【作者】

Clemens Brückner

【摘要】

该论文已在赫尔辛基举行的第28届CIMAC大会上发表，论文的版权归CIMAC所有。ABSTRACT In diesel engine combustion research, the focus has been and will be on the reduction of exhaust Nitric Oxides (NOx) and Particulate Matter (PM), while maintaining or improving fuel economy. In the case of heavy-duty engines this is mainly driven by new emission requirements for non-road and stationary power generation applications (EPA Tier 4), as well as for sea-going vessels (IMO Tier 3), becoming effective in 2015 and 2016 respectively, which require further NOx tailpipe emission reductions of up to 80%. In the effort of reducing both NOx and PM emissions, low temperature combustion (LTC) technologies such as Premixed Charge Compression Ignition (PCCI) have emerged over the recent years. PCCI combustion can be achieved by combining cold end-of-compression temperatures with high levels of recirculated exhaust gas (EGR), causing NOx and PM formation to be mostly or completely avoided. However, this combustion mode is typically solely applicable at lower loads, where low boost pressures and short injection durations result in end-of-injection and sufficient premixing prior to ignition. In this work, we examine the limits of PCCI combustion under a wide range of low to medium loads for early inlet valve closure (i.e. Miller valve timing) and high boost pressures. The experiments are carried out on a 4L single cylinder heavy-duty common-rail DI Diesel engine operating at 1050 rpm in combination with a cooled external EGR system. The engine is equipped with an in-cylinder Optical Light Probe (OLP), providing crank angle resolved information about the in-cylinder soot evolution. The main objectives of this study are to examine the trends of PM, CO, uHC and NOx for varying intake O2 concentrations until PCCI combustion mode is achieved. We show that for all temperatures and loads, a reduction of intake O2 concentration leads to a gradual reduction of NOx emissions. The trend of soot emissions however shows to be different between loads. At higher loads and low loads at high temperatures, soot emissions gradually increase with increasing EGR rate up to a peak value before drastically reducing. At that point, soot formation is supressed and soot luminosity cannot be detected any more. At low loads and low end-of-compression temperatures, the soot emissions show to decrease gradually without the characteristic peak found for all other operating conditions, even though the soot luminosity shows the same gradual reduction until disappearance. This can mainly be attributed to the initial long ignition delay (ID), which is enhanced by increasing EGR rates leading to very lean mixtures prior to combustion. For all loads and temperatures, reducing intake O2 concentration leads to a gradual increase of CO and uHC emissions. However, at a given load and EGR rate, operating conditions at lower temperatures show higher CO emissions, while the uHC emissions are lower compared to hotter cases. Because increasing temperature reduces the ID, worsening mixing prior to the start of combustion, higher EGR rates are required to achieve PCCI combustion with increasing temperature. Moreover, low temperature cases in PCCI mode have higher uHC emissions than hot cases not operating in PCCI mode at the same load and intake O2 concentration. Overall, higher CO and uHC emissions are observed at PCCI conditions for constant load and increasing temperatures due to the higher required EGR rate. A similar effect on the ID is observed for increasing load at constant temperature, requiring higher amounts of EGR to achieve PCCI combustion and thus leading to higher CO and uHC emissions. Overall, this study offers a comprehensive investigation of the limits of PCCI combustion in terms of various engine parameters and the resulting advantages and penalties in emissions from an engine running under these conditions.

【会议名称】

第28届CIMAC会议

【会议地点】

芬兰 赫尔辛基

【下载次数】

3

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