Page 202 - Understanding Automotive Electronics

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SENSORS AND ACTUATORS 6

Variables to be Measured

The set of variables sensed for any given engine is specific to the
associated engine control configuration. Space limitations for this book
prohibit a complete survey of all engine control systems and relevant sensor and
actuator selections for all car models. Nevertheless, it is possible to review a
superset of possible sensors, which is done in this chapter, and to present
representative examples of practical digital control configurations, which is
done in the next chapter.
The superset of variables sensed in engine control includes the
following:
1. Mass air flow (MAF) rate
2. Exhaust gas oxygen concentration (possibly heated)
3. Throttle plate angular position
4. Crankshaft angular position/RPM
5. Coolant temperature
6. Intake air temperature
7. Manifold absolute pressure (MAP)
8. Differential exhaust gas pressure
9. Vehicle speed
10. Transmission gear selector position
In addition to measurements of the above variables, engine control is also
based on the status of the vehicle as monitored by a set of switches. These
switches include the following:
1. Air conditioner clutch engaged
2. Brake on/off
3. Wide open throttle
4. Closed throttle

AIR FLOW RATE SENSOR
In Chapter 5 we showed that the correct operation of an electronically
controlled engine operating with government-regulated exhaust emissions
requires a measurement of the mass flow rate of air (R ) into the engine. The
m
majority of cars produced since the early 1990s use a relatively simple and
inexpensive mass air flow rate (MAF) sensor. This is normally mounted as part
of the air cleaner assembly, where it measures air flow into the intake manifold.
It is a ruggedly packaged, single-unit sensor that includes solid-state electronic
signal processing. In operation, the MAF sensor generates a continuous signal
that varies nearly linearly with true mass air flow R .
m
The MAF sensor is a variation of a classic air flow sensor that was known
as a hot wire anemometer and was used, for example, to measure wind velocity

UNDERSTANDING AUTOMOTIVE ELECTRONICS 189