Development of a Natural Gas Engine with Diesel Engine-like Efficiency Using Computational Fluid Dynamics

Present day natural gas engines have a significant efficiency disadvantage but benefit with low carbon-dioxide emissions and cheap three-way catalysis aftertreatment. The aim of this work is to improve the efficiency of a natural gas engine on par with a diesel engine. A Cummins-Westport ISX12-G (diesel) engine is used for the study. A baseline model is validated in three-dimensional Computational Fluid Dynamics (CFD). The challenge of this project is adapting the diesel engine for the natural gas fuel, so that the increased squish area of the diesel engine piston can be used to accomplish faster natural gas burn rates. A further increase efficiency is achieved by switching to D-EGR technology. D-EGR is a concept where one or more cylinders are run with excess fueling and its exhaust stream, containing H₂ and CO, is cooled and fed into the intake stream. With D-EGR although there is an in-cylinder presence of a reactive H₂-CO reformate, there is also higher levels of dilution. A new piston was designed that can match the high squish burn rates with not much impact from higher dilution and take advantage of the H₂-CO reformate at the same time. The new piston design has a 33% reduced squish area ratio and resulted in a 5.5% point increase in indicated thermal efficiency with D-EGR. Additional efficiency improvement of 2.6% is obtained by increasing the compression ratio by 1.5 points, bringing the total ITE improvement percentage to 8.1%. The goal of the project is to obtain a 10% improvement in efficiency while achieving 0.027 g/kW-hr NOx emissions.