C7 SI Engines

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Engine principles

Topics covered in thischapter

The Otto cycle

Compression ratio

The two-stroke cycle

The Wankel rotary engine

The Atkinson cycle as adapted for use in hybrid vehicles

Valve and ignition timingVariable valve lift and valve timing

The engine is the device that converts the chemical

energy contained in the fuel into the mechanical energy

that propels the vehicle. The energy in the fuel is con-

verted into heat energy by burning the fuel in a process

known as combustion, which is why vehicle engines are

often referred to as internal combustion engines.

The fuel is burned inside the engine cylinders in the

presence of air; when the air is heated its pressure rises

and generates the force that operates the engine. Most

engines used in motor vehicles make use of the piston

and crank mechanism that converts linear motion intorotary motion; the piston moves to and fro in the cylinder

in a reciprocating fashion e because of this the engines

are frequently called reciprocating engines.

The component parts of the simple engine shown in

Fig. 2.1 are:

1. The piston, which receives the gas pressure.

2. The cylinder, in which the piston moves to and fro.

3. The connecting rod that transmits force from the

piston to the crank.

4. The crank that converts the reciprocating movement

of the piston into rotary movement.

5. The flywheel that rotates and stores energy to drivethe piston when gas force is not acting on it.

Engine details

Example of calculating sweptvolume

A single-cylinder engine of the type shown in Fig. 2.2

has a bore of 90 mm and a stroke of 100 mm. Calculate

the swept volume in cm3.

Working in centimetres, bore diameter D 5 9 cm,

stroke length L5 10 cm.

The swept volume5 area of piston crown3 stroke length:

The piston crown is a circle and its area

5pD2

45

3:1423 93 9

45 63:6 cm2:

The swept volume5 63:63 105 636 cm3:

A cycle of operations

In order for the engine to function it goes through

a sequence of events:

1. Getting air into the cylinder.

2. Getting fuel into the cylinder and igniting the

fuel.

3. Expanding the high-pressure air to produce useful

work.

4. Getting rid of the spent gas so that the sequence can

be repeated.

This sequence of events is called a cycle.

The four-stroke Otto cycle

A large proportion of light vehicle engines use petrol as

a fuel and they operate on the Otto cycle. The Otto cycle

is named after Dr A. Otto, who developed the first

commercially successful engines, in Germany, in the

1860s. Otto cycle engines are also called four-stroke

engines because the Otto cycle takes four strokes of

the piston for its completion.

The basic engine

A four-stroke engine (Fig. 2.3) has one end of the

cylinder sealed e this end of the cylinder is called the

cylinder head. In the cylinder head are two valves and

a spark plug that supplies the spark that ignites the

fuel. One valve is called the inlet valve and it is opened

when air and fuel are required; the other valve is the

exhaust valve and this is opened when the spent gas is

removed from the cylinder.

2011 Allan Bonnick and Derek Newbold. Published by Elsevier Ltd. All rights reserved


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The four strokes (see Fig. 2.4)

First strokee

Induction.The inlet valve is openand the exhaust valve is closed. The piston is pulled

down the cylinder by the action of the crank and

connecting rod. As the piston descends it creates

a partial vacuum in the cylinder and this causes the

atmospheric air pressure to force a mixture of air and

fuel that is supplied by a carburettor or fuel injection

system into the cylinder.

Second stroke e Compression.Both valves are now

closed and the piston is pushed up the cylinder by the

action of the flywheel, crank, and connecting rod.

The mixture of air and fuel in the cylinder is now

compressed to a high pressure. A high pressure isrequired to extract the maximum amount of energy

from the fuel.

Third stroke e Power. Both valves are closed and

the spark ignites the fuel. This causes the pressure in

the cylinder to rise and the action pushes the piston

down the cylinder to rotate the crankshaft and deliver

power to the flywheel.

Fourth stroke e Exhaust.The exhaust valve is open

and the inlet valve is closed, the action of the

flywheel and crank pushes the piston up the cylinder

to expel the spent gas. The cycle is now complete and

the engine is ready to start the next cycle.

The four strokes are completed in two revolutions of the

crankshaft, which is equivalent to an angular movement

of 7208.

2

1

5

4

Fig. 2.1 Simple engine mechanism

R

D

Swept

Volume

Piston at top of thestroke. Top DeadCentre TDC.

R = radius of crank.Also called the crank

throw.

Length of stroke= 2 crank radius

D = diameter of thecylinder and piston.Normally referred to asthe cylinder bore.

Stroke

TDC

BDC

Piston at the bottomof the stroke. Bottomdead centre BDC.

The swept volumeis the space that iscreated in thecylinder when thepiston moves fromTDC to BDC.It is also called thecylinder capactiy.

Swept volume = cross sectional area of the pistion length of the stroke.

Fig. 2.2 Single-cylinder engine dimensions (Renault)

Engine principles 9


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Compression ratio

To a certain extent, the more that the mixture of fuel and

air is compressed the greater the amount of power that

can be extracted from the fuel. The amount of compres-

sion that takes place in an engine is determined by the

compression ratio of the engine (Fig. 2.5).

Compression ratio is the total volume inside the

cylinder when the piston is at bottom dead centre

(BDC) divided by the total volume inside the cylinder

when the piston is at top dead centre (TDC).

The total volume inside the cylinder when the piston

is at bottom dead centre is the clearance volume plus the

swept volume. The swept volume is the volume swept

by the piston when it moves from TDC to BDC.

The total volume inside the cylinder when the piston

is at TDC is the clearance volume, or combustion space.

The formula for compression ratio is:

Compression ratio5Vs 1Vc

Vc;

whereVs 5 swept volume and Vc 5 clearance volume.

Example

A certain engine has a swept volume of 400 cm3 and

a clearance volume of 50 cm3

. Calculate the compres-sion ratio.

Compression ratio5Vs 1Vc

Vc5

4001 50

505

450

505 9 :1:

Valve timing

In the four-stroke cycle the valves are required to open

and close at the correct point in the cycle. The inlet

valve normally opens a few degrees before the piston

reaches TDC on the exhaust stroke and closes again

several degrees after BDC on the induction stroke.

The exhaust valve normally opens several degrees

before BDC on the power stroke and closes a few

degrees after TDC on the exhaust stroke.

These events can be shown on a circular display

called a timing diagram. A typical timing diagram is

shown in Fig. 2.6.

Valve timing diagrams display details about valveoperation in terms of degrees of crankshaft rotation,

which also indicates the position of the piston in the

cylinder. In the timing diagram shown in Fig. 2.6 the

following details apply:

The inlet valve opens when the crank is 48 before

TDC and it remains open down the induction stroke

and for 488, part of the way up the compression

stroke.

The number of degrees for which the valve remains

open is called the valve period e in this case, the inlet

valve period is 48 1 1808 1 488 5 2328.

The exhaust valve opens 488

before BDC on thepower stroke and it remains open up the entire

exhaust stroke and for 48 on the induction stroke. The

exhaust valve period is 488 1 1808 1 48 5 2328.

The number of degrees around TDC for which both

valves are open together is called valve overlap.

The number of degrees that the exhaust valve opens

before BDC is called exhaust valve lead. Early

opening of the exhaust valve while there is still some

pressure left in the gas allows gas to escape into the

exhaust system and thus reduces the pressure that the

piston works against on the exhaust stroke. This

improves the efficiency of the engine.

The number of degrees that the inlet valve remainsopen after BDC is calledinlet valve lag. Closing the

inlet valve after BDC allows the momentum of the air

entering the cylinder to overcome the increasing

pressure in the cylinder as the piston moves up the

cylinder on the compression stroke. In this way the

engine is made more efficient.

Valve timing varies from engine to engine and the

actual details are determined by the type of use that

the vehicle is intended for.

The motion of valves is determined by the shape of the

camshaft (cam) and this is designed to open and close

the valves as quickly as possible without causing unduestress on components.

Setting the valve timing

When reassembling an engine after repair it is necessary

to ensure that the camshaft is set in the correct position

relative to the crankshaft. This process is called setting

the valve timing, and most engines carry marks like

those shown in Fig. 2.7 to assist in the process. The

marks are carefully aligned prior to fitting the chain.

Spark plug

Inlet valveExhaust valve

Cylinder head

Fig. 2.3 A four-stroke cycle engine

10 A Practical Approach to Motor Vehicle Engineering and Maintenance


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Engines that use gear or belt drives on the camshaft have

similar marks.

Valve timing and emissions

When the engine is operating at low speed the overlap

that occurs when the inlet valves and exhaust valves

are open simultaneously is a cause of harmful emissions

and various forms of variable valve control are used to

overcome the problem. Two forms of valve control

that are used are:

1. Different amounts of valve lift for low and high

engine speeds.

2. Automatically changing the valve timing while the

engine is running.

The Honda valve system that is outlined here

(Fig. 2.8) is used on engines that have four valves per

Fig. 2.4 The Otto cycle of engine operations (four strokes)

Engine principles 11


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cylinder e two inlet and two exhaust e and it provides

variable valve lift as well as variable valve timing.

Variable valve lift

There are three cams for each pair of valves e two of the

cams provide the low-speed features and the third one

that is placed between the other two provides the lift

and period for high-speed. At low speed the high-lift

cam freewheels until it is required at higher speed.

When the high lift is required the cam is brought into

operation by the movement of the locking pins. Theselocking pins are operated by hydraulic pressure from

the engine lubricating system under the control of the

engine computer. The two low-speed inlet cams that

are called the primary and secondary cams have slightly

different profiles and are designed to produce turbulence

in the combustion chamber. Details of the method for

obtaining variations in valve movement are shown in

Fig. 2.8.

Variable valve timing

The actuator on the inlet camshaft (Fig. 2.10) is

a hydraulically operated device that advances the

opening of the inlet valve at high engine speed to take

advantage of the momentum of the inflowing air and

to maximize volumetric efficiency.Details of valve lift are given in Table 2.1 and the

effect of valve timing is illustrated in Fig. 2.11.

Figure 2.11 shows how, by opening the inlet valve

early, overlap is increased with the effect that the Honda

system varies the amount of overlap, and consequently

the intake closure moment. This strongly influences

engine characteristics: minimum overlap e for smooth

idling and cruising, and excellent fuel economy through

stable combustion; maximum overlap e for power, by

exploiting gas flow inertia to improved cylinder filling.

Ignition timing

The spark at the spark plug is arranged to take place

slightly before TDC on the compression stroke so that

maximum gas pressure is reached at the beginning of

the power stroke. The number of degrees before TDC

that the spark is initiated is called the angle of advance.

In vehicle repair work the action of setting the ignition

timing is called setting the timing and it requires the

piston to be in the correct position when the device

that triggers the spark is also in the correct position.

On most engines there are timing marks on the

Vc

Vs+ VcTDC

Stroke

BDC

1. Piston at TDCTotal volume = Vc

2. Piston at BDCTotal volume = Vs+ Vc

Fig. 2.5 Compression ratio (Renault)

Exhaust valve

Inlet valve

48

4 4

48

BDC

TDC

Fig. 2.6 A timing diagram



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Camshaft chain wheel

Crankshaftchain wheel

Valve timing marks

Fig. 2.7 Valve timing marks

Variable timing

actuator

Exhaust camshaft

Inlet camshaft

Variable lift camsand followers

Fig. 2.8 Variable valve lift and timing (Honda)

The 3 rockers are nowlocked together andthe high-speed camnow operates thevalves.

The high-speedrocker is not lockedto the other two. Thelow-speed camsoperate the valves.The high-speedrocker free wheelsuntil it is locked tothe others.

Locking pinslock the rockerstogether asrequired usinghydraulic pressure.

Camshaft

Low-speed cams

13

2

High-speed cam

Fig. 2.9 Variable valve lift (Honda)



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crankshaft pulley, like those shown in Fig. 2.12, that areused in checking and setting ignition timing.

The two-stroke cycle

In its simplest form the two-stroke cycle offers the

following advantages over the four-stroke cycle:

No valves are used because the piston covers and

uncovers ports through which air and fuel enter the

engine and exhaust products are expelled.

A power stroke occurs once for each revolution of the

cranke in theory this makes a two-stroke engine of

a given size twice as powerful as a four-stroke engine.

Two-stroke engines (Fig. 2.13) have been used in some

light cars and vans from time to time but their main use

has been in motorcycles and mopeds.

The crankcase is sealed because it is used to hold theairefuel mixture at a stage of the cycle of operations. By

using both the top and underside of the piston the four

phases of the cycle (induction, compression, power,

and exhaust) are completed in two strokes of the piston

and one revolution of the crankshaft.

When considering how this type of engine works it is

advantageous to consider events above and below the

piston separately.

First stroke (piston moving down

the cylinder)Events above the piston

The expanding gases that have been ignited by the spark

plug force the piston down the cylinder. About two-

thirds of the way down the cylinder the exhaust port

is uncovered by the piston and the exhaust gases leave

the cylinder. As the piston moves further downwards

the transfer port is uncovered and this allows a fresh

charge of fuel and air from the crankcase to enter the

cylinder above the piston.

Target Wheel

4 Way Pulse WidthModulated (PWM)

Control Valve

Control

Camshaft

drive gear

Camshaft

PowertrainControlModule(PCM)

V1 Supply

V2

Camsensor

Hydraulic pressure applied in these spacesmoves the camshaft relative to the drive gear toadvance the opening point of the valves

Fig. 2.10 Valve timing actuator (Delphi)

Table 2.1 Valve lift

Operation Inlet valve lift Exhaust valve lift

Low-speed Primary cam 7.2 mm Primary cam 6.9 mm

Secondary cam 7.0 mm Secondary cam 7.1 mm

High-speed All cams 12.0 mm All cams 10.7 mm

Valve lift

TDC

EX IN

maximum VTC advance 25

maximum

OVERLAPminimum

Fig. 2.11 The effect of valve timing on valve overlap



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Events below the piston

The descending piston covers the inlet port and

compresses the air and fuel mixture in the crankcase.

Second stroke (piston moving upthe cylinder)

Events above the piston

Compressed fuel and air is forced into the cylinder

from the crankcase, through the transfer port. With

the aid of the deflector on top of the piston the

incoming charge of fuel and air helps to drive exhaust

gas out. When both the transfer port and the exhaust

ports are closed the piston continues to rise and

compress the fuel and air mixture. The spark occurs

at the end of this stroke and the engine begins the

next power stroke.

Events below the piston

As the piston moves upwards the partial vacuum in

the crankcase now draws in fuel and air through the

inlet port as it is uncovered by the bottom of the

piston.

Because the piston is used to control the opening and

closing of the ports the power stroke is effectively short-

ened and this reduces the power output of the simpletwo-stroke engine.

Two-stroke engine with valves

The engine shown in Fig. 2.14 makes use of poppet

valves and direct injection of petrol into the cylinder.

It is equipped with a supercharger that pumps air into

the cylinder rather than relying on crankcase induction

and compression as used in the simple engine. Similar

Timing markson engine

Notch in pulley

for TDC

Fig. 2.12 Ignition timing marks

Exhaustport

Transfer

port

Transfer and exhaust piston at BDC Both ports closed, piston rising on compression

Piston falling,

exhaust open,

transfer port open.

Mixture transferred

from crankcase.

Inlet

port

Compression above

piston

Piston rising,

induction into

crankcase

Fig. 2.13 The two-stroke engine



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types of two-stroke engines operating on the diesel prin-

ciple are used in some large vehicles.

Rotary engines

The rotor in this type of engine (Fig. 2.15) replaces

the piston and crank of the reciprocating engine. The

housing in which the rotor moves has a shape which is

called an epitrochoid and it permits the four steps of

the Otto cycle to be completed in one revolution of

the rotor. On the inside of the rotor is a gear that engages

with a smaller gear on the output shaft and this is the

medium through which the energy from combustion is

transmitted to the engine flywheel.

The Atkinson engine cycle

The theoretical Akinson cycle is shown in the pressuree

volume diagram of Fig. 2.16. There are four processes.

The first starts at point 3 on the diagram, where a mass

of air is compressed up to point 4. At point 4 the air is

heated and the pressure rises while the volume remains

constant. At point 1 the hot air expands on the power

stroke. The power stroke ends at point 2 and the gas is

exhausted at constant pressure up to point 3, where the

cycle starts again.

A point to note is that the power stroke is longer than

the compression stroke because this is the feature that

makes the engine more fuel efficient than the Otto

engine. The original Atkinson engines were made toproduce the four processes in one revolution of the

crankshaft. In order to achieve this it was necessary to

use a complicated toggle mechanism that proved unreli-

able due to excessive wear and the engine fell out of

use.

In recent years the attraction of more efficient use of

fuel and better miles per gallon has led to renewed

Fig. 2.14 Toyota two-stroke engine

Output shaft

Inlet port

Rotor

INDUCTIONCOMPRESSION

Spark plugs

EXHAUSTPOWER

Fig. 2.15 The Wankel-type rotary engine



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interest in the Atkinson engine. The normal four-stroke

engine has been adapted so that it runs on a cycle that

bears a resemblance to the Atkinson. This has been

achieved by keeping the inlet valve open so that some

charge from the induction stroke is pushed back to the

induction system where it is used in other cylinders.

This effectively shortens the compression stroke

because compression does not start until the piston is

someway up the compression stroke. This provides

a power stroke that is of longer duration than the power

stroke and thus provides the feature that produces the

greater fuel efficiency. Unfortunately this type of engine

is only efficient at a fairly narrow range of speeds and it

is necessary to equip vehicles with transmission systems

to counteract this problem.

The theoretical thermal efficiency of the Atkinson

cycle is given by the following equation:

efficiency5 12g

r2a

rg2ag

;

where r is the expansion ratio, a is the compression

ratio, and gis a constant for air.

If we assume a compression ratio of 8:1 and an expan-

sion ratio of 13:1 we can put some figures in this equa-

tion to arrive at a value for thermal efficiency, which we

can then compare with an Otto engine with a compres-

sion ratio of 8:1.

Atkinson efficiency

g for air is approximately 1.4,r5 13, and a58. Putting

these numbers into the equation in place of the symbols

gives the Atkinson thermal efficiency as:

12 1:4

132 8

131:4 2 81:4

5 0:61 or 61%:

The equivalent theoretical efficiency for an Otto

engine with a compression ratio of 8:1 is:

12 1

rg215 12

1

80:45 56:5%:

A value of 61% compared with 56.5% seems a rela-

tively small advantage for the Atkinson cycle over the

Otto cycle, but at a time when emissions and fuel use

are so important some manufacturers consider it worth-

while to make use of the Atkinson principle.

Learning task

See if you can find out how the Toyota Prius Hybrid

vehicle transmission system overcomes the

disadvantages of the four-stroke Atkinson engine.

Self-assessment questions1. Which valve opens near the end of the power

stroke in a four-stroke engine?

2. In a certain engine the cross-sectional area of

the piston crown is 80 cm2 and the stroke

length is 120 mm. The swept volume is:

(a) 9600 cm3

(b) 120 cm3

(c) 960 cm3

(d) 960 cm2.

3. The valve timing details for an engine are:

Inlet valve opens 68 before TDC and closes 388

after BDC

Exhaust valve opens 358 before BDC and closes

58 after TDC.

Calculate in degrees:

(a) The valve overlap

(b) The exhaust valve lead

(c) The inlet valve lag

(d) The period of: (i) the inlet valve, (ii) the exhaustvalve.

4. In a simple two-stroke engine the airefuel mixture

is drawn into the crankcase. What is the name of

the port that is used to get the mixture into the

combustion space above the piston?

5. At the end of which stroke in the four-stroke cycle

does the spark occur?

6. How many degrees of crank rotation does it take to

complete the four-stroke cycle?

7. What is the reason for starting to open the inlet

valve before TDC is reached in a four-stroke

engine?

8. An engine has a bore of 79 mm and a stroke of 100mm. Calculate the compression ratio given that the

clearance volume is 50 cm3.

9. If an engine has a stroke of 120 mm what is the

radius of the crank throw?

10. An engine has a bore diameter of 98 mm and

a stroke length of 90 mm. Calculate its swept

volume.

11. Give a short explanation of the reasons for opening

the exhaust valve before BDC is reached in a four-

stroke cycle engine.

(1)

(2)

(3)VOLUME

PRESSURE

(4)

Fig. 2.16 Pressureevolume diagram for the ideal Atkinson cycle



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12. Figure 2.17 shows two valve timing diagrams.

Which of these would be suitable for a high-speed

engine?

Questions 13e16 relate to the Honda system.

13. 258 of camshaft advance is equal to:

(a) 508 of crankshaft rotation

(b) 1008 of crankshaft rotation

(c) 12.58 of crankshaft rotation(d) 258 of crankshaft rotation.

14. What effect on the closing point of the inlet valve is

brought about by opening it 258 early?

15. The maximum lift of the inlet valve is:

(a) 7.2 mm

(b) 6.9 mm(c) 12.0 mm

(d) 10.7 mm.

16. The difference in lift between the inlet primary and

secondary cams at low speed is:

(a) 4.8 mm(b) 5 mm(c) 0.20 mm

(d) 0.30 mm.

17. Discuss with other students vehicles (other than

the Trabant and motor cycles) that have been fitted

with two-stoke engines in recent years.

18. With the aid of diagrams describe how the

clearance volume of an engine can be measured and

then describe how knowledge of the bore andstroke would enable you to work out the

compression ratio.

19. Make a list of the vehicles equipped with a Wankel-

type rotary engine that are currently available in theUK.

Fig. 2.17 Typical value timing diagrams


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