Engine control systems :
As manufacturers come closer to achieving complete combustion, engines are able to produce more power, use less fuel, and emit fewer pollutants. This has been made possible by technological advances, primarily electronics.
The computer is an engine control system that functions like other computers. It receives inputs, processes information, and commands an output. The primary computer in an engine control system is the engine control module (ECM) or the powertrain control module (PCM) (see photo at the end of the article).
The ECM relies on sensors that convert engine operating conditions such as temperature, engine and vehicle speeds, throttle position, and other conditions into electrical signals that are constantly monitored by the PCM. The PCM also senses some conditions through electrical connections. These include voltage changes at various components.
Based on the input information and the programs, the PCM decides the best operating parameters and sends out commands to various outputs or actuators. These commands are first sent to output drivers that cause an output device to turn on and off. These outputs include solenoids, relays, lights, motors, clutches, and heaters.
The sensors, actuators, and computer communicate through the use of electronic and multiplexed circuits. For example, when the incoming voltage signal from the coolant sensor tells the PCM that the engine is getting hot, the PCM sends out a command to turn on the electric cooling fan. The PCM does this by grounding the relay circuit that controls the electric cooling fan. When the relay clicks on, the electric cooling fan starts to spin and cools the engine. The information may also be used to alter the air-fuel ratio and ignition timing.
The look-up tables (sometimes called maps) contain calibrations and specifications. Look-up tables indicate how an engine should perform. For example, information (a reading of 20 in. Hg) is received from the manifold absolute pressure (MAP) sensor. This information and the information from the engine speed sensor are compared to a table for spark advance. This table tells the computer what the spark advance should be for that throttle position and engine speed. The computer then modifies the spark advance.
When making decisions, the PCM is constantly referring to three sources of information: the look-up tables, system strategy, and the input from sensors. The computer makes informed decisions by comparing information from these sources.
In order to control an engine system, the computer makes a series of decisions. Decisions are made in a step by step fashion until a conclusion is reached. Generally, the first decision is to determine the engine mode. For example, to control air fuel mixture the computer first determines whether the engine is cranking, idling, cruising, or accelerating. Then the computer can choose the best system strategy for the present engine mode. In a typical example, sensor input indicates that the engine is warm, rpm is high, manifold absolute pressure is high, and the throttle plate is wide open. The computer determines that the vehicle is under heavy acceleration or has a wide open throttle. Next, the computer determines the goal to be reached. For example, with heavy acceleration, the goal is to create a rich air-fuel mixture. At wide-open throttle, with high manifold absolute pressure and coolant temperature of 170°F, the table indicates that the air-fuel ratio should be 13:1, that is, 13 pounds of air for every 1 pound of fuel. An air-fuel mixture needed for heavy acceleration.
In a final series of decisions, the computer determines how the goal can be achieved. In our example, a rich air-fuel mixture is achieved by increasing fuel injector pulse width. The injector nozzle remains open longer and more fuel is drawn into the cylinder, providing the additional power needed.