Formaldehyde formation in the endgas of Otto engines: Numerical simulations and quantitative concentration measurements

Formaldehyde is an important intermediate species formed during combustion processes, e.g. inside IC engines. It disappears rapidly during the excitat ion phase of the ignition process and can be measured by means of laser ind uced fluorescence (LIF) with high sensitivity. Due to its important role du ring the combustion of hydrocarbon fuels an accurate knowledge of formaldeh yde concentration fields may improve the detailed understanding of the vari ety of gasdynamical and chemical processes associated with the auto-ignitio n of the endgas. However, LIF as one of the preferred methods in combustion diagnostics cannot quantitatively determine species concentrations as long as the collisional quenching rates are unknown. The approach taken in this study allows an online calibration of the LIF te chnique applied to formaldehyde concentration measurements during the ignit ion phase inside an optically accessible research engine. The method is bas ed on simultaneously recording the LIF signal distribution along the light path and the total absorption of the exciting laser beam. The LIF signal re adings are proportional to the absolute species concentrations assuming tha t the spatial variation of collisional quenching rates is small in the near ly homogeneous endgas. The overall LIF signal can be calibrated by the use of the integral absorption measurement. Absolute concentration measurements of formaldehyde obtained at different c rank angles for the ignition and combustion of n-heptane/iso-octane mixture s inside the SI research engine are compared to results of simulations usin g a detailed reaction mechanism for the specific fuels employed in the expe riments. The measured transient pressure trace is utilised as a time depend ent input for the calculations.