Research On Combustion Process Of Small Air-cooled Diesel Engine Based On Emission Performance

The production and application of small-power non-road diesel engines are large in China.However,the requirements of the current emission standards are very strict.Due to the structural limitation on the single-cylinder air-cooled diesel engine,the application of available technologies for low emissions are limited,as a result,the brake specific emissions are higher,especially for HC and CO.Therefore,it is of great academic and engineering value to carry out researches on combustion process and emission-reduction.In this paper,the 186F air-cooled diesel engine was taken as the research object.The relationship between emission generation and combustion process,low-emission combustion conditions and the generation mechanism of HC and CO were explored by the combination of experiments and simulation.The original machine test illustrates that with the control of NO_X emission in a proper range,the HC and CO emissions are high.While,under the small load condition,HC and CO emission contributions of individual condition are large.Through simulation,the velocity field of fluid,the density field of mixture and the temperature field of working medium in cylinder were analyzed with different combustion chamber structures and compression ratios.Combined with emission generation characteristics,the HC and CO emission levels under small load conditions were obtained.It is related to the temperature in the end of compression stroke and the prophase of pre-mixed combustion.When the temperature in the end of compression reaches to 850 K,it can effectively reduce the HC and CO emissions.With increasement of the temperature in the later period of the combustion,the CO oxidation capacity can be strengthened and the CO emission can be reduced.The overall machine matching experiments were carried out with a combustion chamber structure,which has an increased compression ratio,combined with external EGR(Exhaust Gas Recirculation).The results show that the NO_X emission was controlled well within a certain range while the HC and CO emissions were effectively reduced.Through the analysis and calculation of the measured cylinder pressure and heat release rates,it is concluded that the research schemes can increase the compression temperature and combustion temperature,shorten the ignition delay,improve the mixture of air and fuel,effectively organize combustion,and improve the combustion efficiency.The adoption of EGR can further increase the compression temperature,suppress the development of the maximum combustion temperature,accelerate the vaporization of the fuel droplets in the early stage of combustion,advance the ignition timing,and effectively reduce afterburning.Finally,the emissions of CO,HC and HC+NO_X were reduced by 47.7%,56.8%and 20.6%respectively,compared with the original scheme.In order to further reduce emissions under small and medium loads of the air-cooled diesel engine,thermal management theory was applied to improve the structure of the wind scooper and set a side vent.Through the design of a mechanical linkage,the area of the side vent can be changed automatically with the accelerator.So,the cooling amount of cylinder head was reduced under small and medium loads and the combustion temperature in cylinder was increased in the end.The test results show that the HC and CO emissions were decreased by 7.1%and 10.0%,and the NO_X emission was increased within a narrow range by 2.5%.In summary,the researches improved the combustion process of the single-cylinder air-cooled diesel engine and decreased the overall emissions by means of designing combustion chamber structure,increasing compression ratio,using external EGR,and optimizing the structure of the wind scooper,combined with the matching of fuel injection system.In the end,the brake specific emissions of CO,HC+NO_X and PM were 4.22,6.36 and 0.52 g/(kW·h),reduced by 50.6%,23.3%and 16.1%respectively,compared with the original machine.The diesel engine can meet the requirements of non-road emission standards(ChinaⅢandⅣstages).The research working can provide theoretical guidance and technical reference for the development of the small-power air-cooled diesel engine.