Numerical and optical analysis of heterogeneous gas combustion with diesel pilot ignition in a commercial vehicle engine

Abstract

A new optical measurement technology and a simulation methodology for supporting the combustion system development of a high-pressure gas-diesel combustion system are presented for the first time. With this combustion system, natural gas is directly injected with high pressure into the combustion chamber near top dead center. The simulation methodology is being validated by comparison with a commercial vehicle single-cylinder metal engine and combustion imaging on a commercial vehicle optical engine with full optical access to the combustion chamber through a contoured piston window. For the optical analysis of the gas-diesel combustion system, a measurement setup for the combined two-dimensional imaging of the OH* chemiluminescence and the soot luminescence is being developed. These images allow the ignition characterization and the cyclic stability of combustion to be analyzed. With the support of the simulation methodology, the nozzle-bowl configuration can be designed with regard to optimum mixture formation and low emissions. A thermodynamic validation of the simulation results shows good agreement of the ignition timing and the following combustion phases with heat release rate data obtained from measurements. Furthermore, the end of engine soot concentrations are captured quantitatively well. A local comparison of optical images and simulation data demonstrates that the simulation is able to capture the gas ignition zones well. The combustion intensity and thus the local soot formation are slightly underpredicted in the center of the gas jets. However, the mixture formation and the overall regions of soot formation are in good agreement with the optical images.