was developed for an increased power envelope. It was

considered a light-duty hybrid vehicle application (SUV) as described in chapter 5, in

which a CVPST can be used to bring the power from two sources (engine and electric

motor).The software used for this simulation was LabVIEW version 6i.

Considerations for the development of the program:

The pedal velocity is represented as a knob control, where the user can adjust to the

velocity wanted. This velocity is simulated by a sub-program, which starting from a

random number (close to zero) goes to a number of equations that follow a regular

velocity curve. It has some noise to simulate the variation of speed. This noise vibration

makes it look like a usual speedometer. The appendix has the program code and by

clicking on the icons it opens the subprograms, which show the different equations used


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for the velocity simulation. For training in LabVIEW a tutorial manual is available at

the following web site http://www.physics.utoledo.edu/~alukasz/labview_tutorial.PDF .

Two more knob controls are set for the power adjustment, one for the IC Engine and the

other one for the electric motor. According to the input torque by this two sources and the

velocity set, an equation calculates the change on the CVT and changes the rpms as the

velocity is increasing; trying to maintain the revolutions at a speed of 3 100 rpms at 65

mph (104.6 km/h). At this velocity, by controlling the break mean effective pressure, the

VTO can spend most of the time within the ideal curve as shown in figure 6.4.

Figure 6.4: Specific-fuel consumption map for a V-8 engine 300 in3

(Gillespie, 1992)

A number of input values are required to run the simulation. Depending on them the plots

are going to change and some times show inaccurate results. The

A Continuously Variable Power-Split Transmission in A

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