Analysis of Knock Transient Response to Controlled Coolant Flow Rate Variations in a S.I. Engine

The effects of a controlled coolant flow rate variation on knock tendency are investigated on a small S.I. engine. The analysis concerns the transient behavior of the unburned gas temperature to a controlled variation in the coolant flow rate. This phenomenological investigation aims at preventing knock through a proper thermal management as an efficient alternative to the currently adopted strategies. Moreover, the proposed approach is particularly useful for hybrid-electric powertrain, where the engine is expected to operate in the highest efficiency region by adopting high compression ratios and full stoichiometric map. The analysis was carried out through an experimental campaign, where the control of cylinder wall temperature was achieved by means of an electrically driven water pump. The spark advance and the air/fuel ratio were properly varied in order to operate with advanced spark timing and stoichiometric mixture at full load. Both stationary and transient engine operations were tested. A comparison with the standard conditions involving the adoption of the belt-driven pump and the production ECU combustion parameters is included. Results demonstrate that an increased coolant flow rate leads to a reduction of knock intensity at stationary operating conditions due to the lower wall temperature. When the spark advance is set to the maximum value for the light-knock onset and the mixture is stoichiometric, a major coolant flow rate reduces the knock intensity to values close to the ones achieved with standard spark advance and enriched mixture, recording an increase in efficiency of more than 3% without penalizing engine performance. Moreover, under transient conditions, knock onset is delayed by more than one minute in comparison with standard cooling conditions. This time is long enough to justify the application of the proposed methodology, in considering that the driving conditions, which may cause knock occurrence, are commonly applied for shorter time intervals.