Pressure Oscillations During Rapid HCCI Combustion

Abstract

This work has focused on studying the in-cylinder pressure fluctuations caused by rapid HCCI combustion and determine what they consist of. Inhomogeneous autoignition sets up pressure waves traversing the combustion chamber. These pressure waves induce high gas velocities which causes increased heat transfer to the walls or in worst case engine damage. In order to study the pressure fluctuations a number of pressure transducers were mounted in the combustion chamber. The multi transducer arrangement was such that six transducers were placed circumferentially, one placed near the centre and one at a slight offset in the combustion chamber. The fitting of six transducers circumferentially was enabled by a spacer design and the two top mounted transducers were fitted in a modified cylinder head. During testing a disc shaped combustion chamber was used. The results of the tests conducted were that the in-cylinder pressure experienced during rapid HCCI-combustion is inhomogeneous. Pressure oscillations were experienced which showed good accordance to vibration mode shapes and frequencies suggested by acoustic vibration theory. The pressure waves manifested largest intensities for the first vibration mode, a mode suggesting radial propagation of the pressure waves in the combustion chamber. Experiments showed that the direction of the pressure wave was random which hinted absence of hot spot ignition. Hot spots mean that some part of the combustion chamber is physically hotter than the rest which ignites adjacent mixture. There is no evidence available yet to demonstrate engine damage during rapid HCCI combustion. An explanation to this could be that the violent combustion reactions related to SI knock damage is not present in HCCI combustion due to the diluted mixtures. The local heat released will therefore be lower. Finally, engine tests using two other combustion chamber geometries were conducted. The results showed that altering the geometry of the combustion chamber affects the resulting frequency spectrum. The two geometries were hill- and a bowl shaped respectively. Analytical calculations on the bowl shape vibration frequencies indicate reasonably good accordance to experimental results.