Fuel Effects on Energy Release and Knock Parameters in a SI Engine 952404
Model fuels composed of isooctane, n-heptane, toluene, ethanol and metyl tertiary butyl ether (MTBE) were tested in a modern high speed engine with electronically controlled injection and closed loop, λ=1 control system. The fuel compositions were chosen to represent a spectrum of straight run, catalytically cracked and reformulated gasolines. All fuels were blended in such a way that they had the same research octane number (RON) of 95 as determined by testing in a CFR engine according to the standard ASTM procedure D-2699. 5, 10, 50 and 90 percent energy release and maximum pressure crank angles and their variations, as well as knock probabilities, intensities, locations and characteristics, were determined for all the fuels at two speeds, 25 and 50 rps, at full loads.
The fuel composition has a weak effect on the energy release parameters and their variations, but a strong effect on knock. The latter result was contradictory to expectations, as all the fuels had the same octane ratings. Both the knock probability, and particularly, intensity were substantially lower for fuels containing oxygenates and aromatics, especially those containing MTBE for which the knock intensity in heavy knock cases was more than an order of magnitude weaker and the knock probability reduced by a factor of 2 to 3. A clear inverse correlation was found between the knock intensity and the amount of fuel burned in the flame at the knock onset, showing that a considerable fraction of unburned mixture participates in the knock event. Thus, the low knock probability and intensity for mixtures containing MTBE is principally caused by the increase in autoignition time. A clear correlation between the early flame development time and knock was also observed for all the fuels, indicating that cyclic variations of the flame formation process are a key factor in controlling knock and increasing the spark advance to a thermodynamically optimal position. The knock signature is different for the different fuels, exhibiting different percentages of strong and weak autoignition features. These are characterized respectively by a strong single blast wave, attenuating as a result of multiple reflections, and gradually growing consecutive blast waves autoigniting portions of the end gas after reflections and refocussing. MTBE containing fuels are more likely to exhibit strong autoignition features than the other tested fuels.