Page 192 - Understanding Automotive Electronics

2735 | CH 5 Page 179 Tuesday, March 10, 1998 11:10 AM

THE BASICS OF ELECTRONIC ENGINE CONTROL 5

The intake manifold air density is determined by the absolute pressure
and temperature of the intake air. The intake manifold absolute pressure is
determined by the ambient air pressure (i.e., the air outside the engine), the
throttle position as set by the driver, the RPM, and by the shape and size of
the intake manifold. The intake air temperature is determined by the
ambient air temperature and by the pressure change from ambient across the
throttle.
The intake air density can be computed from the basic physics of air
known as the perfect gas law. The density of any gas (including air) is directly
proportional to pressure and inversely proportional to absolute temperature.
(Absolute temperature is the temperature relative to absolute zero.) Using the
Fahrenheit scale, absolute temperature is the temperature added to 459˚ in
degrees Fahrenheit.
The intake air can be computed relative to a standard condition.
Normally, the standard condition is sea level on a so-called standard day
(SLSD). The SLSD conditions are denoted d , p , T , referring to density,
o
o
o
absolute pressure, and absolute temperature. These parameters are constants for
air for the entire planet and are known to great precision. In mathematical
terms, the intake air density is given by
p T
 o
d = d × ---- × -----
a o p 
o T
i
That is, intake air density is found by multiplying standard density by the
ratio of intake manifold pressure to standard pressure and by the ratio of
standard temperature to intake manifold air temperature. Such a calculation is
readily performed in a digital engine controller based on measurements of
intake manifold absolute pressure (MAP) and intake air absolute temperature
(IAT).
A relatively close estimate of R can be made using inexpensive sensors. As
v
discussed previously, the engine acts like an air pump during intake. If it were a
perfect pump, it would draw in a volume of air equal to its displacement for
each two complete crankshaft revolutions. Then, for this ideal engine, the
volume flow rate would be

D
 RPM  
R = ------------ ---- ideal volume flow rate

v  60  2
where
R is the volume flow rate
v
D is the engine displacement
RPM is the engine speed
For this ideal engine, with D known, R could be obtained simply by
v
measuring RPM.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 179