In-cylinder flow and combustion studies in an air-assisted direct injection gasoline engine
In-cylinder flows and CAI combustion were investigated in a single cylinder, air-assisted gasoline direct injection engine. CAI was promoted and controlled by internal exhaust gas recirculation, achieved by employing short duration camshafts and early exhaust valve closure. The effects of valve and injection timing and engine speed on exhaust emissions, fuel consumption, combustion phasing and operating region were investigated. The results show that valve timing mainly affects engine load and CAI combustion phasing through changes in trapped residual levels and stratification of fresh and residual gases respectively. Injection of fuel into residual gases during the recompression process was found to increase the operating region and reduce uHC emissions though charge cooling effects and increased fuel ignitability via internal fuel reformation. The increased ignitability of the mixture also advanced ignition timing, resulting in increased in-cylinder temperatures and NOx concentrations. It was found that, compared to SI combustion in the same engine, CAI operation reduced NOx emissions by between 98% and 80% while fuel consumption was reduced by between 9% and 17%. The in-cylinder flows of intake air and fuel droplets from the air-assisted injection system and cylinder head were investigated using the PIV technique. No significant large-scale flow structures were found in the in-cylinder airflow and the fuel spray appeared unaffected by the in-cylinder air motion. In addition, the in-cylinder fuel distribution from the air-assisted injection system was measured using laser induced exiplex fluorescence. A combination of naphthalene and DMA in isooctane was used to form an exiplex and simultaneous qualitative images of the liquid and vapour fuel phases were obtained. When using a late injection strategy, a well stratified mixture was formed at the end of the compression stroke, while injection during the intake stroke left a well mixed homogenous charge.