Gasoline direct injection

Gasoline direct injection or GDi is a variant of fuel injection employed in modern two-stroke and four stroke petrol engines. The gasoline or biobutanol is injected right into the combustion chamber of each cylinder, as opposed to conventional multi point fuel injection that happens in the intake manifold, or cylinder port injection in two-strokes.

GDi enables stratified charge (ultra lean burn) combustion for improved fuel efficiency and emission levels at low load. Further improving efficiency and high-load output-power, the engine power is governed by modulating fuel injection, like a diesel engine; as opposed to restricting intake airflow, like a conventional gas internal combustion engine.



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. In addition, there are no throttling losses when compared to a conventional fuel injected or carburated engine, which greatly improves efficiency (only in engines that are using no throttle plate). 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 14.7 to 1 by weight, but in ultra lean mode, it could be as high as 65 to 1. These are leaner mixtures than those ever achieved in the conventional engines are desired because of reduced fuel consumption.

* Ultra lean combustion mode is effective under normal running 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 placed just near the spark plug. This stratified charge is surrounded by mostly air which keeps the fuel away from the cylinder walls for lowest emissions. The 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 ratio, impossible with traditional carburetors or even intake port injection.

* Stoichiometric combustion mode is activated for moderate load conditions. In this mode, fuel is injected during the intake stroke. The air-fuel mixture is homogeneous with the stoichiometric rates necessary for the catalytic converter to remove a maximum of the major pollutants CO and NOx from the exhaust gas.

* In full power mode, the air-fuel mixture is homogeneous as well and contains the minimum mass of fuel over the amount required for stoichiometric that is possible to ignite without knocking out, as defined by the compression ratio of the engine and the mass of air in the combustion chamber. 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. Water injection or EGR can help reduce NOx emissions inevitable when burning ultra lean mixtures.