Fundamental Combustion Research Of The Oil Induced Pre-ignition And Super-knock

Boost is an effective way for energy saving and emission reduction in spark ignition gasoline engines.However,the super-knock caused by pre-ignition may occur at high load low speed operating conditions with the power density increasing.Super-knock can be quite destructive and gasoline engines can be damaged by just one super-knock cycle.Pre-ignition is mainly induced by the oil droplet in the combustion chamber.The work in this paper focuses on the fundamental combustion problems of oil induced pre-ignition and super-knock.A small amount of oil was injected into the cylinder of a thermal research engine to simulate the process of oil intrusion in the combustion chamber of the real turbocharged gasoline engine.Pre-ignition and super-knock were induced by the oil injection in the test,and the pre-ignition timing was closely related to the oil injection timing and oil injection quantity.Three different kinds of product oil were injected,so that pre-ignition were induced by all cases and the pre-ignition timing were different.The effect of oil formation on pre-ignition was further investigated.According to the test result,pre-ignition and super-knock could be induced by a small amount of base oil itself,and the oil additives had effects on the pre-ignition timing.The detergent of calcium sulfonate and the antirust agent of sodium sulfonate tended to promote pre-ignition,while the antioxidants of Zinc dialkyl dithi-phosphate(ZDDP)and Molybdenum dialkyl dithiocarbamate(MoDTC)tended to suppress pre-ignition.Tests were also repeated with the n-hexadecane and n-decane for comparsion.As a result,the oil samples were less effective than n-hexadecane and n-decane on inducing pre-ignition.To make a scientific explanation for the oil induced pre-ignition in the thermal research engine,experiments were conducted in an optical rapid compression machine.The oil droplet was suspended in the high temperature and high pressure oxygen atmosphere in the combustion chamber and its auto-ignition process was observed by the high speed camera.Further experiment results showed that the oil droplet in the high temperature and high pressure iso-octane and oxygen mixture could auto-ignite,and led to the ignition of the mixture and flame propagation,with high pressure and dramatical pressure oscillation.The role of the oil droplet auto-ignition was similar to the spark in this process.The ignition delay for the oil droplet case was shorten with the pressure atthe end of compression increasing,which gave an explanation for the fact that pre-ignition and super-knock prefer to occur in the boost gasoline engine.The three product oil samples were tested under the same thermodynamics condition,and the sequence for their ignition delay agreed with the sequence for the pre-ignition timing in the engine test.It indicated that the different effect of oil samples on the occurrence of pre-ignition and super-knock in the engine test depended on the different oil ignition properties.