Study of jet pressure and timing on emission formation for high-pressure air jet controlled premixed combustion

High-pressure air (HPA) jet controlled compression ignition (JCCI) accomplished in the hybrid pneumatic engine (HPE) was studied in the work. Based on the compound thermodynamic cycle in HPE, it reveals the intensified low-temperature reaction and two-stage high-temperature reaction caused by the high-pressure air jet. The high-pressure air jet compression and mixing of the high-pressure air and original premixture directly affect the combustion and emission characteristics. The numerical simulation was conducted to investigate the effects of high-pressure air jet pressure and start of jet (SOJ) timing on the emission formation process of HPA JCCI combustion. The results indicated that HPA JCCI combustion obtains ultra-low NOx emission, and the first-stage high-temperature reaction has a big effect on the NOx formation due to its relatively high combustion temperature. A high air jet pressure leads to lower NOx emission due to less mixture involved in the first-stage reaction. Relatively advanced or retarded SOJ timing could also lead to lower NOx emission. Because of the mixing effect of the original premixture and high-temperature air with low temperature, the second-stage high-temperature reaction plays a more important role in the CO and THC formation. A lower air jet pressure leads to lower CO and THC emissions, and an optimum SOJ timing can be obtained to get both the lowest CO and THC emissions for each air jet condition. As HPA JCCI combustion has a very low level of NOx emission, a lower air jet pressure with an optimum SOJ timing is more favorable for the overall engine emissions.