Effects of Heat of Vaporization and Octane Sensitivity on Knock-Limited Spark Ignition Engine Performance

Paper #:
  • 2018-01-0218

  • 2018-04-03
The knock-limiting effects of a set of surrogate gasolines all having nominally 100 research octane number (RON), approximately 11 octane sensitivity (S), and a heat of vaporization (HOV) range of 390 – 595 kJ/kg were investigated. A single cylinder engine derived from a GM Ecotec direct injection (DI) engine was used to perform load sweeps at a fixed intake air temperature of 50°C, as well as knock-limited load measurements across a range of intake air temperatures. Both DI and pre-vaporized fuel (supplied by a fuel injector mounted far upstream of the intake valves and heated intake runner walls) experiments were performed to separate the chemical and thermal effects of the fuels’ knock resistance. The DI load sweeps showed no effects of fuel composition or HOV on the knock-limited performance, i.e., the data suggest that HOV may act as a thermal contributor to S under the conditions studied. The increased knock resistance of these high S fuels allowed more advanced combustion phasing at high loads and was characterized with comparative data from isooctane (RON = 100, S = 0) and a 106 RON ethanol-alkylate gasoline blend. Measurements of DI, late combustion phasing, knock-limited loads from sweeps of intake air temperature up to 90°C showed that fuels with higher HOV provide higher knock resistance at the elevated temperatures. Using the pre-vaporized fuel system eliminated the evaporative cooling effects, such that all of the 100 RON / 11 S fuels produced nearly identical knock-limited loads across the range of intake air temperatures studied. Analysis of computed in-cylinder bulk gas temperatures, apparent heat release rates and knock metrics provide insight into the observed fuel property effects on engine performance.
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