Effects of second hydrogen direct injection proportion and injection timing on combustion and emission characteristics of hydrogen/n-butanol combined injection engine

Abstract

Hydrogen and n-butanol are superior alternative fuels for SI engines, which show high potential in improving the combustion and emission characteristics of internal combustion engines. However, both still have disadvantages when applied individually. N-butanol fuel has poor evaporative atomization properties and high latent heat of vaporization. Burning n-butanol fuel alone can lead to incomplete combustion and lower temperature in the cylinder. Hydrogen is not easily stored and transported, and the engine is prone to backfire or detonation only using hydrogen. Therefore, this paper investigates the effects of hydrogen direct injection strategies on the combustion and emission characteristics of n-butanol/hydrogen dual-fuel engines based on n-butanol port injection/split hydrogen direct injection mode and the synergistic optimization of their characteristics. The energy of hydrogen is 20% of the total energy of the fuel in the cylinder. The experimental results show that a balance between dynamics and emission characteristics can be found using split hydrogen direct injection. Compared with the second hydrogen injection proportion (IP2) = 0, the split hydrogen direct injection can promote the formation of a stable flame kernel, shorten the flame development period and rapid combustion period, and reduce the cyclic variation. When the IP2 is 25%, 50% and 75%, the engine torque increases by 0.14%, 1.50% and 3.00% and the maximum in-cylinder pressure increases by 1.9%, 2.3% and 0.6% respectively. Compared with IP2 = 100%, HC emissions are reduced by 7.8%, 15.4% and 24.7% and NOx emissions are reduced by 16.4%, 13.8% and 7.9% respectively, when the IP2 is 25%, 50% and 75%. As second hydrogen injection timing (IT2) is advanced, CA0-10 and CA10-90 show a decreasing and then increasing trend. The maximum in-cylinder pressure rises and falls, and the engine torque gradually decreases. The CO emissions show a trend of decreasing and remaining constant. However, the trends of HC emissions and NOx emissions with IT2 are not consistent at different IP2. Considering the engine's dynamics and emission characteristics, the first hydrogen injection proportion (IP1) = 25% plus first hydrogen injection timing (IT1) = 240°CA BTDC combined with IP2 = 75% plus IT2 = 105°CA BTDC is the superior split hydrogen direct injection strategy.