Investigation of the Effects of Autoignition on the Heat Release Histories of a Knocking SI Engine Using Wiebe Functions 2008-01-1088

In this paper, we develop a methodology to enable the isolation of the heat release contribution of knocking combustion from flame-propagation combustion. We first address the empirical modeling of individual non-autoigniting combustion history using the Wiebe function, and subsequently apply this methodology to investigate the effect of autoignition on the heat release history of knocking cycles in a spark ignition (SI) engine.

We start by re-visiting the Wiebe function, which is widely used to model empirically mass burned histories in SI engines. We propose a method to tune the parameters of the Wiebe function on a cycle-by-cycle basis, i.e., generating a different Wiebe to suitably fit the heat release history of each cycle. Using non-autoigniting cycles, we show that the Wiebe function can reliably simulate the heat release history of an entire cycle, if only data from the first portion of the cycle is used in the tuning process. This concept is then extended to autoigniting cycles whereby, for each cycle, data up to the autoignition angle is used to tune the Wiebe function; the latter is then used to predict the progress of the flame-propagation combustion if autoignition did not occur. The comparison of the actual heat release history with the estimate of the progression of the flame-propagation combustion if autoignition did not occur enables one to investigate the effects of autoignition on the heat release history.

Based on this methodology, we found that at 1200 rpm the fraction of fuel consumed by autoignition is up to 30%, and the duration of combustion is reduced by 3 - 6°, in comparison to the normal (non-autoignition) cycles. Increasing the engine speed reduces the autoignition mass fraction, and the difference in the combustion duration between normal and autoignition cycles.