On knock intensity and superknock in SI engines

Paper #:
  • 2017-01-0689

Published:
  • 2017-03-28
Abstract:
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 paper focuses on knock intensity (KI) which is determined by the evolution of the pressure wave following knock onset in a hot spot 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=(a/u_a ) where u_ais the speed of the autoignition front and α is the speed of sound. 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. Both Z and (∂x/∂T) are affected by stochastic processes. Z increases and hence the probability of high KI increases as Pko increases. For a given Pko, Z is lower for a fuel with higher RON. 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. 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. 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).
Also in:
  • SAE International Journal of Engines - V126-3EJ
  • SAE International Journal of Engines - V126-3
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