Page 343 - Understanding Automotive Electronics

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9 AUTOMOTIVE INSTRUMENTATION

Another important trip parameter that this system can display is the miles
to empty fuel tank, D. This can be found by calculating
D = MPG × Q
where Q is the quantity of fuel remaining in gallons. Since D depends on MPG,
it also changes as operating conditions change (e.g., during heavy acceleration).
In such cases, the calculation of miles to empty based on the above simple
equation is grossly incorrect. However, this calculation gives a correct estimate
of the miles to empty for steady cruise along a highway in which operating
conditions are constant.
Still another pair of parameters that can be calculated and displayed by
this system are distance to destination, D , and estimated time of arrival, ETA.
d
These can be found by computing
D = D – D P
d
T
ETA = T + (D /S )
1
d
where
D is the trip distance (entered by the driver)
T
D is the present position (in miles traveled since start)
P
S is the present vehicle speed
T is the start time
1
The computer can calculate the present position D by subtracting the start
P
mileage, D (obtained from the odometer reading when the trip computer was
1
initialized by the driver), from the present odometer mileage.
The average fuel cost per mile C can be found by calculating
C = (D /MPG) × fuel cost per gallon
P
There are many other useful and interesting operations that can be
performed by the variety of available systems. Actually, the number of such
functions that can be performed is limited primarily by cost and by the
availability of sensors to measure the required variables.

AUTOMOTIVE DIAGNOSTICS
The instrumentation In certain automobile models, the instrumentation computer can perform
computer can also be the important function of diagnosis of the electronic engine control system.
used as a diagnostic aid This diagnosis takes place at several different levels. One level is used during
during vehicle manufac- manufacturing to test the system, and another level is used by mechanics or
turing, operation, or interested car owners to diagnose engine control system problems. Some levels
repair. operate continuously and others are available only on request from an external
device that is connected to the car data link for diagnostic purposes by a
technician. This application is explained in the next chapter.

330 UNDERSTANDING AUTOMOTIVE ELECTRONICS