Internal Combustion Internal Combustion Engine TestingEngine Testing

Kanit WattanavichienKanit Wattanavichien

What is I.C. EngineWhat is I.C. Engine

Internal combustion engines (ICE) Internal combustion engines (ICE) the internal combustion engine is characterised by: the internal combustion engine is characterised by:

““ The working fluid composition changes durin The working fluid composition changes durin g its passage through the engine. Energy is r g its passage through the engine. Energy is r

eleased by combustion and the added heat i eleased by combustion and the added heat i s, in part, converted into work.” s, in part, converted into work.”

strictly speaking, this description includes not only strictly speaking, this description includes not only reciprocating engines, but gas turbines as well. In t reciprocating engines, but gas turbines as well. In t

his chapter we will consider only: his chapter we will consider only:

‘‘ reciprocating internal combustion engines’ or R.I.C reciprocating internal combustion engines’ or R.I.C.E.E

BB asic operation of a four stroke engine: asic operation of a four stroke engine:

CompressionStroke

PowerStroke

ExhaustStroke

A I R

CombustionProducts

IntakeStroke

Air

Fuel Injector

Direct Injectionquiescent chamber

Direct Injectionmulti-hole nozzleswirl in chamber

Direct Injectionsingle-hole nozzleswirl in chamber

Indirect injectionswirl pre-chamber

Engine Geometry Engine Geometry

Vd =

Compression ratiorc = Vc – Vd

Vc

Engine Indicating Diagram Engine Indicating Diagram & Valve Timing Digram& Valve Timing Digram

Valve overlaps

Engine P-V DiagramEngine P-V Diagram

Cl Engine Indicating & P-v Cl Engine Indicating & P-v DiagramDiagram

Engine TestingEngine Testing

Part APart A– Engine InspectionEngine Inspection

Part BPart B– Energy Balance on an Engine and Energy Balance on an Engine and

Engine PerformanceEngine Performance Part CPart C

– Cylinder Pressure MeasurementCylinder Pressure Measurement

Torque and Power

Torque is measured off the output shaft using a dynamometer.

Load cell

Force FStatorRotor

b

N

The torque exerted by the engine is T:

JNmbFT :units

Torque and Power

Torque is measured off the output shaft using a dynamometer.

Load cell

Force FStator

Rotor

b

N

The torque exerted by the engine is T:

W

WattJs

rev

rev

radTNTW

)( :units )2(

JbFT :units

The power delivered by the engine turning at a speed N and absorbed by the dynamometer is:

Note: is the shaft angular velocity in units rad/s

Typical Torque and Power Curv Typical Torque and Power Curvee

Torque is a measure of an engine’s ability to do work and power is the rate at which work is done

The term brake power, , is used to specify that the power is measured at the output shaft, this is the usable power delivered by the engine to the load.

The brake power is less than the power generated by the gas in the cylinders due to mechanical friction and parasitic loads (oil pump, air conditioner compressor, etc…

The power produced in the cylinder is termed the indicated power, .

bW

iW

Indicated Work per Cycle

Given the cylinder pressure data over the operating cycle of the engine one can calculate the work done by the gas on the piston. This data is typically given as P vs V

The indicated work per cycle is given by PdVWi

CompressionW<0

PowerW>0

IntakeW>0

ExhaustW<0

WA > 0

WB < 0

Gross indicated work per cycle – net work deliveredto the piston over the compression and expansion strokes only:

Wi,g =area A + area C (>0)

Pump work – net work delivered to the gas over the intake and exhaust strokes:

Wp =area B + area C (<0)

Net indicated work per cycle – work delivered over all strokes:

Wi,n = Wi,g – Wp = (area A + area C) – (area B – area C)

= area A – area B

Work per Cycle

Indicated power:

where N – crankshaft speed in rev/snR – number of crank revolutions per cycle

= 2 for 4-stroke= 1 for 2-stroke

Power can be increased by increasing:• the engine size, Vd

• compression ratio, rc • engine speed, N

cyclerev

srevcyclekJ

n

NWW

R

ii

))((

Indicated Power

Indicated Work at Part Throttle

At WOT the pressure at the intake valve is just below atmospheric pressure,However at part throttle the pressure is much lower than atmospheric

Therefore at part throttle the pump work (area B+C) can be significant compared to gross indicated work (area A+C)

Pint

Indicated Work with Supercharging

Engines with superchargers or turbochargers can have intake pressuresgreater than the exhaust pressure, giving a positive pump work

Wi,n = area A + area B

Supercharges increase the net indicated work but is a parasitic loadsince they are driven by the crankshaft

Pint

Mechanical Efficiency

Some of the power generated in the cylinder is used to overcome enginefriction and to pump gas into and out of the engine.

The term friction power, , is used to collectively describe these power losses, such that:

gi

f

gi

bm W

W

W

W

,,1

fW

Friction power can be measured by motoring the engine.

The mechanical efficiency is defined as:

bgif WWW ,

• Mechanical efficiency depends on throttle position, engine design and engine speed.

• Typical values for car engines at WOT are:90% @2000 RPM and 75% @ max speed.

• Throttling increases pumping work and thus decreases the brake powerso the mechanical efficiency drops and approaches zero at idle.

• Power varies with speed but torque is “independent” of engine speed

cyclecycle WTTNWWNW so and recall

Mechanical Efficiency (2)

There is a maximum in the brake power versus engine speed called the ratedbrake power (RBP).

At higher speeds brake power decreases as friction power becomes significant compared to the indicated power

There is a maximum in the torque versus speed called maximum brake torque (MBT).

Brake torque drops off: • at lower speeds do to heat losses • at higher speeds it becomes more difficult to ingest a full charge of air.

cyclecycle WTWNW

fgib WWW ,

Max brake torque

1 kW = 1.341 hp

Rated brake power

Power and Torque versus Engine Speed

Indicated Mean Effective Pressure (IMEP)

imep is a fictitious constant pressure that would produce the same work per cycle if it acted on the piston during the power stroke.

R

pp

R

di

d

Ri

d

i

n

UAimep

n

NVimepW

NV

nW

V

Wimep

2

TWT cycle imep so recall

imep does not depend on engine speed, just like torque

imep is a better parameter than torque to compare engines for design and output because it is independent of engine speed, N, and engine size, Vd.

Brake mean effective pressure (bmep) is defined as:

R

d

d

R

d

b

n

VbmepT

V

nT

V

Wbmep

2

2

The maximum bmep of good engine designs is well established:

Four stroke engines:

SI engines: 850-1050 kPa* CI engines: 700 -900 kPa

Turbocharged SI engines: 1250 -1700 kPaTurbocharged CI engines: 1000 - 1200 kPa

Two stroke engines:

Standard CI engines comparable bmep to four strokeLarge slow CI engines: 1600 kPa

*Values are at maximum brake torque at WOT Note, at the rated (maximum) brake power the bmep is 10 - 15% less

Can use above maximum bmep in design calculations to estimate engine displacement required to provide a given torque or power at a specified speed.

Maximum BMEP

• The maximum bmep is obtained at WOT at a particular engine speed

• Closing the throttle decreases the bmep

• For a given displacement, a higher maximum bmep means more torque

• For a given torque, a higher maximum bmep means smaller engine

• Higher maximum bmep means higher stresses and temperatures in the engine hence shorter engine life, or bulkier engine.

• For the same bmep 2-strokes have almost twice the power of 4-stroke

2

d

R

d

b

V

nT

V

Wbmep

VehicleVehicle EngineEngine

typetypeDispl.Displ.

(L)(L)Max PowerMax Power

(HP@rpm) (HP@rpm) Max TorqueMax Torque

(lb-ft@rpm)(lb-ft@rpm)BMEP atBMEP at

Max BTMax BT

(bar)(bar)

BMEP atBMEP at

Rated BP Rated BP

(bar)(bar)

MazdaMazda

Protégé LXProtégé LXL4L4 1.8391.839 122@6000122@6000 117@4000117@4000 10.810.8 9.99.9

Honda Honda

Accord EXAccord EXL4L4 2.2542.254 150@5700150@5700 152@4900152@4900 11.411.4 10.410.4

Mazda Mazda

Millenia SMillenia SL4L4

TurboTurbo2.2552.255 210@5300210@5300 210@3500210@3500 15.915.9 15.715.7

BMWBMW

328i328iL6L6 2.7932.793 190@5300190@5300 206@3950206@3950 12.612.6 11.511.5

FerrariFerrari

F355 GTSF355 GTSV8V8 3.4963.496 375@8250375@8250 268@6000268@6000 13.113.1 11.611.6

FerrariFerrari

456 GT456 GTV12V12 5.4745.474 436@6250436@6250 398@4500398@4500 12.412.4 11.411.4

LamborghiniLamborghini

Diablo VTDiablo VTV12V12 5.7075.707 492@7000492@7000 427@5200427@5200 12.712.7 11.011.0

Typical 1998 Passenger Car Engine Characteristics

Just some part of energy from Fuel Just some part of energy from Fuel are available at engine shaft.are available at engine shaft.

Energy DistributionEnergy Distribution