Knock intensity, superknock and preignition in SI engines

Dr. Gautam Kalghatgi from Saudi Aramco will deliver a special seminar on Tuesday Aug 23, 1500-1630.

The seminar is titled: Knock intensity, superknock and preignition in SI engines

Abstract

Knock is an abnormal combustion phenomenon which limits the efficiency of spark ignition (SI) engines. It is caused by autoignition of the fuel/air mixture in hot spots in the “end-gas” ahead of the advancing flame front which depends on the evolution of pressure (P) and temperature (T) during the cycle and on the fuel anti-knock quality. Most studies on knock focus on the onset of knock which is determined by chemical kinetics and also ignore the stochastic nature of knock. This presentation focuses on knock intensity (KI) which is determined by the evolution of the pressure wave following knock onset and highlights the stochastic processes involved.

KI is defined in this study as the maximum peak-to-peak pressure fluctuation that follows the onset of knock. It depends on ξ=aua where uais the speed of the autoignition front and 𝑎 is the speed of sound, when ua is small. KI can be related to the product of a parameter Z, which depends on, Pko, the pressure at knock onset and the square of (∂x/∂T), which is the inverse of the gradient of temperature with distance in the hot spot. Both Z and (∂x/∂T) were calculated using measured KI and Pko for hundreds of individual knocking cycles for different fuels. The model for ignition delay as a function of pressure P and T in the hot spot and other data needed to calculate Z were available from a previous study (SAE 2016-01-0702).  For a given fuel and operating condition, Z varies because Pko varies because of cyclic variation of combustion – a stochastic process. (∂x/∂T) depends on the evolution of the hot spot during the engine cycle and depends on flow and turbulence – another stochastic process. All else being equal, Z increases and hence the probability of high KI increases as Pko increases, e.g., by more advanced spark timing and or faster flame development. For a given Pko, Z is lower for a fuel with higher RON.

Normally, the primary aim in SI engines is to avoid knock. However, in modern turbo-charged engines extremely high intensity knock, informally termed superknock is observed to occur occasionally. Superknock is caused by developing detonation (DD) which results when the value of 𝜉 decreases and the pressure wave begins to couple with the autoignition front and gets amplified. Autoignition/knock has to be initiated at high P and T for superknock to occur. At practical operating conditions, chosen expressly to avoid knock, this can only happen via another abnormal stochastic phenomenon - preignition, when a flame is established before the spark plug fires. For preignition to occur, the local temperature has to increase beyond a critical value. This cannot happen because of autoignition of the fuel/air mixture and hence does not depend on RON or MON of the fuel. A further initiation criterion has to be satisfied for a stable flame to be established. All else being equal, the chances of preignition (establishing a flame) increase as laminar burning velocity increases. All else being equal, the probability of superknock decreases as fuel RON is increased. However, even with high RON, high KI AND superknock could occur with the right combination of P and (∂x/∂T).