Gasoline Direct Injection
Gasoline Direct Injection or GDI is a variant of fuel injection employed in modern four stroke petrol engines. 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 in a toroidal cavity on the piston's surface. This technique enables the usage of ultra lean mixtures with very high air-fuiel 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.
History
Mitsubishi Motors was the first with a GDI engine in Japan in the 1990s. This engine was brought to Europe in 1998, but high-sulphur fuel led to emissions problems, and fuel efficiency was less than expected. PSA Group also launched a GDI engine in the late 1990s, but both were withdrawn from the market in 2001. DaimlerChrysler produced a special engine for 2000, offered only in markets with low sulphur fuel.
Later GDI engines were tuned and marketed for high performance rather than economy. Volkswagen led the trend with their 2001 GDI engine, and BMW followed with a GDI V12. PSA is cooperating with BMW on a new line of engines for future introduction.