This contribution is focused on the fuel economy improvement of the Miller cycle under part-load characteristics on a supercharged DI (Direct Injection) gasoline engine. Firstly, based on the engine bench test, the effects with the Miller cycle application under 3000 rpm were studied. The results show that the Miller cycle has different extents of improvement on pumping loss, combustion and friction loss. For low, medium and high loads, the brake thermal efficiency of the baseline engine is increased by 2.8%, 2.5% and 2.6%, respectively. Besides, the baseline variable valve timing (VVT) is optimized by the test. Subsequently, the 1D CFD (Computational Fluid Dynamics) model of the Miller cycle engine after the test optimization at the working condition of 3000 rpm and BMEP (Brake Mean Effective Pressure) = 10 bar was established, and the influence of the combined change of intake and exhaust valve timing on Miller cycle was studied by simulation. The results show that as the effect of the Miller cycle deepens, the engine’s knocking tendency decreases, so the ignition timing can be further advanced, and the economy of the engine can be improved. Compared with the brake thermal efficiency of the baseline engine, the final result after simulation optimization is increased from 34.6% to 35.6%, which is an improvement of 2.9%.

Internal deposit formation in the injector nozzle can both restrict the fuel flow and alter the spray characteristics of the injectors, which will lead to a loss in power and fuel economy, as well as an increase of exhaust emissions. Injector temperature is considered as a fundamental element among the parameters influencing the formation of deposits inside the injector. Although all the relative investigations have declared that the injector tip temperature showed significant relations with the formation of deposits inside, the exact relationship between the tip temperature and deposit formation was controversial since they reported quite different results. As a result, how the temperature affects the quantity and positions of deposit formation inside the injector remained unclear. In the present work, an experimental facility that simulated the heat environment when an injector for gasoline direct injection was working in the engine was employed to make internal deposits form inside the injector nozzle as well as to investigate the effects of the injector tip temperature on the formation of internal injector deposits. Also, an experimental investigation concerning the spray behaviors before and after internal deposit forming was reported. According to the results, it can be found that a suitable temperature interval existed, when beyond this temperature interval it was not easy for deposits to form inside the injector. In addition, the positions of deposits that accumulated in the injector were significantly different under different injector tip temperatures, which leads to the conclusion that the higher the tip temperature is, the deeper the positions of deposits are.