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Engine Performance

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**Slide 1:**Engine Performance Some Basics

Torque and Power Torque is measured off the output shaft using a dynamometer. The torque exerted by the engine is T: The power delivered by the engine turning at a speed N and
absorbed by the dynamometer is: Note: w is the shaft angular velocity in units rad/s Torque is a measure of an engine’s ability to do work and power is
the rate at which work is done
Note torque is independent of crank speed.
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, supercharger, etc…)
The power produced in the cylinder is termed the indicated power, . Brake Power 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 Compression
W<0 Power
W>0 Intake
W>0 Exhaust
W<0 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 Compression
W<0 Power
W>0 Intake
W>0 Exhaust
W<0 WA > 0 WB < 0 A C Indicated power:
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 Power 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, , is used to describe collectively these power losses, such that: Friction power can be measured by motoring the engine.
The mechanical efficiency is defined as: Mechanical efficiency depends on pumping losses (throttle position) and
frictional losses (engine design and engine speed).
Typical values for automobile engines at WOT are:
90% @2000 RPM and 75% @ max speed.
Throttling increases pumping power and thus the mechanical efficiency
decreases, at idle the mechanical efficiency approaches zero. Mechanical Efficiency, cont’d 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 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. Max brake torque 1 kW = 1.341 hp Rated brake power Power and Torque versus Engine Speed at WOT 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. imep is a better parameter than torque to compare engines for design and
output because it is independent of engine size, Vd.
Brake mean effective pressure (bmep) is defined as: The maximum bmep of a good engine designs is well established:
Four stroke engines:
SI engines: bmep= 850-1050 kPa*
CI engines: bmep= 700 -900 kPa
Turbocharged SI engines: bmep= 1250 -1700 kPa
Turbocharged CI engines: bmep= 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 and 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 Typical 1998 Passenger Car Engine Characteristics