Low-emission combustion of a pre-chamber-type compression ignition natural gas engine

A pre-chamber-type compression ignition natural gas engine was constructed and its performance and NO emissions were investigated. The pre- and main chambers made of ceramics were connected by a throat valve that opened during the compression stroke. An homogenous fuel air charge mixed with exhaust gas recirculation (EGR) gases was introduced into the main chamber, while a smaller amount of fuel was supplied into the pre-chamber during the intake stroke. The mixture in the pre-chamber was auto-ignited when the compressed hot gases in the main chamber were introduced into the pre-chamber through the throat valve during the compression stroke. The engine was operated without knock under various load conditions by adjusting the throat valve opening timing appropriately. The influences of the throat valve opening timing, compression ratio, and pre-chamber volume ratio on torque and NO emissions were investigated. Test results showed that: (a) at conditions when the thermal efficiencies were high, the patterns of the rate of heat release were similar to each other and independent of the compression ratio and pre-chamber volume ratio; (b) the optimum throat valve opening timing was strongly dependent on compression ratio and pre-chamber volume ratio; and (c) at conditions when the rate of heat release displayed two peaks - first low peak corresponding to the initial combustion in the pre-chamber and second high peak corresponding to combustion in the main chamber - low NO emissions below 20 ppm featuring homogeneous charge compression ignition (HCCI) combustion were achieved.