Applicability of Reducing Valve Timing Overlap for Diesel Engines Under High Exhaust Back Pressure

Abstract

The exhaust back pressure of diesel engines becomes increasing higher nowadays. As an example, the De-NOx system and DE-SOx system necessitated by the increasingly stringent emission standards, would result in increased exhaust back pressure for those diesel engines adopting such systems. Some ships adopt underwater exhaust system to save space on the working deck and to reduce noise and air pollution, while the hydrostatic pressure under water level has made the exhaust back pressure of diesel engines getting much higher. Under high exhaust back pressure, to keep discharging exhaust unhindered and operating smoothly for diesel engine, it often results in large extent of reduction in engine maximum brake output, increased fuel consumption, and lower combustion efficiency with heavy exhaust smokes. To address the above -mentioned disadvantages for diesel engines under high exhaust back pressure, one solution is proposed in this study by reducing valve timing overlap (the overlap crank angle between exhaust valve closing and intake valve opening. In this study, valve timing overlap is varied to study the engine performance characteristics and applicability, especially the improvement of brake output and brake specific fuel consumption of engine. Via engine simulation, 25 kPa, 45 kPa and 65 kPa gauge of exhaust back pressure are studied for a turbocharged diesel engine. The results of engine parameters, including brake output, brake specific fuel consumption, turbine inlet temperature, intake air mass flow rate and exhaust mass flow rate are analyzed to establish empirical equations for the relations with valve timing overlap. The results indicate that the brake output and brake specific fuel consumption are improved by reduced valve timing overlap. An optimal condition of full load operation has been found that a reduced valve timing overlap of 12 degrees CA would lead to highest brake output. Thus, reducing valve timing overlap is proved to be a feasible solution for the diesel engines under high exhaust back pressure, partially resolve the problems of reduced maximum brake output and increased fuel consumption.