Development of a Small-Bore Gasoline Direct-Injection Engine, and Enhancement of Its Performance Using Multiple-Injection Strategies

Gasoline direct-injection (GDI) engines are finding increasing applications due to their potential benefits such as low fuel consumption and superior performance. However, their use in small engines poses challenges in locating the injector and avoiding the fuel impingement on the combustion chamber walls. The selection of suitable injection timing(s) and injection of fuel in multiple pulses can overcome these difficulties. In the present work, a small-bore spray-guided GDI engine was developed by modifying a base carbureted engine. Detailed experiments and supporting CFD simulations were conducted to study the influence of the number of injections per cycle and injection timings on the performance, emissions, and combustion characteristics including cycle-to-cycle variations. It was found that the homogeneity of the fuel-air mixture was improved and fuel impingement was reduced with multiple injections. A computational fluid dynamics (CFD) study revealed that early injections in the case of multiple injections led to a slightly richer mixture near the spark plug at the time of ignition. This enhanced the combustion rate and reduced the flame initiation duration. In-cylinder temperature estimated using CFD simulation was also found to be lower by 40 K to 45 K with multiple injections. This allowed more advanced knock-limited spark timings (KLST) to be used for higher efficiency. Direct injection of fuel extended the lean operating limit of air-fuel ratios and also enhanced the combustion stability as compared to a port-fuel-injection (PFI) mode. Injection timings were limited to the intake stroke for single and multiple injections based on stable engine operation beyond which the engine misfires. On the whole, engine performance and stability were improved with the use of multiple-injection strategies.