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| To comment on this and other articles, visit our blog at fleetservicenorthwest.spaces.live.com/blog Electronic Fuel Injection
A basic electronic fuel injection system for a gasoline/ethanol-powered engine consists of sensors, actuators, and one or more control modules, sometimes referred to as "computers".
Control Modules
The control module receives and interprets data from a variety of sensors, and sends commands to a variety of actuators based on its programming and sensor data. Some control modules are reprogrammable, meaning they can accept software updates via the diagnostic data link connector. Most vehicles use either a Powertrain Control Module, which controls engine and transmission functions, or an Engine Control Module, which does not control transmission functions.
Sensors
Sensors provide information to the control module about intake air flow, various pressures and temperatures, and exhaust oxygen content, among other data. Below are descriptions of some of the major sensors required to manage an EFI system.
- Mass Air Flow Sensor (MAF sensor) - The MAF sensor calculates the actual mass of the air molecules flowing into the engine intake. A wire exposed to the flowing air is heated with an electric current, and the air flowing over the wire cools it, increasing sensor voltage. This voltage is transmitted to the control module, where it is interpreted as a mass per time value, usually grams per second.
- Intake Air Temperature Sensor (IAT sensor) - The IAT sensor measures the temperature of air flowing into the engine intake. These are usually thermostatic resistors, whose resistance varies based on temperature. This provides a reference for the MAF sensor data, since warmer air will cool the MAF sensor wire less than cooler air, and vice versa. Some IAT sensors are incorporated into MAF sensors.
- Throttle Position Sensor (TP sensor) - The TP sensor uses a variable resistor to vary a reference voltage provided by the control module. As the throttle is opened, the resistance lowers and the voltage transmitted back to the control module increases. The control module uses this voltage to determine how far the driver has moved the throttle, and adjusts fuel injector pulse and idle air control motor position accordingly.
- Engine Coolant Temperature Sensor (ECT sensor) - The ECT sensor measures the temperature of the coolant inside the engine block. This sensor is usually a thermostatic resistor. The control module is programmed for different air/fuel mixture ratios for different engine coolant temperatures, as well as different idle speeds. A cooler engine requires a richer air/fuel ratio and higher idle speed to operate efficiently. As the engine warms, a leaner air/fuel ratio and lower idle speed improves fuel economy and lowers harmful emissions.
- Oil Pressure Switch - Some EFI systems use oil pressure to turn on the fuel pump. When the engine begins to crank, a small amount of oil pressure is produced almost immediately. The oil pressure switch closes, allowing electrical current to be sent to the fuel pump relay, which turns on the fuel pump. If the oil pressure switch fails, the control module will command the fuel pump on by sending voltage to the fuel pump relay, once it determines the engine RPM has reached a preset minimum value. This will usually cause a long-cranking condition.
- Crankshaft Position Sensor (CKP sensor) - The CKP sensor uses a hall-effect switch to send a varying voltage to the control module. A hall-effect switch uses an electrical coil with a reference voltage provided by the control module to produce a magnetic field. A toothed ring on the crankshaft moves through this magnetic field, and the disruption in the magnetic field caused by the high and low spots on the metal ring causes the voltage to vary accordingly. Some teeth are different sizes, and act as reference points in the rotation of the crankshaft. This is different from an ABS/traction control system wheel speed sensor (WSS), which generates its own AC voltage. See Antilock Brake Systems.
- Camshaft Position Sensor (CMP sensor) - The CMP sensor works similarly to the CKP sensor. The control module uses CMP sensor data to determine which stroke the crankshaft is on, since the camshaft rotates at ½ the speed of the crankshaft. The CMP sensor data is also used to time ignition spark and fuel injector pulse.
- Oxygen Sensor or Heated Oxygen Sensor (O2S or HO2S) - The O2 sensor measures the amount of oxygen left over in the exhaust after combustion. The O2 sensor is made of materials that, when hot, produce a small electrical voltage in the absence of oxygen. This voltage is transmitted to the control module, which interprets higher voltage as a low exhaust oxygen content, and lower voltage as a high exhaust oxygen content. A lower oxygen content means there was a richer air/fuel mixture than was appropriate for the given engine temperature, and a higher oxygen content means there was a leaner air/fuel mixture than was appropriate. The control module adjusts the injector pulse width, or the length of time the injectors are open, accordingly, to deliver more or less fuel as required. a heated oxygen sensor, or HO2S, has a built-in heater to allow for faster operation after cold engine starts.
Actuators
The control module is able to control engine operation with a variety of devices known as actuators. These actuators supply and deliver fuel, adjust airflow, deliver ignition spark, and perform other engine performance and emissions operations. Below are descriptions of some of the actuators required to control an EFI system.
- Idle Air Control motor (IAC) - The IAC controls airflow when the throttle is closed. A passage inside the throttle body, which contains the throttle plate that is opened or closed by the throttle pedal, allows air to bypass the throttle plate. a valve attached to the IAC opens or closes this passage to regulate airflow into the engine. The IAC is also opened fully under higher throttle application, and closes slowly when the throttle is closed to allow engine speed to reduce smoothly, and prevent an excessively rich air/fuel mixture, which improves fuel mileage and reduces harmful emissions.
- Fuel Pump - An EFI system has an electric fuel pump which remains submerged inside the fuel tank. The control module commands the fuel pump on by sending voltage to the fuel pump relay. The fuel pump is normally commanded on every time the ignition key is turned on, to prime the fuel system. The control module then commands the fuel pump on when engine RPM reaches a preset minimum value. Some systems may use an oil pressure switch to turn the fuel pump on more quickly. Most systems use a fuel pressure regulator to manage fuel rail pressure. The fuel pump flows a high volume of fuel at all times, and any unused fuel bypasses the regulator and returns to the fuel tank. The control module commands the fuel pump off when engine RPM drops below a certain value, or the ignition key is turned off. Some EFI systems have a timed delay which keeps the fuel pump running for several seconds after the engine stops. This allows cooler fuel to be pumped into the fuel rail, helping to prevent vapor-lock, which is caused by fuel vaporizing inside the fuel rail. An oil pressure switch, when closed, will keep the fuel pump running regardless of control module command. For more information, see Fuel Supply Systems.
- Fuel Injector - EFI systems use one or more fuel injectors to deliver fuel to the engine. A typical fuel injector consists of a pintle valve which is pulled open using an electric solenoid. When voltage is applied to the solenoid, it produces a magnetic field, which pulls the pintle valve open. A spring is normally used to return and hold the pintle valve in the closed position. The control module sends an electrical signal to the fuel injector, which is normally timed based on CMP sensor data. The length of time this signal is applied to the injector is known as pulse width. The pulse width is determined based on preset values for the various sensor data available to the control module, and is adjusted based on varying Throttle Position sensor data and oxygen sensor data.
- Ignition Coil - The ignition coil produces ignition spark which may be delivered to the spark plug via a distributor and spark plug wires, or on distributorless ignition systems, either directly to the spark plug or via a spark plug wire. The ignition spark is controlled by a separate ignition control module on some systems.
Summary
The sensors and actuators described above are not used on all EFI systems, and there are many other sensors and actuators that are used in different types of EFI systems, emissions control systems, driver information, and additional engine management requirements. Some EFI systems use multiple control modules which may or may not be connected through a Controller Area Network, or CAN. See additional Tech Info articles for information on these sub-systems and much more, such as Fuel Management.
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