Cooling on-demand for knock prevention in spark-ignition engines: An experimental analysis

Low-speed, high-load is often a short-lasting state for a spark-ignition engine; therefore, the knocking combustion, which is induced by this operating condition, deserves to be analyzed as a dynamic phenomenon in addition to the traditional steady-state approach. That cooling, and, hence engine components temperatures, affects knock tendency is well known. However, a novel interesting point of view, which, to the best of the authors' knowledge, has not been previously analyzed, concerns the analysis of the dynamic response of the engine walls and unburned gas temperature to a variation in load and coolant flow rate. The work presents a novel approach for mitigating knock occurrence by means of cooling on-demand, which can be achieved with the adoption of an electrically driven pump and by the identification of proper cooling control strategies. An experimental campaign was carried out at the test rig, in which the engine metal temperature was controlled under stationary and transient load conditions while modification of spark advance and air/fuel ratio with respect to the production ECU were adopted. The results demonstrate that when passing from a non-knocking to a knocking engine operating condition, the adoption of a controlled coolant flow rate retards the knock onset by more than one minute, which is remarkable when compared to the short-lasting conditions favorable to knock occurrence. This allows the use of more efficient spark advances and air/fuel ratios, which result in an increase in torque and efficiency of more than 3% in comparison to traditional cooling conditions.