【作者】 陈登；

【导师】 郗大光；

【作者基本信息】 天津大学 ， 动力机械及工程， 2005， 硕士

【摘要】 二冲程汽油机具有升功率大、做功密集度大、结构简单、制造成本低以及便于维修等优点,然而其油耗高、排放差的致命缺点,限制了它的进一步应用。造成二冲程汽油机燃油消耗率过高和排放超标的主要原因是低速小负荷时的燃烧不稳定和扫气过程中的燃油短路损失。其中,燃烧不稳定可通过更好地组织扫气以及精确控制空燃比的途径解决;而采用缸内直喷技术,用新鲜空气进行扫气,可以解决燃油短路损失,使得HC排放大大降低。本文使用计算流体动力学(CFD)软件对缸内直喷式二冲程汽油机的扫气和喷油过程进行了多维数值模拟,目的旨在利用理论计算方法对内燃机缸内气体的流动特性进行较为详尽的描述,建立发动机设计参数和运转参数之间的关系,为性能和指标的优化提供依据。发动机低负荷工况下的计算结果表明,当选择合适的喷雾方向和喷油器位置,推迟喷油宜于形成分层稀薄快燃的混合气;高负荷工况下的计算结果表明早喷有益于燃油和新鲜空气的充分混合,从而在点火时刻能够形成预混燃烧的均质混合气。同时,计算结果表明:喷油器的喷射压力是一个很重要的参数,它决定了喷射燃油的液滴直径和喷雾的贯穿距离,而这两个参数的特性又决定着在火花塞点火时刻燃烧室的充量特性。在低负荷工况下,为了避免油滴溅在活塞顶部或燃烧室的壁面上而产生有害油膜,喷射压力不易过大;在高负荷工况下,喷射压力越高,雾化越好,使得混合气越均匀。更多还原

【Abstract】 Two-stroke gasoline engine has a feature of high power density capable of beingmade compact and simple, but has defects such as bad fuel economy and highhydrocarbon emission due to mixture short-circuiting and poor combustion at lightloads, which limit its application. Direct fuel injection into the combustion chamberoffers one of the most promising solutions for the short-circuiting problem whilecharge stratification is a suitable solution for combustion irregularities at light loads.Multi-dimensional numerical simulation is made to analyze the characteristics ofthe scavenging and injection processes in a two-stroke DISI (direct injection sparkignition) engine. The result shows that to get charge stratification at light loads, lateinjections are advantageous when proper injector position and injection direction areselected, but to ensure homogeneous combustion at heavy loads, injection advanceshould be as high as possible, with the limit that excessive advances give rise to lossof fuel from the exhaust port. The computation results also indicate that injectionpressure is a very important parameter. It decides the droplet size distribution andspray penetration, which control the final charge structure present in the combustionchamber at the instant of spark kernel initiation. So a little low injection pressure isnecessary to decrease the mean penetration velocity and thereby avoid a detrimentalwall film formation on the crevices of the combustion chamber, on the other hand, atheavy loads, high injection pressure is beneficial to the evaporation of spray dropletsand the achievement of homogeneous mixture.更多还原