Study On The Regulated Two-Stage Turbocharging System With Miller Cycle For Low Speed Dual-Fuel Engine

With the increasingly stringent emission regulations,diesel engine technology alone has been unable to meet the IMO Tier III emission limits.So natural gas has received extensive attention in the marine sector due to its excellent emission characteristics.The mature dual-fuel engines in the low-speed market include Winterthur’s X-DF series engines and MAN B&W’s ME-GI series engines.The X-DF series dual-fuel engine uses lean-burn technology and can meet IMO Tier III NOx limits without exhaust gas after treatment.However,the functional force is limited due to the limitation of the knocking characteristics,and the maximum average effective pressure of X-DF series engines is limited in 1.73 MPa.In this article,the dual-fuel engine has been optimized to increase the average effective pressure of the dual-fuel engine to 1.9MPa under the premise of ensuring that the knocking characteristics,emission characteristics and efficiency are not deteriorated.Based on the working characteristics of the 7RT-flex50 DF engine,this article established the one-dimensional simulation model for its working process by GT-Power.The model was calibrated with experimental data to match the simulation results with the experimental data.By using knock module of GT-Power,the knock characteristics of the engine gas mode were able to be predicted.Then explored the effects of load,combustion chamber temperature,geometric compression ratio,excess air ratio and ignition advance angle on knock characteristics.Summarized the effective ways to improve knock characteristics.According to the working characteristics of two-stroke engine,the influence law of various parameters on engine performance were studied.Then summarized the effective ways to improve engine efficiency.The knock characteristics of the 7RT-flex50 DF were improved by the Miller cycle technology in the GT-Power.According to the summarized methods of improving efficiency,the geometric compression ratio was optimized to improve the engine efficiency.After optimization,the rated power of the dual-fuel engine was increased from 10080 kW to 11070 kW,and the knocking characteristics of the gas mode were not inferior to the original machine,the gas consumption rate can be reduced by up to 5.25%;the fuel mode NOx emission value was reduced by 6.3%,fuel consumption.The rate can be reduced by up to 3.44%.The research on the adjustable two-stage supercharging system for the engine optimized by the Miller cycle scheme was designed to match the two-stage supercharging system for the low-speed dual-fuel engine,and the difference between the different modes of the low-speed dual-fuel engine and the different requirements for the intake air were considered.Then adjustable secondary turbocharger system solution was designed.According to the position of the bypass valve,it was divided into the high-pressure stage turbine front bypass scheme and the exhaust manifold bypass scheme.The effects of the bypass rate on the boosting system under the two schemes were compared,and the adjustment capabilities and characteristics of the two schemes were compared.After the comparison,the exhaust manifold bypass scheme was selected as an adjustable scheme.Under this scheme,the engine was in a high load in the fuel mode,and the high load in the gas mode needs to be opened to adjust the bypass valve.At this time,the turbocharger system had high and low pressure superchargers,and the turbocharger system was able to maintain efficient work and have sufficient surge margin.The adjustable secondary supercharged dual-fuel engine still has the problem of lack of gas at low load.Below 30% load in diesel mode and below 25% load in gas mode,it is necessary to turn on auxiliary fan to make up for the problem of insufficient intake.