A Study of H2, CH4, C2H6 Mixing and Combustion in a Direct-Injection Stratified-Charge Engine 971710
The combustion characteristics of three gaseous fuels (hydrogen, methane and ethane) in a direct-injection stratified-charge single-cylinder engine with a centered square head-cup operated at 800 rpm (compression ratio = 10.8, squish ratio = 75%, nominal swirl ratio = 4) were studied to assess the extent to which the combustion is controlled by turbulent mixing, laminar mixing and chemical kinetics. The injection of gaseous fuels was via a Ford AFI injector, originally designed for the air-forced injection of liquid fuel. Pressure measurements in the engine cylinder and in the injector body, coupled with optical measurements of the injector poppet lift and shadowgraph images of the fuel jets provided both quantitative and qualitative information about the in-cylinder processes.
To make the cases comparable, the total momentum of the fuel jets and the total heat released by the three fuels was kept the same (equivalence ratio = 0.316, 0.363, 0.329 for H2, CH4 and C2H6, respectively). The total momentum of the gas jets was calculated using the measured values of the flow coefficient, under the assumption of a quasi-steady flow through the injector, coupled with the measured values of the cylinder and injector body pressures. With a spark timing of 14° BTDC, combustion was fast and repeatable for all fuels (COV = 1.2%, 4% and 2.2% for H2, CH4 and C2H6, respectively).
All indications suggest that the combination of direct injection, high squish, high swirl and square cup in the engine head produced fast mixing and a high degree of mixture uniformity within the head cup at combustion time; thus turbulence intensity, molecular diffusion and chemical kinetics were the main contributors to the establishment and propagation of the turbulent flame. The effect of chemical kinetics was particularly significant in the early stages of combustion and the laminar flame speed remained relevant during the major part of the combustion event.