Effect of Oil and Gasoline Properties on Pre-Ignition and Super-Knock in a Thermal Research Engine (TRE) and an Optical Rapid Compression Machine (RCM)

High boost and direct injection are effective ways for energy saving in gasoline engines. However, the occurrence of super-knock at high load has become a main obstacle for further improving power density and fuel economy. It has been known that super-knock can be induced by pre-ignition, and oil droplet auto-ignition is found to be one of the possible mechanisms. In this study, experiments were conducted in a single-cylinder thermal research engine (TRE), in which different types of oil and surrogates were directly injected into the cylinder and then led to pre-ignition and super-knock. The effect of oil injection timing, oil injection quantity, different gasoline and different oil were tested. All the oil in this work could induce pre-ignition, even though their combustion phasing was much later than that in the case of n-hexadecane. Additionally, the effect of oil additives was investigated and the test results indicated that the addition of CaSulfonate and NaSulfonate tends to promote pre-ignition, while the addition of ZDDP and MoDTC tends to retard pre-ignition at certain conditions. Besides the tests in the single cylinder gasoline engine, experiments were carried out in a rapid compression machine (RCM) to investigate the auto-ignition process of a single oil droplet. Both the images acquired by the high speed camera and pressure traces showed that a single oil droplet could self-ignite in the stoichiometric mixture of iso-octane.