Computer controlled engines and the need for electronic diagnostic equipment. Before looking at the diagnostic equipment a basic understanding of engine control is useful.

Due to the ongoing drive to cut down on harmful exhaust emissions a greater need of control over the engine was needed. By legislation a catalytic converter was fitted to the exhaust of all petrol cars and light vans since 1990. (See catalytic convector) this greatly reduces the amount of carbon monoxide and unburned hydrocarbons (petrol) but for it to perform it's duty properly over many tens of thousands of miles the amount of air and the amount of fuel introduced into the engine has to be controlled as close as possible to a ratio of 14.7 pounds of air to 1 pound of fuel.

To enable this to be carried out accurately most manufacturers were using some form of electronic control. Electronics can control the ignition and fuelling. Most systems used either a single injector mounted in the inlet manifold where the carburettor would previously had been or one injector per cylinder fitted so that it injects the fuel at the back of the inlet valves. Injectors are simpler and so much easier to control than an electronically controlled carburettor.

By using a computer to control the vehicle engine a far greater control over the air/fuel ratio can be achieved. A computer program (map) was written to account for every situation that the engine would encounter. For example when the engine is cold it requires more fuel. This would have previously been achieved by the choke on a carburettor. In the electronic injection system a number of sensors are employed. When the ignition is switched on the computer gets a wake-up call, just like your home PC it goes into a self check routine (boot up). This has to be done very quickly so that it is ready to check and receive the information from the engine's sensors. One of the sensors is the water temperature sensor (CTS coolant temperature sensor) the information from this sensor tells the computer how cold or hot the engine is so that along with additional information received from other sensors the right amount of fuel can be injected. There are many other sensors that feed information to your car computer that enables it to run properly.

All this information going into the car computer has to be turned into an action. For example to inject fuel. These actions are carried out by actuators. The main actuators are the injector/s and the control device that maintains the engines idle speed. Other actuators could control the venting of the fuel tank and the process of returning a small proportion of the exhaust gas into the inlet manifold to reduce excessive heat in the combustion chamber. (EGR)

Early engines fitted with catalytic converters in the exhaust relied on the program that had been written for the computer (map) this program would regulate the fuelling only by using the information from the various sensors. This was known as an unregulated system. The cars computer did its job well, but it was unaware of the final result. Manual adjustment of the idle speed and fuel mixture was normal.

To assist in achieving a greater accuracy of fuelling and to increase the efficiency of the catalytic converter a new system was introduced. This is known as a closed loop system. This employs a device called an oxygen sensor. This sensor is fitted into the exhaust pipe before the catalytic converter and measures the amount of oxygen that is in the exhaust gases. The information from the oxygen sensor is fed back to the car computer and using this information the computer can now add or reduce the amount of fuel to keep the engine at 14.7/1 working to perfection over a long period of time. Vehicles fitted with this system have a different emission test on the annual M.O.T. Manual adjustment is not usually possible.

When you receive your M/O.T. pass or failure you should also receive a copy of the emission test report. This will indicate the amount of carbon monoxide and hydrocarbons that are present in your exhaust. It will also show a value called lambda. This value shows whether engine is working correctly. The value of lambda 1 is used to represent the 14.7/1 air fuel ratio previously mentioned. The amount of carbon monoxide and hydrocarbons will vary if measured before or after the catalytic converter. This is due to the catalytic converter doing it's job and reducing the harmful gases as they pass through the exhaust system As the efficiency of the catalytic converter relies on the engine running at 14.7/1 ratio or Lambda 1 also represented as 1 this calculation is required because a reading of lambda 1 before the catalytic converter will also be lambda 1 after the catalytic converter. In the M.O.T. test a small window is allowed. Usually .97 to 1.03.

Now that the computer can see the results of its action further enhancements can be made to the system. For example, as the engine wears or as the engine sensors change their values over time, the computer will be come aware that it is having to increase the quantity of fuel injected more often than decrease or vice versa. So the computer has now got the ability to store a new value to keep the system at 1. Also it can store a fault code in its memory if a problem has occurred with a sensor even if the driver was unaware.

From year 2000 EOBD (European onboard diagnostics) were fitted to all new cars. This system monitors the sensors that are primarily responsible for the car emissions. If a fault is detected that might affect the emissions it turns on a light to warn the driver of and at the same time records the date and time this occurs. A diagnostic socket is fitted to allow the technician to access the codes stored in the computers memory and a snapshot of the live data immediately before this occurred.

For a more detailed look. http://www.howstuffworks.com/car-computer.htm
To learn more about. Click on picture.