Fundamental analysis on auto-ignition condition of a lubricant oil droplet for understanding a mechanism of low-speed pre-ignition in highly charged spark-ignition engines

Abstract

This article presents a study of the mechanism that the lubricant oil droplet initiates low-speed pre-ignition in highly boosted downsized gasoline engines. Low-speed pre-ignition is a phenomenon that the fuel-air mixture ignites before the spark timing, leading to flame propagation that results in a heavy knock. The ignition of lubricant oil droplets is thought to be one possible mechanism for low-speed pre-ignition. However, the oil droplet ignition conditions are not yet well understood. First, the conditions under which a single oil droplet initiates the combustion of a fuel-air mixture were investigated using a rapid compression and expansion machine. When an initial droplet temperature was above 250°C, the vaporized oil ignited before the gasoline-air mixture, in which case the combustion of the gasoline-air mixture around the droplet was initiated. The numerical results showed that the oil droplet temperature increases above 250°C if the droplet is heated by burned gas remaining in the combustion chamber from the previous cycle. A directinjection single-cylinder research engine was operated under the condition that no residual gas exists in the combustion chamber. In this case, no low-speed pre-ignition occurred even if gross indicated that mean effective pressure was 2.5 MPa. These results indicate that an oil droplet does not cause low-speed pre-ignition if any droplet flies into the combustion chamber unless it remains in the chamber over the exhaust stroke.

Experimental assessment of ignition characteristics of lubricating oil sprays related to low-speed pre-ignition (LSPI)

“…Hence, LSPI seems to be explained by a combination of several mechanisms, including engine-related factors together with the tendency of an oil droplet to stay in the chamber for the next cycle after the exhaust stroke. 41…”

“…Hence, LSPI seems to be explained by a combination of several mechanisms, including engine-related factors together with the tendency of an oil droplet to stay in the chamber for the next cycle after the exhaust stroke. 41 Influence of different additive types content on the lubricant oil reactivity Figure 8 shows the comparison among Oils #3 and #9 with their versions with higher content of specific additive types: anti-wear (AW), anti-oxidants (AO), and friction modifiers (FM), as shown in Table 3. Results show no significant differences among substances for any of the suppliers at any temperature.…”

“…30,38,39 An explanation is that some oil components may generate solid deposits and flakes that could act as hot spots. 40 However, Ohtomo et al 41 demonstrated in a rapid compression expansion machine (RCEM) that an oil droplet itself could be responsible for LSPI if it remained in the chamber over the exhaust stroke, staying hot in the next cycle. In this sense, Ullal et al 42 have recently shown the importance of the oil drop residence time on its pre-ignition through a numerical investigation.…”

Mechanism Triggering Pre-Ignition in a Turbo-Charged Engine

“…To summarize, it is possible to have no pre-ignition cycle if (i) there are no residuals carrying pre-ignition precursors from the previous cycle (shown previously by [24]) or (ii) reducing the intake temperature to be very low, hence preventing precursors from triggering pre-ignition. These two cases represent two extremes.…”

“…A recent study from Toyota used a fully flexible valve control system achieving an ideal scavenging phenomenon (by partially opening the intake valve during exhaust valve opening duration). By doing so, the researchers were able to suppress pre-ignition completely [24].…”