Temperature Fluctuations in the Unburned Mixture: Indirect Visualisation Based on LIF and Numerical Simulations

We apply a method for the visualization and semi-quantitative estimation of small spatial temperature fluctuations in internal combustion engines with premixed loads. It is based on laser-induced fluorescence (LIF) of formaldehyde (CH₂O), which is formed in the unburned gas near the end of the compression stroke. The chemical reactions leading to formaldehyde formation during the phase before auto-ignition are strongly temperature-dependent. The concentration of CH₂O therefore acts as a natural, very sensitive tag for local gas temperature variations. A correlation between temperature fluctuation and formaldehyde concentration fluctuation is assessed by using numerical simulations involving a detailed treatment of chemical reactions leading to formaldehyde formation in the unburned gas. Formaldehyde is detected in the unburned gas of an optically accessible test SI engine by laser-induced fluorescence (LIF) along a line. The LIF-signal trace displays pronounced spatial fluctuations, which can not be attributed to inhomogeneities of the fuel/air ratio or to measurement noise, but which must be due to local temperature inhomogeneities in the unburned gas. By combining the observed spatial LIF-signal fluctuations with the computed formaldehyde/temperature correlations, temperature fluctuation amplitudes can be estimated. It is shown that the technique is capable of detecting fluctuations well below ±10 K before a mean temperature background of 800 K and higher.

The results obtained by applying the method in a simple test engine give evidence that considerable temperature fluctuations exist in the unburned gas, both with respect to their amplitude and to their geometrical size. The information delivered by the method may be important especially in the field of auto-ignition in HCCI-engines, since local temperature inhomogeneities in the unburned gas can be of paramount influence for the subsequent development of self-ignition and combustion.