| 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. |
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