Research On Pre-chamber Jet Ignition Based On Rapid Compression Machine And Natural Gas Engine Experiments

Spark ignition(SI)engines are widely used as automotive power,which usually adapt conventional spark ignition mode.However,the thermal efficiency of conventional SI engines is not comparable to that of compression ignition engines due to its relatively low combustion velocity,resulting from the combustion mode composed of single-point ignition and flame propagation.Natural gas engines possess promising perspective among SI engines but the combustion velocity is even lower.Hence,due to increasingly stringent regulation in automotive fuel economy,optimizing the combustion characteristics and improving the thermal efficiency are both inevitable direction of development for SI engines,especially for natural gas engines.Pre-chamber jet ignition is proposed as a potential approach to improve the performance of SI engines.The prechamber serves as an ignition source for the main chamber,providing flame jets spatially distributed inside the main chamber.Meanwhile,the turbulent intensity in the combustion chamber is evidently enhanced.As a result,the combustion velocity is increased,providing the capacity to improve the thermal efficiency.Utilizing the connecting nozzles to generate the jets is the classical form of prechamber jet ignition.However,the combustion process of classical pre-chamber jet ignition is rather complicated,which needs to be further recognized.Experiments were conducted in the rapid compression machine(RCM)with full optical accessibility to analyze classical pre-chamber jet ignition.Two combustion modes were found by the experiments of several pre-chamber designs with different nozzle diameter: double stage mode and single stage mode.Double stage combustion mode takes place in the condition with relatively low nozzle dimension,showing relatively long ignition delay and extremely short combustion duration.The jets could not ignite the mixture directly.Ignition happens at a central position in the main chamber after a lag time followed by the rapid development of the flame with similar speed in each direction.However,the double stage combustion mode has poor combustion stability due to the high randomness of the ignition process inside the main chamber.With single stage combustion mode,the ignition delay and the combustion duration can be shortened simultaneously with satisfying combustion stability.The combustion processes inside the pre-chamber and the main chamber take place continuously.The flame jet develops from the nozzle,composed of long and thin fire near the nozzle and approximately cylindrical fire in the tip.The speed of flame jet raises with the increase of load,exceeding 15 times than that of flame propagation under all load conditions.In addition,the experiments of classical prechamber with the combination of two different nozzles were conducted to study the relationship and discrepancy of two flame jets.According to the concept of pre-chamber jet ignition and the phenomenon of flame acceleration in tunnel,a new ignition method named flame accelerated ignition is proposed.The flame acceleration tunnel can be regarded as a pre-chamber,where the flame is accelerated inside.Then the combustion in the main-chamber can be induced by the flame jet rushed out of the tunnel.The RCM experiments indicated that the combustion could be evidently enhanced by flame accelerated ignition.The flame jet maintains nearly cylindrical with favorable speed characteristic.Higher indicated thermal efficiency is gained by applying flame accelerated ignition compared to conventional spark ignition in the engine.In addition,the misfire limit is expanded with the introduction of residual gas cavity in the flame acceleration tunnel.