An engine computer, or an engine control unit (ECU) is a type of electronic control unit that controls a series of actuators on and internal combustion engine,in order to ensure the optimum running of the engine. It does this by reading values from a multitude of sensors. The ECU has five basic functions, control of air to fuel ratio, control of ignition timing, control of idle speed, and control of variable valve timing.
1: Control of air to fuel ratio For an engine with fuel injection, the ECU will determine the quantity of fuel to inject. As you push down on the pedal, the mass flow sensor measures the amount of air that is being sucked into the engine and the ECU will inject more fuel into the engine. Likewise if the engine coolant temperature sensor is showing that the engine is not warmed up yet, the ECU will inject more fuel until the engine is warmed up. Air to fuel ratio on computer controlled carburetors work basically the same way but with a mixture control solenoid incorporated in the float bowl of the carburetor.
2: Control of ignition timing An engine requires a spark to initiate combustion in the combustion chamber. The ECU will adjust the exact timing of the spark to provide better power and economy. This is called the ignition timing.
3:Control of idle speed Most engine systems have idle speed control built into the ECU. Idle speed is controlled by a programmable throttle stop or an idle air bypass control stepper motor. Carburetor based systems use a programmable throttle stop using a bidirectional DC motor. Throttle body injection systems use an idle air control stepper motor. Idle speed control must anticipate the engine load at idle. The “load “ varies because of all the other systems using the motors power, for instance, the HVAC systems, power steering, power brakes, and the electrical charging and supply systems. It also provides cruise control functions and top speed limitation.
4:Control of variable valve timing. Some engines have variable valve timing. In such an engine, the ECU controls the timing in the engine cycle at which the valves open. The valves are usually opened sooner at higher speed than at lower speed. This can optimize the flow of air into the cylinder, increasing power and economy.
A special category of ECUs are those which are programmable. These units do not have a fixed behavior and can be reprogrammed by the user. Programmable ECUs are required where significant aftermarket modifications have been made to a vehicles engine. These can be programmed with a laptop connected using a serial or USB cable, while the engine is running.
Hybrid digital designs were widely used in the mid 1980s. They used analog techniques to measure and process input parameters from the engine, then used a look-up table stored in a digital ROM chip to yield precomputed output values. Modern ECUs use a microprocessor which can process the inputs from the engine sensors in real time.
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