Computational fluid dynamics modeling of a large bore two-stroke natural gas engine

Abstract

Natural gas fueled engines have been used for many years in stationary applications such as gas compression and electric power generation. In the United States alone, there are over 8,000 large bore (bore >30 cm) slow speed (speed < 500 rpm) natural gas engines in use. The most common configuration is a two-stroke cycle with direct injection of natural gas into the cylinder. The specific engine modeled in this study, the GMV Cooper Bessemer engine, is widely used in the gas compression industry, primarily in 10 cylinder versions. As air emission regulations have been enacted, reducing exhaust emission levels from pipeline engines has become increasingly important. Insufficient in-cylinder mixing due to ineffective fuel delivery is believed to be problematic in these natural gas engines. Cyclic combustion instability is a key-contributor to NOx and CO. Therefore, enhancement of fuel-air mixing using high pressure pipeline gas for fuel injection and use of alternative ignition systems such as pre-combustion chamber ignition and laser ignition are being considered as engine retrofit technologies. The characteristics of natural gas fuel jets emanating from a low pressure conventional poppet valve and from a retrofitted high pressure valve are investigated and compared. The fuel-air mixing in the engine is characterized and quantified by defining the parameters such as flammable volume fractions. At low injection pressures the gas flow around a typical poppet valve collapses to axis of symmetry of the valve downstream of the poppet. At high pressure, the gas flow from this simple poppet valve does not collapse, but rather expands outward and eventually flows along the cylinder wall, producing poor mixing in the cylinder. A Poppet valve is not an efficient valve in delivering momentum to the cylinder. Stagnation pressure losses occurring while the fluid passes through the valve are quantified and classified. Pressure base valve injection efficiency was defined and used to show the valve injection performance. Comparison of the results indicates that it is possible to make remarkable improvements of injection performance in momentum delivery by substituting well designed valves for the conventional poppet valves. The development of a compatible virtual valve which reproduces downstream jet characteristics of the jet issuing from actual valves is described. Instead of including the complex detail of a real valve, a simple converging-diverging type virtual valve is suggested for three dimensional engine simulation with high pressure injection. The results indicated that the suggested converging-diverging nozzle type virtual valve produces practically identical downstream fuel jet with the real valve injection jets. The simulation results for three dimensional overall engine simulations for a two-stroke natural gas engine are presented and discussed. The interaction between the scavenging flow and the injected fuel jet causes the jet to deflect enough to impact the piston top off center, producing non-symmetric mixing in the combustion chamber. As a result, the combustion mixture is not fully mixed at the time of ignition. There is a lean region in the core, and richer regions around the edge of the cylinder, with the richest region in the crevice around the edge of the cylinder. The flame propagation during combustion is non-uniform, with a greater flame speed in regions with a near stoichiometric equivalence ratio and of the engine flow fields favoring the direction of flame propagation. Retrofitted operation of the engine with high pressure fuel injection, precombustion chamber ignition and laser spark ignition are all investigated computationally. The simulation results indicate that high pressure fuel injection enhances the cylinder mixing. The alternative ignition methods examined change the flame propagation pattern so that the duration of heat release is shortened.