A CFD Study of Fuel Evaporation and Related Thermo-fluid Dynamics in the Inlet Manifold, Port and Cylinder of the CFR Octane Engine

Knock in Spark Ignited (SI) engines has received significant research attention historically since this phenomenon effectively restricts the compression ratio and hence the thermal efficiency of the engine. The latent heat of vaporization (LHV) of a fuel affects its knock resistance in production engines as well as affecting its Research Octane Number (RON) rating. The reason for this is that evaporative cooling of the fuel lowers in-cylinder gas temperatures resulting in reduced tendency for end-gas auto-ignition. Controlling of the fuel-air mixture temperature to 422 K at the inlet port as per the Motor Octane Number (MON) test method ensures full evaporation of the liquid fuel, and hence LHV is assumed to have little effect during this procedure. LHV therefore has a strong influence on a fuel's Octane Sensitivity (OS) - the difference between its RON and MON values. Since the chemical auto-ignition characteristic of a fuel also strongly influences its octane rating performance, it is of interest to examine the relative effect that LHV has on knock resistance of fuels. LHV is also expected to influence the strength of thermal gradients which develop in the cylinder. These thermal gradients are important, since they have recently been shown to have a strong influence on the rate of pressure rise during end-gas auto-ignition in the Co-operative Fuels Research (CFR) engine used for octane testing. Since the CFR knock meter sensor is triggered by the rate of pressure rise (rather than pressure fluctuation) in a knocking CFR engine, LHV may possess an additional mechanism of influence on the RON rating of a fuel. The current work uses Computational Fluid Dynamics (CFD) engine modeling techniques to provide further insight into the influence of LHV and heat transfer during octane rating by the RON and MON test methods. Fuel LHV and the test engine speeds are shown to influence the in-cylinder temperature gradients at Inlet Valve Closure (IVC). The development of these thermal gradients during compression is shown to be strongly influenced by the combustion chamber wall temperatures and the compression heating behavior of the fuel-air mixture constituents, in particular the ratio of specific heats, commonly referred to using the symbol γ.