Study On The Expansion Of High Efficiency And Stable Operation Range Of Compression Ignition Natural Gas Engine

Facing increasingly severe energy environment situtation and severe challenges faced by traditional internal combustion engine,the use of clean alternative fuels and new combustion mode is one of the ways to improve the problem.In the paper,the clean fuel natural gas and efficiency combustion mode of homogeneous charge compression ignition(HCCI)are the research subjects.Aiming at the problems of difficult control of combustion process and narrow operation range of HCCI engine,the research objective is to expand the operating range of natural gas HCCI engine and realize stable operation in all working conditions.Theoretical analysis,bench test and simulation calculation are used as research methods.Natural gas HCCI single cylinder engine,diesel/natural gas dual-fuel multi cylinder engine,three-dimensional simulation calculation platform and detailed chemistry quasi-dimensional multi-zone simulation calculation platform were constructed.A series of studies including the combustion characteristics of natural gas HCCI engine with sensitive boundary conditions,the measures to expand engine operating boundary to high load and low load,and stable operation range of engine after expansion were conducted.The main contents and conclusions are as follows:(1)Natural gas HCCI engine is limited by knock and misfire limits,and its stable operation range is narrow.The knocking limit and misfiring limit move to lean mixture with the increase of speed,there is no obvious change in the stable fire range.The change of intake temperature(T_in)has little effect on the knocking limit and the range of stable fire becomes wider and moves to lean mixture.Brake thermal efficiency decreases with the increase of excess air coefficient(?)and increase of T_inat 1000r/min.At low speed,start of combustion(SOC)is sensitive to?,sensitivity coefficient decreases as the increase of speed.Sensitivity coefficient of SOC to?increases when speed reaches 1800r/min.With the increase of speed,sensitivity of SOC to Tin first increases and then decreases.(2)The three EGR strategies of external exhaust gas recirsculation(EGR),negative valve overlap(NVO)and late exhaust valve close(LEVC)all have the potential of delaying ignition,prolonging combustion duration and expanding the natural HCCI engine operation to high load boundary.However,the potential of internal EGR under LEVC strategy is the least.Internal EGR strategy of NVO is easier to realize the expansion of natural gas HCCI engine to high load operation boundary than external EGR strategy.With the increase of EGR rate,the in-cylinder temperature decreases under external EGR strategy,and cylinder axial temperature changes from the obvious temperature stratification phenomenon of upper,middle and lower corresponding to three temperature levels of low,medium and high to upper and lower corresponding to low and medium temperature region.In-cylinder temperature of NVO strategy is almost unchanged,and the in-cylinder temperature presents obvious temperature stratification phenomenon of upper,middle and lower corresponding to three levels of low,medium and high temperature region,representing the transform of high temperature zone,which thansfers from intake side to exhaust side.Under the LEVC strategy,in-cylinder temperature increases,and the axial temperature changes from low,medium and high levels to medium and high temperature.At the appropriate i-EGR rate of NVO strategy,the distribution of natural gas is rich at top and thin at bottom.The high temperature region in cylinder is concentrated in pits on both sides of the combustion chamber.The distribution of intake and exhaust is consistent.Ignition position is located in the pits on both sides.Combustion phase retards and IMEP increases.It can realize the controlled combustion of natural gas HCCI engine and expand to high load boundary.Compared with the three EGR strategies,NVO strategy is an effective control strategy to achieve high indicated thermal efficiency.(3)Study on the effect of intake water addition on combustion characteristics,important reaction species and ringing intensity under high load were examined through detailed chemistry reaction dynamics.In addition,physical characteristics of water added into cylinder were changed to explore the effects of dilution effect,thermal effect and chemical effect on natural gas HCCI engine.The results show that with the increase of water mass fraction,in-cylinder pressure decreases and the crank angle corresponding to its peak retards.Under the condition of lower T_in and leaner mixture,combustion process is more sensitive to water addition and tolerance of natural gas HCCI engine to water decreases.Under the high load condition of abnormal detonation combustion,start of combustion(SOC)retards and ringing intensity reduces with the increase of water addition,which will normalize the combustion process.Thermal effect of water is greater than chemical and dilution effects on SOC.It is an effective way to realize the controlled combustion of natural gas HCCI engine by changing intake temperature to realize the coordinated control of combustion boundary conditions and fuel chemistry while water addition to adopting water spray to slow down the chemical reaction,relieve the knock and expand the low load.Water addition can effectively reduce the production and consumption rate of radical H,OH,H₂O₂,CO and HCO,slow down chemical reaction but does not alter reaction pathways.Compared with the external EGR strategy,NVO strategy,LEVC strategy and water addition strategy,NVO strategy can obtain high thermal efficiency and can be used as an optimal control strategy to expand the engine knock boundary.(4)As additive,hydrogen,ozone,dimethyl ether(DME)and diesel all can change the autoignition characteristics of natural gas and air mixture,enhance ignition ability.With the increase of additive mass fraction and diesel substitution rate,combustion phase is advanced and during of combustion is shortened.All the four additives have the ability to extend the low load operation boundary of natural gas HCCI engine.From the combustion supporting mechanism of the four additives,radical OH is an important component to enhance natural gas HCCI combustion.The combustion supporting mechanism of hydrogen and ozone are increase OH concentration and accelerate reaction rate of methane consumption.While combustion supporting mechanism of DME and diesel are ignite natural gas.In terms of mixture formation,engine thermal efficiency and cost,hydrogen is an excellent additive which is environmentally friendly,sustainable,low cost and easy to expand the low load misfire boundary of natural gas HCCI engine.(5)Based on the control strategies to extend stable range of natural gas HCCI engine,the stable operation range of natural gas HCCI engine at different speeds has been expanded,which shows that NVO strategy and hydrogenation strategy are effective strategies to expand the stable operation range of natural gas HCCI engine.However,the specific i-EGR rate of NVO strategy and hydrogen mass fraction matching different conditions should be carried out through engine test in the future..