Numerical Simulation Researches On In-cylinder Cleaning Technologies For Low-speed Two-stroke Marine Diesel Engines

Diesel engines as the main propulsion of ships, they have been widely used but limited by the strict emissions regulations. The IMO Tier III emission regulations will be implemented in 2016. This is undoubtedly a huge challenge for marine diesel engine. In order to quickly capture the market, the world famous marine diesel engine manufacturers are working on the technical route of reducing NOx emission. Under the background of environmental protection and strict energy saving, this paper is focusing on two stroke diesel, do some researches about the emission control technology, which will provide reference for the optimization of the engine.One-dimensional and three-dimensional CFD simulations are coupled on the work process of the engine. AVL BOOST and AVL FIRE simulation software were used to set up the model of the original diesel engine. According to the actual structure and parameters, the flow and combustion model of cylinder are set up and the developed model is validated against the test result. Based on the accurate model, the intake and exhaust flow, pressure, temperature, heat release rate, NOx distribution are analyzed for subsequent optimization.In the simulation of emission control technology, the design of a variety of Miller cycle, high compression ratio and the rate of EGR scheme were carried out. Miller cycle was applied through exhaust valve closing timing delay. Six different Miller cycle were used. The results show that:Miller cycle can reduce NOX emissions, and NOx gradually decreases with the delay of exhaust valve closing timing. With the scheme of M30, NOx can reduce 45%, but will result in the deterioration of fuel economy. Only the BSFC of M5 is same to the original engine. In order to improve the economy of Miller cycle, two-stage turbocharging system was used. After the using of two-stage turbocharging system, the BSFC of Miller cycle is obviously improved, especially the economy of M5 and M10 are better than the original engine.To provide more methods for EGR combining optimization, the increase of geometric compression ratio were calculated based on two-stage supercharged Miller cycle. By one-dimensional scanning calculation in AVL BOOST software, when geometry compression ratio increases 0.6, fuel consumption can be reduced by about 0.7g/kWh. The schemes of lower fuel consumption are selected from the one-dimensional simulation results, and then the three-dimensional calculations are carried out in AVL FIRE. NOx emissions increase with the increasing of geometry compression ratio, but the peak FIRE pressure is lower than the limit value of the original engine, which meets the design requirements. In-order to further reduce NOx emissions, the Miller cycle, high compression ratio and high pressure EGR were combined. Under the simulation and analysis of different EGR rate, the optimal trade-off between NOx-BSFC was selected. The research shows that, EGR combined with Miller cycle and high compression ratio can further reduce NOx under the conditions of maintaining fuel economy, compared with the original machine reaches 50% decrease.