Analysis of Combustion Cycle-to-Cycle Variation in an Optical Single Cylinder Dual-Fuel Engine

Abstract

This study aims to improve the dual fuel combustion for low/zero carbon fuels. Seven cases were tested in a single cylinder optical engine and their ignition and combustion characteristics are compared. The baseline case is the conventional diesel combustion. Four cases are diesel-gas (compressed natural gas) dual-fuel combustion operations, and two cases are diesel-hythane combustion. The diesel fuel injection process was visualized by a high-speed copper vapour laser. The combustion processes were recorded with a high-speed camera at 10000 Hz with an engine speed of 1200 rpm. The high-speed recordings for each case included 22 engine cycles and were postprocessed to create one spatial overlapped average combustion image. The average combustion cycle images were then further thresholded and these images were then used in a new method to analyze the cycle-to-cycle variation in a dimensionless, for all cases comparable value. Furthermore, the ignition delay and heat release profile of each case are analyzed. The results showed the lowest deviation from the complete overlap for the pure Diesel case and the Hythane Cases since the flames are more concentrated in these. From these studies, it can be concluded that the cyclic variation for the pure diesel combustion is mostly caused by the different swirl speeds in the piston bowl. The diesel-gas dual-fuel combustion with earlier pilot injections have lower cyclic variation due to a wider spread of the combustible mixture. The usage of hythane as main fuel instead of methane results in a about 10% faster combustion and more concentrated flames areas.