Check out this link:
http://www.rexresearch.com/tour/tour.htmThe goal is to use a smaller piston/cylinder to perform the intake and compression while a larger piston/cylinder waits. Then, when the compression cycle is complete, the air/fuel mixture is transferred to the larger cylinder and ignited. Once the mixture is ignited in the larger cylinder, it starts to move, which is when power is produced. Both cylinders are connected either through gearing or rods that allow the pistons to wait until it is the appropriate time to move.
This is roughly the same concept as closing the intake valve partway through intake so that you only fill the cylinder with a partial charge. The cylinder goes through the normal compression cycle and power cycle, but the piston travels much further than the original volume so that more work can be extracted from the "working fluid". This theoretically results in less power/cylinder, but better efficiency. I've seen a few car companies do this with variable valve timing.
The dual cylinder idea is neat, but very inefficient due to the extra weight, extra parts, extra losses due to the coupling system, and plain annoyance of having to deal with all of that extra maintenance. The VVT idea is much more elegant, and allows greater control of the engine.
Camelman
