Optical Diagnostics of Isobaric and Conventional Diesel Combustion in a Heavy-Duty Diesel Engine

Abstract

Compared to conventional diesel combustion (CDC), isobaric combustion can achieve higher thermal efficiency while lowering heat transfer losses and nitrogen oxides (NOx). However, isobaric combustion suffers from higher soot emissions. While the aforementioned trends are well established, there is limited literature about the high-temperature reaction zones, the liquid-phase penetration distance, and the flame tip propagation velocity of isobaric combustion. In the present study, the line-of-sight integrated imaging of Mie-scattering, combustion luminosity, and CH* chemiluminescence were conducted in an optically accessible single-cylinder heavy-duty diesel engine. The engine was equipped with a flat-bowl-shaped optical piston to allow bottom-view imaging of the combustion chamber. The experiments were conducted using n-heptane fuel for CDC and isobaric combustion modes. The peak cylinder pressure (PCP) and the fuel mean effective pressure (Fuel MEP) for both combustion modes are kept as 70 bar and 19 bar, respectively. For a given combustion mode, flame image velocimetry (FIV) analysis was performed on the consecutive combustion luminosity image pairs to obtain in-flame flow-field details, and the liquid-phase penetration distance and flame tip propagation velocity were also estimated. The combustion luminosity and FIV analysis show that compared to CDC, isobaric combustion has resulted in less prominent flame-flame interactions, which explained the lower heat release rate. Double-injection-based CDC has resulted in a shorter spray penetration distance compared to triple-injection-based isobaric combustion; however, shorter than the boundary of piston-bowl wall. In addition, it was found that CDC has a sudden increase in flame tip propagation velocity preceded by flame propagation due to flame-piston bowl impingement from the second injection and successive movement towards the squish region. However, isobaric combustion showed a trend of constant or slightly decreased flame tip propagation velocity for which the second injection flames remain within the piston bowl wall.