Miller Cycle Regulatable, Two Stage Turbocharging System Design for Marine Diesel Engines

Abstract

The increasingly stringent NOx emission regulations of the International Marine Organization (IMO) have demanded new design concepts and architectures for diesel engines. The Miller cycle, which reduces the incylinder combustion temperature by reducing the effective compression ratio, is the principal measure used for reducing NOx specific emissions; however, this is at the cost of volumetric efficiency and engine power. Therefore, it is essential to combine the Miller cycle with a highly boosted turbocharging system, twostage turbocharging for example, to recover the power. While much work has been done in the development of Millercycle regulatable two stage turbocharging system for marine diesel engines, there are nonetheless few, if any, thorough discussions on system optimization and performance comparison. This study presents a theoretical optimization design process for a Millercycle regulatable, twostage turbocharging system for marine diesel engines. First, the different scenarios and regulation methods of twostage turbocharging systems are compared according to the system efficiency and equivalent turbine flow characteristics. Then, a multizone combustion model based on a onedimensional cycle simulation model is established and used for the optimization of valve timings according to the IMO NOx emission limits and fuel efficiencies. The highand lowstage turbochargers are selected by an iterative matching method. Then, the control strategies for the boost air and highstage turbine bypass valves are also studied. As an example, a Millercycle regulatable, twostage turbocharging system is designed for a highly boosted highspeed marine diesel engine. The results show that NOx emissions can be reduced by 30% and brake specific fuel consumption (BSFC) can also be improved by a moderate Miller cycle combined with regulatable twostage turbocharging.