Study of a Stratified-Charge DISI Engine with an Air-Forced Fuel Injection System

A small-bore 4-stroke single-cylinder stratified-charge DISI engine using an air-forced fuel injection system has been designed and tested under various operating conditions. At light loads, fuel consumption was improved by 16∼19% during lean, stratified-charge operation at an air-fuel ratio of 37. NOx emissions, however, were tripled. Using EGR during lean, stratified-charge operation significantly reduced NOx emissions while fuel consumption was as low as the best case without EGR. It was also found that combustion and emissions near the lean limit were a strong function of the combination of injection and spark timings, which affect the mixing process. Injection pressure, air injection duration, and time delay between fuel and air injections also played a role. Generating in-cylinder air swirl motion slightly improved fuel economy. Additionally, it was found that stratified-charge operation at medium-high load was not constrained by soot emissions, and speeds up to 3000 rpm could be achieved with stratified charge operation. At full load, the knocking tendency was reduced and the maximum torque output was increased versus PFI operation.