Electronic Controls

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 efficient engines.

Inputs

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 other things.
  • 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 intake system). The engine speed sensor often uses the same device.
  • 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.

Outputs

The computer, after processing data from the various input signals, produces several output signals. Among them are:

  • 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.

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