چكيده به لاتين
Abstract:
Low temperature combustion (LTC) is an emerging engine technology that has ability to yield low NOx and soot emissions while maintaining high fuel efficiency. LTC strategy includes homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI), reactivity controlled compression ignition (RCCI) and partially premixed combustion (PPC). These LTC strategies use early fuel injections to allow sufficient time for air/fuel mixing before combustion. According to the literature, some LTC strategies are not promising strategies for future automotive and power generation applications due to difficulties in controlling the heat release rate (HRR) and the lack of a combustion phasing control mechanism. To mitigate these problems, the RCCI combustion concept was introduced. RCCI is a dual-fuel PPC concept which uses port fuel injection (PFI) of a low reactivity fuel (e.g., gasoline, natural gas and alcohol fuels) and direct injection (DI) of a high reactivity fuel (e.g., diesel and biodiesel) blending inside the combustion chamber to increase the combustion duration and to provide phasing control. Combustion phasing is controlled by the relative ratios of the two fuels and the combustion duration is controlled by spatial stratification between the two fuels.
In the first part of the present dissertation, the effects of diesel injection strategies, engine initial temperature and engine speed on the combustion and pollutant emissions characteristics of a modified heavy-duty reactivity controlled compression ignition engine fueled with natural gas/diesel are studied. Natural gas with low reactivity is assumed to be inducted into the engine through the intake port, while diesel fuel with high reactivity is directly injected into the engine using a double injection strategy. Several parameters were studied including the premixed natural gas amount, the first and second injection timings and the injected diesel mass split between the two injections. The results showed improved engine efficiency with reductions in soot and oxides of nitrogen emissions could be achieved with the injection strategies studied, but hydrocarbons and carbon monoxide emissions were deteriorated. Three factors, namely first start of injection timing, second start of injection timing and the diesel injection fuel fractions, had pronounced effects on reactivity controlled compression ignition engine combustion performance and emissions. To reduce soot and oxides of nitrogen emissions, increasing the natural gas percentage, advancing first and second starts of injection timing beyond a certain point and increasing fuel fraction in first start of injection timing are preferred, but they had an adverse effect on hydrocarbons and carbon monoxide emissions. Then, the effects of natural gas composition and engine speed on combustion and emissions characteristics of an RCCI engine are studied. It is shown that Wobbe number (WN) of gases and engine speed significantly affects RCCI engine combustion and emissions. The gas with higher WN displayed higher peak pressure, temperature and NOx emissions, and lower unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions.
Finally, the effects of piston bowl geometry showed that the optimum piston bowl design from peroformance and emissions point is the bathtub design. In addition, it was reported that the piston bowl depth and chamfer size can influence engine out UHC emissions.
Keywords: Reactivity controlled compression ignition (RCCI); combustion; efficiency; emissions; natural gas; piston bowl geometry
