增压直喷乙醇-汽油发动机超级爆震模拟

Simulation Study of Super Knock in Turbocharged Direct Injection Engine with Ethanol-Gasoline Blends

采用在压缩行程上止点前向燃烧室内直接喷入一定量机油液滴,模拟了悬浮在燃烧室内的机油液滴引燃可燃混合气诱发低速早燃(LSPI)现象的过程.试验验证了选用的计算模型及计算方法的可行性后,数值模拟了不同低速运转条件下、不同乙醇掺混比(体积分数)的乙醇-汽油混合燃料时,小缸径增压直喷发动机燃烧室内由机油液滴引发的低速早燃现象以及后续的超级爆震过程.结果表明:乙醇掺混比分别为10%和20%(E10、E20)时,发动机缸内依次发生了超级爆震燃烧;当乙醇掺混比为30%(E30)时,即使发生了早燃现象(1200 r/min)并导致随后的爆震燃烧,但压力升高幅度明显降低,此时没有发生超级爆震燃烧;随着发动机转速提高(1600 r/min),使用E30燃料时发动机缸内也仅发生了早燃现象,而没有发生爆震燃烧;当乙醇掺混比高于50%(E50)后,不同工况条件下发动机缸内已经没有低速早燃现象.使用乙醇-汽油混合燃料的小缸径增压直喷发动机在超级爆震发生前一定有低速早燃现象发生,但低速早燃现象不一定导致超级爆震过程.

The process of low-speed pre-ignition(LSPI)caused by the oil droplets suspending in the combustion chamber was simulated by the method of injecting a certain amount of oil droplets directly into the chamber at the top dead center of compression stroke.After verifying the feasibility of calculation models and methods,the processes of LSPI and super knock induced by oil droplets were simulated numerically with different operating conditions and different ethanol blending rates(volume fraction)of ethanol-gasoline blends in a downsized turbocharged direct injection(DI)engine.The results demonstrate that super knock occurs in the engine cylinder with 10%or 20%ethanol(E10,E20)blending rates fuels.When ethanol blending rates is up to 30%(E30),although the LSPI phenomenon occurs and subsequently leads to a regular knocking combustion,the pressure rise amplitude is obviously reduced and the super knock does not appear.With speed up to 1600 r/min,LSPI but no any knock occurs in engine cylinder.As blending rates are higher than 50%(E50),no LSPI happens in the cylinder under different operating conditions.There must be a LSPI phenomenon before super knock happens in the downsized turbocharged DI engine with ethanol-gasoline blends,but the LSPI phenomenon does not necessarily lead to a super knock process.