Fuel Sensitivity Affects on the Knock and CoV Limits of a Spark Ignited Engine

Engine knock is one of the limiting factors in determining the compression ratio and engine efficiency for spark ignited engines. Using the Southwest Research Institute Knock-CoV test method, it was previously shown that the knock limited load versus combustion phasing (CA50) has a constant slope. All of the knock mitigation strategies tested provided a shift to these knock limited loads but also increased the slope. That is, for the same CA50 retard the knock limited load could be increased more. Our hypothesis was that due to fuel sensitivity, or the difference between the RON and MON, the reactions that lead to knock will behave differently as the pressure-temperature history changes with engine speeds and loads. The fuel affects on the knock and CoV limits were studied by testing fuels with various sensitivities including methanol, E85 (85% ethanol) and Iso-octane. Methanol and E85 have higher sensitivities compared to the baseline gasoline fuel and as a result showed a steeper slope of their respective knock limited load line. A similar CA50 retard enabled a larger increase in the knock limited load. Iso-octane by definition has a sensitivity of zero. The results showed it had a smaller knock limited load increase for the same CA50 retard. The pressure temperature histories of the tests were compared to temperature and pressure histories for the standardized RON and MON tests. As the CA50 was retarded and the load was increased to knocking conditions, the cylinder pressure history is elevated moving further into the beyond RON region. In the beyond RON region high sensitivity fuels show a higher knock resistance and explains why baseline gasoline had a larger slope than iso-octane and E85 had a larger slope than each.