Influence of Gas Exchange and Volumetric Efficiency on Knock Behavior in a Spark-Ignition Engine

THE INFLUENCE OF INLET MANIFOLD DESIGN on knock in a naturally aspirated single-cylinder S-I engine under wide-open-throttle operating conditions has been investigated by carrying out computer simulations and engine tests. The different inlet manifold configurations were compared at engine speeds where they had equal volumetric efficiency.

COMPUTER SIMULATIONS showed that when the mass flow into the cylinder is enhanced during the early part of the intake stroke, there is less tumble and lower in-cylinder temperature after compression. The reason for the lower temperature is that the heat transfer from the cylinder walls to the charge is reduced when tumble is decreased. The lower temperature of the charge should have a positive effect in reducing knock.

ENGINE TESTS were carried out on a single-cylinder research engine. Different inlet manifold lengths and diameters were tested. All other parameters were kept constant. The knock sensitivity, measured as ignition advance at detonation borderline, DBL, was compared to volumetric efficiency. The engine tests gave as results that the ignition advance tolerance correlated to volumetric efficiency over the whole engine speed range. There was no measurable difference in knock sensitivity at speeds where the manifolds had equal volumetric efficiency. The conclusion of this is that the knock sensitivity is controlled predominantly by volumetric efficiency if no other parameters are changed.