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In
internal combustion engines,
gasoline direct injection (GDI), also known as
petrol direct injection or
direct petrol injection, is a variant of
fuel injection employed in modern
two-stroke and
four-stroke gasoline engines. The
gasoline is highly pressurized, and injected via a
common rail fuel line directly into the
combustion chamber of each
cylinder, as opposed to conventional
multi-point fuel injection that happens in the
intake tract, or cylinder port.
In some applications, gasoline direct injection enables a stratified fuel charge (ultra
lean burn) combustion for improved
fuel efficiency, and reduced emission levels at low load.
Theory of operation
The major advantages of a GDI engine are increased
fuel efficiency and high
power output. Emissions levels can also be more accurately controlled with the GDI system. The cited gains are achieved by the precise control over the amount of fuel and injection timings that are varied according to the load conditions. In addition, there are no throttling losses in some GDI engines, when compared to a conventional
fuel-injected or
carbureted engine, which greatly improves efficiency, and reduces 'pumping losses' in engines without a throttle plate. Engine speed is controlled by the
engine control unit/engine management system (EMS), which regulates fuel injection function and ignition timing, instead of having a throttle plate that restricts the incoming air supply. Adding this function to the EMS requires considerable enhancement of its processing and memory, as direct injection plus the engine speed management must have very precise algorithms for good performance and drivability.
The engine management system continually chooses among three combustion modes: ultra
lean burn,
stoichiometric, and full power output. Each mode is characterized by the
air-fuel ratio. The
stoichiometric air-fuel ratio for
gasoline is 14.7:1 by weight, but ultra lean mode can involve ratios as high as 65:1 (or even higher in some engines, for very limited periods). These mixtures are much leaner than in a conventional engine and reduce fuel consumption considerably.
- Ultra lean burn mode is used for light-load running conditions, at constant or reducing road speeds, where no 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 placed near the spark plug. This stratified charge is surrounded mostly by air, which keeps the fuel and the flame away from the cylinder walls for lowest emissions and heat losses. The combustion takes place in a toroidal (donut-shaped) cavity on the piston's surface.[1] The cavity is displaced to one side of the piston, the side that has the fuel injector. This technique enables the use of ultra-lean mixtures that would be impossible with carburetors or conventional fuel injection.
- Stoichiometric mode is used for moderate load conditions. Fuel is injected during the intake stroke, creating a homogenous fuel-air mixture in the cylinder. From the stoichiometric ratio, an optimum burn results in a clean exhaust emission, further cleaned by the catalytic converter.
- Full power mode is used for rapid acceleration and heavy loads (as when climbing a hill). The air-fuel mixture is homogenous and the ratio is slightly richer than stoichiometric, which helps prevent knock (pinging). The fuel is injected during the intake stroke.
Direct injection may also be accompanied by other engine technologies such as
variable valve timing (VVT) and tuned/multi path or
variable length intake manifolding (VLIM, or VIM).
Water injection or (more commonly)
exhaust gas recirculation (EGR) may help reduce the high
nitrogen oxides (NOx) emissions that can result from burning
ultra lean mixtures.
It is also possible to inject more than once during a single cycle. After the first fuel charge has been ignited, it is possible to add fuel as the piston descends. The benefits are more power and economy, but certain octane fuels have been seen to cause exhaust valve erosion. For this reason, most companies have ceased to use the Fuel Stratified Injection (FSI) operation during normal running.
Tuning up an early generation FSI power plant to generate higher power is difficult, since the only time it is possible to inject fuel is during the induction phase. Conventional injection engines can inject throughout the
4-stroke sequence, as the injector squirts onto the back of a closed valve. A direct injection engine, where the injector injects directly into the cylinder, is limited to the suction stroke of the piston. As the RPM increases, the time available to inject fuel decreases. Newer FSI systems that have sufficient fuel pressure to inject even late in compression phase do not suffer to the same extent; however, they still do not inject during the exhaust cycle (they could but it would just waste fuel). Hence, all other factors being equal, an FSI engine needs higher-capacity injectors to achieve the same power as a conventional engine.
