Engine Rapid Shutdown: Experimental Investigation on the Cooling System Transient Response

Experimental measurements have been taken on a production four-cylinder, multipoint (fuel) injection spark-ignition engine, 1.2 dm3 displacement with a four-valve per cylinder aluminum head, and a 60 kW at 5500 rpm rated power. The aim of the investigation was to understand the behavior of the cooling system of a small automotive engine, which was operated for a prolonged period at high speed under full or part load, then brought to idle for a short period and finally shut down. In this study, the effects of different loads, idle operation time, and lengths of the engine-radiator piping were analyzed. In particular, experimental tests were carried out with the engine running at 4000 rpm under different brake mean effective pressure values in the range 496 to 1133 kPa. In all experimental tests the engine was brought to idle in 5 s, and measurements were repeated for different values of the idle operation time ranging from 1 s to 80 s. Test data of coolant conditions and metal temperature at 26 points of the engine head and liner were recorded. The cooling circuit was instrumented with transparent tubes at the radiator inlet and photographs of the vapor phase moving to the radiator were taken during experimental tests. The volume of leaked coolant as a function of time was also measured. Additional tests were carried out to evaluate the effects of different lengths of the engineradiator piping on the after-boiling phenomenon. Finally, in order to make the results applicable also to nonautomotive engines, measurements were repeated without the standard cabin heater and the associated piping. The investigation results show that as the engine is shut down and coolant flow stops, the head metal may be hot enough to vaporize a fraction of the coolant contained in the cylinder head passages, causing the pressure within the cooling circuit to rise above the threshold value of the radiator cap pressure valve and, consequently, an important quantity of the coolant to be expelled.