Experimental And Numerical Study On The Effects Of Hydrogen Peroxide On The Combustion And Emission Characteristics Of An N-Butanol HCCI-DI Engine

Homogeneous charge compression ignition?HCCI?has been considered as a promising combustion mode due to the ultra-low NOx emissions,near zero soot emissions,and diesel engine-like efficiency.Nevertheless,the HCCI combustion process is mainly controlled by the in-cylinder thermal environment and chemical kinetics,which results in uncontrollable combustion phasing and narrow operating range,hindering the commercial application of HCCI combustion technology.In order to overcome the above-mentioned difficulties,two new operation modes are proposed in this paper:one is the hydrogen peroxide?H₂O₂?/n-butanol HCCI mode with variable volume fraction of H₂O₂ solution,and the other is the HCCI-DI mode with pipe injection of H₂O₂ solution/n-butanol blend and direct injection of n-butanol.In this paper,the effects of these two modes on the combustion characteristics,emission characteristics and performance of H₂O₂/n-butanol HCCI engine were studied through experiments.In addition,the effects of H₂O₂ on the oxidation process of HCCI engine at low temperature range were studied by numerical simulation.Firstly,the effects of H₂O₂ solution volume fraction,intake temperature(T_in)and fuel/air equivalence ratio???on the combustion process of H₂O₂/n-butanol HCCI engine were investigated using CHEMKIN software.Simulation results show that the H₂O₂ additive promotes the elementary reactions of n-butanol at low temperature,which facilitates the formations of hydroxyl?OH?radical and hydroperoxyl?HO₂?radical,advancing the auto-ignition timing of the engine.Meanwhile,the H₂O₂ additive inhibits the formations of enol and olefin during the low temperature oxidation process.The increased volume fraction of H₂O₂ solution advances the auto-ignition timing of n-butanol.In addition,the intake temperature has a great influence on the auto-ignition timing of n-butanol.With the rise of intake temperature,the auto-ignition timing of n-butanol is continuously advanced,and the concentrations of OH and HO₂ radicals increase during the low temperature oxidation process.However,the fuel/air equivalence ratio has little effect on the auto-ignition timing of n-butanol.Secondly,for investigating the effects of H₂O₂ additive on the combustion process of n-butanol HCCI engine and verifying the simulation results,the HCCI combustion was conducted on a modified two-cylinder diesel engine.And,five H₂O₂ solution/n-butanol blends with different volume fraction of H₂O₂ solution?H₂O₂ volume fraction are 0%,1%,3%,5%and 7%?are used as fuel for the HCCI engine.In this paper,the effects of H₂O₂solution volume fraction,T_in,and?on the combustion and emission characteristics as well as the performance of an HCCI engine were studied.The experimental results show that the combustion phasing of n-butanol HCCI engine could be controlled by changing the concentration of H₂O₂ in a given operating condition to some extent.With the increased volume fraction of H₂O₂ solution,the peaks of cylinder pressure,heat release rate and in-cylinder temperature first rise and then reduce,combustion phasing advances.At the same time,the engine combustion cyclic variations slightly increase,the indicated mean effective pressure?IMEP?and the indicated thermal efficiency??_i?initially increase and then reduce.When the volume fraction of H₂O₂ solution is 3%,the engine comprehensive performance is the best.In addition,the T_in has a significantly effect on the H₂O₂/n-butanol HCCI engine.As the T_in increases,for the five test fuels,the peaks of in-cylinder pressure and heat release rate increase and then reduce,the combustion phase advances,the IMEP and?_i firstly increase and then decrease.The HC and CO emissions reduce,and the NOx emissions slightly increase.With the increase of?,the peak in-cylinder pressure,the temperature,and the heat release rate of the five test fuels show monotonously increasing trend.At the same time,the change of combustion timing?CA10?is not obvious,the combustion duration?CD?decreases,and the HC and CO emissions reduce.The IMEP monotonously increase,however,the?_i firstly increases and then slightly reduces.Finally,the injection strategy,pipe injection of H₂O₂ solution/n-butanol blend combined with direct injection of n-butanol,is applied to investigate the effects of five factors(pre-injection energy Ratio Rp,direct injection timing?_in,total fuel/air equivalence ratio?_t,pre-injection equivalence ratio?_p,in-cylinder injection equivalence ratio?_d)on the combustion and emission characteristics as well as the performance of an H₂O₂/n-butanol HCCI-DI engine.The experimental results show that,at a given operating condition,when Rp increases from 0.44 to 0.70,the peak in-cylinder pressure and temperature gradually increase.The change of CA10 is not evident,CA50 continuously advances,and CD significantly decreases.The best Rp is 0.62,where the IMEP and?_i achieve their maximum?0.346 MPa and 39.31%,respectively?.Furthermore,the?_in has a little effect on the in-cylinder pressure and temperature.With the constant delay of?_in,CA10 and CA50 are retarded,CD firstly extends and then shortens.The Coefficient Of cyclic Variation of peak in-cylinder(COV_Pmax)firstly reduces and then increases,and the minimum of it is 2.91%.The CO and HC emissions increase.The IMEP and?_i initially increase and then reduce.With the increase of?_t,CA10gradually advances,but the change is not obvious.The CA50 firstly advances and then retards,COV_Pmax exhibits the opposite trend.Meanwhile,the IMEP gradually increases,and the?_i initially increases and then reduces.With the increase of?_p,the CA10 and CA50 advance,the CD gradually shortens,and the COV_Pmax gradually reduces.The IMEP and?_i initially increase and then reduce.However,the?_d has a little effect on in-cylinder peak pressure,in-cylinder peak temperature and CA10.According to these experimental results,it can be seen that the strategy of pipe injection of H₂O₂/n-butanol mixed fuel combined with direct injection of n-butanol can effectively expand the operating range of n-butanol HCCI combustion to higher loads,and control the combustion phasing of the H₂O₂/n-butanol HCCI-DI engine to some extent.In general,compared with the n-butanol HCCI mode,the H₂O₂/n-butanol HCCI mode with variable volume concentration of H₂O₂ solution is more conducive to extend the operating range to lower loads,and the?_i is higher,but is not beneficial to extend the operating range to higher loads.The H₂O₂/n-butanol HCCI-DI model can obviously extend the operating limits of high loads,and the?_i is also higher than that of n-butanol HCCI model.