射流控制压缩着火发动机不同负荷下燃烧及排放特性

针对柴油预混合气着火相位难以直接控制的问题,提出射流控制压缩着火(jet controlled compression ignition,JCCI)方式。将一台单缸农用柴油机改造为JCCI发动机:压缩比降至12,增加一个带液化石油气(liquefied petroleum gas,LPG)供给通道和火花塞的点火室,并进行了JCCI发动机全负荷特性试验研究。试验结果表明:采用射流控制压缩着火方式可以有效控制发动机的燃烧相位和排放。在平均有效压力低于0.44 MPa的工况范围,NOx排放比原机降低较多,燃烧始点相位CA10与滞燃期几乎不随负荷增加而改变;在平均有效压力高于0.44直至0.54 MPa负荷范围内,燃烧始点相位迅速前移,滞燃期迅速减小,柴油提前自燃,射流对着火相位控制作用减弱,NOx排放迅速增加并超过原机;在全负荷范围,烟度始终维持在低水平,HC和CO排放较高。该研究可为柴油预混合燃烧着火相位控制提供参考。

Combustion and emission characteristics of jet controlled compression ignition engine at different loads

Abstract: One of the key points in diesel premixed combustion is the realization of combustion phasing control. The sensitivity of diesel to temperature and equivalent ratio was the crucial obstacle. In order to directly control the diesel premixed combustion phasing, a novel method called the jet controlled compression ignition (JCCI) for diesel premixed compression ignition was proposed. A single cylinder diesel engine was modified to study the JCCI system. First, a small ignition chamber comprising a gas fuel injector and a spark plug was mounted on the cylinder head in this diesel engine. Six small orifices were used to connect the ignition chamber and the main chamber. Furthermore, the compression ratio was reduced to 12 to avoid the auto-ignition of the premixed diesel fuel. Experiments were conducted on the JCCI engine under overall loads at a constant speed to study the trend of combustion and the emission characteristics of JCCI system. The results showed that the combustion phasing and emissions could be controlled by using the JCCI system. At medium load region whose brake's mean effective pressure was below 0.44 MPa, there was little change in the start phase of combustion and the midpoint phase of combustion and the combustion duration with the variation of load. The timing of jets issued from ignition chamber was controlled by spark timing. Therefore it meant that a direct control of the combustion phasing of diesel premixed combustion by employing spark timing in JCCI system could be expected. Due to a reduction of compression ratio and the relatively longer mixing time the combustion temperature was greatly reduced compared with that of original diesel engine. Then nitrogen oxide (NOx) was reduced accordingly. The maximum reduction of NOx was over 80% compared with that of original diesel during the medium high load region. At high load region whose brake's mean effective pressure was above 0.44 MPa, the controllability of jet was weakened with the load increasing as well as the linearity between spark timing and start phase of combustion and midpoint phase of combustion. The cylinder temperature increased as the load increased. When approaching full load, the cylinder was too high to avoid the auto-ignition of diesel mixture. It meant that the combustion of diesel mixture had begun before it was triggered by the jets issued from the ignition chamber. The combustion phasing was advanced greatly due to an auto-ignition of diesel mixture. Therefore the start phase of combustion and the midpoint phase of combustion and the combustion duration were reduced quickly with the load increasing. Furthermore, the acceleration of cylinder temperature increase could be expected and a great increase of NOx was caused as well. When approaching full load, NOx in JCCI was higher than that of original diesel engine. During overall load region, the smoke was maintained at a low level due to longer mixing time of diesel mixture compared with that in traditional diesel engine. The high emission of hydrocarbon (HC) and carbon monoxide (CO) could be seen during overall load region due to incomplete oxidation of fuel. The research can provide a reference for the control of diesel premixed combustion phasing.