Study On Combustion And Emission Characteristics Of Hydrogen/N-butanol Combined Injection Engine Based On Hydrogen Direct Injection

Due to the increasing Energy crisis and environmental pollution,seeking clean and renewable alternative energy is an effective way to solve the energy shortage and alleviate the industry crisis.Butanol,as an oxygenated renewable biofuel,is one of the ideal alternative fuels for internal combustion engines.With the development of butanol production technology,low-cost raw materials such as corn and wild plants can be used for biological fermentation to produce butanol.The low production cost provides more powerful support for butanol as an engine alternative fuel.Due to the low saturated vapor pressure and high latent heat of vaporization of butanol,the performance of internal combustion engines fueled with pure butanol is not ideal.Therefore,the application of butanol fuel at this stage is still limited to the use of mixed with traditional fuels.Blending with other fuels to optimize the properties of butanol fuel and improve the performance of internal combustion engines fueled with butanol is an effective way to realize the practical application of butanol as an alternative fuel.As another clean alternative energy source,hydrogen has many advantages such as low ignition energy,fast flame propagation speed,high combustion temperature and short quenching distance.Therefore,in order to solve the problem of ignition difficulty and poor lean combustion performance of n-butanol engine,a hydrogen/n-butanol combined injection mode was proposed in this study.The combination of hydrogen direct injection in cylinder and n-butanol inlet injection improves the performance of butanol engine by virtue of the excellent physical and chemical properties of hydrogen and the local hydrogen-rich mixture distribution formed by hydrogen direct injection in cylinder.The research method combining simulation model and engine bench test is adopted in this paper,which is used to analyze the layered state of hydrogen in the cylinder,and explore the combustion and emission performance of hydrogen/n-butanol compound injection engine based on direct injection in the cylinder.The main research work and conclusions are as follows:(1)In view of the fact that the hydrogen stratification mechanism and hydrogen control strategy of hydrogen/n-butanol combined injection engine are not clear,this paper conducts a simulation study on the impact of direct injection strategy on hydrogen distribution and combustion status in a hydrogen cylinder.The simulation results indicate that due to the fast diffusion rate of hydrogen,premature injection of hydrogen will cause uniform distribution of hydrogen in the cylinder,while too late hydrogen injection time can lead to uneven distribution of hydrogen in the cylinder.At 100°CA BTDC hydrogen injection time,hydrogen forms an ideal hydrogen stratification state in the cylinder,that is,locally rich hydrogen near the spark plug,while there is a certain distribution of hydrogen in the cylinder.And under this condition,the optimal combustion state in the cylinder can be obtained.In addition,good hydrogen stratification state can be formed at ignition time under different hydrogen mixing ratio.With the increase of hydrogen addition ratio,the hydrogen concentration in the cylinder increases,the expansion speed of the temperature field in the cylinder increases,and the high temperature region in the cylinder becomes larger.However,the optimum ignition range is different with different hydrogen addition ratio.The optimum ignition range is 10°CA BTDC～14°CA BTDC under 5%hydrogen addition ratio,and 8°CA BTDC～10°CA BTDC under 20%hydrogen addition ratio.(2)To reflect the actual and complex working process of the engine,this article conducts a detailed study on the combustion and emission characteristics of hydrogen/n-butanol combined injection engine through bench testing.A typical combined injection engine has been modified to achieve a combined injection mode of hydrogen direct injection/n-butanol port injection.The effects of hydrogen injection timing,ignition advance angle,speed and load on the combustion and emission characteristics of n-butanol engines were studied.The test results show that:First,at 100°CA BTDC hydrogen injection time,the engine combustion characteristics are the best,the power performance is the best,and the emission is low.At MBT injection time,the engine has the shortest flame development period,low CO emission and HC emission and no serious deterioration of NOx emission,which can be regarded as the ignition moment to obtain better power performance and emission.Secondly,after blending hydrogen,all combustion parameters are improved on the whole,the maximum torque is increased,CO emission and HC emission are reduced,but NOx emission is increased.Atλ=0.9,hydrogen addition can more effectively reduce CO emission and HC emission,and NOx emission does not increase significantly.While under the lean burn conditions,the improvement of power performance is more obvious,but it leads to a surge in NOx emission.In addition,at different speeds and intake manifold absolute pressure(MAP),hydrogen addition can effectively improve combustion characteristics and CO and HC emissions,but it leads to an increase in NOx emissions.At the speeds of 1500 to 2000 r/min,hydrogen addition has a more significant improvement effect on engine combustion speed and cylinder pressure.When the MAP is 43-49k Pa,the power performance at medium speed is better;When MAP=52～55k Pa,the higher the speed within the test range,the better the power performance.(3)In order to further explore the potential of hydrogen direct injection in improving the lean combustion stability and reducing emissions of n-butanol engine,the lean combustion characteristics of hydrogen/n-butanol combined injection engine are studied based on the optimal hydrogen injection time and ignition timing mentioned above.The results show that with the increase ofλ,the fuel injected into the cylinder decreases,and the indicated mean effective pressure(IMEP)decreases.With the increase of hydrogen addition ratio,the IMEP and combustion parameters are gradually improved,and the cycle-by-cycle variations of the engine gradually decreases.Whenλ=1.0,the improvement effect of 5%hydrogen addition ratio on the engine is the most significant.Whether hydrogen is added or not has a more significant effect on the performance of the butanol engine and is not sensitive to the size of the hydrogen ratio.With the increase ofλ,the sensitivity of the engine combustion performance to the hydrogen addition ratio increases,which indicates that a higher hydrogen addition ratio is needed to ensure the good stability of the n-butanol engine under lean burn conditions.In terms of emissions,with the increase ofλ,CO emissions and NO_x emissions are significantly reduced,but HC emissions increase;with the increase of hydrogen blending ratio,CO emission and HC emission decrease,but NOx emission increases.The lean burn condition ofλ=1.1 combined with 5%hydrogen blending ratio,although sacrificing a small part of the engine power,can significantly reduce CO and NOx emissions and obtain lower HC emissions.(4)In order to solve the problem of high NOx emission of n-butanol engine caused by hydrogen direct injection,a coordinated control strategy of hydrogen direct injection+EGR was proposed in this study.The effects of hydrogen addition ratio and EGR rate on the combustion and emission performance of hydrogen/n-butanol combined injection engine under different excess air coefficients were studied.The results show that whenλ=1.0,the 10%hydrogen addition ratio combined with 10%EGR rate can keep the IMEP,combustion performance and cycle-by-cycle variations of the pure butanol engine at the same level without EGR,which indicates that the introduction of a certain proportion of EGR under stoichiometric conditions can still achieve stable combustion.In lean combustion mode,the pure butanol engine is sensitive to the change of EGR rate.With the increase of EGR rate,the stability of the engine deteriorates rapidly.10%hydrogen addition rate can make up for the loss caused by 15%EGR rate.When the hydrogen addition ratio reaches 20%,the cycle-by-cycle variation with EGR rate less than 15%can be maintained below the level of pure butanol engine without EGR.In terms of emissions,the addition of hydrogen significantly reduced HC and CO emissions.When the EGR rate is less than or equal to 15%,10%hydrogen addition ratio can offset the increase of HC emissions caused by 15%EGR rate,so that HC emissions remain below the original engine emission level.Only whenλ=1.0+0%～5%hydrogen addition ratio+20%EGR rate,the CO emission is higher than that of the original machine,and the CO emission remains at a very low level under other conditions.EGR can significantly reduce the high NOx emissions caused by hydrogenation.The EGR rate of 10%can offset the effect of 5%hydrogen addition ratio on NOx emission.The of 15%EGR rate and the less than 15%hydrogen addition ratio can make the NOx emission much lower than the original engine.The cooperative control strategy of hydrogen direct injection and EGR can not only effectively reduce NOx emission by using EGR,but also improve combustion stability by using hydrogen direct injection,thus improving the engine’s tolerance to EGR.