Large Bore Diesel Engine Performance Research Based On Thermodynamic Simulation

As energy crisis and environmental regulations progresses with increasing restriction,economic performance and emission of internal combustion engine are further restricted throughout the world,which require better engine design and development.WEICHAI-7L21/31 is an a four-stroke large-bore marine genset diesel engine,which has a modular design and is easy to repair and maintain later.Due to its high reliability and low operating costs,this engine is widely used in inland container ships,oil tankers and bulk carriers,etc.In order to meet the IMO requirements for improving marine engine emissions,engines have to be upgraded and optimized,including the optimum design of key components such as intake and exhaust systems,fuel supply systems,superchargers,camshafts,etc.In this thesis,engine performance of WEICHAI-7L21/31 four-stroke diesel engine generator was thermodynamically simulated and analyzed,mainly focusing on the impacts and optimization of engine valve system parameters and Miller cycle.Firstly,based on the structural data of the WEICHAI-7L21/31 four-stroke diesel engine,thermodynamic simulation model was built in GT-POWER.Under D2working conditions,key engine performance parameters such as power,fuel consumption,maximum combustion in-cylinder pressure,inlet pressure after intercooling,exhaust pressure before supercharger,intake flow and NO_x emission were simulated,and this numerical model was calibrated according to the experimental data.After calibration,the calculated results of this model was within3%of the error compared to the experimental data,which indicates that the simulation results were in good agreement with the experimental data,showing that the model was accurate.Secondly,utilizing the calibrated simulation model,the engine performance of changing exhaust valve opening timing(EVO),intake and exhaust valve overlap timing(VO)and the intake valve closing timing was studied under the constant excess air ratio conditions.Impacts of different EVO(113°CA,118°CA,123°CA,128°CA,133°CA,138°CA),VO(79°CA,89°CA,99°CA,109°CA,119°CA,129°CA)and IVC(536°CA,546°CA,556°CA,566°CA,576°CA,586°CA)on engine fuel consumption and emissions were studied and compared.Simulated results revealed that:when EVO equals to 118°CA and VO equals to 89°CA,the fuel consumption was the lowest;when IVC equals to 556°CA,the comprehensive performance of fuel consumption and emissions is optimum;Comparing to the original engine valve timing(EVO=128°CA,VO=109°CA and IVC=586°CA),1%to 3%reduction in fuel consumption and 20.5%to 22%drops in NO_x emissions were achieved under all conditions.At last,according to the optimal simulation results,the intake and exhaust camshaft were redesigned,and the diesel engine performance and emission experiments were carried out.The test results show that the fuel consumption is reduced by 1.8%-3%,and the NO_x emissions was decreased by 20%-22%,compared with the original engine working conditions.The specific emissions of NO_x was decreased from 11 g/(kW·h)to 8.59 g/(kW·h),which can meet the IMO Tier II emissions requirements.