Experimental study of the impact of port and direct fuel injection strategies on the efficiency, performance and emissions of a downsized GDI engine

Abstract

In recent years after the introduction of gasoline direct injection (GDI) engines, gasoline engine downsizing has been widely adopted to reduce fleet CO2 emissions of passenger cars. These engines are typically boosted direct injection gasoline engines equipped with variable valve timing systems for both intake and exhaust valves. Fuel consumption reduction in these downsized engines is achieved by operating more at higher brake mean effective pressure (BMEP) area of the engine map in order to reduce pumping losses and through reducing cylinder numbers to decrease total friction losses. However, the degree of downsizing and compression ratio (CR) of these engines are constrained by thermal stresses and knocking combustion as well as the low speed pre-ignition phenomena. In addition, combustion efficiency and emissions in these GDI engines can be improved further by better in-cylinder mixture preparation (in terms of homogeneity and temperature). To overcome these limitations, technologies such as dual injection systems, cooled external exhaust gas recalculation (EGR), Atkinson and Miller cycle, variable compression ratio (VCR) and water injection have been found to be highly effective in improving the combustion processes and reducing pollutant emissions. The present work investigates the impact of port and in-cylinder fuel injection strategies as well as intake port injection of water on boosted downsized GDI engine combustion, efficiency and emissions. A single cylinder direct injection gasoline engine and its testing facilities were used for extensive engine experiments. Various PFI / DI injection strategies were tested, and the results compared to the baseline PFI only and DI only strategies. Intake port injection of water also was investigated at different water/fuel ratios and with gasoline with three different research octane numbers (RON). The experiments were performed at several steady state points to determine the optimal strategy for improved engine fuel economy in real applications. The results show that PFI / late DI and early DI / late DI strategies can reduce the net indicated specific fuel consumption (NISFC) significantly by a maximum of 9% at low speed / mid-high load compared to the baseline due to the reduction of end of compression temperature and therefore advancement in knock limited spark timing. Smoke emissions were also lower under PFI / late DI, PFI / early DI, and PFI only operations compared to early DI / late DI, and DI only operations due to the improvement in mixture preparation. In addition, the results showed that PFI / late DI and early DI / late DI extend the lean limit from 1.5 to 1.7 at 1000 rpm / 8.83 bar net indicated mean effective pressure (NIMEP) due to a more advanced combustion phasing and shorter combustion duration compared to the baseline PFI only and DI only operations. Water injection results show net indicated efficiency improved significantly by a maximum of around 5% at medium load and around 15% at high load when increasing the injected water mass. Improvement in efficiency was mainly due to the increased heat capacity of charge (higher specific heats of water and increased in-cylinder mass) and the cooling effect of the injected water evaporation which reduced the in-cylinder pressure and temperature. Thus, knock sensitivity was reduced and more advance spark timings could be used which shifted the combustion phasing closer to the optimum point. However, increasing the water ratio further (more than 1 at medium load and more than 1.5 at high load) deteriorated the combustion efficiency, prolonged the flame development angle and combustion duration, and caused a reduction in the net integrated area of the P-V diagram. Comparison of fuels of different RON also reveals that water injection can virtually increase the RON of fuel, therefore makes it possible to run on a low octane number fuel and achieve higher efficiency by adjusting the water mass. In terms of other, harmful, non-CO2 emissions, water injection was effective in reducing the NOx (by a maximum of around 60%) and particle emissions significantly but increased the HC emissions as the water/fuel ratio increased. In addition, water injection also reduced the exhaust gas temperature by around 80 °C and 180 °C at medium and high loads, respectively.