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 excitation phase of the ignition process and can be measured by means of laser induced fluorescence (LIF) with high sensitivity. Due to its important role during the combustion of hydrocarbon fuels an accurate knowledge of formaldehyde concentration fields may improve the detailed understanding of the variety of gasdynamical and chemical processes associated with the auto-ignition 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 technique applied io formaldehyde concentration measurements during the ignition phase inside an optically accessible research engine. The method is based on simultaneously recording the LIF signal distribution along the light path and the total absorption of the exciting laser beam. The LIF signal readings are proportional to the absolute species concentrations assuming that the spatial variation of collisional quenching rates is small in the nearly 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 crank angles for the ignition and combustion of n-heptane/iso-octane mixtures inside the SI research engine are compared to results of simulations using a detailed reaction mechanism for the specific fuels employed in the experiments. The measured transient pressure trace is utilised as a time dependent input for the calculations