Fuel System

Trends in development of gasoline engine fuel introduction system are shown in Fig 5.3. Until 1980 practically all gasoline SI engines employed carburettors. Only a few premium model gasoline cars employed electronic fuel injection (EFI) system.  Initially, a single point EFI injecting fuel at the throttle body was used. It was also known as throttle body injection (TBI) system and it provided more precise fuel metering than the carburettor. However, from 1990 onwards electronically controlled multi-point port fuel injection (MPFI or PFI) system replaced the carburettor on all the production cars in the USA. Port fuel injection systems require one injector per cylinder that injects fuel into intake port of each cylinder. Some engines use an additional injector to supply extra fuel required during starting and warm-up.

The PFI has the following advantages over the carburettor and TBI system;

• Increased power and torque due to improved volumetric efficiency: the venturi system needed for carburettor is eliminated and less fuel vaporizes in the intake manifold, which increase  the volumetric efficiency
•  Faster fuel evaporation: Improved fuel atomization leads to smaller droplets and a faster fuel evaporation.
• More uniform fuel distribution: for each cylinder the fuel is metered separately by the corresponding  port fuel injector
• More rapid dynamic response to changes in throttle position and hence transient operation:  Faster fuel evaporation leads to rapid mixture formation and faster dynamic response. Hence, less fuel enrichment is required during acceleration.
• More precise control of air-fuel ratio: use of closed loop feedback control system that employs exhaust oxygen sensor very precise control of air-fuel ratio is obtained so the unnecessary fuel enrichment of the engine can be prevented
• More precise control of air-fuel ratio cold-start and warm up: less extra fuel compared to carburettor is necessary during cold start and warm up.