Overview of the main mechanisms triggering low-speed pre-ignition in spark-ignition engines

Abstract

Low-speed pre-ignition has become of great concern since it represents a strong limit to any further downsizing of gasoline engines. The increase of pressure and temperature inside the combustion chamber at high loads can indeed lead to a premature auto-ignition of the mixture and to severe engine damages. Several hypotheses have been formulated but there is no consensus to explain and demonstrate the cause of low-speed pre-ignition. This article provides an overview of the mechanisms that might lead to low-speed pre-ignition. A general discussion and a synthesis of the favourable conditions for auto-ignition are first introduced. Fundamentals of auto-ignition in homogeneous gaseous phase are discussed and confronted to concrete experimental observations of low-speed pre-ignition. The real operating conditions of modern gasoline engines complicate the analysis of auto-ignition because spatial and cycle-to-cycle fluctuations are linked to various physicochemical phenomena involved in the mixture preparation process. At the same time, the increase in temperature and pressure with load enhances the mixture reactivity and any perturbation at the end of the compression can unbalance the mixture and lead to uncontrolled low-speed pre-ignition. A classification of these mechanisms into three groups is proposed to categorize pre-ignitions resulting from auto-ignitions in the gaseous phase, or from auto-ignitions resulting from interactions between either a liquid or a solid phase and the gaseous mixture. Finally, the impact of mixture dilution with burned gases, mixture temperature, liquid films and deposits is examined for different engine settings, fuels and charge motions to illustrate each of these three groups.

Recent Progress in Automotive Gasoline Direct Injection Engine Technology

“…Because gasoline dilution reduces the viscosity and surface tension of the oil, the droplet is likely to be released from the crevice because of the inertial forces during deceleration of the piston just before it reaches TDC. Currently, this explanation regarding the ingress of oil into the combustion chamber is widely accepted [72,75]. The release of oil droplets from the piston crevice at the end of the compression stroke has also been observed by Kassai et al [76] Lauer et al [77] confirmed that pre-ignition frequently leads to follow-up events.…”

“…Fuel composition and properties have been shown to have a relatively large impact on the frequency of pre-ignition occurrence [75,[96][97][98][99]. It is clearly evident and well established that aromatic content has a large positive correlation with the pre-ignition frequency [96,99], although the associated mechanism is not well understood.…”

“…However, in the case of a high RON fuel, the pre-ignition frequency did not show an obvious correlation with the intake temperature and could even increase at lower temperatures. To summarize [75], a higher intake temperature may promote auto-ignition, but the same effect can also be experimentally observed when lowering the intake temperature. Low temperatures have a positive effect on ignition delay for pure gaseous mixtures, but can also promote the formation of liquid films and rich gas mixtures, thus producing significant local decreases in ignition delay.…”

“…The effect of the intake temperature on pre-ignition is not as significant as that of pressure. An investigation by Zaccardi et al [75] showed that, when using a low RON fuel with a minimal concentration of heavy components, the pre-ignition frequency increased along with intake temperature. However, in the case of a high RON fuel, the pre-ignition frequency did not show an obvious correlation with the intake temperature and could even increase at lower temperatures.…”

Pressure Oscillations Generated by Knocking Combustion of C2H2 in a Constant Volume Vessel

“…Super knock usually happens before the spark ignition, which drives researchers to relate it to pre-ignition. Therefore, endeavors were made to dig the cause of pre-ignition [1][2][3][4]. On the other hand, although preignition is reckoned to be the indicator, it actually doesn't necessarily result in super knock.…”