Energy Analysis of Low-Load Low-Temperature Gasoline Combustion with Auxiliary-Fueled Negative Valve Overlap 2017-01-0729
In-cylinder reforming of injected fuel during an auxiliary negative valve overlap (NVO) period can be used to optimize main-cycle auto-ignition phasing for low-load Low-Temperature Gasoline Combustion (LTGC), where highly dilute mixtures can lead to poor combustion stability. When mixed with fresh intake charge and fuel, these reformate streams can alter overall charge reactivity characteristics. The central issue remains large parasitic heat losses from the retention and compression of hot exhaust gases along with modest pumping losses that result from mixing hot NVO-period gases with the cooler intake charge. Accurate determination of total cycle energy utilization is complicated by the fact that NVO-period retained fuel energy is consumed during the subsequent main combustion period. For the present study, a full-cycle energy analysis was performed for a single-cylinder research engine undergoing LTGC with varying NVO auxiliary fueling rates and injection timing.
A custom alternate-fire sequence with 9 pre-conditioning cycles was used to generate a common exhaust temperature and composition boundary condition for a cycle-of-interest, with performance metrics recorded for each custom cycle. The NVO-period reformate stream and main-period exhaust stream of the cycles-of-interest were separately collected, with sample analysis by gas chromatography used to identify the retained and exhausted fuel energy in the respective periods. To facilitate gas sample analysis, experiments were performed using a 5-component gasoline surrogate (iso-octane, n-heptane, ethanol, 1-hexene, and toluene) that matched the molecular composition, 50% boiling point, and ignition characteristics of a research gasoline. The highest total cycle thermodynamic efficiencies occurred when auxiliary injection timings were early enough to allow sufficient residence time for slow reforming reactions to take place, but late enough to prevent significant fuel spray crevice quench. Increasing the fraction of total fuel energy injected into the NVO-period was also found to increase total cycle thermal efficiencies, in part due to a modest reduction in NVO-period heat loss from a combination of fuel-spray charge cooling and endothermic fuel decomposition by pyrolysis. The effect was most pronounced at the lowest loads where larger charge mass reformate fractions increased overall specific heat ratios and main-period combustion phasing advanced closer to top dead center. These effects improved both expansion efficiency and combustion stability.