Idle Stalling Phenomena in High Performance Spark Ignition PFI Engines: an Experimental Analysis

High performance Spark Ignition (SI) Port-Fuel Injected (PFI) internal combustion engines are usually optimized to deliver high power output at high speed in Wide Open Throttle (WOT) conditions. However, they also have to run consistently at idle, possibly with stoichiometric Air-Fuel Ratio (AFR), in order to limit tailpipe emissions. The two requirements are sometimes conflicting, as it is difficult to match high-speed volumetric efficiency with lowspeed turbulence: the intake runner size and shape are often designed for performance, meaning that usually they do not guarantee a satisfying air-fuel mixing at idle. The consequence of poor mixture formation may be high cycle-tocycle variation or misfiring, with obvious consequences on pollutant emissions and driveability. In the worst cases, however, the consequence could be even more serious: stalling phenomena have been observed on the test bench. While running at idle, the engine suddenly stops: the event is so quick that the idle controller is not able to react. The paper shows a detailed experimental analysis of stalling phenomena, based on engine speed, intake pressure, incylinder pressure, ion current information. Intake and incylinder pressure data show that stalling phenomena are related to anomalous combustions taking place during the compression stroke: the negative torque generated by such combustions is able to stop the engine. Further analysis show that these phenomena are triggered by defined conditions: a partial combustion releasing little heat and leading to a constant pressure exhaust stroke seems to be a necessary condition to ignite the undesired combustion. Ion current signals show that the combustion extends during the exhaust stroke, and continues throughout the following intake and compression strokes. The sensitivity of the phenomenon to changes in the injection layout suggests that its origin is related to the process of mixture formation. The presence of a large amount of liquid fuel in the cylinder could lead to diffusive combustions, maintained throughout the exhaust stroke and the subsequent intake stroke, thus resulting in a backfire. The Rate Of Heat Release (ROHR) analysis based on incylinder pressure confirms that the frequency of the phenomenon is higher in the cylinder where more liquid fuel is likely to be accumulated.