Page 171 - Understanding Automotive Electronics

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5 THE BASICS OF ELECTRONIC ENGINE CONTROL

Figure 5.7
Major Controller
Outputs to Engine

DEFINITION OF ENGINE PERFORMANCE TERMS
Several common terms are used to describe an engine’s performance. Here
are a few.

Power
Power is a measurement The most common performance rating that has been applied to
of an engine’s ability to automobiles is a power rating of the engine. It normally is given in kilowatts or,
3
perform useful work. formerly, in horsepower (note: / kilowatt = 1 horsepower). Power is the rate at
4
Brake power, which is which the engine is doing useful work. It varies with engine speed and throttle
measured with an engine angle. Power may be measured at the drive wheels or at the engine output shaft.
dynamometer, is the It is more convenient and useful to the designer of an electronic engine control
actual power developed system to know the output power of only the engine. This permits realistic
by the engine minus comparisons of engine data as engine controls are varied. To make such
losses due to internal measurements, an engine dynamometer is used. This dynamometer is similar to
friction. the one in Figure 5.1 except that the engine output shaft drives the
dynamometer directly instead of coupling the output through wheels and
rollers.
The power delivered by the engine to the dynamometer is called the brake
power and is designated P . The brake power of an engine is always less than the
b
total amount of power that is actually developed in the engine. This developed
power is called the indicated power of the engine and is denoted P . The
i
indicated power differs from the brake power by the loss of power in the engine
due to friction between cylinders and pistons, and other friction losses. That is,

P = P – friction and other losses
b
i

158 UNDERSTANDING AUTOMOTIVE ELECTRONICS