DIRECT INJECTION
Appearing on regular road cars only since being introduced by Mitsubishi in 1996, gasoline direct injection (GDI) was widely used on WW2 aircraft engines. Its first use in a car was in the 1955 Mercedes-benz 300 SL Gullwing coupé and roadster and provided a significant power increase.
With direct injection, the fuel mixture is formed internally; the fuel is injected directly into the combustion chamber where it mixes with air entering via the inlet valve. Compared to port injection MPI engines, this leaves a short time for vaporisation, which necessitates high injection pressures of up to 350 bar to create the fine fuel atomisation and precise injection timing required. The cooling benefit of the fuel phase change occurs within the combustion chamber, lowering surface temperatures and the potential for premature ignition of the air/ fuel mixture (detonation). This enables the use of higher compression ratios and turbo boost pressures to improve efficiency and performance.
Two combustion strategies can be employed in GDI engines: homogeneous or stratified charge. With the homogeneous operation, fuel is injected early in the intake stroke allowing a homogeneous mixture to be formed, cooling the combustion chamber, and minimising the possibility of detonation. With stratified charge the fuel is injected towards the end of the compression stroke; swirl injection and shaped-top pistons create an ideal air-fuel mixture around the spark plug while the rest of the combustion chamber has a lean mixture. This improves efficiency at lower speeds but owing to the resulting slower combustion, the engine switches to homogeneous combustion at higher speeds. Most GDI engines now use homogenous operation as the overall efficiency advantages of stratified charge is not considered sufficient.
While direct injection systems improve fuel efficiency and reduce CO2 emissions, they do have their downsides.the operation of the high-pressure injectors can be noisy and the high-pressure fuel pump and other fuel system components are all more expensive than the low-pressure equivalents. Intake valves do not have the benefit of being cleaned by the fuel additives and this can lead to a build-up of carbon deposits.the short fuel vaporisation time may result in incomplete combustion leading to increased soot or particulate emissions. Particulates are similar but smaller than those produced by diesel engines and particulate filters are sometimes required for GDI engines to meet the most stringent emission regulations.