Phenomena that determine knock onset in spark-ignited engines

Abstract

Experiments were carried out to collect in-cylinder pressure data and microphone signals from a single-cylinder test engine using spark timings before, at, and after knock onset for four different octane-rated toluene reference fuels. This data was then processed and analyzed in various ways to gain insight into the autoignition phenomena that lead to knock. This was done to develop a more fundamentally based prediction methodology that incorporates both a physical and chemical description of knock. The collected data was also used to develop a method of data processing that would detect knock in real time without the need to have an operator listening to the engine. Bandpass filters and smoothing techniques were used to process the data. The processed data was then used to determine knock intensities for each cycle for both the cylinder pressure data and microphone signal. Also, the rate of build-up before reaching peak amplitude in a bandpass filtered pressure trace was found. A trend was found showing that cycles with knock intensities greater than 1 bar with rapid build-up (5-10 oscillations) before reaching the peak are the type the cycles whose autoignition events lead to engine knock.

(cont.) The cylinder pressure knock intensities and microphone knock intensities were plotted and then fit with a linear trendline. The R2 value for these linear trendlines will transition from considerably lower values to values greater than 0.85 at the spark timing of knock onset. It is believed that the higher cylinder pressure knock intensities, in conjunction with the faster build-up of 5-10 oscillations before reaching peak, helps to explain the knock phenomena. It supports conclusions from previous works that the end gas contains one or more hot spots that autoignite in sequence causing pressure gradients that can trigger rapid pressure oscillations. These pressure oscillations can cause block and head vibrations that lead to audible noise outside the engine.