Combined effects of intake oxygen enrichment, intake pressure and temperature on combustion behavior and emission characteristics of a small-scaled rotary engine

Intake oxygen enrichment has a promising potential to improve the combustion performance of a small scaled rotary engine (RE) operating at high altitudes. However, the process is complicated because the intake temperature and pressure are sensitive to altitudes as well as the elongated rotor chamber. In this study, a three-dimensional CFD simulation model coupling with a suitable turbulent model and a reduced chemical kinetic mechanism was established and validated. The combined effects of intake oxygen enrichment as well as intake pressure and temperature under different altitudes on early flame growth, combustion behavior, and emission characteristics of small-scaled REs were numerically investigated. Results showed that intake oxygen enrichment promotes the formation of OH, O, H radicals, which leaded to the significant improvements of the early flame growth, especially for the flame near leading spark plug. The promotion effect due to intake oxygen enrichment became weak with increasing altitude. Moreover, the early flame growth duration at high altitudes was more dramatically shortened by intake oxygen enrichment than that at low altitudes. It is interesting to note that when oxygen concentration of 25% and 29% were applied to the small-scaled RE working at 2000 m and 3000 m altitude, respectively, the engine performance could be restored to the level of that working at sea level at the price of a smaller increment of NO emissions. Under high intake oxygen concentration conditions, the effect of altitude on indicated thermal efficiency of the engine could be ignored. Besides, increasing oxygen concentration significantly reduces HC and CO emissions, especially more significant reduction for HC emissions at higher altitudes.