Analysis of OH Radical Injection and Production Behavior by Pulsed Flame Jet in HCCI Combustion

抄録

Homogeneous Charge Compression Ignition (HCCI) combustion with Pulsed Flame Jet (PFJ) and Exhaust Gas Recirculation (EGR) was studied using Rapid Compression Expansion Machine (RCEM). PFJ is a jet of burning gas issuing from a small cavity facing a main combustion chamber. Small amount of fuel-rich premixed gas is introduced to the cavity through the injector and ignited by a spark plug. The jet issuing from the cavity supplies heat and radicals volumetrically and initiates combustion in the main combustion chamber. Thus PFJ can advance the ignition timing of HCCI combustion. On the other hand, EGR retards the ignition timing and reduces pressure rise rate. In the previous study, the authors confirmed the possibility of reducing pressure rise rate, controlling the ignition timing and improving the thermal efficiency by using PFJ and EGR together. In the present study, OH radical injection and production behavior by PFJ was experimentally observed through chemiluminescence. Then, temperature and composition of the burned gas in the cavity of PFJ igniter were numerically evaluated by zero dimensional (0D) equilibrium calculation. OH and H are the main radicals. The combustion in the main chamber was also roughly estimated by 0D transient calculation. It was assumed that the burned gas issued from the cavity and the unburned gas in the main chamber are locally mixed at the front of the jet plume, and the autoignition of the mixture of PFJ burned gas and the unburned main gas was calculated. In the optical observation, the chemiluminescence of OH radical temporarily increased in the early stage of PFJ injection and then decreased. This was consistent with the tendency of the calculation, regardless of the PFJ/(PFJ+main) volume ratio. The optical observation also showed that the brightness of the OH radical chemiluminescence decreased with increasing EGR ratio. The calculations showed that the peak value of the mole fraction of OH radical decreased with increasing EGR ratio regardless of the PFJ/(PFJ+main) volume ratio. In order to estimate the effect of radicals supplied by PFJ on the initiation of HCCI combustion, calculations were also performed without the inclusion of radicals in the PFJ burned gas. The results imply that the heat supply effect of PFJ is dominant for the initiation of HCCI combustion in the main chamber, while the radical supply effect is not negligible in negative-temperature-coefficient (NTC) region.