Relation between hydroxyl and formaldehyde in a direct-injection heavy-duty diesel engine

Formaldehyde and the hydroxyl radical have been recorded simultaneously in a diesel spray in anoptically-accessible heavy-duty diesel engine. Formaldehyde (CH2O) was excited by 355 nm radiation,while hydroxyl (OH) was excited around 284 nm. Both laser beams were focussed into a sheet, ofwhich the overlap while traversing the spray was monitored. The experimental results have beencompared to simulations. The ignition phase was modeled by a set of homogeneous reactors, whilethe diffusion combustion phase was simulated by a flamelet approach, viz. a rich partially premixedcounterflow diffusion flame. Two situations were tested, one in which the combustion started onlyafter the end of injection (short and early injection) and one with a longer injection duration whereinjection and combustion partly overlapped in time. In the former, OH initially shows up far from theinjector and then quickly proceeds towards the injector. The position of OH varied significantly fromcycle to cycle, suggesting that the combustion is partly governed by turbulence. When combustionand injection do partly overlap, the flame structure is much more stable. In both cases, OH andCH2O generally form complementing spatial distributions, indicating that CH2O is consumed locallyduring the hot combustion