Although fuel injection implies fuel control, modern systems include ignition control and more accurately should be referred to as engine management systems. There are two basic types of electronic fuel injection (EFI): Throttle Body Injection (TBI) and Multiport Fuel Injection (MPFI). TBI is considered a wet system in that the intake manifold is wetted by the fuel carried by the air. MPFI is considered a dry system because the intake manifold carries only air. Older fuel injection systems were mechanically controlled by throttle position and the injectors were located at the intake ports. These systems are extremely rare as they were almost always used in racing applications. Direct Injection is a variation of MPFI where fuel is sprayed directly into combustion chamber.
Both electronic systems use injectors that quickly pulse on and off to spray a fine mist of fuel into air. The fuel mixture is precisely metered by the injectors. The pulse length is controlled by the computer or electronic control unit (ECU) or powertrain control module (PCM). The amount of oxygen in the exhaust gas is what the computer uses to determine the correct fuel mixture. The ECU also uses other parameters that it measures such as manifold vacuum, air temperature, engine temperature, and engine speed for a best guess of the correct fuel mixture. Under light load cruise conditions, the ECU uses the exhaust oxygen (O2) content to correct its best guess.
There are 2 types of exhaust O2 sensors: narrow-band and wide-band. Narrow band O2 sensors can only measure when the fuel mixture is either slightly rich or slightly lean, which is why they are commonly used to control stoichiometric fuel mixtures. A stoichiometric fuel mixture is one where there is exactly enough oxygen molecules to combust with all of the hydrogen and carbon atoms in the fuel molecules. Basically, it is the ideal fuel mixture for perfect combustion. A wide-band exhaust O2 sensor can measure a wide range of exhaust O2 content but is much more expensive than a narrow-band O2 sensor.
The continual correction of the best guess of fuel mixture is known as closed loop control. Conversely, open loop control is the best guess of the correct fuel mixture that is uncorrected. The O2 sensor needs to be hot to determine the stoichiometric. Rich mixtures and lean mixtures are not accurately measured by narrow-band O2 sensors. Maximum power requires a rich mixture which cannot be measured by the O2 sensor and so cannot be under closed loop control. Similarly, because an O2 sensor is cold when the engine is started, the engine also runs in open loop mode until it reaches operating temperature.
Newer wide band O2 (WBO2)sensors have a linear response to O2 content and allow the ECU to control the engine in closed loop mode. However, the WBO2 sensor requires its own controller and can not simply be plugged into ECUs that don't support it.
TBI is the next step in the evolution of the fuel system from the carburetor. In fact, it very much resembles a carburetor. The obvious visual difference is that there are injectors where venturis would have been located. For people wanting to upgrade to EFI, TBI is the simpler and cheaper way to go. The disadvantage of TBI is that it is still a wet system and the fuel mixture is still subject to cylinder to cylinder variations. The advantage of TBI over carburetion is that it is a turn-the-key-and-go system that provides crisp throttle response under all operating conditions.
MPFI is the most precise way of delivering the correct fuel mixture to each cylinder. Because the injectors are located at the cylinder head's intake port, there is no cylinder to cylinder variation. Custom intake manifolds can be fabricated for any engine because the manifolds are simply air delivery devices. The disadvantage of MPFI is that it is more expensive and complex to install than a TBI system.
MPFI systems come in three basic varieties:
- Batch systems are those which fire all of the injectors simultaneously with every crankshaft rotation.
- Bank to bank systems fire half the injectors, alternating between halves, with every crankshaft rotation.
- Sequential systems fire each injector prior to the opening of its intake valve.
EFI systems need to measure the amount of air flowing into the engine to be able to calculate the correct amount of air to mixture in it. There are two means of do this: Mass Flow and Speed Density. Mass flow systems use a device to measure air flow. Early systems used a lever-mounted paddle that deflected proportionally to the air flow but later system used a hot-wire anemometer. The speed density systems use a manifold air pressure sensor to imply air flow from the engine's volumetric efficiency and speed. Mass flow systems work well but put a restriction in the induction system that reduces air flow which in turn reduces maximum power.
The cheapest way of upgrading to an OEM EFI system is to scavenge one from a later model car. Most aftermarket parts are based on GM designs so GM systems are the easiest to adapt. The simplest (and most expensive) way to upgrade to EFI is to buy a prepackaged kit from any one of a number of suppliers. There are also DIY EFI systems, which are relatively inexpensive but require a high degree of technical understanding of electronic fuel injection.