Effect of Injection Timing on Mixture and CAI Combustion in a GDI Engine with an Air-Assisted Injector

The application of controlled auto-ignition (CAI) combustion in gasoline direct injection (GDI) engines is becoming of more interest due to its great potential of reducing both NOx emissions and fuel consumption. Injection timing has been known as an important parameter to control CAI combustion process. In this paper, the effect of injection timing on mixture and CAI combustion is investigated in a single-cylinder GDI engine with an air-assisted injector. The liquid and vapour phases of fuel spray were measured using planar laser induced exciplex fluorescence (PLIEF) technique. The result shows that early injection led to homogeneous mixture but late injection resulted in serious stratification at the end of compression.

CAI combustion in this study was realized by using short-duration camshafts and early closure of the exhaust valves. During tests, the engine speed was varied from 1200rpm to 2400rpm and A/F ratio from stoichiometric to lean limit. The injection timing was varied from the end of the exhaust process to the middle stage of the compression stroke. The experimental results show that injection timing had important effect on CAI combustion. Early injection gave faster and more stable combustion, less HC and CO emissions, but more knocking and higher NOx emissions. The CAI operation range could be extended significantly by early injection. Late injection led to slower and unstable combustion, higher HC and CO emissions but lower noise and NOx emissions.