No abstract

No abstract

No abstract

Pre-ignition is an abnormal combustion event that may occur in boosted direct injection gasoline engines, where one or more auto-ignition events are observed before spark ignition. Due to the direct injection of fuel into the cylinder, some liquid fuel may splash off the walls, pulling lubricating oil with it. The auto-ignition of liquid fuel/lubricant droplets is considered as one of the possible sources of pre-ignition. To assess this stochastic phenomenon in a controlled way, the autoignition of a single droplet of a hexadecane-fuel mixture was investigated, with hexadecane serving as a surrogate for the lubricating oil. This experiment included suspending a single hexadecane-fuel mixture droplet on a thermocouple bead in preheated air, at a temperature of 300 ± 4°C, in a constant volume chamber over a wide range of pressures (4−30 bar). Four different fuels with a range of research octane number (RON) between 70 and 120 were mixed with hexadecane at a volume percentage of 75% hexadecane to 25% fuel to investigate the time to ignition of the droplet, designated as TI. The TI was measured by recording the droplet temperature history simultaneously with high-speed droplet imaging. The droplet ignition is triggered by the auto-ignition of the combustible mixture formed by the hexadecane-fuel mixture's vapor that mixes with the hot ambient air around the droplet. An empirical model was proposed to predict the TI in terms of pressure and the mixture's RON.At constant pressure, the rate of evaporation of the mixture's droplet increases with increasing RON. The time to ignition is seen to increase exponentially as the fuel's RON used in the mixture increases. On the other hand, for a given fuel RON, the time to ignition decreases with increasing ambient pressure. The empirical model shows that at pressures above 20 bar, the dilution of hexadecane by the different fuels has no significant effect on delaying the auto-ignition of the droplet.

The auto-ignition of liquid fuel and lubricant oil droplets is considered as one of the possible sources of pre-ignition. Researchers are continually finding new ways to form advanced lubricant oil by changing its composition and varying different oil additives to prevent the occurrence of this event. In this study, three sets of mixtures are prepared. The first set of mixtures was prepared by adding different alcohols namely ethanol, methanol and propanol, to the commercial lubricant oil (SAE 15W-40) in ratio of 1 -5 % by vol The second set of mixtures were prepared by mixing SAE 15W-40 with aforementioned alcohols (1 % vol.) and H2O (1 % vol.). Lastly, third set of mixtures were prepared by adding SAE 15W-40 in toluene (1 % -5% by vol.). Two experimental setups were used in the current work. An Ignition Quality Tester (IQT) was used investigate the mixtures' ignition delay time (IDT) following standard ASTM D6890 procedure, while a constant volume combustion chamber (CVCC) was used to investigate the combustion characteristics of burning of a suspended single oil droplet. For the CVCC, experiments were carried out in an ambient air environment at 300 °C with varying pressure ranging from 4 bar -22 bar at 6 bar interval pressures.IDT of lubricant oil was considered as the base IDT, which was compared to those of other mixtures. Addition of alcohols and water in lubricant oil showed a significant increase in IDT compared to other mixtures. However, the addition of toluene resulted in a decrease in IDT. Among the alcohols, methanol addition showed highest IDT compared to ethanol. However, alcohols had to be added >4 % by vol., to show a significant increase in IDT compared to pure lubricant oil.