Throttle-body injection (called TBI by General Motors and Central Fuel Injection (CFI) by Ford) or single-point injection was introduced in the mid-1980s as a transition technology toward individual port injection. The TBI system injects fuel at the throttle body (the same location where a carburetor introduced fuel). The induction mixture passes through the intake runners like a carburetor system, and is thus labelled a "wet manifold system". The justification for the TBI/CFI phase was low cost. Many of the carburetor's supporting components could be reused such as the air cleaner, intake manifold, and fuel line routing. This postponed the redesign and tooling costs of these components. Most of these components were later redesigned for the next phase of fuel injection's evolution, which is individual port injection, commonly known as MPFI or "multi-point fuel injection". TBI was used extensively on American-made passenger cars and light trucks in the 1980 to 1995 timeframe and some transition-engined European cars throughout the early and mid 1990s.
Bosch's K-Jetronic (K for kontinuierlich, German for "continuous") was introduced in 1974. In this system, fuel sprays constantly from the injectors, rather than being pulsed in time with the engine's intake strokes. Gasoline is pumped from the fuel tank to a large control valve called a fuel distributor, which separates the single fuel supply pipe from the tank into smaller pipes, one for each injector. The fuel distributor is mounted atop a control vane through which all intake air must pass, and the system works by varying fuel volume supplied to the injectors based on the angle of the air vane, which in turn is determined by the volume flowrate of air past the vane, and by the control pressure. The control pressure is regulated with a mechanical device called the control pressure regulator (CPR) or the warm-up regulator (WUR). Depending on the model, the CPR may be used to compensate for altitude, full load, and/or a cold engine. On cars equipped with an oxygen sensor, the fuel mixture is adjusted by a device called the frequency valve. The injectors are simple spring-loaded check valves with nozzles; once fuel system pressure becomes high enough to overcome the counterspring, the injectors begin spraying. K-Jetronic was used for many years between 1974 and the mid 1990s by BMW, Lamborghini, Ferrari, Mercedes-Benz, Volkswagen, Ford, Porsche, Audi, Saab, and Volvo. There was also a variant of the system called KE-Jetronic with electronic instead of mechanical control of the control pressure.
In piston aircraft engines, continuous-flow fuel injection is the most common type. In contrast to automotive fuel injection systems, aircraft continuous flow fuel injection is all mechanical, requiring no electricity to operate. Two common types exist: the Bendix RSA system, and the TCM system. The Bendix system is a direct descendant of the pressure carburetor. However, instead of having a discharge valve in the barrel, it uses a flow divider mounted on top of the engine, which controls the discharge rate and evenly distributes the fuel to stainless steel injection lines which go to the intake ports of each cylinder. The TCM system is even more simple. It has no venturi, no pressure chambers, no diaphragms, and no discharge valve. It simply uses a butterfly valve for the air which is linked by a mechanical linkage to a rotary valve for the fuel. Inside the control unit is another restriction which is used to control the fuel mixture. From the control unit, fuel flows to the flow divider, then through the lines to the injectors.
Central port injection (CPI)
General Motors implemented a system called "central port injection" (CPI) or "central port fuel injection" (CPFI). It uses tubes with poppet valves from a central injector to spray fuel at each intake port rather than the central throttle-body. The 2 variants were CPFI from 1992 to 1995, and CSFI from 1996 and on. CPFI is a batch-fire system, in which fuel is injected to all ports simultaneously. The 1996 and later CSFI system sprays fuel sequentially.
Multi-point fuel injection
Multi-point fuel injection injects fuel into the intake port just upstream of the cylinder's intake valve, rather than at a central point within an intake manifold, referred to as SPFI, or single point fuel injection. MPFI (or just MPI) systems can be sequential, in which injection is timed to coincide with each cylinder's intake stroke, batched, in which fuel is injected to the cylinders in groups, without precise synchronisation to any particular cylinder's intake stroke, or Simultaneous, in which fuel is injected at the same time to all the cylinders.
All modern EFI systems utilize sequential MPFI. Some Toyotas and other Japanese cars from the 1970s to the early 1990s used an application of Bosch's multipoint L-Jetronic system manufactured under license by DENSO.
Many diesel engines feature direct injection (DI). The injection nozzle is placed inside the combustion chamber and the piston incorporates a depression (often toroidal) where initial combustion takes place. Direct injection diesel engines are generally more efficient and cleaner than indirect injection engines. See also High-pressure Direct Injection (HDi).
Some recent petrol engines utilize direct injection as well: Ford, Mazda(DISI), BMW, GM(SIDI), Hyundai (GDI), Kia (GDI), Nissan, Lexus, Subaru, Saab, Saturn, Mitsubishi(GDI), Volkswagen and Audi (FSI) (for Fuel Stratified Injection). This is the next step in evolution from multi port fuel injection and offers another magnitude of emission control by eliminating the "wet" portion of the induction system. Direct fuel injection can optimize each of the three important engine operational variables...horsepower, emissions levels, and fuel efficiency. By virtue of the more ideal dispersion and homogeniety of the directly injected fuel, the cylinder and piston are cooled, thereby permitting higher compression ratios and more aggressive ignition timing...thus enhanced power. More precise management of the fuel injection event also enables better control of emissions. Finally, the homogenity of the fuel mixture allows for leaner air/fuel ratios under any/all circumstances, and along with more precise ignition event management, can improve fuel efficiency. One recent advance in fuel injection has been that of the Piezo Electronic injector. With ultra fast acting and even more precisely controlled injection cycles...multiple injection events can occur during each power stroke of the engine. Disadvantages are in the cost of DFI..inasmuch as the injector is exposed to more heat/pressure than in port injection. More exotic materials and exact electronic management are required...the latter being addressed by the Piezo Electronic injectors.
Fuel injection introduces potential hazards in engine maintenance due to the high fuel pressures used. Residual pressure can remain in the fuel lines long after an injection-equipped engine has been shut down. This residual pressure must be relieved, and if it is done so by external bleed-off, the fuel must be safely contained. If a high-pressure diesel fuel injector is removed from its seat and operated in open air, there is a risk to the operator of injury by hypodermic jet-injection, even with only 100 psi pressure. The first known such injury occurred in 1937 during a diesel engine maintenance operation.