Bowl piston geometry as an alternative to enlarged crevice pistons for rapid compression machines

Abstract

Thermal inhomogeneity and physical processes like fluid dynamics reduce the utility of rapid compression machine (RCM) facilities to accurately study fuel combustion phenomenon relevant to internal combustion engines. Most current RCMs incorporate a large crevice volume in the piston to capture roll-up vortices that encroach into the combustion zone during compression. In this work, a bowl piston design similar to those used in diesel engines is proposed as an alternative to enlarged creviced pistons for creating a sufficiently thermally homogenous gas mixture prior to ignition without undesirable fluid motion found in flat piston configurations. The bowl piston also eliminates the possibility of cold unreacted gases entering the combustion chamber when the piston is retracted in rapid compression and expansion machines (RCEMs) like in creviced piston designs. In the work, a bowl piston was compared to creviced piston and flat piston configurations numerically and experimentally. Through non-reacting computational fluid dynamics simulations, the bowl piston reduced the roll-up vortex found for the flat piston and led to a higher temperature and more thermally uniform core of gas at peak compression compared to the enlarged crevice piston. Experimentally, three pistons were studied in a RCM facility with ethanol and n-butane as fuels. Results showed that the bowl piston yielded benefits over conventional piston geometries including: reduced heat loss due to lower surface area, higher turbulent Reynolds Number, stronger ignition, and higher heat release rate and combustion efficiency as estimated using heat release analysis. Based on the findings presented here, we conclude that bowl piston geometries are a promising alternative to creviced pistons for conducting fuel ignition studies in RCM and RCEM facilities.