EFI Terminology

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Description and explanation

Electronic Control Unit (ECU)

The computer or brains of the EFI system. This is where the fuel maps exist and all fuel calculations are made. The fuel map provides the ECU with the information to adjust the air/fuel ration of the engine. All OEM ECUs are able to control both the fuel injection and ignition systems. This combination of electronic control systems is sometimes referred to as an Engine Management System (EMS). Aftermarket EFI systems may be a complete EMS or only control the fuel injection.

Engine Temperature Sensor

The engine temperature sensor lets the ECU know what the operating temperature of the engine is. On air cooled engines, the sensor is usually a Cylinder Head Temperature (CHT) sensor. On water cooled engines, the sensor is usually located at the radiator.

Intake Air Temperature (IAT)

The temperature of the air going into the engine is important to the ECU for making fine adjustments to the fuel mixture. When the air temperature is cold the amount of fuel going to the engine is increased. As the air temperature goes up, the amount of fuel can be reduces. This can also be referred to as the Manifold Air Temperature (MAT).

Oxygen Sensor (O2)

Narrow Band O2 sensor NBO2

Wide Band O2 sensor WBO2

The Oxygen Sensor determines the air/fuel ratio (AFR) of the engine by reading the hot gases in the exhaust system. O2 sensor can be one of two types. There is a narrow band (NBO2) sensor that is only able to accurately determine a vary narrow range of AFR’s. This is the type of sensor found on the majority of vehicles on the road today, but is not suitable for high performance work. The second type of O2 sensor is a wide band (WBO2) or UEGO (universal exhaust gas oxygen). This type of sensor is able to accurately determine AFR over a very wide operating range. These O2 sensors are suitable for use in high performance EFI engines. The newer ‘self learning’, no tuning required aftermarket EFI systems use this type of O2 sensor.

When an EFI system is operating in "closed loop" mode, it uses an O2 sensor to maintain the most efficient fuel mixture for reduced emissions and fuel mileage. While O2 sensor are currently rare on motorcycle EFI systems, governmental regulations for reduced emissions may result in more engines with these sensors on them in the future. All 2007 Harley-Davidson's use NBO2 sensors in closed loop operation mode.

Manifold Absolute Pressure (MAP)

The MAP sensor provides the ECU with an indication of how much vacuum is in the intake manifold. A high vacuum indicates small throttle openings or low loads. A low vacuum indicates large throttle openings or high loads. As the vacuum approaches zero, the engine has a higher load on it.
If the engine is turbo-charged or super-charged, this signal can become a positive pressure. The fact that MAP sensors understand positive and negative pressures in the intake manifold make them ideal for forced induction fuel systems.

Throttle Position Sensor (TPS)

The throttle position sensor provides the ECU with an indication of how much the throttle has been opened. The TPS sensor is a simple potentiometer that provides a 0-5 volt signal to the ECU with the higher voltages indicating more throttle.

Crank Position Sensor

This sensor provides the ECU with the exact position of the engine as it rotates through the "Otto" cycle and the RPMs of the engine. On a sequential fuel injection system the crank position sensor allows the ECU to precisely time the opening of the fuel injectors to the opening of the intake valves.

In a batch fire fuel injection system the crank position sensor may be nothing more than a connection to the ignition coil or tachometer output signal from the ignition module. Figure 1 shows a ignition coil trigger signal.

Throttle Body

The throttle body is nothing more than the butterfly that opens and closes based on how much throttle is given to the engine.

Fuel Injector

Fuel injectors is an electro-mechanical valve that provides metering of the fuel into the engine. The ECU can turn the valve on and off with great accuracy, allowing a precise amount of fuel into the engine. Based on the operating parameters of the engine, a specific amount of time for the injector to be opened is determined by the ECU. While the ECU determines this time in milliseconds, often the amount of time the injector is opened is referred to as the "duty cycle" or percentage of time the injector is open. The higher the duty cycle, the more fuel that is allowed into the engine.

Fuel injectors are sized to flow a specific amount of fuel within a specific time frame at a specified fuel pressure. The standard measurements are either pounds per hour or cc’s per minute. The sizing of the injectors does set an ultimate horsepower limit on an engine. If the size of the fuel injectors is too small, an engine will stop making power at a predetermined limit which is based on the injector size.

Fuel Pump

A high volume and pressure of fuel is required to run an EFI system. EFI fuel pumps are sized in liters (LPH) or gallons (GPH) and are capable of producing 60-90 pounds of pressure in the fuel line. A typical automotive fuel pump for a carburetor produces 5-7 pounds of pressure. A typical motorcycle carburetor is gravity fed with no pressure.

Fuel Pressure Regulator

The fuel pressure regulator is required to reduce the fuel pump pressure at the fuel injectors. While the fuel pump may be capable of 90 PSI, the pressure regulator reduces this pressure to 39-45 PSI.

Because the pressure (vacuum) varies in the intake manifold, a line is run from the manifold to the pressure regulator to maintain a consistent pressure at the fuel injectors.

Fast Idle Control


Idle Air Control (IAC)

Fast Idle Control is a valve that allows more air into the engine while it is cold. The Idle Control valve is opened or closed based on the engine temperature.

Idle Air Control is a more sophisticated version of idle control that maintains a consistent engine idle RPM under a wide variety of conditions. The IAC is a stepper motor, or simpler terms an valve that can allow varying amounts of air into the engine to keep the idle stable.

Knock Sensor

When a knock sensor is present, it can sense detonation in the engine and will enrich the fuel mixture. If an EMS system is on the engine, the ignition timing can also be retarded. One type of knock sensor is know as "ion sensing". This type of knock detection monitors small changes in the current required to fire the coil and spark plug.

Vehicle Speed Sensor (VSS)

A fancy name for the wheel speed sensor of an electric speedometer. An engine management system can use this information to fine tune fuel and ignition requirements.

Bank Angle Sensor (BAS)

This sensor determines the lean angle of the motorcycle. If the "bank angel" exceeds designated levels, the engine can be shut down. This sensor is a safety feature in OEM EFI systems.

Sequential Fuel Injection

A sequential fuel injection system times the opening of the injectors to match the opening of the intake valve into each cylinder. This type of fuel injection requires the precise timing of a crank position sensor to allow the ECU to precisely fire each injector. Most OEM EFI systems are sequential injection systems.

Batch Fuel Injections

A batch fuel injection system simply allows all injectors to fire at once. This type of fuel injection system does not require the precise timing of the Sequential Fuel Injection system, but does allow fuel metering with the same degree of accuracy. Many aftermarket injection systems are batch fuel injection systems because of the simplicity of the electronics and the ability to precisely control the fuel mixture.

Port Injection

Port injection systems individual fuel injectors located at or near the intake valves of each cylinder.

Throttle Body Injection

Throttle body injection has one or more injectors located at or near the throttle plate in the throttle body of the injection system. Think "electronic" carburetor.

Closed Loop

Close Loop operation refers to a time when the ECU is controlling the fuel mixture based on input signals received from the Oxygen Sensor. Fuel injection systems typically only operate in closed loop mode during cruising and low engine loads. When a heavy load is placed on the engine, the ECU reverts to Open Loop operation. Not all Fuel Injection systems operate in closed loop mode.

The 2007 Harley-Davidson EFI systems use close loop mode of operation with a narrow band oxygen sensor for each cylinder.

Open Loop

Open Loop operation refers to the time when the ECU is reading the engine sensors and using the internal fuel maps to determine the appropriate amount of fuel to inject into the engine. The O2 sensor is not used in this mode of fuel injection operation.

Table 1 - Electronic Fuel Injection Terminology and Definitions

Copyright 2003-2006 Stephen Mullen, Oldsmar, FL