【作者】 马帅营；

【导师】 尧命发；

【作者基本信息】 天津大学 ， 动力机械及工程， 2013， 博士

【摘要】 以均质压燃（HCCI）为代表的新一代燃烧方式可取得极低的NOx和颗粒物排放并保持高热效率，但由于化学反应动力学在燃烧过程中起主导作用，其燃烧控制困难及运行工况范围狭窄的难题至今没有得到有效的解决，限制了HCCI的广泛应用。为此，本文提出了一种基于燃料化学活性及混合气分层协同控制的燃烧模式—汽油/柴油双燃料高预混合低温燃烧（Highly Premixed Charge Combustion,HPCC），其供油方式为进气道喷射汽油、缸内直喷柴油，根据发动机运行工况通过调整汽油/柴油比例、EGR率及柴油喷油策略控制实现高比例的预混合燃烧，从而实现HPCC负荷工况范围内的高效清洁燃烧。本文主要针对HPCC燃烧技术的相关基础问题及其负荷工况范围内的燃烧控制策略开展了系统的研究。本文首先进行了燃烧控制参数对HPCC燃烧、性能及排放影响的试验研究，结果表明，柴油喷油时刻影响HPCC的燃烧放热模式，柴油早喷与晚喷时分别表现出双燃料的HCCI和柴油引燃汽油预混合气的准均质压燃（QHCCI）燃烧放热特性。通过调整汽油/柴油比例、EGR率及柴油喷油策略可改变缸内混合气的化学活性及浓度分层，实现对不同燃烧模式的控制；进气压力影响HPCC的着火滞燃期、燃烧反应速度和“失火”与“爆震”燃烧的汽油比例限值，提高进气压力可以提高汽油比例，从而大幅度降低NOx和碳烟排放；提高柴油喷油压力仍是改善HPCC模式下NOx和soot排放的重要途径；通过优化燃烧控制参数，在平均指示压力（IMEP）为0.9MPa工况，NOx和soot比排放分别低于0.4g/(kW·h)和0.003g/(kW·h)，并保持较高的热效率，但HC和CO排放偏高。柴油早喷是在中低负荷实现HPCC的主要控制策略，但早喷燃油的“湿壁”会导致碳烟排放升高，而且碳烟排放峰值出现在某一特定的喷油时刻区域。本文针对燃油早喷时刻下的碳烟排放特性开展了试验研究，结果表明，在特定的喷油时刻区域碳烟排放升高的根本原因是“湿壁”燃油在缸内形成了局部过浓区。燃油早喷情况下的碳烟排放特性不同于传统柴油机，如提高进气压力、控制进气温度、冷却水温度和提高喷油压力等均不能有效降低碳烟排放，而提高EGR率、采用高挥发性燃料或进气道结合缸内直喷的供油方式均可显著降低碳烟排放。柴油喷油策略是控制燃烧模式、扩展高效清洁运行工况范围的重要途径。本文分别对单次喷油（晚喷：L-single，早喷：E-single）和两次喷油（第二次晚喷：L-SOI2，第二次早喷：E-SOI2）策略进行了试验研究，并以L-SOI2为例，采用KIVA-CHEMKIN软件从混合气活性及其分层角度对HPCC燃烧及排放的控制机理进行了数值模拟研究。结果表明，在单次喷油策略中，E-single的NOx和soot排放远低于L-single；在两次喷油策略中，对于L-SOI2，着火时刻主要由第二次喷油时刻控制，高温放热分两阶段进行，可有效降低最大压力升高率（MPRR）及最大爆发压力，但由于较晚的第二次喷射的柴油存在部分扩散燃烧导致碳烟排放升高。对于E-SOI2，着火时刻主要由第二次喷油时刻与化学反应动力学共同控制，燃烧持续期短，热效率高，但MPRR也较高。为了使高效清洁运行工况范围向高负荷扩展，本文对不同转速（900～2500r/min）下的高负荷扩展技术途径进行了试验研究，结果表明，在满足一定的MPRR及排放等限定条件下，对各运行参数进行优化，采用E-single策略时，高负荷边界点的指示比油耗在1900r/min时达最低值为168.6g/(kW·h（)即50%的指示热效率），各边界点的NOx、soot排放均分别低于0.4g/(kW·h)、0.003g/(kW·h)。边界点IMEP值随转速的升高而增大，最高达到1.2MPa。低、中、高转速下的高负荷扩展分别受到较高的NOx排放、MPRR及最大爆发压力的限制。针对这些限制，在900r/min下，通过提高进气压力由0.12MPa至0.18MPa，最大IMEP由0.67MPa提高至0.79MPa；在1500r/min下，通过采用L-SOI2策略，并减少第二次喷射柴油量、提高汽油比例及柴油喷射压力，最大IMEP由1.026MPa提高至1.391MPa。在上述研究基础上，本文最后对HPCC运行工况范围内的燃烧控制策略进行了试验研究，结果表明，在低负荷工况，采用纯柴油低温燃烧（LTC）模式更有利于提高热效率；在中等负荷工况，宜采用汽油/柴油双燃料HCCI燃烧模式；而在高负荷工况，宜采用汽油/柴油双燃料QHCCI燃烧模式。采用上述燃烧控制策略，汽油/柴油双燃料HPCC可在其运行工况范围内实现极低的NOx和碳烟排放，有满足欧Ⅵ排放法规的潜力，并可实现高热效率。HPCC是一种极有前途的高效清洁燃烧新技术，且适用于其它高十六烷值与高辛烷值双燃料。更多还原

【Abstract】 Homogeneous charge compression ignition (HCCI), as the representative of newgeneration combustion mode with high thermal efficiency and lower NOx andparticulate matter (PM) emissions, has been widely investigated. However, HCCIcombustion is controlled by chemical kinetics, which makes it difficult to control thecombustion and expand operating range. This difficulty has not been effectivelyresolved up to now, so the wide use of HCCI is limited. Therefore, a new promisingcombustion strategy based on the cooperated control between fuel chemical reactivityand mixture stratification has been proposed, which is named gasoline/diesel dual-fuelhighly premixed charge combustion (HPCC). HPCC proposes port fuel injection ofgasoline and direct injection of diesel fuel with rapid in-cylinder fuel blending. Thehigh-efficiency clean combustion of HPCC characterized of highly premixed chargecould be realized over the whole load region by varying parameters of gasoline-to-dieselratio, EGR rate and injection strategy according to the engine operating condition.Systemic investigations were carried out in this thesis mainly focusing on HPCC relatedfoundation problems and its combustion control strategy over the whole load region.First, the effects of combustion control parameters on HPCC combustion,performance and emissions characteristics were experimental investigated. The resultsshow that injection timing of diesel affects heat release mode. The heat release of dieselearly-injection/late-injection represents the characteristics of dual-fuel HCCI andQuasi-HCCI (QHCCI) with premixed gasoline and air mixture triggered by diesel,respectively. These different combustion modes can be controlled by the cooperation ofmixture chemical reactivity and mixture concentration stratification, which can bechanged by varying parameters of gasoline-to-diesel ratio, EGR rate and diesel injectionstrategy. Intake boosts have effects on ignition delay, burning rate andgasoline-to-diesel ratio of mission fire and knocking limit. Increasing intake chargepressure could increase gasoline-to-diesel, so as to dramatically decrease NOx and sootemissions. Increasing injection pressure is still an important measure to decrease NOxand soot emissions for HPCC. At the point of0.9MPa indicated mean effectivepressure (IMEP), by optimizing the above parameters, HPCC achieves NOx and sootspecific emissions levels below0.4g/(kW·h) and0.003g/(kW·h) respectively, whilemaintaining high thermal efficiency, however, HC and CO emissions increase.Early injection strategy of diesel is the main control measure to realize HPCC at middle and low load. However, wall-impingement of diesel caused by early injectiontiming will lead to the increase of soot emissions. The peak values of soot emissionsarise at a certain injection timing regime. In response to this problem, the characteristicsof soot emissions caused by wall-impingement of diesel were especially investigated inthis thesis. The results show that the primary cause of the increased soot emissions atcertain injection timing regime is the combustion of local fuel-rich region provided bythe evaporation of wetted-wall fuel. Therefore, the characteristic of soot emissions inearly injection timing regime is different from that of conventional diesel engine. Forexample, increasing intake pressure, controlling intake temperature, coolant temperatureand increasing injection pressure could not be able to effective decrease soot emissionsin early injection timing regime, which could be decreased dramatically by means ofincreasing EGR rate, employing fuels with good evaporability or fuels delivery of portinjection combined with direct injection in-cylinder.Diesel injection strategy is an important way in controlling combustion mode andexpanding the operation range with high efficiency and low emissions. Single(including late injection timing of single (L-single) and early injection timing of single(E-single)) and double (including late second injection timing (L-SOI2) and earlysecond injection timing (E-SOI2)) injection strategies were investigated. Furthermore,take L-SOI2strategy for example, the control mechanism of combustion and emissionsof HPCC was simulated by using KIVA-CHEMKIN codes, viewed from the mixturereactivity and reactivity stratification point. The results show that, for single injectionstrategy, E-single strategy is more superior to L-single strategy in decreasing NOx andsoot emissions. For double injection strategy, in L-SOI2strategy, the ignition is mainlytriggered by the SOI2timing. The combustion process is characterized by two-stagehigh temperature heat release, which contributes to decreasing the maximal pressure riserate (MPRR) and cylinder pressure. While the soot emissions increase due to thediffusion combustion of the later second injected diesel. In E-SOI2strategy, the ignitiontiming was mostly dominated by joint action of the second injection timing andchemical kinetics, the combustion duration is short, and the thermal efficiency is highbut with the penalty of the MPRR.For the purpose of expanding operation range with high-efficiency clean combustionof HPCC to higher load, technology approaches of expanding higher load at differentspeeds (900～2500r/min) have also been investigated in this thesis. The results showthat on the basis of optimizing the operating parameters and satisfying a given criteria of MPRR and emission etc., by using E-single injection strategy, the optimal points of highload obtain the lowest indicated specific fuel consumption (ISFC) value of168.6g/(kW·h)(corresponding to50%indicated thermal efficiency) at1900r/min, and theNOx and soot emissions at different speeds are lower than0.4g/(kW·h) and0.003g/(kW·h) respectively. The IMEP of optimal points of high load increase with theincrease of speed, the maximum value can reach1.2MPa. High load expansion at low,middle and high speeds are limited by high NOx emissions, MPRR and peak cylinderpressure, respectively. Aiming at these limits, at900r/min, the maximum IMEPincreases from0.67to0.79MPa by boosting the intake pressure from0.12to0.18MPa.At1500r/min, the maximum IMEP increases from1.026to1.391MPa by employingL-SOI2strategy, minimizing the second injection mass of diesel, increasing gasolineproportion and diesel injection pressure.Based on the researches above, finally combustion control strategies over the HPCCload region have been experimental investigated in the thesis. The results show that, atlow load, employing low temperature combustion (LTC) mode fuelled with singlediesel fuel benefits the improvement of thermal efficiency. At middle load,gasoline/diesel dual-fuel HCCI combustion mode should be employed. And at high load,gasoline/diesel dual-fuel QHCCI combustion mode should be realized. By using abovehybrid combustion control strategies, ultra-low NOx and soot emissions meanwhile highefficiency could be achieved simultaneously with gasoline/diesel dual-fuel HPCC modeover its operating load range. HPCC engines possess the potential of meeting NOx andPM emissions standard in Euro VI heavy-duty regulations. HPCC is a new exceedinglypromising combustion technology, it fits other dual-fuel combinations of high-cetaneand high-octane fuels.更多还原