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Electronic Fuel Injection - an Introduction

Electronic fuel injection (EFI) is simple, in theory. The design goal for an EFI system is to deliver the correct air fuel ratio for varying load, speed and temperature. Most modern EFI systems are  incorporated in an ECU which also controls the ignition system, and may control may other functions such as anti-lock brakes, traction control systems, the transmission, etc. A fuel injection system itself consists of a source of pressurized fuel, fuel injectors and the electronic controller. The fuel system is composed of the fuel tank, a high pressure pump, and some method for regulating fuel pressure. The injectors can be though of as electrically operated valves. The amount of fuel delivered to the engine is determined by size of the injectors, the amount of time that the injectors are open, and on the fuel pressure. The electronic controller to determines how much fuel is needed and commands the injectors to remain open for the necessary amount of time to deliver the needed fuel volume. The amount of time that the injector is open is called the pulse width and is usually expressed in milliseconds (one thousandths of a second).

A typical fuel injector.

Electronic fuel injection has come into its' own in the last 10-15 years for the same underlying reason as most of the other amazing technological changes - the development of the microchip. The microchip allows the engine control unit (ECU) to make the necessary calculations to determine how much fuel is needed. A lot of computing power is required to do this. The Electromotive TEC3, one of our favorite aftermarket ECU's, can make more calculations per second than the desktop computers of just a decade ago. Once these computations are accomplished, relatively simple circuitry is used to open and close the injectors for the requisite time. 

The fuel requirements of an engine are determined by the displacement, rpm, load, and inherent engine efficiency. In order to operate effectively, the ECU needs inputs that represent these variables. Depending on the design, the minimum inputs required include the sensors listed below.

                               MAP sensor             throttle position sensor           mass air sensor

• Mass Air Flow (MAF - mass of air flow per unit of time such as grams/second)

• Manifold Absolute Pressure (MAP - an electronic pressure gauge to measure vacuum or positive pressure in the intake)

• Throttle Position Sensor (TPS - a rheostat that varies out put according to how far open the throttle is)

A simple EFI system can operate fairly effective with just these three inputs. However, many systems include additional inputs from the following types of sensors. Most sensors are of one of two types. Some are simple on-off switches or counters, such as a gear indicator or tachometer. Most measure continuous variables like temperature or airflow. These sensors produce a 0-5v output. This signal is read by the ECU (computer). By referring to a calibration table, the ECU translates the signal into a value representing the physical parameter which is being measured, such as airflow in units of mass/time or temperature. The tuning process boils down in most cases to changing the values in these tables. 

• Mass Air Flow (MAF - a sensor which directly measures airflow into the motor)

• Intake Air Temperature (IAT - an electronic temperature sensor to measure the temperature of the air entering the motor)

• Coolant Temperature Sensor (ACT - an electronic temperature sensor to measure the temperature of the engine coolant)

• Vehicle Speed Sensor (VSS - an electronic speedometer)

• Oxygen sensor (O2 sensor - measures the amount of oxygen in the exhaust which reflects the A:F (air:fuel) ratio)

• Knock sensor (a microphone which picks up the characteristic sound of engine knock)

• Crankshaft position (a magnetic sensor that tells the ECU which cylinder is ready to fire or receive fuel)

• Transmission sensors (these may include gear selection, pressure, and temperature sensors)

• Emission control device operation (a variety of functions may be monitored by different sensors)

                           Electromotive TEC3 ECU and direct fire ignition. The toothed wheel is part of the crank position sensor.