The Impact of Intake Valve Dynamics on Knock Propensity in a Dual-Fuel SI Engine 2017-01-2236
In this study, the impact of the intake valve timing on knock propensity is investigated on a dual-fuel engine which leverages a low octane fuel and a high octane fuel to adjust the fuel mixture’s research octane rating (RON) based on operating point. Variations in the intake valve timing have a direct impact on residual gas concentrations due to valve overlap, and also affect the compression pressure and temperature by altering the effective compression ratio (eCR). 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. At 2000 rpm and 6 bar IMEP, the fuel RON requirement ranges from 80 to 90 as a function of the intake valve timing, and the valve timing can change the RON requirement from 98 to 104 at 2000 rpm and 14 bar IMEP. These 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. Due to the wide range of fuel RON requirements with respect to the valve timing, a more sophisticated fueling control strategy is required in order to ensure that knock is properly suppressed despite varying valve timing. 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.