Effects Of Hydrogen Injection Strategy On Combustion And Emission Characteristics Of A Lean Burn Gasoline Engine With Hydrogen Direct Injection

As the auto industry faces great pressure of energy shortage and environmental pollution, searching a clean and renewable energy instead of fossil fuel is the most effective way. As a completely renewable and environmentally friendly energy, hydrogen becomes an ideal alternative energy in the auto industry. There are some key technical problems with hydrogen fuel cell and hydrogen engine and their cost is quite high. The conventional engine with small amounts of hydrogen addition makes hydrogen apply to autos more possibly, because it has the advantages of low hydrogen consumption, small change with the original engine and low dependence on the hydrogen filling stations.The effects of hydrogen injection strategy on combustion and emission characteristics of a lean burn gasoline engine with hydrogen direct injection were studied, based on the project of the National Natural Science Foundation “Study on Energy-saving Mechanism of HEV Equipped with Lean-burn Gasoline Engine with Hydrogen Direct Injection System and with Local Hydrogenous Mixture”. Hydrogen injection process and distribution in cylinder with different injection timings through the method of CFD simulation were investigated. The effects of hydrogen injection timing, fraction and distribution on combustion and emission characteristics of the engine through changing hydrogen injection timing, quantity and excessive air coefficient in the engine test bench were studied.The main conclusions are as follows:(1)With 300°CA BTDC hydrogen injection timing, as hydrogen moves for quite a long time before ignition, the hydrogen is well-distributed at ignition time. It is not good for stratified combustion. With 90 to 105°CA BTDC hydrogen injection timing, as the strength of tumble and time of hydrogen moving before ignition are appropriate, hydrogen completes a clockwise circle movement following the tumble and is concentrated around the spark plug at ignition time. Meanwhile some hydrogen also spreads to other space of the cylinder through the circle movement so that it forms the ideally stratified hydrogen distribution that hydrogen is from concentrated to dilute in the area from spark plug to cylinder wall.(2)With different excessive air coefficient, as hydrogen fraction increases, the maximum cylinder pressure and heat release rate rise and the corresponding crank angle advances. With hydrogen injecting in the compression stroke, as the hydrogen injection timing advances, the maximum cylinder pressure and heat release rate first rise then fall and the corresponding crank angle first advances then lags. With different excessive air coefficient and hydrogen fractions, the maximum cylinder pressure and heat release rate of 105°CA BTDC hydrogen injection timing are higher than 300°CA BTDC hydrogen injection timing and the corresponding crank angle of 105°CA BTDC hydrogen injection timing advances. The trend becomes more obvious as excessive air coefficient increases.(3)With lean mixture combustion, the IMEP increases significantly after adding hydrogen and keeps increasing constantly as hydrogen fraction increases. With hydrogen injecting in the compression stroke and quite lean mixture, as the hydrogen injection timing advances, the IMEP first increases then decreases. With different excessive air coefficient and hydrogen fractions, the IMEP of 105°CA BTDC hydrogen injection timing are higher than 300°CA BTDC hydrogen injection timing, because hydrogen in cylinder is stratified of 105°CA BTDC hydrogen injection timing and richer ignitable hydrogen is concentrated around the spark plug at ignition timing. The stratified hydrogen distribution is conducive to high flame propagation speed. Then the combustion duration shortens, the maximum cylinder pressure rises and the corresponding crank angle advances so that engine power performance improves.(4)The flame development duration and rapid combustion duration decrease significantly after adding hydrogen and keeps decreasing constantly as hydrogen fraction increases. With hydrogen injecting in the compression stroke, the flame development duration and rapid combustion duration from 90 to 105°CA BTDC hydrogen injection timing are shortest in most excessive air coefficient and hydrogen fractions. With different excessive air coefficient and hydrogen fractions, the flame development duration and rapid combustion duration for 105°CA BTDC hydrogen injection timing are shorter than 300°CA BTDC hydrogen injection timing and the trend becomes more obvious as excessive air coefficient increases, because hydrogen in cylinder is stratified for 105°CA BTDC hydrogen injection timing and richer ignitable hydrogen is concentrated around the spark plug at ignition timing. The stratified hydrogen distribution contributes to rapid combustion so that heat release is intensive and the flame development duration and rapid combustion duration shorten.(5) With different excessive air coefficient, NOX emission increases as hydrogen fraction increases. NOX emission decreases significantly when excessive air coefficient exceeds 1.4. Especially at 1.6 to 1.7 excessive air coefficient, NO X emission decreases to a very low level. Burning the quite lean mixture can reduce NOX emission effectively.(6) With excessive air coefficient below 1.4, HC emission keeps at quite low level. When excessive air coefficient increases to 1.6, the mixture is too lean and combustion process becomes worse so that HC emission increases significantly, but HC emission decreases significantly as hydrogen fraction increases. With small hydrogen fraction, the stratified hydrogen distribution for 105°CA BTDC hydrogen injection timing improves combustion in cylinder, so HC emission is the lowest. As hydrogen fraction increases, the well-distributed hydrogen can reduce HC emission resulted from wall quenching and crevices, so HC emission of 300°CA BTDC hydrogen injection timing is lower than that of 105°CA BTDC hydrogen injection timing.(7)At 1.0 excessive air coefficient, CO emission is high and decreases as hydrogen fraction increases. CO emission keeps at quite low level when excessive air coefficient exceeds 1.0.