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Engine Testing

Testing of ICEs is an important part of research, development and teaching

Engine tests are performed to -

- find out performance before mass production and flitting it into a vehicle

- improve the design and configuration, to integrate new materials and technology

- find out the power and fuel consumption, also to test effectiveness of cooling,

vibration and noise, lubrication, controllability, etc.

Basic Instrumentation for Engine Test

Power/torque measurement

Engine speed measurement

Air flow rate measurement

Fuel flow rate measurement

Test Equipment and instruments

- Emission equipment, Thermocouples, Pressure transducers (in cylinder measurement),

Turbine flow meters, Smoke measurement, Fuel measurement, Blow-by measurement, Air

flow measurement

The fundamental output of the engine is engine torque, usually expressed in N-m

Torque/power is measured by a dynamometer or an in-line device

The principle is rather simple - typically the engine flywheel has a band of friction material

around its circumference, and the torque reaction on the friction material corresponds to the

torque output of the engine

The term Brake Horse Power (bhp) derives from the simplest form of engine dynamometer,

the friction brake (or Prony brake)

Dynamometer Types

Another type of dyno is the electric dyno which acts as a generator to absorb power from

the engine

- Advantage of this is that it can be used as motor for starting engine, and for

motoring tests (when the engine is run at operating speeds without combustion) to

determine the mechanical losses in the engine

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Torque output or load absorbed by the dyno is controlled by the dyno field strength

Other common type of dynamometer is the water brake

- A vaned rotor tums adjacent to a pair of vaned stators

- The sluice gates separate the stators from the rotor, and these control the toad

absorbed by the dyno

Dynamometers can be classified by the type of absorption unit or absorber/driver that they

use

Some of these are as follows:

- Eddy current or electromagnetic brake (absorption)

used in modern chassis dynos

provide the quick load change rate for rapid load setting

air cooled, but some are designed to require external water cooling systems

require an electrically conductive core, shaft or disc, moving across a

magnetic field to produce resistance to movement

use variable electromagnets to change the magnetic field strength to control

the amount of braking

- Magnetic powder brake (absorption)

similar to an eddy current dynamometer, but a fine magnetic powder is

placed in the air gap between the rotor and the coil

The resulting flux lines create “chains” of metal particulate that are

constantly built and broken apart during rotation creating great torque

typically limited to lower RPM due to heat dissipation issues

- Hysteresis Brake dynamometers (absorption)

use a steel rotor that is moved through flux lines generated between magnetic

pole pieces

as in the usual “disc type” eddy current absorbers, allows for full torque to be

produced at zero speed, as well as full speed

Hysteresis and “disc type” EC dynamometer are one of the most efficient

technologies in small (200 hp (150 kW) and less) dynamometer

A hysteresis brake is an eddy current absorber that unlike most “disc type”

eddy current absorber puts the electromagnet coils inside a vented and ribbed

cylinder and rotates the cylinder, instead of rotating a disc between

electromagnets

The potential benefit for the hysteresis absorber is that the diameter can be

decreased and operating RPM of the absorber may be increased

- Electric motor/generator dynamometer (absorb or drive)

absorption/driver unit can be either an AC motor or OC motor

electric motor/generator dynamometer can be configured as universal

dynamometers

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universal dynamometers can not only absorb the power of the engine but

also, drive the engine for measuring friction, pumping losses and other factors

generally more costly and complex than other types of dynamometers

- Fan brake (absorption)

A fan is used to blow air to provides engine load

- Hydraulic brake (absorption)

consists of a hydraulic pump (usually a gear type pump), a fluid reservoir

and piping between the two parts

the fluid used was hydraulic oil, but recent synthetic multi-grade oils may be

a better choice

the engine is brought up to the desired RPM and the valve is incrementally

closed and as the pumps outlet is restricted, the load increases and the throttle

is simply opened until at the desired throttle opening

power is calculated by factoring flow volume (calculated from pump design

specs), hydraulic pressure and RPM

renowned for having the absolute quickest load change ability, just slightly

surpassing the eddy current absorbers

- Water brake (absorption)

noted for their high power capability, small package light-weight, and

relatively low manufacturing cost as compared to other, quicker reacting

“power absorber” types

drawbacks are that they can take a relatively long period of time to 'stabilize'

their load amount and the fact that they require a constant supply of water to

the “water brake housing” for cooling

The housing attempts to rotate in response to the torque produced but is

restrained by the scale or torque metering cell that measures the torque

Hydraulic Dynamometer

Torque absorbed by the dyno increases with speed

By varying the throttle and sluice gate setting, any operating point should be attainable

For stable operation the dyno operating lines and throttle lines should intersect as close as

possible to 90"

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Dynamometer Types

- Compound dyno (usually an absorption dyno in tandem with an electric/motoring dyno)

Torque measurement is somewhat complicated since there are two machines in

tandem; an inline torque transducer is the preferred method of torque measurement in

this case

An eddy current or water brake dynamometer with electronic control combined with

a variable frequency drive and AC induction motor is a commonly used configuration

of this type

Disadvantages include requiring a second set of test cell services (electrical power

and cooling) and a slightly more complicated control system

Fuel Consumption Measurement

A common measurement system for fuel consumption is to time the consumption of a fixed

volume and then converted to a gravimetric consumption by using the density

In a fuel-injection system, if the spill flow is fed back to the tank this must be measured

separately

Orifice-type flow meter gives instantaneous readings, but is less accurate

Automatic flow meter adopts both of these

A continuous measurement system employs a hydraulic equivalent of a Wheatstone bridge

Another system uses Coriolis acceleration principle to measure fuel flow rate. It uses a U-

shaped tube

Air Flow Measurement

A simple system to measure the air flow rate is obtained by connecting the air intake to a

large rigid box with an orifice at its inlet

- The box should be large enough to damp out the pulsations in the flow and be free or

resonances in the normal speed range of the engine

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Temperature and Pressure Measurements

Mercury-in-glass thermometers and thermocouples both provide economical means of

measuring temperature, with the potential of achieving and accuracy of about ±0.1 K - same

level of accuracy can be obtained from platinum resistance thermometer

Bourdon pressure gauges and manometers provide a cheap and accurate means of

measuring steady pressures

- Pressures in the range of 1 - 100 kN/m2

can be measured with an accuracy of about 1

Percent

Pressure transducers utilize a piezo-resistive effect

In-cylinder Pressure Measurement

The simplest form of engine indicator is the Dobbie Mclnnes mechanical indicator

The area of the diagram corresponds to the indicated work per cylinder

imep = k hd = k (Ad/Id)

where Ad = diagram area

Id = diagram length

hd = mean height of diagram

Mechanical indicators can only be used at speeds of up to 600 rpm

Electronic systems are now very common for recording indicator diagrams - the pressure is

plotted on a time instead of a volumetric basis

A pressure transducer is used to measure the pressure inside the cylinder, the output of

which is sent to an oscilloscope

An inductive pick-up coil is connected to record the position of tdc accurately

- Since tdc occurs during the period of maximum pressure change, a 1" error in

position of tdc can cause a 5% error in imep

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The location of tdc is not straightforward because of

- the finite stiffness of the crank-slider mechanism

- the flexibility of the crankshaft is such that at full load there can be about 1° twist at

certain speeds

To convert the time base to a piston displacement base it is usual to assume constant

angular velocity throughout each revolution

The output from an oscilloscope can be plotted in an x-y plotter

Techniques for Estimating Indicated Power

Very often a pressure transducer cannot be readily fitted to an engine, so alternative means

of deducing imep are useful

The difference between indicated power and brake power is the power absorbed by friction

- often assumed to be dependent only on engine speed

However, the friction power also depends on the indicated power since the increased gas

pressures cause increases in piston friction

If the friction power is assumed to be independent of the indicated power, then the friction

power can be deduced from the Morse test

- This is applicable only to multi-cylinder engines, as each cylinder is disabled in turn

- When a cylinder is disabled the load is reduced so that the engine returns to the test speed

the reduction in power

correspond to the indicated power of that cylinder

For a n-cylinder engine

∑indicated power - ∑friction power = (brake power)n

With one cylinder disabled

∑indicated power - ∑friction power = (brake power)n-1

Subtracting:

indicated power of disabled cylinder = reduction in brake power

A method for estimating the friction power of compression ignition engines is Willans’ line

A more accepted figure used to compare the performance of engines of differing capacities

is the BMEP - Brake Mean Effective pressure

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Engine Test Conditions

Various standards authorities (BS, DIN, SAE, ASTM) are involved with specifying the test

conditions for engines, and how allowances can be made for variations in ambient conditions

Corrections for datum conditions vary, and in general they are more involved for CI

engines

Energy Balance

Experiments with engines very often involve an energy balance on the engine

Energy is supplied to the engine as the chemical energy of the fuel and leaves as energy in

the cooling water, exhaust, brake work and extraneous heat transfer

- The heat transfer to the cooling water is found from the temperature rise in the

coolant as it passes through the engine and the mass flow rate of the coolant

- The energy leaving in the exhaust is more difficult to determine

- Heat transfer from the engine cannot be readily determined

- Brake power should be used in the energy balance

Experimental Accuracy

Whenever an experimental reading is taken there is an error associated with that reading

There are three main sources of error

- the instrument is not measuring what is intended

- the instrument calibration is inaccurate

- the instrument output is incorrectly recorded by the observer