예혼합 압축착화 엔진에서 함산소 연료의 연소 및 배기배출물 특성에 관한 연구

Abstract

In this study, one gaseous and three liquid oxygenated fuels such as biogas, bioethanol, biodiesel, and dimethyl ether (DME) as the alternative fuels for conventional gasoline and diesel fuel were experimentally investigated under various operating conditions for the evaluation and comparisons of atomization and combustion performance in a compression ignition (CI) diesel engine. This study focused on the investigation and realization of premixed charge compression ignition (PCCI) fueled with oxygenated fuels in order to achieve the effective reduction of exhaust emissions as a part of possible solution to meeting future exhaust emissions regulations. In this study, various methods are employed for PCCI combustion by supplying premixed fuel into intake chamber (dual-fuel), early direction injection with narrow angle direct injection (NADI), multiple injection strategy, and low temperature combustion (LTC) with high EGR rates. The combustion performance, exhaust emissions characteristics and particle number and size distributions were examined in a single-cylinder direct injection (DI) diesel engine and a four-cylinder indirect injection (IDI) diesel engine. In addition, experimental analysis of the macroscopic and microscopic spray characteristics of conventional diesel and alternative fuels in the common-rail injection system were also investigated under various test conditions. At early injection timings and multiple injection strategy, the wall impingement of the fuel spray was suppressed by using a narrow angle spray injector fueled with diesel. NADI combustion showed significant decrease in ignition delay than conventional engine at advanced injection timings. In multiple injections, the optimal injection timing for simultaneous reduction in NOx and soot emissions measured with 30 % EGR and early first injection timing. In the case of gasoline premixed fuel combustion, at retarded direct injection timing and high premixed ratio, the simultaneously reductions of soot and NOx emissions could be achieved with high combustion performance. Total particle number and volume of conventional diesel combustion indicated fairly higher than those of gasoline premixed combustions at all direct injection timings. The droplet size of neat biodiesel at local droplet was slightly higher than diesel due to its higher viscosity and surface tension. Also, biodiesel showed a slightly longer spray tip penetration compared to diesel due to larger droplet and higher momentum, resulted in longer spread out. Biodiesel emitted relatively higher NOx and the remarkably low levels of soot emissions. The HC emissions for biodiesel were also obviously lower than that of diesel in all injection mass. In the case of CO emissions, biodiesel emitted somewhat low and nearly same levels compared to diesel combustion. Biodiesel-bioethanol blends emitted relatively lower NOx and soot emissions levels than diesel. However, blends were slightly increased HC and CO emissions at low load conditions. Besides, split injection cases for blend, the number concentrations of larger size particles were significantly reduced. In the case of split injection case, the total volume and number of particle for blended fuel for split injection case also emitted much lower levels than those of diesel fuel. In the case of biogas premixed combustion, peak pressure for biogas-biodiesel are slightly lower compared to biogas-diesel at low load condition. For medium load, biogas-biodiesel combustion showed the slightly higher peak combustion pressure, ROHR and IMEP than those of diesel. Significantly lower NOx emissions are emitted under biogas premixed operation for both pilot fuels (diesel, biodiesel) compared to single fuel combustion at all conducted test ranges. Also, biogas-biodiesel provided superior performance in reduction of soot emissions. Multiple injection strategy of DME; it could be achieved simultaneous reduction of NOx and soot emissions compared to single injection results, and NOx emissions decreased with the advance of fist injection timing without increasing of soot emissions due to lower C-H ratio in the fuel. At retarded first injection timing, HC and CO emissions for DME indicated relatively low levels compared with the single injection levels. Narrow-angle injectors and multiple injections for DME could be achieved low NOx emissions and similar levels of PM emissions, as compared to single-injection cases at advanced first injection timings. In addition, for the case of high EGR rate, DME combustion exhibited extremely low level of soot emissions at all injection timing, because the high oxygen contents, the absence of soot precursors and the short diffusion combustion phase suppressed the soot formations. 50 % of EGR combustion indicated very low NOx emissions. This result is mainly due to the decrease in combustion temperature of dilution, thermal and chemical effects of EGR.