Page 161 - Understanding Automotive Electronics

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5 THE BASICS OF ELECTRONIC ENGINE CONTROL

Unfortunately, the combustion of the SI engine is not perfect. In
addition to the CO and H O, the exhaust contains amounts of carbon
2
2
monoxide (CO), oxides of nitrogen (chemical unions of nitrogen and
oxygen that are denoted NO ), unburned hydrocarbons (HC), oxides of
x
sulfur, and other compounds. Some of the exhaust constituents are
considered harmful and have come under the control of the federal
government. The exhaust emissions controlled by government standards are
CO, HC, and NO .
x
Automotive exhaust emission control requirements started in the United
States in 1966 when the California state regulations became effective. Since
then, the federal government has imposed emission control limits for all states,
and the standards became progressively more difficult to meet through the
decade 1970–1980. Auto manufacturers found that the traditional engine
controls could not control the engine sufficiently to meet these emission limits
and maintain adequate engine performance at the same time, so they turned to
electronic controls.

Fuel Economy
Everyone has some idea of what fuel economy means. It is related to the
number of miles that can be driven for each gallon of gasoline consumed. It is
referred to as miles per gallon (MPG) or simply mileage. Just like it improves
emission control, another important feature of electronic engine control is its
ability to improve fuel economy.
Electronic engine con- It is well recognized by layman and expert alike that the mileage of a
trol is used to reduce vehicle is not unique. It depends on size, shape, weight, and how the car is
exhaust emissions and driven. The best mileage is achieved under steady cruise conditions. City
improve fuel economy, driving, with many starts and stops, yields worse mileage than steady highway
both of which have lim- driving.
its set by the govern- The government fuel economy standards are not based on just one car,
ment. but are stated in terms of the average rated miles per gallon fuel mileage for the
production of all models by a manufacturer for any year. This latter
requirement is known in the automotive industry by the acronym CAFE
(corporate average fuel economy). It is a somewhat complex requirement and is
based on measurements of the fuel used during a prescribed simulated standard
driving cycle.

FEDERAL GOVERNMENT TEST PROCEDURES
For an understanding of both emission and CAFE requirements, it is
helpful to review the standard cycle and how the emission and fuel economy
measurements are made. The U.S. federal government has published test
procedures that include several steps. The first step is to place the automobile
on a chassis dynamometer, like the one shown in Figure 5.1.

148 UNDERSTANDING AUTOMOTIVE ELECTRONICS