At the heart of the modern car engine rests a
computer. The effects of having a computer control
several elements of a car's operation have been
phenominal. Prior to the advent of the computer,
automobile engine operations were controlled primarily by
mechanical devices, which were rather imprecise depending
on driving conditions. For example, carburators were used
almost exclusively to meter the correct amount of fuel
that was mixed with the engine's intake air (see intake system). Ignition
timing was controlled by flywheel weights on the
distributor along with manifold vacuum (see ignition system). Finally,
emissions control devices (see emissions)
that were dependent on mechanical controls were
inefficient and served to significantly reduce the fuel
efficiency and power output of engines of that era.
What does the computer do?
The computer works like pretty much any other
computer. It takes inputs, processes them, and produces
outputs. The complexity and calculating capacity along
with the speed of modern computers are the primary
attributes that make them useful for controlling certain
aspects of a car's engine. There are still many
mechanical systems that cannot be readily controlled, but
the computer is now essential for today's cleaner, more
There are several pieces of data that a car's computer
collects for processing. Among them are:
- Air Flow sensor - The air flow sensor determines
the volume of air being drawn into the engine
through the intake system (see intake system).
- Oxygen sensor - This device tells the computer
how much oxygen is present in the car's exhaust.
An overabundance of oxygen indicates that the
air-fuel ratio is too lean (not enough fuel per
unit air) or too rich. A lean mixture will result
in a loss of power and, surprisingly, fuel
economy. An overly rich mixture will result,
again, in poor economy, as well as greatly
increased hydrocarbons in the exhaust (primarily
unburned fuel). The information provided by the
oxygen sensor also helps the computer determine
the optimum timing for ignition.
- RPM sensor - This sensor tells the computer how
fast the engine is turning. Different engine
speeds require changes in ignition timing among
- Crankshaft Position sensor - This device senses
the position of the engine's crankshaft at any
given moment. It is useful in engines that don't
have a mechanically connected distributor (see ignition system)
for determining spark timing. It also is used to
determine when to trigger each fuel injector (see
The engine speed sensor often uses the same
- Camshaft Position sensor - This sensor works in
unison with the crankshaft position sensor. Since
the camshaft turns at half the speed of the
crankshaft, its position is essential in
determining the timing of fuel injector pulses
and ignition pulses (see ignition
system). If only the crankshaft position
were monitored, the computer would likely send a
spark signal to each cylinder on both the power
and the intake stroke (see engine).
- Intake Air Temperature sensor - This device
monitors the temperature of the air as it enters
the intake system (see intake
system). This helps to more accurately
meter the amount of fuel needed for combustion.
- Engine Coolant Temperature sensor - This sensor
monitors the temperature of the engine's coolant,
which is a directly related to the engine's
temperature. Since a colder engine requires a
richer mixture to operate, this sensor is
essential in helping the computer duplicate the
function of the choke used on carburated engines
by increasing the amount of fuel per unit air
when the engine is cold.
- Knock sensor - This sensor is used to determine
when pre-ignition is occuring and sends a signal
to the computer which will retard or delay the
spark to all cylinders (see ignition system).
Pre-ignition is sometimes referred to as knock,
and is potentially harmful to the engine. It
occurs when the fuel/air mixture burns before the
piston starts its downward travel in the power
stroke. If unchecked, this pre-ignition can cause
physical damage to the piston, connecting rod, or
other internal part.
- Vehicle Speed sensor - As its name implies, this
device monitors the road speed of the vehicle. It
measures this through a magnetic pickup device
that senses the rotational speed of the output of
the transmission. The computer then determines
the speed through calculations. One function of
this sensor is to govern the vehicle's maximum
speed. When the car reaches a predetermined top
speed, the computer will partially shut down the
fuel supply, preventing the vehicle from
travelling any faster. The information from this
sensor is also used to help determine when an
electronically controlled transmission should
shift gears. Finally, some speedometers are
driven electronically instead of through the
speedometer cable. This sensor provides the
information for such speedometers.
The computer, after processing data from the various
input signals, produces several output signals. Among
- Signals to initiate spark - In cars that do not
have a mechanical distributor, the signal that
initiates firing of each spark plug originates
from the computer. This allows greater precision
in the timing of the spark (see ignition system).
- Signals to initiate fuel injectors - The timing
and duration of open valve condition at each fuel
injector is initiated by the computer, allowing
for very accurate metering of the fuel (see intake system).
- Signals to the automatic transmission - Modern
automatic transmissions (see transmission) are
controlled electronically by the car's computer.
The throttle position as well as engine RPM are
used to determine when the transmission will
shift to another gear ratio.
- Various emission control devices - A complete
description of these devices can be found under emissions.
Drivetrain Emission Controls, Engine,
Formula, Gear Math, Ignition System,
System, Suspension, Torque and Horsepower, Transmission