Future requirements for emission reduction from combustion engines in ground vehicles might be met by using the HCCI combustion concept. In this concept a more or less homogenous air fuel mixture is compressed to auto ignition. This gives good fuel consumption compared to a normal SI engine and its ability to burn lean mixtures at low temperatures has a positive impact on exhaust emissions. However, there are challenges associated with this concept, for instance its limited operating range and combustion control.
The objective of this work is to investigate a hybrid concept, based on a combination of HCCI combustion of n-heptane and SI combustion of hydrogen. The basic idea is to initiate HCCI combustion with a spark ignited stratified lean hydrogen mixture. To verify that the combustion sequence consists of flame front combustion followed by HCCI combustion, photographs of OH chemiluminescence from the combustion were taken. This was made in a single cylinder engine with optical access through a quartz window in the piston. The performance of the hybrid combustion was compared to that of pure HCCI combustion.
Chemiluminescence images show an expanding flame front initiated by the spark plug. It is shown that the flame front propagation through the hydrogen charge can be used to expand the operating range of HCCI combustion, especially towards lower loads. The hybrid combustion concept gives greater scope for controlling the combustion than the pure HCCI concept. By varying the amount of hydrogen the crank angle when 50% of the energy is burned, CA50, can be phased further away from TDC.