FUEL INJECTION BASICS
by Lance Wright

In the last twenty five years, automotive fuel and emission systems have undergone constant transformation in order to meet federal emission standards and corporate average fuel economy requirements. The most significant change has been the use of catalytic converters and fuel injection. Much of this has been driven by California demands for cleaner air. Give that California is the most populous state and thereby the largest market for automobiles in the United States, California sets the agenda of Detroit and of foreign auto makers.

Catalytic converters are fantastic emission control devices because they have the ability to change the harmful emissions of a vehicle into carbon dioxide and water vapor. In order to work efficiently, catalytic converters require a precise air/ fuel mixture. The ratio of fourteen parts air to one part fuel, has been found to provide the most efficient catalytic converter operation. Any variation to the air/fuel mixture, too much or too little fuel, will affect the function of the catalytic converter, resulting in costly auto repairs. Because of this, engineers had to design a fuel system that had the ability to maintain a constant air/fuel ratio throughout the engines operating range and conditions. Computer controlled carburetors were first used, but their ability to provide fuel control to meet all vehicle operating conditions was limited. Electronic fuel injection systems were found to provide a wider range of fuel control and the added benefit of better fuel economy and increased performance over carbureted systems.

There are two basic types of fuel injection systems used on passenger cars and light trucks, multiport fuel injection and throttle body fuel injection. Because they provide lower emissions and more efficient operation, most vehicles today are equipped with multiport fuel injection systems.

Multiport fuel injection systems use one injector per cylinder. The injectors are mounted in the intake manifold and positioned to direct a fine spray of fuel directly at the intake valve. Ignition voltage is supplied to each injector and the ground circuit is connected to the engine control module. Fuel delivery requirements for the engine are met by controlling the amount of time the injector is turned on. Fuel injector control is a function of the control module and is accomplished by fuel injector drivers. The fuel injector drivers are located inside the engine control module and are used as tiny switches to complete the ground circuit to the injectors. The quantity of fuel delivered to the engine is determined by the amount of time the fuel injector is commanded on. This time period is referred to as injector pulse width and is measured in milliseconds. Some port fuel injection designs will vary fuel pressure instead of pulse width to control fuel delivery.

Throttle body injection systems use an injector or a pair of injectors placed in the throttle body to deliver fuel to the engine. Fuel is sprayed into the intake air of the engine and is distributed to the individual cylinders by the intake manifold. As in the multiport fuel injection system, the injectors are pulsed width operated to maintain proper air/fuel ratio. This system was used as a less expensive alternative to multiport fuel injection but was not as efficient. Automotive emission control regulations that went into effect for the 1996 model year, all but eliminated the use of throttle body injection on automobiles and light trucks.

(Lance owned his own auto repair shop for 30 years before retiring in 2006.)