A New Methodology For Diesel Surrogate Fuel Formulation And Its Basic Researches In New Combustion Mode

In the continuous pursuit of internal combustion engines with high efficiency and near-zero emissions,it is necessary to comprehensively understand the physical and chemical processes of the in-cylinder combustion.However,fossil fuels?including gasoline,diesel and kerosene?are of complex compositions,including hundreds or even thousands of hydrocarbon components.Given the complexity and uncertainty of diesel compositions,studying the fundamental combustion processes and mechanism of diesel fuel presents a great challenge,in particular the gaseous and particulate pollutant formation mechanisms.A reasonably designed diesel surrogate is proved to be an effective way to study the fundamental combustion mechanism of diesel fuel.However,most existing diesel surrogates mainly adopt light hydrocarbon components,increasing the difficulty of accurately reflecting the physical and chemical properties of the commercial diesel fuel or the combustion and emissions characteristics of diesel engines.Therefore,in this study,a methodology to construct diesel surrogates with C₁₀^C₁₈ hydrocarbon components based on fuel properties and engine combustion and emissions characteristics was proposed.First,the key physical and chemical fuel properties that affect fuel injection,atomization,ignition,combustion,engine efficiency and emissions were discussed in detail.Second,13 candidate components were chosen to represent n-alkanes,iso-alkanes,cycloparaffins and aromatics and blended with commercial diesel fuel in different proportions.Fuel injection,spray,ignition and combustion phase,engine efficiency and emissions versus changed blending component and proportion were systematically investigated.In particular,the effects of the cetane number and fuel volatility on combustion and emissions were investigated under fixed injection timing and fixed ignition timing conditions,respectively.Afterward,the key physical and chemical parameters of the surrogate fuels were defined,and the constraint equations were constructed for different parameters.Finally,three diesel surrogates were proposed,including a 3-Components surrogate?41.3%n-hexadecane,36.8%2,2,4,4,6,8,8-heptamethylnonane?HMN?,21.9%1-methylnaphthalene,by mol.?,a 5-Components surrogate?21.6%n-hexadecane,15.5%n-octadecane,26.0%HMN,20.7%1-methylnaphthalene,16.2%decalin,by mol.?and a 7-Components surrogate?21.5%n-hexadecane,15.4%n-octadecane,25.8%HMN,13.7%1-methylnaphthalene,8.1%decalin,8.1%n-butylbenzene,7.4%n-butylcyclohexane,by mol.?.Considering the diversity of parameters during the actual engine operation?including engine loads,fuel injection strategies,exhaust gas recirculation,etc.?and the complexity of the actual diesel components,this paper further validates the surrogate fuels under different engine operating conditions,and compares the combustion process and main exhaust emissions of diesel and multi-component surrogates.In addition,considering the effects of trace components in diesel fuel including olefins,halogenated hydrocarbons,polycyclic aromatic hydrocarbons and hydrocarbon oxygenates on the combustion characteristics of diesel fuel,the above components were blended into the surrogate fuel in a certain proportion to study the effect on surrogate fuel combustion and emissions.The results show that the three-component surrogate fuel and the five-component surrogate fuel can accurately reproduce the diesel combustion process under the wide range of engine load and direct injection timing.Under different direct injection timings,the CO and NOx emissions of surrogate fuels and diesel are all very close.The HC emissions and the total number of particulate emissions of five-component surrogate fuel are closer to diesel than those of three-component fuel.The surrogate fuel can also reproduce the low-temperature combustion phenomenon caused by the early injection of diesel as the two-stage injection stgrategy is used.The CO emissions of surrogate fuel are higher than those of diesel when the pilot injection is earlier,but this difference is reduced with the delay of pilot injection timing.The HC emissions of surrogate fuels are higher than those of diesel under different pilot injection timings,while the NOx emissions are very close to each other.As the earlier pilot injection timing,the surrogate fuels produce a large number of nucleation mode particles,resulting in the total particle number larger than that of diesel,but with the delay of pilot injection timing,the total particle number concentrations of surrogate fuels and diesel are roughly the same.When the EGR rate is less than 60%,the pressure curve and combustion process of surrogate fuels are in good agreement with diesel.However,the surrogate fuel combustion phase is delayed by about 0.5^oCA.Correspondingly,CO emissions are very close to diesel at low EGR rates,but when the EGR rate reaches 60%,the surrogate fuel CO emissions are higher than those of diesel.At different EGR rates,HC emissions of surrogate fuels are slightly higher than those of diesel fuel,and NOx emissions are very close.Blending the five-component surrogate fuel with trace-components in diesel have trivial effect on the in-cylinder pressure and combustion process.Only when methyl nonyl ketone and phenanthrene are blended into surrogate fuel,the CO emissions are slightly higher in the small and medium load.Blending dodecane,and 1-tetradecene with surrogate fuel will improve the particulate matter emissions,and adding methyl nonyl ketone has little effect on the total number of particulate matter,but the addition of phenanthrene will lead to a sharp increase in particulate emissions.Premixed Charge Compression Ignition?PCCI?mode adopts high EGR ratio combined with in-cylinder multi-stage injection to form the combustible mixture.The reactivity of fuel is important for the in-cylinder activation process of pilot-injected diesel and the combustion delay of main-injected diesel.The volatility of the fuel is very critical to mitigate the impingement of the pilot-injected diesel fuel and the overall distribution of the fuel in the cylinder.Therefore,based on the previous constructed surrogate fuel,the effects of fuel cetane number and volatility on combustion and emissions are further investigated by modulating the composition and ratio of the five-component diesel surrogate fuel in the PCCI mode.The results show that the cetane number of the fuel plays a decisive role in the combustion phase at the same direct injection timing.Low cetane number fuel helps to prevent premature combustion of pilot injection fuel in PCCI mode,and with the decrease of cetane number of direct injection fuel,the combustion duration is shortened and the heat release process is more concentrated,which results in a higher pressure rise rate.In the aspect of emissions,reducing the cetane number of the direct injection fuel will result in an increase in CO and HC emissions.The relationship between the cetane number of direct injection fuel and the peak value of particle size distribution is not fixed,however,the reduction of fuel cetane number has a significant effect on the inhibition of particles greater than 20nm.Under the single injection strategy,improving fuel volatility only has an inhibitory effect on CO emissions and has minor impact on HC emissions and NOx emissions.When the pilot injection ratio is low,improving the volatility can effectively inhibit CO emissions resulting from the pilot injection fuel.However,with the increase of the pilot injection ratio,the effect of improving fuel volatility on CO emission improvement is weakened.When the pilot injection timing is earlier,the effect of fuel volatility on CO emission is obvious,but when the pilot injection timing is later than 40^oCA,the ability of fuel volatility to reduce CO emission is decreased.At different pilot injection timings,the improvement of fuel volatility decreases the number concentration of particles with the diameter between 20nm to 200nm.The normalization analysis reveals that the improvement of fuel volatility can decrease CO and HC emissions and reduce indicated fuel consumption,while the reduction of fuel cetane number will lead to the increase of CO and HC emissions and indicated fuel consumption.Under the test conditions,adjusting the fuel injection strategy can achieve low NOx emissions and low fuel consumption.Under the premise that NOx emission is lower than 200ppm,improving fuel volatility has a great potential for reducing fuel consumption.Reactivity Control Compression Ignition?RCCI?has become an international research hotspot because of the dual fuel injection strategy of injection in both cylinder and inlet port,which is more adaptable to the fuel type.Based on the proposed diesel surrogate fuel combined with the three-component gasoline surrogate fuel?TRF?,the effects of the reactivity of port-injected fuel,direct-injected fuel and the in-cylinder mixture?the ratio of port-injected fuel?on the combustion process in RCCI mode were systematically studied by modulating the cetane number of diesel surrogate fuel and the octane number of gasoline surrogate fuel.The main conclusions are as follows:When the octane number of the port-injected fuel is high,the cetane number of direct-injected fuel plays a dominant role in the combustion phase of RCCI mode.As the octane number of port-injected fuel decreases,the combustion phase is gradually advanced and the combustion duration is gradually shortened.With the same premixed ratio,the octane number of port-injected fuel plays a leading role in CO emissions.When the premixed ratio is low,the CO emissions gradually decrease as the octane number increases.When the premixed ratio is 0.6 and the octane number is about 80,CO emissions are the lowest,and both the increase and decrease in octane number result in the increase of CO emissions.When the premixed ratio is high,the CO emissions gradually increase with the increase of octane number.The octane number of port-injected fuel and cetane number of direct-injected fuel have complex effects on HC emissions.When the premixed ratio is low,HC emissions are higher under both low octane number and low cetane number conditions.As the octane number decreases and cetane number increases,the HC emissions gradually decrease.When the premixed ratio is 0.6 and 0.8,there is a high HC emissions region,respectively.On both sides of the region,HC emissions gradually reduce towards the low fuel reactivity direction,which is mainly due to the lower combustion temperature in the cylinder and fuel is converted to CO emissions.The increase in the premixed ratio will increase the CO and HC emissions,at the same time suppress the NOx and particulate matter emissions and reduce the particulate matter geometric mean diameter.In terms of the un-regulated emissions,hydrocarbon oxygenates,alkenes and alkynes are primarily controlled by premixed ratio in RCCI mode.The hydrocarbon oxygenates emissions are lower at low premixed ratio.With the increase of premixed ratio,the hydrocarbon oxygenates emissions gradually increase,and in the combustion deterioration area,the hydrocarbon oxygenates emissions increase sharply.The indicated fuel consumption has a decreasing trend as the premixed ratio increases in RCCI mode.At the premixed ratio of 0.4 and 0.6,the indicated fuel consumption decreases as the in-cylinder fuel reactivity increases.At the premixed ratio of 0.8,port injection with RON70 and direct injection with CN30,the indicated fuel consumption decreases to 185g/kWh and NOx emissions decrease to 250ppm.The LLRCCI mode,port injection with low RON fuel and direct injection with low CN fuel,is expected to achieve efficient clean combustion.Compared to DI and PCCI modes,LLRCCI mode has the lowest NOx emissions and can improve the particulate matter emissions of DI mode.Besides,the indicated fuel consumption of LLRCCI mode is between DI mode and PCCI mode,while HC,CO and harmful un-regulated emissions of LLRCCI mode are higher.