Simultaneous OH- and Formaldehyde-LIF Measurements in an HCCI Engine

Simultaneous OH- and formaldehyde LIF measurements have been performed in an HCCI engine using two laser sources working on 283 and 355 nm, respectively. Two ICCD camera systems, equipped with long-pass filters, were used to collect the LIF signals. The simultaneous images of OH and formaldehyde were compared with heat-release calculated from the pressure-trace matching the cycle for the LIF measurements.

The measurements were performed on a 0.5 l single-cylinder optical engine equipped with port-fuel injection system. A blend of iso-octane and n-heptane was used as fuel and the compression ratio was set to 12:1. The width of the laser sheet was 40 mm and hence covered approximately half of the cylinder bore.

At some 20 CAD BTDC low temperature reactions is present and formaldehyde is formed. The formaldehyde signal is then rather constant until the main heat-release starts just before TDC, where the signal decreases rapidly to low values. From some 15 CAD to 5 CAD BTDC the formaldehyde is uniformly distributed in the imaged area. As formaldehyde decreases, OH increases and follows the main rate of heat release curve, though with a slight lag in phase. Thereafter OH is formed in the areas from which the formaldehyde has disappeared and the OH signal is present to some 20 CAD ATDC.