Theoretical And Experimental Research On The Efficiency And Knock Boundaries Of Argon Cycle Hydrogen Engine

Since the Paris Agreement,achieving carbon neutrality and controlling temperature rise at the earliest possible date has become a global consensus issue.By March 2022,198 countries around the world have proposed a vision of carbon neutrality or plan to reduce emissions to varying degrees.The carbon neutrality goal has profoundly affected the energy sector,posing a new challenge to the power industry and bringing new opportunities.The replacement of fossil energy with clean electricity,mainly wind and solar power,is an important measure to achieve carbon neutrality.However,there are three main problems facing clean electricity represented by wind and solar power.In terms of time,there is a variation in the clean electricity generation between day and night,and there are also seasonal fluctuations.Spatially,clean energyrich areas are usually far from areas with high energy demand.In addition,not all sectors can be fully electrified.In 2021,the State Council released the "Action Plan for Peaking Carbon Emissions by 2030",which considers hydrogen as a key carrier of clean energy along with batteries.Internationally,the European Union and Germany have also released dedicated hydrogen energy strategies.The production of hydrogen by electrolysis of water through clean electrical energy can smooth out the diurnal variations and inter-seasonal fluctuations,enabling the storage of clean energy,which in turn can be transported in the form of hydrogen.By 2050,it is predicted that 20～70%of the transportation sector will not be electrified.The portion of the transportation sector that is difficult to be electrified can be decarbonized through hydrogen.Considering the urgency of the task of carbon peaking by 2030,hydrogen-fueled internal combustion engines(hydrogen engines)can be chosen as the method to utilize hydrogen energy.The hydrogen engine is a technology that can be put into use in the short term because it has fewer modifications to the conventional gasoline engine and can follow the existing R&D and manufacturing system.In recent years,key technologies for hydrogen engines have continued to achieve breakthroughs.In passenger cars,70 MPa high-pressure hydrogen storage technology has been applied and is the state-of-the-art technology for automotive hydrogen storage.At present,the latest hydrogen engine adopts technologies such as hydrogen direct injection,lean combustion,and exhaust gas turbocharging,which can generally achieve brake thermal efficiencies of 42%and control CO,HC,NOx emissions below 10×10-6 vol under small and medium loads.Hydrogen engines face significant challenges to further improve efficiency and reduce emissions,or even achieve zero emissions,based on the current state.In terms of efficiency,like other Otto-cycle engines,the efficiency limit of hydrogen engines is the Otto cycle efficiency,which is mainly limited by two parameters:compression ratio and specific heat ratio.The specific heat ratio of conventional engines is usually around 1.33 at room temperature due to the main component of the engine’s working fluid,namely air,and can decrease to below 1.20 as the cylinder temperature increases.Raising the compression ratio is a common means to increase engine efficiency,but the heat transfer loss that increases with the compression ratio makes the efficiency of the engine cannot increase indefinitely with the increase of compression ratio.In terms of emissions,the presence of N2 in the air makes it impossible for conventional hydrogen engines to completely eliminate NOx emissions.For these reasons,the concept of Argon Cycle Hydrogen Engine is proposed by Prof.Robert Dibble,in which hydrogenoxygen-argon mixture is the working fluid.Argon is a noble gas,which is not involved in combustion.The only product of hydrogen-oxygen combustion is water vapor,thus leading to zero emissions.Argon can be recycled by separating water vapor through a condenser.Noble gases are monoatomic molecules with a temperature-independent specific heat ratio as high as 1.67,which elevates engine efficiencies.Argon has the highest abundance of all noble gases in the air,at 0.934%.Therefore,Argon Cycle Hydrogen Engine is a promising novel technology to achieve high efficiency and zero emission.In addition,there are also technological routes that combine argon-oxygen mixtures with other fuels.These engines and Argon Cycle Hydrogen Engines are collectively called Argon Cycle Engines.Argon has a high specific heat ratio as well as a low specific heat capacity,which increases the in-cylinder pressure and temperature,thus promoting the occurrence of knock.Knock is currently the main obstacle to achieving high efficiency in Argon Cycle Hydrogen Engines,limiting the compression ratio to 5.5:1.This paper focuses on the knock suppression and efficiency improvement of Argon Cycle Hydrogen Engine,and the main research contents and related findings are as follows.(1)Efficiency analysis with Variable Kappa Otto cycle based on thermochemistry.The Otto cycle uses a constant specific heat ratio higher than the actual situation,thus overestimating the thermodynamic efficiency,i.e.,overestimating the upper limit of engine efficiencies.In this paper,on the basis of Otto cycle,thermochemistry is introduced.In such a way the modified Otto cycle can adopt the real specific heat ratio changing with the change of species and temperatures.The modified Otto cycle is named Variable Specific Heat Ratio Otto cycle,or,in short,Variable Kappa Otto(VKO)cycle,and provides a new method for the study of the upper limit of engine efficiencies which is more precise.According to validation by computer programming,with an excess oxygen ratio of 1.0 and Ar dilution ratios in the Ar-O2 mixture ranging from 79%to 90%,the Otto cycle overestimates the thermodynamic efficiency by 7%to 17%compared to the VKO cycle.This overestimation effect is insensitive to the compression ratio,but the lower the Ar dilution ratio,the more severe the overestimation.With knock tendency evaluated in terms of the ignition delay times of H2-O2 auto-ignition,combined with the efficiencies of the VKO cycle,the effects of several measures on the knock suppression and efficiency improvement of Argon Cycle Engines are evaluated.With a constant amount of total intake gas,increasing the Ar ratio or using lean combustion can suppress knock and improve thermodynamic efficiencies.Replacing H2 with CH4 or NH3 can extend the ignition delay times by one to two orders of magnitude respectively,and slightly improve the thermodynamic efficiencies.Water Port Injection has a significant knock suppressing effect,but at the cost of a reduction in thermodynamic efficiencies.(2)Energy flow analysis and constructing energy flow efficiency equations of engines based on experimental data.Based on a large amount of experimental data of a methane-fueled Argon Cycle Engine,energy flow analysis is performed to establish the energy flow efficiency equation:ηb=ηC ·ηHT ·ηVKO·ηT·ηGE ·ηM.This equation shows that the factors affecting the brake thermal efficiency ηb include combustion efficiency ηC,heat transfer efficiency ηHT,thermodynamic efficiency(Variable Kappa Otto cycle efficiency)ηVKO,time efficiency ηT,gas exchange efficiency ηGE,and mechanical efficiency ηM.Net indicated thermal efficiency ηin and gross indicated thermal efficiency ηig and be calculated with the following equations:ηin=ηC·ηHT·ηVKO·ηT ·ηGE,ηig=ηC·ηHT · ηVKO ·ηT.These equations provide a detailed and comprehensive analytical method for the increase of engine efficiencies.Besides,according to the experimental data,the following laws and typical values can be summarized for reference in the subsequent study.According to the experimentally achievable operating conditions,ηVKO can exceed 70%at a compression ratio of 12.5:1,which is the theoretical efficiency boundary of the Argon Cycle Engine at this compression ratio.Through lean combustion(λ>1.1),a ηC near 100%is achievable.A volumetric efficiency close to 100%can result in aηGE close to 100%.The highest ηin is obtained in the interval of CA50 from 5 to 15℃AATDC,with CA50 defined as the crankshaft angle of 50%cumulative released heat.At a compression ratio of 12.5:1 and a speed of 1000 r/min,in the high-efficiency CA50 interval,a typical ηHT is about 75%which is the main factor limiting ηin,and a typical ηM is about 80%which is the main factor limiting ηb.ηT has a typical value of about 100±10%in the high-efficiency CA50 interval,reflecting the degree of deviation of the real combustion process from the constant-volume process.In this experiment,the highest ηinreached is 51.7%.The purpose to utilize CH4 in this research is to inhibit knock under most conditions through the anti-knock features of CH4,thus avoiding the disturbance of efficiency as far as possible.(3)Chemical kinetic research on the auto-ignition of H2-O2 in an Ar/N2 diluted environment.The ignition delay times of H2-O2 in an Ar/N2 diluted environment are systematically measured on a rapid compression machine.The auto-ignition of H2-O2 is found to be very temperature sensitive.Under various test conditions,the ignition delay time is reduced from 40 ms to 1 ms with only a 40 K temperature rise.This suggests that the actual cause of some random abnormal combustion presented by hydrogen engines may lie in the high temperature sensitivity of H2-O2 auto-ignition.With a constant amount of the overall working fluid,the inhibition of H2-O2 autoignition by dilution and lean combustion is confirmed by experiments.Experimental data from the rapid compression machine are further used to validate 17 most important or recent H2-O2 chemical kinetic mechanisms.The comparison confirms that the mechanism published by Varga et al.in 2015 can predict H2-O2 ignition delay times at different pressures with the highest accuracy at present.Sensitivity analysis reveals that H+O2(+M)=HO2(+M)and H+O2=O+OH are the most sensitive elementary reactions with the pressure at the end of compression at 1.0 MPa and temperature no lower than 1000 K.The competition between these two reactions dominates the lengths of ignition delay times.At 1.0 MPa with a temperature no higher than 950 K,or at 3.0 and 5.0 MPa with a temperature interval of 900～1050 K,20H(+M)=H2O2(+M)and H+H2O2=H2+HO2 are the most sensitive and ignition delay time shortening reactions.The above-mentioned elementary reactions are important reactions dominating ignition delay times.(4)Experimental research on the effects of water injection on the knock inhibition and efficiency improvement of Argon Cycle Hydrogen Engines.Both Water Port Injection and Water Direct Injection can guarantee the normal operation of an Argon Cycle Hydrogen Engine with a compression ratio of 9.6:1.Previously,the compression ratio of a normal running Argon Cycle Hydrogen Engine was about 5.5:1 due to the limitation of knock.Without water injection,a maximum ηin of 53.50%is obtained in the experiment,while the knock intensity reaches 3.43 MPa,higher than the knock threshold of 0.1 MPa.Through Water Port Injection,with a molar ratio of water to ArO2 mixture of 6%,the knock intensity can be lower than 0.1 MPa,knock can totally be eliminated,and a maximum ηinof 52.41%can be achieved.Water Direct Injection can also suppress knock with a maximum obtained ηin of 50.32%.With Water Port Injection,CO,CO2,HC,and NOx emissions are all below the lower detection capability of the five-gas analyzer,with a reading value of 0.With Water Direct Injection,CO,CO2,and HC emissions are also below the lower detection capability limit,with a reading value of 0.The maximum detected value of NOx is 8×l0-6 vol.