Effects of propeller load fluctuations on performance and emission of a lean-burn natural gas engine at part-load conditions

Providing stable combustion of lean-burn natural gas engines was always a big challenge, particularly during a low load operation. In transient sea conditions, there is an additional concern due to irregular time-varying loads. Therefore, this study aimed at investigating the part-load operation of a marine spark-ignition lean-burn natural gas engine by simulating the entire engine. The engine's essential components are modeled, including air manifold, intake valves, fuel system, controllers, combustion chamber, exhaust valves, exhaust manifold and turbocharger. In steady-state, the results of emission compounds from modeling have been compared to measured data from 25% to 100% loads. For transient conditions, for the sample time of about 50 min, the fuel flow and turbocharger output are selected from the vessel logged data and compared with the simulation results. The model has shown the great potential of predicting the engine response throughout the steady-state and transient conditions. Simulating the engine at part-load transient condition showed that the unburned hydrocarbon formation, known as methane slip in lean-burn gas engines, is more than the part-load steady-state. This increase of methane slip is due to the combustion instability in lower loads and flame extinguishing in such transient conditions. The engine measured data shows a double amount of methane slip in a 25% load than the 100% load in steady-state. However, the simulation output in the transient conditions confirms an increase in methane slip over four times than equivalent steady-state load. Moreover, the lean-burn gas engine releases less NOX in part-load operation in a steady-state due to lower in-cylinder temperature. In transient conditions, there is remarkable instability in excess air ratio. Due to this instability, there is a rich mixture in instantaneous time steps during loads up. Therefore, it will result in an unusually high amount of NOX, and more than two times in comparison with the equivalent steady-state output.