A Study of a Glow Plug Ignition Engine by Chemiluminescence Images

An experimental study of a glow plug engine combustion process has been performed by applying chemiluminescence imaging. The major intent was to understand what kind of combustion is present in a glow plug engine and how the combustion process behaves in a small volume and at high engine speed. To achieve this, images of natural emitted light were taken and filters were applied for isolating the formaldehyde and hydroxyl species.

Images were taken in a model airplane engine, 4.11 cm³, modified for optical access. The pictures were acquired using a high speed camera capable of taking one photo every second or fourth crank angle degree, and consequently visualizing the progress of the combustion process. The images were taken with the same operating condition at two different engine speeds: 9600 and 13400 rpm. A mixture of 65% methanol, 20% nitromethane and 15% lubricant was used as fuel.

The experiments show that glow plug combustion is a propagating autoignition combustion and that the homogeneity of the oxidation process increases with the engine speed. It was also observed that at low speed, the low temperature reactions start together with the rate of heat release and once they are over the high temperature reactions appear. On the other hand at high speed there is no time for low temperature reactions followed by high temperature reactions. This means that formaldehyde formation is partially skipped and hydroxyl shows up almost at the same time but not in the same location as formaldehyde.