Internal Combustion
Engine Testing
Kanit Wattanavichien
What is I.C. Engine
Internal combustion engines (ICE)
 the internal combustion engine is characterised by:
“The working fluid composition changes during its
passage through the engine. Energy is released
by combustion and the added heat is, in part,
converted into work.”

strictly speaking, this description includes not only
reciprocating engines, but gas turbines as well. In this
chapter we will consider only:

‘reciprocating internal combustion engines’ or R.I.C.E
Basic operation of a four stroke engine:
A
Fuel Injector
I
R
Air
Intake
Stroke
Compression
Stroke
Combustion
Products
Power
Stroke
Exhaust
Stroke
Direct Injection
quiescent chamber
Direct Injection
multi-hole nozzle
swirl in chamber
Direct Injection
single-hole nozzle
swirl in chamber
Indirect injection
swirl pre-chamber
Engine Geometry
Vd =
Compression ratio
rc = Vc – Vd
Vc
Engine Indicating Diagram &
Valve Timing Digram
Valve overlaps
Engine P-V Diagram
Cl Engine Indicating & P-v Diagram
Engine Testing

Part A
– Engine Inspection

Part B
– Energy Balance on an Engine and Engine
Performance

Part C
– Cylinder Pressure Measurement
Torque and Power
Torque is measured off the output shaft using a dynamometer.
b
Force F
Stator
Rotor
N
Load cell
The torque exerted by the engine is T:
T  F b
units : Nm  J
Torque and Power
Torque is measured off the output shaft using a dynamometer.
b
Force F
Stator
Rotor
N
Load cell
The torque exerted by the engine is T:
T  F b
units : J
The power W delivered by the engine turning at a speed N and
absorbed by the dynamometer is:
W    T  (2  N )  T
 rad  rev 
units : 

( J )  Watt
 rev  s 
Note:  is the shaft angular velocity in units rad/s
Typical Torque and Power Curve
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, Wb , 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, Wi .
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 Wi   PdV
WA > 0
WB < 0
Compression
W<0
Power
W>0
Exhaust
W<0
Intake
W>0
Work per Cycle
Gross indicated work per cycle – net work delivered
to 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
Indicated Power
Indicated power:
Wi N

Wi 
nR
(kJ cycle)(rev s)
rev cycle
where N – crankshaft speed in rev/s
nR – 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
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
Pint
Therefore at part throttle the pump work (area B+C) can be significant
compared to gross indicated work (area A+C)
Indicated Work with Supercharging
Engines with superchargers or turbochargers can have intake pressures
greater than the exhaust pressure, giving a positive pump work
Pint
Wi,n = area A + area B
Supercharges increase the net indicated work but is a parasitic load
since they are driven by the crankshaft
Mechanical Efficiency
Some of the power generated in the cylinder is used to overcome engine
friction and to pump gas into and out of the engine.
The term friction power,W f , is used to collectively describe these power
losses, such that:
W f  Wi , g  Wb
Friction power can be measured by motoring the engine.
The mechanical efficiency is defined as:
W f
Wb
m 
 1
Wi , g
Wi , g
Mechanical Efficiency (2)
• 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 power
so the mechanical efficiency drops and approaches zero at idle.
• Power varies with speed but torque is “independent” of engine speed
recall W  N Wcycle
and W  N  T
so T  Wcycle
Power and Torque versus Engine Speed
W  N Wcycle
Rated brake power
1 kW = 1.341 hp
T  Wcycle
There is a maximum in the brake power
versus engine speed called the rated
brake power (RBP).
At higher speeds brake power decreases as
friction power becomes significant compared
to the indicated power Wb  Wi , g  W f
Max brake torque
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.
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.
Wi Wi  nR
imep 

Vd Vd  N
imep  Vd  N
 Wi 
nR

imep  Ap  U p
2  nR
imep does not depend on engine speed, just like torque
recall T  Wcycle
so imep  T
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:
bmep 
Wb 2  T  nR

Vd
Vd
 T
bmep  Vd
2  nR
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 kPa
Turbocharged CI engines: 1000 - 1200 kPa
Two stroke engines:
Standard CI engines comparable bmep to four stroke
Large 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
bmep 
Wb 2  T  nR

Vd
Vd
• 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
Typical 1998 Passenger Car Engine Characteristics
Vehicle
Engine
type
Displ.
(L)
Max Power
(HP@rpm)
Max Torque
(lb-ft@rpm)
BMEP at
Max BT
(bar)
BMEP at
Rated BP
(bar)
Mazda
Protégé LX
L4
1.839
122@6000
117@4000
10.8
9.9
Honda
Accord EX
L4
2.254
150@5700
152@4900
11.4
10.4
Mazda
Millenia S
L4
Turbo
2.255
210@5300
210@3500
15.9
15.7
BMW
328i
L6
2.793
190@5300
206@3950
12.6
11.5
Ferrari
F355 GTS
V8
3.496
375@8250
268@6000
13.1
11.6
Ferrari
456 GT
V12
5.474
436@6250
398@4500
12.4
11.4
Lamborghini
Diablo VT
V12
5.707
492@7000
427@5200
12.7
11.0

Just some part of energy from Fuel are
available at engine shaft.
Energy Distribution