Experimental Study On The QHCCI Combustion Technology Of Diesel Micro-pilot Injection Natural Gas Engines

Natural gas is a low-carbon fossil fuel and its efficient and clean use in internal combustion engines can reduce CO₂ emissions and alleviate energy crisis.The research object of this paper is the quasi homogeneous charge compression ignition（QHCCI,Quasi homogeneous charge compression ignition）process of diesel micro-pilot injection natural gas engines.Based on the LFEE(load–Φ_oxy–EGR–effective compression ratioε_e)strategy proposed by the research group early on,combined with pilot injection timing,an important parameter for controlling the combustion phase,a LPFEE(load–Pilot–Φ_oxy–EGR–effective compression ratioε_e)strategy suitable for optimizing natural gas QHCCI combustion is proposed.This article studies the influence of key combustion boundary control parameters in LPFEE strategy at low speed,medium and high load,including pilot diesel injection timing,intake pressure,EGR,and LIVC technology on QHCCI combustion characteristics and emissions.And under medium and high load,explored the use of LPFEE strategy to optimize the combustion heat efficiency of natural gas QHCCI.First,the impact of the key combustion boundary parameters in the LPFEE strategy on the QHCCI combustion process and emissions is studied.The experiment on the influence of the key combustion boundary parameters in the LPFEE strategy on the combustion process is carried out on a single-cylinder engine.The single-cylinder engine is modified by Weichai WP12 HPDI gas engine.The natural gas injection method uses port injection,and the pilot diesel injection method uses direct injection.The study found that:pilot diesel injection timing affects the combustion phase.Advance diesel injection timing is beneficial to increase the combustion rate,shorten the combustion duration,and increase ITE_g,but it will cause the problem of rough combustion(the limit of rough combustion in this study is RI<5 MW/m²,PRR_max<1.5MPa/deg),so the pilot diesel injection timing cannot be too advanced;Under the same load,reducing the intake pressure can increase the oxygen equivalent ratioΦ_oxy,thereby accelerating the combustion rate,which is beneficial to increase the indicated thermal efficiency ITE_g,but will increase the degree of rough combustion;Increasing the EGR rate will increase the heat capacity in the cylinder,reduce the combustion temperature,and at the same time reduce the oxygen content in the cylinder,so that the combustion rate is reduced,the combustion phase is delayed,and P_max,PRR_max and RI are reduced,which is beneficial to the expansion of QHCCI combustion to high load.However,increasing the EGR rate will reduce the specific heat ratio of the working fluid in the cylinder and increase the incomplete combustion loss,which is not conducive to the improvement of ITE_g;Through LIVC technology,reducing the effective compression ratio can delay the combustion phase,reduce the P_max during combustion,avoid coarse explosion combustion,reduce PRR_max and RI,which is conducive to the stable operation of QHCCI combustion under heavy load.Then,based on the influence of LPFEE control parameters on the combustion process,efficient and clean combustion strategies for different load conditions are explored.The results showed that:Under low speed and medium load（1000rpm,IMEP_g=12bar）,the key to increasing ITEg is to reasonably increase the oxygen equivalent ratioΦ_oxy（to about 0.5）to increase the combustion rate and reduce the incomplete combustion loss,and adjust the pilot diesel injection Timing,under the maximum engine explosion pressure(P_max<18MPa)and rough combustion(PRR_max<1.5MPa/deg,RI<5MW/m2)limits,the combustion phase is controlled to maintain the front combustion phase（CA50 is around 5 deg ATDC）,To reduce exhaust loss.After optimization,under the condition that the intake pressure P_in=1.8bar(Φ_oxy=0.5)and the pilot diesel injection timing is-10 deg ATDC,the indicated thermal efficiency ITEg can reach 49.79%;Under low-speed and high-load conditions（1000rpm,IMEP_g=18bar）,the charge temperature and pressure in the cylinder are high,and EGR is required to suppress rough combustion.After LIVC is used,the demand for EGR rate can be reduced and the specific heat of the working fluid can be reduced.Specific loss,and reducing the effect of reducing the large EGR rate on the excessive combustion rate,reducing the incomplete combustion loss,and combining the pilot diesel injection timing to make the combustion phase at the front level under the maximum explosion pressure and knock limit（CA50 at 11 deg ATDC）to reduce exhaust loss and maintain high indicated thermal efficiency under heavy load.Finally,using LIVC technology,P_in=3.0bar,Φ_oxy=0.79,EGR rate 15%,and pilot diesel injection timing is-3 deg ATDC,the indicated thermal efficiency ITE_g reaches 47.5%.