Fuel Injection
American automobiles with carburetors were produced for the last time during the 1986 model year and were replaced by designs equipped with electronic fuel injection (EFI) systems. Clean air standards were the driving force that pushed manufacturers to reduce pollutants by re-engineering fuel delivery methods. Fuel injection, however, is not a new idea. Rudolf Diesel included it in his diesel engine patented in 1892 and 1893. Automotive engineers have analyzed means of maintaining car performance while meeting emissions standards; and sophisticated electronics, materials, and designs have made both performance and improved air quality possible through fuel-injected vehicles. Fuel injection typically improves horsepower, drivability, and fuel economy over carburetor-equipped cars.
All electronic fuel injection systems consist of sensors that feed data to an Electronic Control Unit (ECU). The ECU processes and controls the electrical operations of the engine and can adjust directions sent to the fuel injectors based on operating conditions at any given time while the automobile is running. The fuel injectors are meters for fuel flow. Engines with continuous combustion like gas turbines and liquid-fueled rockets must use injection systems because there is no suction from the motion of pistons.
At the inlet end of the injector, fuel flows in through a filter screen. The injector is activated by an electrical solenoid that receives its commands from the ECU. The solenoid lifts a ball off the injector seat, the injector sprays a cone-shaped spray of fuel above the throttle valve, and the fuel mixes with air and is atomized around the intake valve before the mixture is combusted. Various sensors monitor the amount of oxygen in the exhaust gas, the coolant temperature, and the manifold pressure. Inputs from these sensors tell the ECU how to adjust the air/fuel mixture. The throttle position sensor monitors throttle loads (ranging from idling while stopped to rapid acceleration), and it signals fuel needs to the ECU. Fuel temperature, air density, vehicle speed, and other characteristics are also monitored. Ports in the throttle body signal the air cleaner housing, air injector, the manifold, the spark advance, and purge systems to help the system clean itself and control air flow.
There are two types of EFI systems called throttle body injection and port injection, although all EFI systems have both throttle bodies and injectors. The throttle body system is similar to the carburetor in that the injectors spray the fuel directly into the intake manifold in this "wet manifold" method. The injectors are mounted above the throttle plates which regulate the passage of fuel through the throttle bores to the manifold where it mixes with air and then enters the combustion chambers. Trucks and off-road vehicles tend to have throttle body injection systems because they perform better at high altitudes, give better throttle response, and improve drivability.
In port injection, each cylinder has a dedicated injector that sprays fuel directly into the ports. A fuel rail feeds the injectors. This system is also called a "dry manifold" system because fuel is not injected into the intake manifold. Car manufacturers have their own names for variations of port injection, such as Tuned Port or Sequential Port Fuel Injection, depending on fine points of the operation. The port injection system dominates the industry because of efficiency; fuel sprayed directly into the combustion chamber burns optimally. Because more parts are required (eight injectors for a V-8 engine), however, maintenance costs may be higher.
Fuel injection systems operate best with quality gasolines containing additives that prevent varnish from plugging the tiny injectors and containing detergents that clean the whole system. Higher octane gasolines usually have the best combination of detergents and additives even if the engine itself doesn't need the octane level. Fuel system cleaners are also commercially available.
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