| Due to the increasingly strict emission regulations of marine engines,the fluctuation of international fuel prices and the high price of high-quality fuel such as low-sulfur fuel,the marine industries have the urgent need to seek new marine energy.Natural gas has broad application prospects in the field of marine engine for its advantages of economy,green,environmental protection and safety.Using diesel as pilot fuel to ignite HPDI(high-pressure direct injection)natural gas is one of the most advanced natural gas combustion technologies.However,the main difficulty of the technology is to improve fuel economy and reduce NOx emissions to meet the strict IMO Tier Ⅲ phase regulations.Therefore,it is necessary to study the efficient and clean combustion strategy of the HPDI natural gas low-speed marine engine.Firstly,the effects of scavenging boundary conditions,scavenging port parameters and combustion chamber shapes on the combustion and emissions of HPDI natural gas marine engine were investigated through the three-dimensional simulation model of HPDI natural gas marine engine based on CONVERGE.The results show that increasing scavenging pressure is conducive to improving engine economy and power performance,and the upper limit of scavenging pressure is mainly limited by the strength of cylinder block.By increasing the scavenging temperature with interval of 10K,the peak pressure is reduced by about 0.13MPa,but the NOx emission is increased.Increasing the scavenge port angles can significantly strengthen the swirl ratio,which is conducive to the charge replacement near the cylinder wall,but the charge backflow areas in the central cylinder against the improvement of scavenging effect.Increasing the swirl ratio properly is conducive to expanding the ignition range of natural gas,but the shape of natural gas jets tends to narrow and the spray penetration shortens when the swirl ratio is too high,which is adverse to the mixture formation and combustion of natural gas.As a result,it is best for the combustion when the scavenge port angle is 20°.Compared with the deep pit combustion chamber,the shallow bowl combustion chamber and the shallow ω combustion chamber have the larger turbulence intensity and the wider distribution range,which are beneficial to the diffusion and mixing of fuel with air.The effect of combustion chamber shapes on HPDI natural gas marine engine is less obvious than that of scavenge port angles.Based on the above studies,aiming at reducing NOx to meet Tier Ⅲ regulations,the influence of EGR,Miller cycle and natural gas injection strategies on the HPDI natural gas engine were studied.The results show that only the 30%EGR rate can meet the Tier Ⅲ emission standards,but the indicating fuel consumption and soot emissions get worse.Miller cycle is not as effective as EGR on reducing NOx emissions.Excessive later exhaust valve closing angle caused the increase of compressor workload and result in considerable increase of fuel consumption.With the coupling of the mild Miller cycle(retard exhaust valve closing timing 5℃A)with 25%EGR rate and the proper advance of natural gas injection timing(2℃A),the NOx emissions can be decreased by 77%and meet Tier Ⅲ emission regulations with a slight increase of the fuel consumption by 1.58%.The double injection strategy with 2℃A natural gas pre-injection interval and the 20%pre-injection mass,coupled with the 25%EGR rate can reduce the NOx emission to the Tier Ⅲ emission limit without penalty on fuel consumption,but the soot emissions increase to 10.3 mg/kWh,and the peak pressure in the cylinder is higher,which has higher requirements for the cylinder strength.In conclusion,the present work has studied the influence of key structural parameters on the combustion and emission of HPDI natural gas marine engine,and put forward the emissions control strategies to meet the IMO Tier Ⅲ regulations,which has great guiding significance and engineering application value for the development of HPDI natural gas low-speed marine engine. |
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