Numerical Researches On Combustion And Emission Characteristics Of A Hydrogen Enriched Natural Gas Engine With Pre-chamber

The application of internal combustion engine brings great convenience to human production and life,but it also causes some problems such as oil shortage and environment pollution.In order to alleviate these problems,clean alternative fuels become an important solution.Therefore,the clean and efficient natural gas used in internal combustion engine is increasingly noticed.Natural gas has the advantages of low price,low pollution and rich storage.At the same time,it has the disadvantages of high ignition temperature and slow flame propagation speed which lead to difficulties in ignition under lean condition.To solve these problems,a pre-chamber stratified lean combustion method is used on a large-bore natural gas engine,which forms near-equivalent mixture in the pre-chamber and lean mixture in the main chamber.The combustion in the pre-chamber generates turbulent flame jets which ignite the lean mixture in the main combustion chamber.This combustion mode can improve the ignition reliability and accelerate the flame propagation rate.It can also achieve ultra-low NO_x emissions and meet the strict NO_x emission standards of non-road and ship engine.In addition,hydrogen enrichment in natural gas is also an important method to reduce the ignition temperature,increase the flame propagation speed and extend the lean combustion limit.In the high-power natural gas engine,the combination of pre-chamber stratified lean combustion technology and hydrogen enrichment in natural gas technology is expected to further improve engine performances.In this paper,the 6ACD320G marine pre-chamber natural gas engine was taken as the research object.The coupled numerical simulation of computational fluid dynamics and chemical reaction kinetics is employed to investigate the combustion process of engine.Firstly,the geometric structure of the pre-chamber was optimized.Then the optimized structure of pre-chamber was used to research the effects of parameters such as hydrogen fraction ratio,excess air coefficient and ignition timing on the performances and emissions of the natural gas engine.For the optimization of pre-chamber geometric structure,the pre-chamber volume ratio,the pre-chamber orifice angle and the pre-chamber orifice diameter are considered as independent variables,and the indicated thermal efficiency and the NO_x emission were considered as the optimization targets.In the optimization study,the varied range of the pre-chamber volume ratio,the orifice angle and the orifice diameter were from 1.1%to 4.0%,60°to 80°,and 2.5mm to 3.5mm,respectively.The multi-parameter multi-objective optimization design was carried out by using the response surface method,and the optimized parameters for the geometric structure were:pre-chamber volume ratio of 2.1%,pre-chamber orifice angle of 77.5°,and pre-chamber orifice diameter of 3.06mm.Under these conditions,the highest indicated thermal efficiency was obtained and the NO_x emissions was not acceptable.It is found that when the pre-chamber volume ratio was relatively larger,it is difficult to form pressure difference,leading to a lower flame jet velocity at the outlet of orifice.Moreover,when the pre-chamber orifice angle was relatively small,the flame jet will impinge with the top surface of the piston,causing flame quenching.And when the pre-chamber orifice diameter was relatively small,the throttle effect was very strong and more heat will loss through the walls of the pre-chamber orifice.Based on the optimized pre-chamber geometry,the effect of hydrogen enrichment in natural gas on engine combustion and performance was studied,in which the hydrogen fraction ratio varies from 0%to 30%.The results show that the enrichment of hydrogen in natural gas accelerated the flame propagation,leading to increased heat release rates.The mechanism was found to be that the hydrogen enrichment in natural gas greatly increases the production of active radicals such as H and OH in the flame edges,which promote the combustion reaction.It is also found that the hydrogen enrichment in natural gas shortens the combustion duration.More specifically,the first half of the main combustion duration seemed to be not sensitive to hydrogen fraction ratio,indicating that the first half of the combustion process was mainly dominated by the turbulent disturbances induced by the flame jets.While the second half of the main combustion duration decreased with the hydrogen fraction ratio,indicating that the latter combustion process was mainly dominated by the chemical dynamics of the fuel.Hydrogen enrichment in natural gas reduced the CO and HC emissions,but the NO_x emissions increase,and the main component of HC emissions was methane which accounts for about 90%.Hydrogen enrichment in natural gas decreases the indicated specific fuel consumptions and improves the indicated thermal efficiency.Considering both fuel economy and emissions,the hydrogen fraction ratio of 15%was deemed to be an optimized solution.Furthermore,the effect of excess air ratio on engine combustion and performance was studied.It is found that with the increase of excess air ratio,the in-cylinder peak pressure,heat release rates,the average temperature,and the local flame temperature decreased,and the phase of the peak pressure phase firstly delayed and then advanced.When the excess air ratio is very high,the flames of natural gas were easy to quench.With the enrichment of hydrogen in natural gas,this trend was highly suppressed.With the increase of excess air ratio,the combustion durations become significantly longer.When the excess air ratio was high,incomplete combustion will occur which results in the increasing of HC and CO emissions.But the NO_x emissions are significantly reduced.Through comprehensive comparison,for the natural gas fuel,the fuel economy and emissions of engine were more sensitive to the change of excess air ratio.The main reason was that the combustion efficiency declines obviously when the excess air ratio exceeds 2.2.While for the hydrogen enriched natural gas,the combustion efficiency significantly improved at high excess air ratio,and the lean burn limit could be extended.By advancing the ignition timing,it was possible to accelerate in-cylinder combustion process under lean combustion condition and improve engine performances and emissions.