Auto questions: how car computers work

Today's cars make intense use of modern electronics. Here's a primer on how the computer in your automobile works.

The computer in your car is actually very similar in function to the computer on your desktop. The difference is that while your desktop PC is a multifunction machine capable of word processing, Internet connection, etc., the automobile computer is specialized to perform only one very complicated function, controlling your car.

The capabilities of the car computer vary widely depending on the make and model of automobile. In some cars, the computer may control only the fuel and ignition systems while in others it can also control the temperature in the passenger compartment, the instrument panel and even the braking system. Let’s look at exactly how the computer performs some of these functions.

Computers first made their mark in the auto industry in the mid-‘70’s when engineers were seeking ways to control automobile exhaust emissions. They realized that they needed a way to more precisely control the introduction of fuel into the engine and began experimenting with electronic fuel injection. An automobile must operate over a wide range of conditions from idling at the stop light to full throttle passing acceleration on the highway. Controlling the fuel flow over such a huge variation in requirements seemed a tailor made job for a computer. Before fuel injection, cars used carburetors to control the flow of gasoline. Changing the characteristics of a carburetor required actual physical changes to the design. Often making a change to improve performance in one area would have and undesirable effect on another area. In a computer controlled system, changing the operation required only a change in program rather than a mechanical change. Several years of experimentation and improvement led to reliable inexpensive computer controlled fuel injections systems that significantly improved automobile performance. Their success with this effort along with the increase in speed and power of computers encouraged them to try controlling other auto functions in a similar manner.

It turns out that having more precise control of the ignition system also led to better performance and control of exhaust emissions. When coupled with high energy ignitions and spark plugs, they also achieved significant decreases in required maintenance. As with home computers, the automobile computer continued to increase in power and speed and decrease in price placing more computer power at the disposal of the engineers. They responded by developing antilock braking systems, digital instrumentation, automatic climate control and a host of other computer controlled features.

But this revolution would not have been possible without advances in many other fields. In order to perform these control functions, the computer must receive and output information. On your home computer, you input information via your mouse and keyboard and receive output from a printer. An automobile computer receives its input from sensors and sends out signals that control fuel injectors, spark coils or digital speedometers. As computers were developing there was a simultaneous development in sensors that greatly improved the quality and reliability of the information input to the computer. As an example, to control a fuel injection system the computer must know how much air is entering the engine at any particular time. This can either be measured directly by an air flow sensor or calculated from measurements of such things as air temperature, pressure and engine speed. In either case, these pieces of information are supplied by sensors attached to the engine and connected to the computer. The computer then calculates the proper signals to send to the fuel injectors that actually allow fuel to enter the engine. Computer controlled ignition systems require sensors that measure the engine speed and piston position. The computer then calculates the precise instance at which to send a signal to fire the spark plug and ignite the gasoline. Sensors mounted on each wheel send signals to the antilock braking system. If the computer detects that one wheel is beginning to move more slowly than the rest (in other words, it’s skidding), the computer signals the braking system to release pressure on that wheel to stop the skid. It does this on each wheel separately and simultaneously and it obviously does it much faster than the driver could.

Every function that the computer performs is controlled by a program. These programs are written by the engineers who design the systems. Unlike the programs on your personal computer which are stored on disk drives, these programs are stored in special electronic circuits called ROMs (Read Only Memory). This provides for instant startup and high reliability.
When the use of computers in cars was first being considered, computer manufacturers were very confident of success. After all, they had sent computers to the moon. A series of humiliating failures soon convinced them that this project wasn’t as easy as it seems. The automobile computer must operate reliably from -40 degrees to 140 degrees. It must be impervious to water, oil, dirt and a variety of other contaminants. It must not fail and leave the car without brakes or lights or any other safety related items. It must not malfunction when subjected to electrical interference from radios or ignition systems and it must start immediately and operate continuously and reliably under all conditions. These are just a few of the issues that automobile electronics designers faced. Despite these huge challenges, the computer and automobile engineers persisted and today’s cars are safer, cleaner and more reliable than they’ve ever been. And the use of automobile computers is behind most of these advances.