Page 781 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 2 3. 1 Automotive instrumentation and telematics

The computer reads the binary number from the A/D fuel quantity because usually it is not important for the
converter that corresponds to sensor voltage and uses it to driver to know the actual temperature at all times. For safe
address a particular memory location. Another binary operation of the engine, the driver only needs to know that
number corresponding to the actual fuel quantity in thecoolant temperatureislessthanacriticalvalue. A block
gallons for that sensor voltage is stored in that memory diagram of the measuring system is shown in Fig. 23.1-11.
location. The computer then uses the number from The coolant temperature sensor used in most cars is
memory to generate the appropriate display signaldeither a solid-state sensor called a thermistor; the resistance of
analog or digital, depending on display typedand sends this sensor decreases with increasing temperature.
that signal via DEMUX to the display. Fig. 23.1-12 shows the circuit connection and a sketch of
Computer-based signal processing can also compen- a typical sensor output voltage versus temperature curve.
sate for fuel slosh. As the car moves over the road, the The sensor output voltage is sampled during the ap-
fuel sloshes about and the float moves up and down propriate time slot and is converted to a binary number
around the average position that corresponds to the equivalent by the A/D converter. The computer com-
correct level for a stationary vehicle. The computer pares this binary number to the one stored in memory
compensates for slosh by computing a running average. It that corresponds to the high temperature limit. If the
does this by storing several samples over a few seconds coolant temperature exceeds the limit, an output signal is
and computing the arithmetic average of the sensor generated that activates the warning indicator. If the
output. The oldest samples are continually discarded as limit is not exceeded, the output signal is not generated
new samples are obtained. The averaged output becomes and the warning message is not activated. A proportional
the signal that drives the display. It should be noted that display of actual temperature can be used if the memory
this is actually a form of digital filtering. contains a cross-reference table between sensor output
voltage and the corresponding temperature, similar to
23.1.5 Coolant temperature that described for the fuel quantity table.
measurement
23.1.6 Oil pressure measurement
Another important automotive parameter that is mea-
sured by the instrumentation is the coolant temperature. Engine oil pressure measurement is similar to coolant
The measurement of this quantity is different from that of temperature measurement in that it uses a warning

Fig. 23.1-11 Coolant temperature measurement.

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