The Impact of Intake Valve Dynamics on Knock Propensity in a Dual-Fuel SI Engine

Paper #:
  • 2017-01-2236

  • 2017-10-08
In this study, the impact of the intake valve timing on knock propensity is investigated on a dual-fuel SI engine leveraging a low octane fuel and a high octane fuel to adjust the fuel mixture’s octane rating (RON) based on operating point. Variations in the intake valve timing have a direct impact on residual gases concentration due to valve overlap and changes in the compression pressure and temperature due to variations of the effective compression ratio. In this study, it is shown that the fuel RON requirement for a non-knocking condition at a fixed operating point can vary significantly solely due to variations of the intake valve timing. The fuel RON requirement at 2000 rpm and 6 bar BMEP ranges between 80 to 90 as a function of VVT, and between 98 to 104 at 2000 rpm and 14 bar BMEP. The significant changes in the required fuel RON are attributed to increases in the charge temperature due to high residual concentration as well as increased cyclic variability in combustion phasing. The wide range of fuel RON requirement with respect to the valve timing represents a controls challenge for finding an adequate fueling strategy that will simultaneously suppress knock and minimize the consumption of the high octane fuel. The impacts of valve dynamics are investigated in this study to evaluate the effect of response lag in which there is a mismatch between the commanded and actual valve position. The simulation of a WLTC drive cycle is used to illustrate the resulting effect on fuel consumption, which shows a 6% excess consumption of the high octane fuel during valve mismatch periods of the cycle.
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