摘 要

随着汽车保有量的增加，石油危机和排放问题愈发引人关注，为了在满足日益严格排放法规的同时保证汽油机的动力经济性，主机厂和发动机厂商将汽油机朝小型化、高压缩比等方向进行强化。尽管混合气加浓、废气再循环(Exhaust Gas Recirculation, EGR)等手段能降低汽油机在强化过程中的爆震倾向，但会使汽油机排放恶化。喷水作为一种利用水的高比热容和高汽化潜热的技术，通过吸收缸内热量的方式抑制爆震的发生，同时不会使排放恶化。

本文使用CONVERGE软件建立某款直喷汽油机的数值模型，通过减少喷油质量的方式使汽油机在高压缩比高转速工况下发生爆震，建立汽油机的爆震数值模型并评价其爆震的强度。在爆震模型的基础上进行进气歧管喷水系统的数值模拟，分析进气歧管喷水抑制爆震的原理及方式。之后在进气歧管内设计不同喷水位置和不同喷水质量的喷水方案，从缸内压力、放热率、缸内温度变化情况等方面分析不同喷水位置、喷水质量对直喷汽油机喷水后的影响。通过本文研究得出以下结论：

1. 在高压缩比高转速工况下将直喷汽油机每循环喷油量减少至48.5mg（过量空气系数达到0.9），汽油机在此工况下发生了强烈的爆震现象。通过分析燃烧室内温度、压力变化等结果数据得出由于火焰前锋面向燃烧室内空间较为狭窄的区域传播时受到阻碍，导致火焰前锋面传播至此处的时间长于末端混合气发生自燃的时间，使得该区域发生爆震现象，压力变化最为剧烈。
2. 通过进气歧管喷水系统模型的模拟结果数据，说明了在直喷汽油机上搭载喷水系统能有效地降低爆震倾向。由于水的高比热容以及高汽化潜热，喷水后缸内末端混合气被冷却，在燃烧过程中不会因自身温度达到燃点而发生自燃，所以汽油机的爆震倾向被降低。但水蒸气对混合气具有稀释作用，从而阻碍了火焰前锋面的传播，增加了燃烧持续时间，降低了汽油机的热效率。
3. 不同喷水位置的喷水方案对直喷汽油机性能的影响程度不同，原因是喷水位置对水蒸气的运动情况具有重大的影响。不良的喷水位置将影响到水蒸气与进气充量的混合情况，从而降低喷水对进气充量的冷却效果，在进气歧管内壁上形成的水膜也将腐蚀汽油机。良好的喷水位置不仅可以促进水蒸气与进气充量的混合，还能在保证冷却进气的能力不变的情况下减少喷水质量或采用低压喷水器进行喷水，从而减少成本。
4. 不同喷水质量也对喷水后的直喷汽油机性能有较大的影响。与汽油机较为匹配的喷水系统能让汽油机在不发生爆震的情况下在高转速工况下采用理论空燃比的混合气进行燃烧，减少燃油消耗，提高燃油经济性。当喷水位置、喷水角度等参数不可更改时，可通过改变喷水质量、喷水温度、喷水时刻等参数提升喷水后汽油机的动力经济性。

关键词：直喷汽油机，爆震，喷水技术，喷水位置，喷水质量

Abstract

With the increase in car ownership, the oil crisis and vehicle emissions are becoming more and more noticeable. In order to meet the increasingly stringent emission regulations and ensure the power economy of gasoline engines, OEMs and engine manufacturers have shifted gasoline engines toward miniaturization and high compression ratios Strengthen the direction. Although enrichment of mixed gas and exhaust gas recirculation (EGR) and other methods can reduce the knocking tendency of the gasoline engine during the strengthening process, it will make the gasoline engine emissions worse. Water injection as a technology that utilizes the high specific heat capacity and high latent heat of vaporization of water, suppresses the occurrence of knocking by absorbing the heat in the cylinder, and does not worsen the emissions.

In this paper, CONVERGE software used to establish a numerical model of a direct injection gasoline engine. By reducing the quality of fuel injection, the gasoline engine knocked when it is operated under high compression ratio and high speed. The knock model of the gasoline engine was established to evaluate its knock. Strength of. Based on the knock model, the numerical simulation of the water injection system of the intake manifold was carried out, and the principle and method of suppressing the knock of the water injection of the intake manifold were analyzed. Then designed different water injecting schemes with different injecting positions and different injecting quality in the intake manifold, and analyze the different injecting positions, injecting quality and direct injection gasoline engine from the aspects of in-cylinder pressure, heat release rate, and temperature change in the cylinder. Effects after water injected. Through the research in this paper, the following conclusions were drawn:

1. Under the condition of high compression ratio and high rotation speed, the fuel injection quantity per cycle of the direct injection gasoline engine is reduced to 48.5mg (excess air coefficient reaches 0.9). When the gasoline engine operates under this condition, a strong knock phenomenon occurs. By analyzing the results data such as temperature and pressure changes in the combustion chamber, it is concluded that the flame front is hindered when it propagates towards the narrow area of ​​the combustion chamber, resulting in the end mixture spontaneous combustion time is shorter than the flame front propagation time, making this Detonation occurs in the area, and the pressure changes most drastically.

2. The simulation result data of the water injection system model of the intake manifold shows that the water injection system installed on the direct injection gasoline engine can effectively reduce the knocking tendency. Due to the high specific heat capacity of water and high latent heat of vaporization, the gas mixture at the end of the cylinder is cooled after water injection, and spontaneous combustion will not occur due to its own temperature reaching the ignition point during combustion, so the knocking tendency of the gasoline engine is reduced. However, water vapor has a dilution effect on the mixture, which hinders the spread of the flame front, increases the duration of combustion, and reduces the thermal efficiency of the gasoline engine.

3. The degree of influence of the water injection schemes of different water injection positions on the performance of the direct injection gasoline engine is different, because the water injection position has a significant impact on the movement of water vapor. Poor water inject position will affect the mixing of water vapor and intake charge, thereby reducing the cooling effect of water injecting on the intake charge, and the water film formed on the inner wall of the intake manifold will also corrode the gasoline engine. A better water inject position not only promotes the mixing of water vapor and intake air charge, but also reduces the quality of the water injection or uses a low-pressure water injector to inject water while ensuring the ability to cool the intake air, thereby reducing costs.

4. Different water injection qualities also have a greater impact on the performance of the direct injection gasoline engine after water injection. The water injection system that matches the gasoline engine allows the gasoline engine to burn with a mixture of theoretical air-fuel ratios under high-speed conditions without knocking, reducing fuel consumption and improving fuel economy. When the parameters such as water inject position and water inject angle cannot be changed, the power economy of the gasoline engine after water injected can be improved by changing the water inject quality, water inject temperature, water inject time and other parameters.

Key Words: Gasoline Direct Injection engine, Knock, Water Injection, Water Inject position, Water Inject quality