Idling Performance Study And Numerical Simulation Of An HCNG Engine

Hydrogen enriched natural gas mixture fuel，as a new kind alternative fuel，whichhas combined the advantage features of natural gas and hydrogen，will contribute toalleviate the dependence on oil fuel and can be adopted as interim solution to hydrogenpowered vehicles. Idling condition is very common while driving in the city and hasgreat influence on the engine economics and emissions. This paper experimentallyresearches the idle performance of an HCNG engine and numerically studies thecombustion process in the engine.This research conducted experimental study of the effects of hydrogen ratio andcompression ratio on the idle performance of HCNG engines，where hydrogen ratios are0%，30%，55%and compression ratios are separately10and11. The results show thathydrogen enrichment can increase the flame speed，decrease the cycle coefficient ofvariants and improve the combustion stability. Besides，the improvement correlatesgreatly with ignition advance angle. While the ignition advance angle is larger than16°CA，the smaller hydrogen ratio（30%）plays a greater role in improving combustionstability; while it is smaller than16°CA，the higher hydrogen ratio（55%）plays agreater role. With the increase of hydrogen ratio，CO and THC emissions decreases asthe C/H ratio decreases. The NOxemission increases because of the higher combustiontemperature. Increasing compression ratio can also decease the cycle coefficient ofvariants，increase mixture combustion speed and improve combustion stability. With theincrease in compression ratio, both thermal efficiency and power output increase. TheNOx，THC and CO emissions increase as the compression ratio increases.In addition，this research conducted numerical study of combustion process of aHCNG engine including the quasi-dimensional model and KIVA three-dimensionalprogram. In the quasi-dimensional model research，a fractal geometry based combustionmodel is set up with a newly developed fractal dimension equation. Simulation resultsshow that fractal based quasi-dimensional model can be used to simulate cylinderpressure curve. Besides，great improvement in predicting cylinder pressure can beachieved with the newly developed equation. In respect to the KIVA program，hydrogenenriched natural gas mixture is added to the fuel library. Computational grids of different compression ratios are established to study combustion process with differenthydrogen ratios and compression ratios. Simulation results show that with higherhydrogen ratio or compression ratio, the combustion temperature is higher. Because oflarger area to volume ratio, heat release loss is larger and the temperature gradient islarger for the higher compression ratio. Besides, NO emissions increase with theincrease of cylinder temperature.