Occurrence of sporadic super-knock is the main obstacle in the development of advanced gasoline engines. By utilizing a rapid compression machine, events of pre-ignition and super-knock in a closed system under high temperature and high pressure were captured by synchronous high-speed direct photography and pressure measurement. Two different types of engine super-knock could exist. The first type is the super-knock induced by pre-ignition followed by deflagration of the end-gas. This type of super-knock is quite similar to conventional knock and usually causes moderate pressure oscillation. The second type of super-knock exhibits significantly higher magnitude of pressure oscillation than that of the first type due to the detonation of the end-gas. The second type of super-knock is designated as ‘‘Deto-knock’’.
Three conditions must coexist for deto-knock to occur. First, pre-ignition triggers the combustion. Second, end-gas pressure and temperature are high enough to cause detonation. Third, local hot-spot exists in the end-gas that triggers the detonation of the end-gas. The mechanism of deto-knock could be described as hotspot-induced deflagration followed by hot-spot-induced detonation in the end-gas.
The above mechanism obtained in the RCM can also be used to explain the phenomena of super-knock in boosted gasoline engines, as shown in the following Figure. First, pre-ignition occurs before TDC due to a local hot-spot (oil, deposit, oil-gasoline, etc.) in the combustion chamber during the compression stroke. A pre-ignition-triggered flame propagates from the hot-spot to the rest of the mixture. Then, the spark ignition occurs, and the 2nd flame front may propagate if the spark ignition is in an unburned zone. The rapid expansion of the burned gas rapidly compresses the unburned mixture to higher temperature and pressure (about 1000 K, 10 MPa). Finally, a second hotspot (or multiple hot-spots) in the end gas induces the detonation of the un-burned mixture at high temperature and high pressure.
As the timing of the “hot spot” combustion in the unburned mixture is crucial to detonation, this mechanism also helps to explain why an earlier pre-ignition does not always lead to a higher knock intensity. If the “hot spot” appears too early, the in-cylinder pressure and temperature are relatively low. It may turn out to be a deflagration, similar to the combustion processes in the 1st stage. If the “hot spot” appears too late, the majority of the mixture has already been consumed by the deflagration, and pressure tends to decrease with the downward movement of the piston. As a result, the pressure rise and pressure oscillation will be smaller. If the “hot spot” starts near TDC, it is likely to trigger detonation under high pressure and high temperature conditions.
The energy density of the unburned end-gas mixture at the onset of knock was identified as a criterion for super-knock. For gasoline fuel in the test engine, when the energy density of the unburned end-gas mixture exceeded 30 MJ/m3, super-knock was always observed. For lower energy densities, knock or non-knock was observed.
- Y. Qi, Y. Xu, Z. Wang, J. Wang, The effect of oil intrusion on super knock in gasoline engine, SAE Technical Papers (2014).
- Z. Wang, H. Liu, T. Song, Y. Xu, J.X. Wang, D.S. Li, T. Chen, Investigation on pre-ignition and super-knock in highly boosted gasoline direct injection engines, SAE Technical Papers (2014).
- Z. Wang, F. Wang, S.J. Shuai, Study of Engine Knock in HCCI Combustion using Large Eddy Simulation and Complex Chemical Kinetics, SAE Technical Papers (2014).
- Z. Wang, H. Liu, T. Song, Y. Qi, X. He, S. Shuai, J. Wang, Relationship between super-knock and pre-ignition, International Journal of Engine Research 16 (2015) 166-180.
- Z. Wang, Y. Qi, X. He, J. Wang, S. Shuai, C.K. Law, Analysis of pre-ignition to super-knock: Hotspot-induced deflagration to detonation, Fuel 144 (2015) 222-227.
- Y. Qi, Z. Wang, J. Wang, X. He, Effects of thermodynamic conditions on the end gas combustion mode associated with engine knock, Combustion and Flame 162 (2015) 4119-4128.
- Z. Wang, H. Liu, R.D. Reitz, Knocking combustion in spark-ignition engines, Progress in Energy and Combustion Science 61 (2017) 78-112.