Experimental Research On Super Knock Mechanism Of Gasoline Engine

Direct injection combining boosting and downsizing is one of the main methods to reduce fuel consumption for gasoline engine. However, the consequently occasional super knock occurring at low speed and high load conditions is greatly destructive to the engine, which hinders the way to further increase the engine power density and improve the fuel economy. Previous researches have shown that super knock is caused by preignition, but two issues, namely the origin of pre-ignition and the evolution from preignition to super knock are not yet fully understood. This dissertation focuses on these two issues, which is discussed in three aspects, i.e. the test bench study of pre-ignition origin, the optical analysis of super knock and the essential investigation of combustion.In the test bench study of pre-ignition origin, a super knock resist single-cylinder gasoline engine is built. A small amount of oil and hot particles are separately introduced into the combustion chamber, and their impact on pre-ignition and super knock are compared with those triggered by advanced spark ignition. The test results show that both oil and hot particles can cause pre-ignition and lead to super knock. However, only the oil is introduced into the combustion chamber before top dead center of compression stroke, and the hot particles are required to have a sufficient size, can they initiate preignition. The pre-ignition and super knock caused by different ways shows similar behaviors.In the optical analysis of super knock, a rapid compression machine(RCM) is optically refitted, therefore, high speed photography can be used to record the combustion process in the RCM. Isooctane-air mixture is used in the RCM study. The comparison between end gas combustion modes in RCM and the knock type in real engine reveals the detonation nature of super knock. The test results show that the possibility of detonation occurrence increases with energy density. Besides direct initiation by local explosion in the end gas, the detonation can also be initiated either by deflagrationdetonation- transition(DDT) or shock wave reflection. The damage of super knock to materials is due to the near wall initiation of the detonation which is independent of the negative temperature coefficient(NTC) property of the fuel. In addition, based on the luminosity change of the high speed image sequence, a method to analyze the luminosity spectrum is proposed, which is also used to reconstruct the resonant mode shapes of the pressure wave oscillation.In the essential investigation of combustion, a one-dimensional constant volume bomb(ODCVB) is built. Hydrogen-oxygen mixture is used instead of isooctane-air to study the combustion mode and detonation initiation process under low pressure conditions. The test results showed that detonation can be triggered either by local explosion of hot spot in turbulent flame brush or by shock wave reflection. In ODCVB, detonation tends to occur at high energy density, which is consistent with that observed in the RCM. Furthermore, the combustion processes initiated by hot spots with different temperature gradient are numerically analyzed. The results showed that with the increase of the temperature gradient within the hot spot, the combustion mode gradually transits from supersonic deflagration to detonation, during which the shock wave compression plays an important role.