Page 173 - Understanding Automotive Electronics

2735 | CH 5 Page 160 Tuesday, March 10, 1998 11:10 AM

5 THE BASICS OF ELECTRONIC ENGINE CONTROL

Calibration

The definition of engine calibration is the setting of the air/fuel ratio and
ignition timing for the engine. With the new electronic control systems,
calibration is determined by the electronic engine control system.

ENGINE MAPPING
Engine mapping is a The development of any control system comes from knowledge of the
process by which mea- plant, or system to be controlled. In the case of the automobile engine, this
surements are made of knowledge of the plant (the engine) comes primarily from a process called
important engine vari- engine mapping.
ables while the engine is For engine mapping, the engine is connected to a dynamometer and
operated throughout its operated throughout its entire speed and load range. Measurements are
speed and load ranges. made of the important engine variables while quantities, such as the air/fuel
ratio and the spark control, are varied in a known and systematic manner.
Such engine mapping is done in engine test cells that have engine
dynamometers and complex instrumentation that collects data under
computer control.
From this mapping, a mathematical model is developed that explains the
influence of every measurable variable and parameter on engine performance.
The control system designer must select a control configuration, control
variables, and control strategy that will satisfy all performance requirements
(including stability) as computed from this model and that are within the other
design limits such as cost, quality, and reliability. To understand a typical engine
control system, let’s look at the influence of control variables on engine
performance.
Effect of Air/Fuel Ratio on Performance

Figure 5.8 illustrates the variation in the performance variables of torque
(T) and brake power (BSFC) as well as engine emissions with variations in the
air/fuel ratio with fixed spark timing and a constant engine speed.
In this figure the exhaust gases are represented in brake-specific form.
This is a standard way to characterize exhaust gases whose absolute emission
levels are proportional to power. The definitions for the brake-specific
emission rates are

BSHC = brake-specific HC concentration
r
HC
= --------
P b
BSCO = brake-specific CO concentration
r CO
= --------
P
b

160 UNDERSTANDING AUTOMOTIVE ELECTRONICS