Experimental Investigation On NO_x Formation And Control Strategies Of A Hydrogen-fueled Engine

Hydrogen is one of the ideal fuels for internal combustion engines because of its excellent physical and chemical properties.However,large amounts of NO_x emissions will generate during the operation of hydrogen-fueled engine due to high combustion temperature caused by combustion of hydrogen.Therefore,reducing NO_x emissions is a major challenge restricting the development of hydrogen-fueled engine.In this paper,the effects of different control strategies on combustion and emission characteristics of a hydrogen-fueled engine under different operating conditions were studied experimentally to reduce NO_x emissions.The cold start test of hydrogen-fueled engine showed that when the excess air ratio(?)is slightly less than 1,the flame speed increased,and the flame development and propagation durations shortened.The peak cylinder pressure(P_max)and the relative maximum rate of pressure rise increased,the successful starting time of hydrogen-fueled engine shortened.The maximum engine speed increased with the decline of?.In addition,as the oxygen concentration in the cylinder decreased with the reduction of?,the oxygen-enriched areas in the cylinder decreased,therefore,NO_x emissions significantly reduced.When?gradually decreased from 1.4 to 0.7,the peak value of NO_x emissions significantly reduced from 1356 ppm to 89 ppm,and the average NO_x emissions within the first 6 s decreased by about 84.3%.At the same time,the delay of spark timing under rich combustion also contributed to the further reduction of NO_x emissions.Because of the good reducibility of hydrogen,NO_x emissions can be reduced by hydrogen in the catalytic unit.Therefore,this paper studied the influences of rich combustion strategy on the conversion efficiency of NO_x emissions in the catalytic unit at idle condition.The results showed that when?was less than 1,the conversion efficiency of NO_x emissions in the catalytic unit was nearly 100%.It indicated that,when?was slightly less than 1,near zero emission of hydrogen-fueled engine can be achieved with the catalytic device.Since the state of fuel-air mixtures can affect on the combustion process,the effects of different hydrogen injection strategies on combustion and emission characteristics of a hydrogen-fueled engine under partial load conditions were also studied.It was found that,delaying the hydrogen injection timing(HIT)shortened the mixing time of hydrogen and air in the combustion chamber,thus,rich mixtures would be formed in the area around the spark plug.It resulted in the increased cylinder pressure and indicated mean effective pressure(IMEP),and it shortened the flame development and propagation durations.At the same time,since the average highest cylinder temperature(T_max)increased first and then decreased with the delay of HIT,NO_x emissions increased first and then decreased.In addition,similar to the results of delayed HIT,under the condition of secondary hydrogen injection,the delay of secondary hydrogen injection timing(SHIT)also caused NO_x emissions to increase first and then decrease.However,when SHIT was later than 60°CA BTDC,the locally over-concentrated mixtures in the cylinder leaded to the reduction of cylinder pressure and IMEP.It indicated that the change of hydrogen injection strategy also had important effects on the performace of hydrogen-fueled engine.For hydrogen-fueled engines,in-cylinder temperature had a decisive effect on the formation of NO_x emissions.Therefore,this paper further studied the influence of the in-cylinder water injection(WI)strategy on the performance of hydrogen-fueled engine based on the in-cylinder WI test bench.It was found that under the same?or ignition timing,WI could reduce the cylinder pressure and the maximum pressure rise rate of hydrogen-fueled engine.At the same time,since the heat absorption of water,the temperature and pressure in the cylinder reduced,the flame development and propagation durations increased.Because water absorbed heat and produced work,both IMEP and indicated thermal efficiency(ITE)improved under WI conditions.The WI strategy could also significantly reduce NO_x emissions of hydrogen-fueled engine.When the amounts of injected water(AIW)increased from 1.95 mg/cycle to 4.50mg/cycle,NO_x emissions gradually decreased from 1333 ppm to 617 ppm,with a decrease of about 53.7%.Also,the earlier water injection timing(WIT)had a positive effect on reducing NO_x emissions of hydrogen-fueled engine.Similar to the results of in-cylinder WI,when the relative humidity of intake increased,both cylinder pressure and the relative maximum rate of pressure rise decreased due to the cooling effect caused by the water vapor in the combustion chamber.The flame development and propagation durations increased,and the heat release per unit time decreased in the combustion process.At the same time,due to the increase of working medium in the cylinder after inlet humidification,intake humidification was beneficial to the improvement of IMEP and ITE.Besides,NO_x emissions of hydrogen-fueled engine significantly reduced after intake humidification.When?was 1.15 and the relative humidity of intake gradually increased from 40%to74%,NO_x emissions significantly reduced from 1821 ppm to 1000 ppm,with a decrease of about 45.1%.