| With the pursuit of the ultimate thermal efficiency of internal combustion engines,the potential shown by lean combustion in thermal efficiency has received a lot of attention from researchers.Turbulent Jet Ignition(TJI)is an important method to achieve lean combustion in SI engines by using a pre-chamber.The reliability of ignition is ensured by additional fuel supply in the pre-chamber,and a large amount of highly reactive intermediate products produced after the jet can well ignite the mixture in the cylinder and increase the combustion rate.However,there is not enough research on TJI in terms of ignition,misfire,knocking combustion and strategy optimization for different load conditions.Therefore,the present work designed and developed a set of TJI device with fuel supply system,and carried out a study on the combustion characteristics of TJI and its effect on lean combustion in an engine.First,the combustion characteristics of TJI under different boundary conditions were carried out based on a visualized constant volume combustion bomb.The results show that compared with spark ignition,jet ignition combustion does not go through the laminar combustion stage,and the jet flame develops directly into high-speed turbulent combustion,which has a significant acceleration effect on flame propagation.Different initial pressure leads to different combustion phenomena: when the initial pressure is high,jet flame with fast burning rate will compresses the unburned gas in the combustion chamber,producing pressure waves,and the reflected pressure waves by the wall will cause the flame to slow down or even push the flame back.When the initial pressure is low,the jet products entering the main chamber might be not hot enough to ignite the fuel-air mixture,thus causing misfire and delayed combustion.Under lean combustion conditions,the ignition reliability and combustion rate of jet ignition are significantly reduced.The pre-chamber fuel supply is necessary to ensure the reliability of TJI ignition and to increase the jet flame speed,therefore greatly extending the lean burn limit.Further research on the influence of key structural parameters such as different jet hole diameters,number of holes and hole distribution on flame structure and propagation process of TJI was carried out.The results show that the structural parameter of the jet hole has an important influence on flame propagation: a small hole diameter will lead to complete quenching of the jet flame,and the quenched hightemperature active radicals will re-ignite the mixture in the main chamber,although the combustion rate is fast after ignition,the ignition timing is delayed;a large hole diameter will lead to a slower jet velocity,resulting in a slower flame propagation velocity in the middle and late stages of combustion;an appropriate hole diameter can simultaneously maintain proper ignition timing and flame propagation speed.Then,based on the constant porosity of the optimal jet hole,the effect of different number of holes and hole distribution on flame propagation was studied,and it was found that a three-hole structure with one large and two small holes could obtain a larger flame area while maintaining the flame propagation speed as much as possible.The study provides a basis for the selection and optimization of jet hole structure parameters for TJI engines.Next,a study on the effect of pre-chamber injection strategy on combustion and performance under lean combustion conditions was carried out based on a singlecylinder engine.The results show that fuel injection in the pre-chamber can expand the lean burn limit from excess air factor of 1.3 to over 2.0;when the fuel injection timing in the pre-chamber is near the bottom dead center,it can not only ensure the sufficient mixing of fuel and air,but also avoid fuel diffusion to the main chamber;excessive fuel injection in the pre-chamber will lead to deterioration of engine performance,thus the amount of fuel injection in pre-chamber should be reduced under the premise of ensuring stable combustion.Based on the results,this paper proposes an injection strategy with low fuel amount at bottom dead center in the pre-chamber to obtain the best engine performance and emission results.Based on the optimal injection strategy of the pre-chamber,an optimization study on the key parameters,such as pre-chamber volume,jet hole diameter and number of orifices,under lean burn conditions was carried out.The results show that a small prechamber volume is beneficial to obtain higher thermal efficiency and lower emissions due to lower heat losses;the jet orifice diameter has a significant effect on the combustion rate,too large jet orifice leads to too slow jet velocity and low combustion rate,too small jet orifice leads to too strong quenching effect and reduced combustion rate,therefore,a suitable jet orifice diameter is needed to achieve the ideal combustion process.As for the number of jet holes,it is beneficial to increase the combustion rate by appropriately increasing the number of holes,but the specific parameters such as the number of holes,hole angle and hole distribution need to be designed according to the combustion chamber structure in order to obtain the optimized combustion process.According to the above results,taking into account engine performance,knocking and emissions,an unequal three-hole small volume pre-chamber was designed in this work.With this pre-chamber,a stable combustion process with excess air coefficient in the range of 1.0-2.2 is achieve,which reduces fuel consumption rate by up to 9.6% under part load compared with SI,and the NOx emission is reduced by about 95%simultaneously.However,under low and medium load conditions,TJI combustion is too lean resulting in a significant reduction in combustion efficiency;under high load conditions,lean combustion is difficult to maintain the required power output.Furtherly,to solve the problems of low efficiency at medium-to-low loads and the incompatibility of power output and thermal efficiency under high load conditions emerged in TJI lean combustion,this paper proposes a technical solution of using different air charging strategies(including boost,Miller cycle,throttle adjustment,and negative valve overlap)for different loads,so as to flexibly regulate the air/fuel ratio and maintain the in-cylinder excess air coefficient in the optimal range(1.5-2.0)at all times.The results show that under high load conditions,the fuel consumption rate is reduced by 10%-12.5% compared with the baseline engine after adopting the boosted lean combustion strategy;under medium-to-low load conditions,the use of a reasonable charging strategy to reduce the intake air mass can avoid excessive lean combustion,and the fuel consumption is reduced by more than 9.6% compared with the baseline engine;among different charging strategies under medium-to-low loads,the Miller cycle has the lowest fuel consumption rate and the throttle adjustment method has the lowest implementation cost.Finally,based on the optimized charging strategy,this paper explores a technical approach to further improve thermal efficiency by using high compression ratio combined with negative valve overlap to achieve spark induced compression ignition.In this combustion mode,the high initial in-cylinder thermodynamic state and the high combustion rate of jet ignition ensure the stability of induced compression ignition,while the compression ignition stage promotes the combustion rate at the later stage of flame propagation and shortens the combustion duration;compared with SI combustion,jet induced compression ignition can improve thermal efficiency by 4.4%,reduce fuel consumption rate by 12.5% and NOx emission by 96.7% under part load conditions,which has very high potential for energy saving and emission reduction. |
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