HCCI engines have long been seen as the holy grail of internal combustion engines. These engines would combine a very high efficiency with ultra-low emissions. The bottleneck of HCCI engines is the lack of control over the ignition timing of the homogeneous air-fuel mixture. In this paper, a new engine concept is presented to solve this problem. In this new concept, next to the main compression of the engine, a secondary piston is provided in the combustion chamber to give an additional compression for a short duration close to the top dead center of the main piston.

As a result, this new internal combustion engine concept is called "Piston Assisted Compression Ignition" or "PACI". This secondary piston provides the additional compression that is needed to ignite the air-fuel mixture at the right timing during the engine cycle for a range of different loads and different inlet temperatures. In the same way as the combustion is initiated by a spark in a spark ignition engine, this concept tries to initiate and control the combustion in a compression ignition engine by an additional mechanical compression near the top dead center.

First, insights are given about the possibilities of this new engine concept and secondly, calculations of the ignition timings for a load range at different inlet temperatures are presented. The ignition timings are compared with the ignition timings that would be possible without the secondary piston. Calculations with methanol (single-stage autoignition) and PRF95 (two-stage autoignition) have shown the potential of this concept to better control the autoignition timings for a load and temperature range for both these fuels compared to the usual HCCI concept with a single compression stroke. To conclude, this research shows the potential of a radical new engine concept that could be an interesting way to solve the control problem of HCCI engines if possible mechanical challenges of this concept can be overcome.

Dr. Louis Sileghem, Ghent University, BELGIUM