Experiment And Numerical Study Of Injection Flow Rate,Fuel Stratification And Combustion Characteristics In The Low Temperature Combustion Mode

Low emission and high efficiency can be simultaneously achieved in internal combustion engines if combustion takes place under homogeneous or slightly stratified conditions,e.g.in homogeneous charge compression ignition(HCCI)or partially premixed combustion(PPC)conditions.The key technology for these advanced combustion concepts is the accurate and flexible fuel injection systems.Common-rail injection system inject fuel as a larger number of smaller droplets,giving a much higher ratio of surface area to volume.This provides improved vaporization from the surface of the fuel droplets,and so more efficient combining of atmospheric oxygen with vaporized fuel delivering more complete combustion.Direct-injection compression ignition(DICI)engine exhibit multiple combustion regimes.HCCI can be realized by early injection and PPC by later injection and they have quite different characteristics in combustion and emissions.In the DICI engine,the fuel is injected directly into the cylinder and injection strategy is commonly manipulated to control the stratification level.The injection rate profile,which describes the velocity/mass flow rate of the liquid fuel as a function of time during the injection period,is one of the most important parameters of the injector,which affects the evaporation rate,the fuel/air mixing,and hence the combustion process and pollutant emissions.Therefore,the bench experiments were carried out to investigate the influence of injection pressure and injection timing on the temporal evolution of the injection rate and injection duration in a specially designed experiment rig equipped with a common rail injection system.It is well known that the injection signal from the electronic control unit(ECU)of the injection system,which is often the only injection information available in engine operation and experiments,gives little information about the actual injection rate profile.It is shown in the present experiments that the actual injection duration is usually longer than the energizing time(ET).The time delay between the actual injection of the fuel and the ECU signal is about 0.3–0.4 ms,and the time delay appears to be insensitive to the injector geometry and injection pressure condition.The injection process can be characterized as five stages,a fast injector valve opening stage,a slow valve opening stage,a valve fully open stage,followed by a slow valve closing stage,and finally a rapid valve closing stage.A new injection model was developed for the common rail injection system,which was capable of simulating the instantaneous fuel injection rate and injection duration for a range of injection pressure and injection duration.The model was shown to be able to replicate the experimental injection rate profile of the present experiments and experiments found in the literature for common rail injection system.Both numerical simulations and experiments were also conducted in a heavy-duty DICI engine,with PRF81 as a gasoline surrogate,to investigate how the fuel stratification,combustion,emission and engine performance are affected by the start of injection(SOI)and intake air temperature.The injection timing was swept from-100 to-20 crank angle(CA)after top dead centre(ATDC)to achieve different levels of stratification of the charge in the cylinder while the intake air temperature was adjusted accordingly to keep the same combustion phasing at 3 ^oCA ATDC.The combustion process can be divided into different regimes,HCCI,PPC,and transition from HCCI to PPC,depending on SOI.In the DICI engine,the fuel stratification includes mixture stratification and temperature stratification.It is found that in the earlier injection HCCI regime the combustion process is less sensitive to the variation of SOI and piston geometry since the fuel/air mixture is fairly homogeneous owing to the long mixing time.The fuel/air mixture is under fuel-lean condition and the required intake temperature for a constant CA50 is the highest.The mixture stratification in the HCCI regime is lowest while the temperature stratification is high.The mixture in the squish region is uncomplete combustion due to the low temperature,thus a high carbon monoxide(CO)emissions.During the transition from HCCI to PPC regime,there are two SOI windows.In the first SOI windows(Zone 2),the fuel is injected towards the piston head in the squish region.The amount of the fuel in the squish region and in the crevice is the highest.The combustion efficiency and the engine thermal efficiency are the lowest among the studied cases,since combustion in the squish region is less complete due to the wall heat loss.The mixture and temperature stratification in this SOI windows Zone 2 are higher than that in the HCCI regime.In the another SOI windows(Zone 3)in the transition regim,the injections hit on the rim of the piston bowl and fuel split into two directions:inside the piston bowl and towards the squish region.The combustion process is highly sensitive to SOI due to the high sensitivity of fuel distribution in the bowl and the squish region to SOI.In the PPC regime there is an optimal SOI window,within which the required intake temperature is the lowest to maintain a constant CA50 and the engine thermal efficiency is the highest.The optimal operation window starts at the SOI when all fuel is injected into the piston bowl and ends when the fuel injection is towards the bottom wall of the piston bowl,which results in a poor mixing of the fuel and oxidizer and high heat transfer losses.The SOI window for optimal engine operation is expected to be fuel injector and piston bowl geometry dependent.Two distinctive classes of in-cylinder combustion temperature distributions could be found from the simulation results for the studied engine:one was for the SOI range from-100 to-46 ^oCA ATDC,which was the HCCI regime and/or the transition from HCCI to PPC regime,where the mean effective in-cylinder temperature was lower than 1800 K.The second class was for the SOI range from-44 to-20 ^oCA ATDC,where the combustion temperature was higher than 1850 K.This corresponded to the PPC regime.The combustion process is of a sequential manner:first-stage ignition is the low temperature reactions and the fuel is consumed to the intermediate species such as CH₂O,IC₄H₈,C₃H₆,H₂O₂,C₂H₄,etc;The formation of CO is accompanied by the consumption of intermediate species in the first stage and releases the radical of OH,H₂,CH₄ and oh,etc.CO is oxidized to CO₂ through the water-gas shift reaction.The NOx emission was not only affected by the mean temperature but also the distribution of temperature in the cylinder.The NOx come from the mixture with the equivalence ratio of 0.8?1.2.The main source of unburned hydrocarbon(UHC)emission in the HCCI and transition regimes was the fuel trapped in the crevice region where the oxidation process could not function properly.Carbon monoxide(CO)emissions showed a non-monotonic variation with the injection timing.The main source of CO emission was in the low temperature and fuel-lean region in the cylinder.In the present engine operation,the pressure rise rate(PRR)in the PPC regime was higher than that in the HCCI regime,which is contrary to most results reported in the literature.This is a combined effect of low equivalence ratio in the piston bowl and in the squish region,and the stratification of the ignition delay time in the mixture.The stepped-lip profile piston improves the engine performance for the SOI in the zone 3,within which the engine efficiency is high with the ultra-low emissions to meet the Euro VI regulation.Low emission and high efficiency can be simultaneously achieved in the case that the fuel/air mixture have the low mixture and temperature stratification and less fuel distribution in the equivalence ratio between 0.8 and 1.2.