摘 要
本文通過對內(nèi)燃機(jī)理想工作循環(huán)進(jìn)行理論分析，得出壓燃式內(nèi)燃機(jī)實(shí)現(xiàn)等壓循環(huán)時熱效率最高，同時運(yùn)用數(shù)值計(jì)算的方法對二甲醚（DME）發(fā)動機(jī)中摻混液化石油氣（LPG）燃料的燃燒過程進(jìn)行了深入研究，揭示了預(yù)混合壓燃式發(fā)動機(jī)混合氣的形成與著火特點(diǎn)，并分析了噴油參數(shù)對實(shí)現(xiàn)預(yù)混合燃燒過程的影響。綜合各方面因素，實(shí)現(xiàn)了DME發(fā)動機(jī)預(yù)混合等壓燃燒過程，為進(jìn)一步提高DME發(fā)動機(jī)效率提供參考。
理論研究發(fā)現(xiàn)：受機(jī)件強(qiáng)度的限制，壓燃式內(nèi)燃機(jī)必須限制其循環(huán)最高壓力，為得到較高的熱效率，需按定壓加熱循環(huán)工作，因此柴油機(jī)燃燒模式需遵循預(yù)混合近似等壓燃燒理論。其燃燒方式要求燃燒放熱在上止點(diǎn)附近，并且在著火前盡量將全部燃油噴入缸內(nèi)，實(shí)現(xiàn)預(yù)混合燃燒以代替?zhèn)鹘y(tǒng)的空間霧化燃燒方式的擴(kuò)散燃燒。
針對某機(jī)型進(jìn)行了幾何模型的建立和動態(tài)網(wǎng)格的劃分及生成，之后對標(biāo)定工況下?lián)交觳煌壤齃PG燃料的DME發(fā)動機(jī)的氣相流動過程、噴霧過程和燃燒過程等缸內(nèi)過程進(jìn)行了數(shù)值仿真研究，比較詳細(xì)地展示了缸內(nèi)流場、噴霧發(fā)展形態(tài)、及其燃燒的壓力及放熱率變化。研究表明：
（1）通過對DME發(fā)動機(jī)摻燒不同比例LPG后氣缸內(nèi)混合氣濃度場、溫度場及燃料蒸發(fā)率（量）曲線的比較發(fā)現(xiàn)，摻混LPG燃料后，燃油霧化程度有所提高，預(yù)混合氣量增大，分布趨于均勻，缸內(nèi)溫度隨LPG量增加而有所下降；
（2）通過其燃燒特性參數(shù)如著火始點(diǎn)、燃燒持續(xù)期、燃燒壓力及放熱率的計(jì)算比較發(fā)現(xiàn)：隨混合燃料中LPG添加量的增加，其燃燒滯燃期延長，著火始點(diǎn)延后，燃燒持續(xù)期縮短。缸內(nèi)燃燒壓力略有升高，燃燒放熱時刻和放熱率峰值時刻推遲，同時放熱率峰值大幅度增加。燃燒放熱規(guī)律中擴(kuò)散燃燒部分亦隨LPG含量增加而減少；
（3）在研究噴油參數(shù)對預(yù)混合燃燒影響的基礎(chǔ)上，通過選擇合適的噴油參數(shù)，添加高熱值的燃料LPG等措施，使燃燒放熱規(guī)律實(shí)現(xiàn)近似等腰三角形燃燒模式，實(shí)現(xiàn)了預(yù)混合等壓燃燒理論。對于提高壓燃式發(fā)動機(jī)的效率具有重要意義。

關(guān)鍵詞：等壓燃燒；數(shù)值模擬；DME/LPG混合燃料；預(yù)混合氣；燃燒特性

ABSTRACT
Diesel ideal motive cycle is theoretical analyzed in this dissertation and it is concluded that the heat efficiency is highest when compression-ignition engines realize equipressure cycle. Based on the theory, the combustion process of LPG-DME blends on the dimethyl ether (DME) engine is deeply investigated via the simulative calculation method. The mixture formation and inflammation characteristics are obtained and the injection parameters’ effects on the realization of premixing combustion process are also analyzed. In conclusion, synthesizing omnifaceted factors the premixing isopiestic combustion process of DME engine is realized. And the result provides certain reference for further improvement of DME engine efficiency.
By theoretical study, maximum combustion pressure of compression-ignition engine must be limited according to the restriction of parts’ force. To get high thermal efficiency, the engine should be operated according as constant-pressure heating cycle. Consequently, the combustion mode should be followed premixed approximate isostatic combustion theory. This mode requests combustion heat release in BDC nearly and most of fuel should be injected into the cylinder as far as possible before ignition. By this, the premixed combustion would be achieved in place of traditional diffusion combustion which adopts space atomization combustion mode.
By numerical simulation method, the moving mesh model of DME engine is built. And then in standard running condition the process of air fluid, atomization and combustion in cylinder with different LPG ration are investigated. Velocity distribution in-cylinder, spray development, pressure and heat release rate of combustion are shown in detail. The results are shown that:
(1) Through the comparison of Mixture concentration field, temperature field and fuel evaporation rate/volume curve, with the increase of LPG content not only the fuel atomization is improved but also the pre-mixed gas concentration is increased and more evenly distributed. On the contrary, the temperature in-cylinder is decreased with the increase of LPG content.
(2) Comparison of combustion characteristics parameters such as combustion duration, combustion pressure and heat release rate is investigated. With the increase of LPG content, ignition delay is prolonged; the time of ignition is delayed; the combustion duration of blend fuel is reduced. Combustion pressure in-cylinder is slightly higher and the time reaching peak of heat release rate is delay while the peak of rate is significantly increased. The diffusion combustion part of combustion heat release is decreased with the increase of LPG content.
(3) Injection parameters’ effects on premixed combustion are also investigated. Based on the above conclusion, pre-mixed constant pressure combustion theory is able to realized by the way of selecting applicable Injection parameters, the content of LPG and other measures which make the combustion model similar to premixed combustion of isosceles triangle of heat release. The above conclusion has important significance to improve the efficiency of compression ignition engine.

Key word: isobaric combustion, numerical simulation, DME/LPG fuel combination, premixed gas, combustion characteristics -..