Indirect injection
Typical gasoline engines are usually equipped with indirect injection systems. They 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 intake 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.
Throttle-body injection
Electronic
throttle-body injection (normally called
TBI, though Ford Motor Company 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. This system also resulted in 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 cleaners 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 poppet valves.
Direct injection
Gasoline direct injection or GDI is a variant of fuel injection employed in modern four strokespark ignition engines. Some hi-tech gasoline/petrol engines utilise this system as it gives a better volumetric efficiency as only air is drawn in through the induction system, increasing amount of air induced and minimising fuel losses. The injector also features several spray modes so that the fuel is better distributed and a powerful air-fuel mixture is created.The gasoline is injected right into the combustion chamber of each cylinder, as opposed to conventional multi point fuel injection that happens in the intake manifold.
Theory of operation
The major advantages of a GDI engine are increased fuel efficiency and high power output. This is achieved by the precise control over amount of fuel and injection timings which are varied according to the load conditions. Basically, the engine management system continuously chooses between three different modes of combustion: ultra lean burn combustion, stoichiometric combustion and high power output mode.
Each mode is characterized by air-fuel ratio, the amount of fuel in the air-fuel mixture; the stoichiometric ratio for petrol is 14.6 to 1, but in ultra lean mode, it could be as high as 65 to 1, resulting in much leaner mixtures than those ever achieved in the conventional engines.
- Ultra lean combustion mode is effective under normal conditions, when little acceleration is required. The fuel is not injected at the intake stroke but rather at the latter stages of the compression stroke, so that the small amount of air-fuel mixture is optimally stratified just below the spark plug. The initial combustion takes place in a toroidal cavity on the piston's surface. This technique enables the usage of ultra lean mixtures with very high air-fuel rates, impossible with traditional intake valves.
- Stoichiometric combustion mode is activated for moderate load conditions. In this mode, fuel is conventionally injected furing the intake stroke to obtain stoichiometric rates.
- In full power mode, the air-fuel mixture is homogeneous and consists of maximum amount of fuel that is possible to ignite without knocking out, as defined by the compression ratio of the engine. The fuel is injected during the intake stroke. This mode activates at high load conditions and provides maximum output and torque.
Direct injection can also be accompanied by traditional methods such as VVT and VLIM, which provide conventional control over airflow swirl patterns at stoichiometric and full power modes. EGR can help reduce NOx emissions inevitable when burning ultra lean mixtures.
