【作者】 倪培永；

【导师】 姚春德；

【作者基本信息】 天津大学 ， 动力机械及工程， 2007， 博士

【摘要】 冷起动排放控制是目前车用汽油机排放控制的关键环节之一。汽油机冷起动排放高的根本原因是燃油蒸发差,使混合气形成质量不良,进而导致燃烧质量下降,汽油机冷起动排放恶化。因此,减少冷起动排放可以从改善燃油蒸发入手。为减少汽油机冷起动排放,提出了采用快速加热进气空气来提高起动时进气温度的技术路线。在初期试验与理论论证的基础上,开发了快速空气加热器,并应用在发动机台架及整车上,进行了冷起动排放试验研究。在初期试验中,在发动机台架上研究了在不同进气温度情况下汽油机的冷起动排放规律。试验结果表明,提高进气温度有利于减少汽油机HC和CO冷起动排放,并存在一个最佳进气温度(70℃),当加热到这个温度时,排放最低。最初吸入发动机的新鲜空气的温度对HC排放有重要影响,对发动机进气歧管内的冷空气进行加热可以大幅度降低HC排放。这些研究结果为进一步研究冷起动排放提供了科学的依据。为了更有效地减少发动机在冷起动时排放及改善对应工况的燃烧质量,在模拟仿真中,基于燃油蒸发理论,使用计算流体力学软件对汽油机冷起动燃油蒸发进行计算。模拟结果表明,进气温度、燃油温度和发动机转速对燃油蒸发均有重要影响,其中属进气温度影响最大,油滴初直径和喷射定时对燃油蒸发率影响不大,油滴初速度对燃油蒸发率影响很小。提高进气温度,不仅可以提高进气道内的燃油蒸发,而且还能提高缸内温度,促进缸内燃油的蒸发,为可燃混合气的制备创造有利的条件。基于快速加热的策略,使用低压喷射高压点火的方案,采用空心圆锥并配以来自配风器的气流,开发了一种低成本的快速空气加热器用来提高低温环境下的进气温度,进而控制汽油机冷起动排放。加热器采用独立电子控制系统。使用计算流体力学软件对加热器燃烧及传热过程的数值模拟,对加热器进行结构优化,试验结果表明,加热器实现了快速加热和清洁燃烧,但排气温度升高。在台架试验中,通过对发动机进油管路进行简单改造以及暖风出口与发动机进气管的连接,空气加热器可燃烧来自于汽油泵的燃油并成为发动机进气系统的一部分。试验结果表明,加热器工作32 s,能达到较满意的进气初始温度,发动机起动后,在较几分钟内进气温度仍保持较高。燃用汽油和乙醇汽油均能使得HC和CO排放分别降低33%和11%以上。整车低温冷起动排放测试表明,对只能达到欧Ⅱ排放指标的微型汽车,使用空气加热器后,THC和CO排放均低于欧Ⅲ、Ⅳ冷起动排放限值,可以满足未来对汽油机的严格排放法规要求。此外,使用加热器还可以很好地改善发动机低温冷起动性能。更多还原

【Abstract】 The emission control during cold start is one of the key processes of exhaust emissions from vehicle gasoline engines at present. The essential reason for high cold-start emissions can be traced to one factor: low fuel volatility, resulting in low quality of mixture and poor combustion. Therefore,reduction of cold-start emissions can start with improving fuel volatility.To reduce cold-start engine emissions, the method was presented to preheat the intake air rapidly during cold start. On the bases of the preliminary test and the simulation, a rapid air heater was developed and then applied to the engine bench and the vehicle. The tests were conducted to study the influence on cold-start emissions with the heater heating engine intake air.In the preliminary test, effects of engine emissions at different intake temperature were investigated. The testing results showed that increasing intake temperature was favorable to the decrease of HC and CO emissions. HC emissions were remarkably decreased by heating the cold air in the intake manifold. The optimal intake temperature of 70℃could reduce emissions most. The temperature of the initially inducted fresh air significantly influenced HC emissions. HC emissions were remarkably decreased by heating the cold air in the intake manifold. These results provide scientific evidence for further studying emissions during cold start.To reduce cold-start emission and improve combustion process more effectively, based on fuel evaporation theory, three-dimensional numerical simulation of fuel evaporation in a gasoline engine during cold start was employed using the CFD software. The simulation results indicated that the most influential parameters on the fuel evaporation were: intake temperature, fuel temperature and engine speed. The intake temperature was the most influential among the three. The initial droplet size and injection timing had some effect on the percentage of evaporated fuel. The initial droplet velocity had a relatively little effect on the fuel evaporation. Increasing intake temperature not only conduced to fuel evaporation in the intake port, but also increased in-cylinder temperature, which promoted the evaporation in the cylinder fuel. It also created a favorable condition for the formation of ignition mixture.Based on rapidly heating, adopting low pressure injection and high ignition, employing a hollow cone swirl nozzle with the flow from a swirler, a rapid air heater with low cost was developed to increase the intake temperature for the reduction of cold-start emissions of gasoline engines. The heater was independently controlled by an ECU. Through the numerical simulation of the combustion and heat transfer of the air heater using CFD software and the optimization of its construction, the testing result showed that the heater realized fast heating and clean burning, but the exhaust temperature was excessively high.Through the simple modification of the oil inlet pipe and the connection between the hot air outlet of the heater and the intake pipe of the engine, the heater consumed the fuel from the fuel pump of the engine and became part of the engine air inlet system. The results indicated that a satisfied intake air temperature before engine start was obtained in 32 seconds. After engine start, the intake temperature kept high for several minutes. Preheating intake air could reduce HC by more than 33% and CO more than 11% by fueling both gasoline and ethanol-gasoline during engine cold start.A test on low temperature start-up emissions from a micro subminiature car of EuropeⅡstandard was carried out. The vehicle test showed that both THC and CO emissions were lower than the emission limit value in the EuropeⅢandⅣby using the air heater, meeting more stringent gasoline exhaust limits. In addition, the heater substantially improved the low temperature start-up performance.更多还原