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5 THE BASICS OF ELECTRONIC ENGINE CONTROL

Figure 5.5
Engine Functions and Control

DEFINITION OF GENERAL TERMS
Before proceeding with the details of engine control systems, certain
definitions of terms must be clarified—first some general terms and then some
specific engine performance terms.
Parameters

Design parameters such A parameter is a numerical value of some engine dimension that is fixed
as engine size, compres- by design. Examples of engine design parameters include the piston diameter
sion ratio, and so forth, (bore), the distance the piston travels on one stroke (stroke), and the length of
are fixed; therefore, they the crankshaft lever arm (throw). The bore and stroke determine the cylinder
are not subject to any volume and the displacement. Displacement is the total volume of air that is
engine operating con- displaced as the engine rotates through two complete revolutions. Compression
trol. ratio is the ratio of cylinder volume at BDC to the volume at TDC. Other
parameters that engine designers must specify include combustion chamber
shape, camshaft cam profile, intake and exhaust valve size, and valve timing. All
of these design parameters are fixed and are not subject to control while the
engine is operating.

Variables
A variable is a quantity that changes or may be changed as the engine
operates, typically under the control of the electronic control system. Some of

156 UNDERSTANDING AUTOMOTIVE ELECTRONICS