Experimental Research On The Effects Of Fuel Properties And Injection Strategies On Low Temperature Diesel Combustion

Low temperature combustion(LTC) has been widely investigated in recent years due to its enormous potential in realizing clean and high-efficiency combustion. Fuel reformulation and advanced injection strategy are important technical measures for improvement of LTC. In this research, the pure diesel was used as base fuel, and three other test fuels, diesel/gasoline(G30), diesel/n-butanol(B30) and diesel/gasoline/n-butanol(DGB) were formulated by blending gasoline or/and n-butanol into diesel. The effect of four fuels on LTC characteristics and emissions was investigated at different exhaust gas recirculation(EGR) rates. Then under high EGR rate condition, the effects of single injection parameters(injection timing and injection pressure) and two-stage injection strategies(pilot/main injection and main/post injection) on LTC and emissions characteristics were experimentally studied.Compared to diesel, diesel/gasoline blend can improve smoke and indicated thermal efficiency(ITE) due to its higher volatility and lower cetane number. Besides above advantages, addition of n-butanol brings presence of oxygen into blended fuel, inducing further reduction in smoke emission. And fairly low smoke emission can be achieved over the entire EGR range by adopting diesel/gasoline/n-butanol blend in this research. Under high EGR rate conditions, advancing injection timing and increasing injection pressure is beneficial for reduction of smoke, CO and THC emissions. Further advancing injection timing for blended fuels is restricted by maximum pressure rise rate(MPRR). The combustion process and smoke emissions can be improved obviously by increasing injection pressure, even for blended fuels with high volatility and low cetane number.Then two-stage injection strategies were studied. The results show that pilot injection timing close to main injection is effective for the reduction of premixed heat release rate and MPRR. Compared to diesel, the effect of pilot injection on MPRR is not that obvious for blended fuels with high blending ratio(i.e. DGB). It has a negative effect on smoke emission when a small pilot-main interval is employed. While for a large pilot-main interval, CO and THC emissions increase sharply, along with limited improvement in MPRR. Therefore, pilot injection parameters for blended fuels should be carefully optimized in consideration of fuel economy, emisions and MPRR. As for post injection, it presents little effect on combustion of main injection fuel. As main-post interval increases, the ITE decreases and this trend is more obvious with increasing post injection rate. Moreover, the smoke emission for blended fuels is more sensitive with variation of post injection. Using post injection with appropriately small quantity and main-post interval is effective for the reduction of smoke emission and slight improvement in ITE can be achieved as well. In conclusion, the combination of optimization on fuel properties and injection strategies is an effective method for smoke emission reduction and fuel economy improvement, which proposes an important technical approach for realizing high-efficiency and clean low temperature combustion.