Fuel Effects on Knock in a Highly Boosted Direct Injection Spark Ignition Engine 2012-01-1634
Extensive tests have been carried out in a single-cylinder Direct Injection Spark Ignition (DISI) engine using up to fifteen different fuels at inlet pressure of up to 3.4 bar abs. to study fuel effects as well as inlet pressure effects on knock. In addition fuel effects on particulate emissions at part-throttle were measured. Fuel anti-knock quality does not correlate with MON and is best described by the Octane Index, OI = RON-KS where S = RON -MON is the sensitivity of the fuel and K is a constant depending on the engine pressure/temperature regime. The RON of the fuels considered was in the range between 95 and 105 and the sensitivity between 8 and 13. K is negative at all the conditions tested, i.e., for a given RON, a higher sensitivity fuel has better anti-knock quality. K decreases with increasing intake pressure and more generally, decreases as Tcomp₁₅, the temperature of the unburned gas at a pressure of 15 bar decreases. If the temperature of the unburned gas at a given pressure is lower than that in the RON test conditions, K is negative. Knock becomes more likely with increasing intake pressure and increasing compression ratio with all else being equal. Higher anti-knock quality of the fuel enables higher load to be reached. However the gain in load with increasing OI decreases as OI increases - there are fast diminishing returns on increasing RON above 100 as long as the sensitivity is 10 or greater. Reducing the compression ratio enables greater boosting levels and hence a higher potential for down-sizing. In addition, compared to the pump gasoline (96.4 RON), a higher OI fuel can be run at a higher intake pressure, if such boosting levels can be provided by the hardware, so that the potential for down-sizing can be increased, e.g., a 3.2-liter engine could be down-sized to 1.5 liters using a 2-stage turbo-charger. A vehicle simulation over urban, road and highway conditions suggests that, with the pump fuel, this would lead to around 16% reduction in fuel consumption. This reduction could be around 19% with a fuel of RON of 99.6 and MON of 86.8 compared to a 95 RON fuel. The time taken for the first 10% burn is greater than the time taken for burning the next 80% of the charge. At cold idle conditions, the particulate emissions are higher than at full throttle conditions and they increase as the fuel aromatic content increases. Such emissions considerations as well as fuel effects on pre-ignition and the well-to-wheel implications need to be considered along with knock in designing optimum fuels for future SI engines.