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Electronic Fuel Injection 101: Injector Timing

Fuel level sending unit schematic

Fuel injected systems in a modern combustion engine use fuel injectors to control the relativel richness or leanness of the fuel mix. The amount of fuel injected is determined by the duration of the injector's pulse, called the "pulse width." The larger the pulse width, the more fuel is delivered and the richer the fuel:air mixture.

The engine's computer, usually called the ECM, controls the injector timing for the cylinders as part of the overall spark and air delivery timing for engine operations.

The Computer Matrix For Precise Control

In most fuel injected engines, the computer uses input from several sensors along with input from the throttle to determine how much pulse width to use on the next firing cylinder.

Typically, all of this information is pulled together and "mapped" against a preset grid of scenarios. These are all stored on a read-only chip on the computer. Maps for engine warmup, highway cruising, and various other engine load scenarios are pre-programmed and followed as the vehicle is used. Timing and pulse width are set according to the map's directions.

Different engines will use different mapping and even engines that are identical but being used on different platforms will differ.

Two Types of EFI Systems

There are two general types of electronic fuel injection systems: Speed-Density and Mass Airflow. Both work differently and have advantages and disadvantages.

Speed-Density systems are often found on Chrysler and General Motors engines. They measure airflow into the engine by estimating it based on readings from the Throttle Position Sensor, Manifold Absolute Pressure, and engine RPM. This bypasses the need for a mass airflow sensor and gives more tolerance for air leaks and slight vacuum losses.

Mass Airflow systems uses a mass airflow sensor (MAF) to directly measure airflow into the engine. There are several types in use, such as the Hot Wire sensor used by many Ford engine designs.

Further, fuel pulse will be determined by the last oxygen sensor in the line (usually called the Heated O2 Sensor), located on or after the manifold. Newer engines often call these the Air/Fuel sensor). This sensor measures the oxygen leaving the combustion chambers into the exhaust. The EFI will usually increase fuel to burn off excess oxygen (enrich the air:fuel mix) if too much oxygen is in the exhaust.

Controlling the Injectors

Control of the injectors is simple. The injector itself is a simple valve controlled by a solenoid. When the solenoid receives an impulse from the computer, the valve opens and stays open for as long as the impulse is received. Cycling and duration are usually measured in milliseconds.

Fuel Pressure

Finally, the last thing controlling the pulse width and the amount of fuel it sends into the chamber is fuel pressure. The higher the fuel pressure to the injector, the more fuel it will send out when pulsed.

Fuel pressure is generated by a fuel pump which draws fuel from the tank. Different vehicles will have different pump types and outputs. A check valve, either on the pump itself or near the fuel rail on the engine, will moderate the pressure and maintain it when the pump is not operating (engine is off).

Most engines today use multiport EFI systems which vary the pressure using engine vacuum levels. Higher vacuum means lower need and fuel pressure is lowered via the pump or pressure regulator on the fuel rail. Often through a combination of both.