Fuel injection is a technology used in internal combustion engines to mix the fuel with air prior to combustion.
As in a traditional carburettor, fuel is converted to a fine spray and mixed with air. However, where a traditional carburettor forces the incoming air through a venturi to pull the fuel into the air stream, a fuel injection system forces the fuel through nozzles under pressure to inject the fuel into the air stream without requiring a venturi.
The use of a venturi reduces volumetric efficiency by approximately 15%, which results in a reduction in engine power. Thus, a fuel injection system increases the power that an engine with the same engine displacement will produce. Additionally, fuel injection allows for more precise control over the mixture of fuel and air, both in proportion and in uniformity.
The fuel injection may be purely mechanical, purely electronic or a mix of the two. Early systems were mechanical but from about 1980 onward more and more systems were completely electronic. By the middle of the decade, nearly all new passenger vehicles were equipped with electronic fuel injection. The 1990 Subaru Justy was the last passenger car sold in the United States with a carburettor.
The modern electronic systems that cars are equipped with today utilise a number of sensors to monitor engine conditions, and an electronic control unit (ECU) to accurately calculate the needed amount of fuel. Thus fuel injection can increase fuel efficiency and reduce pollution.
Fuel injection systems may be single point where the fuel is injected using one nozzle, usually in the throttle housing, or multi point where each cylinder has its own injector in the inlet manifold. The nozzles may be opened using the pressure in the fuel system or there may be a solenoid on the injector that will pulse it open and closed in a duty cycle according to the desired fuel requirement.
Frederick William Lanchester joined the Forward Gas Engine Company Birmingham, England in 1889. He carried out what were possibly the earliest experiments with fuel injection. Fuel injection has been used in diesel engines since the mid 1920s, almost from their introduction (due to the higher energy required for diesel to evaporate). It was adapted for use in petrol-powered aircraft during World War II and a system developed by Bosch was first used in a car in 1955 with the introduction of the Mercedes-Benz 300SL. An electronic fuel injection system was also developed by the Bendix Corporation.
Fuel injection became widespread with the introduction of electronically controlled fuel injection systems in the 1980s and the gradual tightening of emissions and fuel economy laws. Meeting modern emissions standards whilst retaining acceptable performance would be impossible without it. In addition, the development of microprocessor technology made it possible to control the amount of fuel injected precisely.
Many modern diesel engines use direct injection, in which the injection nozzle is located inside the combustion chamber. Some petrol engines utilise this system as well since it gives a better volumetric efficiency since only air is drawn in through the induction system, not fuel which takes up space, which means more power.
Some diesel engines have a very highly pressurised common rail fuel supply line to allow the injection process. For the diesel engine this replaces the older mechanically more complicated, noisy combined pump and selector valve assembly.
Electronic throttle-body injection (normally called TBI, though Ford used the abbreviation, CFI) was introduced in the early 1980s as a transition technology to fully-electronic port injection. The system injects fuel into the throttle-body (a wet system), so fuel can condense and cling to the walls of the intake system, harming emissions. Computer-controlled TBI was inexpensive and simple, however, and lasted well into the 1990s.
Central port injection
General Motors developed a new "in-between" technique called central port injection or CPI. It uses tubes from a central injector to spray fuel at the intake port rather than the throttle-body (it is a dry system). However, fuel is continuously injected to all ports simultaneously, which is less than optimal.
Sequential central point injection
GM refined the CPI system into a sequential central port injection (SCPI) system in the mid-1990s. It used valves to meter the fuel to just the cylinders that were in the intake phase. This worked well on paper, but the valves had a tendency to stick. Fuel injector cleaner sometimes worked, but the system remained problematic.
Multi-port fuel injection
The goal of all fuel injection systems is to carefully meter the amount and timing of fuel to each cylinder. This is achieved with the more sophisticated fuel injection systems, often called multi-port fuel injection (MFI) or sequential port fuel injection (SFI). It uses a single injector per cylinder and sprays the fuel right above the intake valves.
The newest method for petrol engines now is direct injection or DI. It has a special fuel injector inside the combustion chamber itself, along with the valves and spark plugs. This system is just appearing in the mid-2000s, and like most systems before, it is being pioneered in Diesel applications. This method was used in various WWII aircraft as well. Notable engines included the Daimler Benz DB 605 and later versions of the Wright R-3350 used in the B-29 Super fortress.
In direct injection, the piston incorporates a depression (often toroidal) which is where initial combustion takes place. Direct injection diesel engines are generally more efficient than indirect injection engines, but tend to be noisier.