The dynamic effects of a coolant flow rate variation on knock tendency are experimentally investigated on a small S.I. engine. The analysis concerns the transient response of the unburned gas temperature and the knock onset to a step variation in load and 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 may result 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 is carried out through an experimental campaign, where the control of cylinder wall temperature is achieved by means of an electrically driven water pump. The spark advance and the air/fuel ratio have been properly varied in order to operate with advanced spark timing and stoichiometric mixture at full load. 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, under transient conditions, an increase in coolant flow rate retards the knock onset, thus allowing the adoption of higher spark advance and the use of stoichiometric mixture at full load. Under these conditions, an increase in engine efficiency of about 3%, without penalizing engine performance, is achieved.