M100点燃式发动机未燃醇醛排放及TWC失活的机理研究

M100 means pure methanol. M100 spark ignition engine usually gets in trouble of high formaldehyde emission and a shorter life of three-way catalytic converters (TWC). Methanol can be oxidized in the exhaust when its temperature is higher than 450℃. It can be fast transformed over 600℃ that is why formaldehyde emission decreases. In most conditions, the temperature of the exhaust in the expansion stroke and exhausted into the pipe is higher than 600℃, due to the heat dissipation along with the pipe, its temperature descends rapidly, as a result, methanol is oxidized to be formaldehyde in TWC, instead of being oxidized in the pipe. The applied project is aimed at the mechanism and the control of the unburned methanol and formaldehyde emissions from the exhaust of M100 methanol engine. To study the effects of the boundary conditions of the exhaust gas composition, temperature, flow rate and oxygen concentration on methanol and formaldehyde oxidation, the variation of the concentrations of methanol and formaldehyde along with the exhaust pipe will be investigated. The low temperature oxidation mechanism in the exhaust pipe will be understood. To study the control strategies, low heat rejection exhaust pipe will be applied. Their influences on methanol and formaldehyde oxidation will be clarified. Furthermore, with the help of TWC, ultra-low unregulated emissions can be achieved. Although, methanol consists of no sulphur, TWC is still short life. TWC deactivation mechanism caused by methanol engine exhaust will be studied. Based on the researches carried out in this project, the oxidation mechanism and control strategy of unburned methanol and formaldehyde will be established, which will support the development of M100 methanol spark ignition engine theoretically and technically.

纯甲醇发动机常存在未燃醇醛排放高和常规尾气三效催化反应器(TWC)寿命短等现象。发动机排气中甲醇在450℃开始氧化生成甲醛，600℃以上甲醇、甲醛则快速氧化。发动机大部分工况下排气管进口温度不低于600℃，但由于沿程传热排气温度快速下降，甲醇甲醛不能被高温氧化，甲醇可能在TWC内氧化成甲醛排放。本项目重点研究排气沿程甲醇、甲醛浓度随时间变化历程，试验和数值模拟相结合探索排气成分、温度、流速和氧浓度等参数对醇、醛氧化的影响规律，阐明排气系统中醇醛低温氧化机理；利用排气余热强化甲醇、甲醛氧化，探索改善氧化边界条件对醇、醛排放的影响规律，结合TWC协同控制，优化醇醛排放控制策略；通过采集已老化的TWC，对不同部位的催化剂样品进行表征与分析，探索有害金属和非金属元素等对催化剂活性下降的影响规律，揭示甲醇发动机三效催化反应器短寿命失活的机理。本研究为甲醇发动机实现超低醇醛排放提供理论依据和技术支持。

甲醇发动机排气中未燃甲醇、甲醛排放较高，三效催化反应器存在使用寿命短等问题。针对甲醇发动机排气中未燃甲醇、甲醛的氧化机理和三效催化反应器的失活问题，本研究从理论和实验两方面开展了M100点燃式发动机未燃醇醛排放及TWC失活的机理研究。首先模拟计算得到发动机排气环境下的排气温度和排气流速。基于温度、流速和氧浓度等边界条件对排气系统中甲醇的氧化特性进行了研究。利用搭建的流反应器试验装置，流反应器材质分别为不锈钢和石英玻璃，在不同材质的流反应器中进行了对比研究。研究结果表明不锈钢流反应器中的金属对甲醇氧化存在明显的催化作用，不锈钢流反应器中甲醇的起始氧化温度明显低于石英玻璃流反应器和数值模拟中甲醇的起始氧化温度。研究结果表明甲醇发动机排气中的未燃甲醇、甲醛在温度约600K以上的排气温度下可发生氧化反应。基于甲醇在排气管中低温氧化的机理研究，开展了利用排气余热优化边界条件，强化甲醇、甲醛氧化及控制的研究。研究表明排气余热利用可有效促进甲醇氧化，排气温度和高温下的反应时间对甲醇在排气系统中的氧化较为关键。结合三效催化反应器的控制，可以有效降低甲醇发动机的醇醛排放。通过采集老化后的三效催化反应器，研究了未燃甲醇、甲醛、甲酸、水蒸汽等对三效催化反应器失活的影响。研究表明甲醇发动机冷启动和暖机过程中排气中未燃甲醇排放量较高，未燃甲醇、甲醛、甲酸、水蒸汽共同作用对三效催化反应器载体反复浸润腐蚀，引起三效催化反应器的转化效率降低。