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Engine Performance Some Basics. b. Force F. Stator. Rotor. N. Load cell. 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

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Engine Performance Some Basics

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Engine performance some basics l.jpg

Engine PerformanceSome Basics


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b

Force F

Stator

Rotor

N

Load cell

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


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Brake Power

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, .


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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

Power

W>0

Compression

W<0

Exhaust

W<0

Intake

W>0


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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

WA > 0

A

C

WB < 0

Power

W>0

Compression

W<0

Exhaust

W<0

Intake

W>0


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Indicated Power

  • 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


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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:


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Mechanical Efficiency, cont’d

  • 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.


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Power and Torque versus Engine Speed at WOT

Rated brake power

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

1 kW = 1.341 hp

  • 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


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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:


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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.


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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


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Typical 1998 Passenger Car Engine Characteristics


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