By adjusting the slope of the monohead in relation to the engine block and its internal reciprocating components, the compression ratio is varied. This alters the volume of the combustion chamber with the piston at top dead center (highest position of the piston in the cylinder), which, in turn, changes the compression ratio.
Interesting as the notion of variable compression is, Saab's Smith warns that it has to be used in conjunction with other technologies, that on its own it would improve fuel economy by just 4 to 5 percent in a conventional, naturally-aspirated engine.
"The full potential of variable compression can only be realized when it's used in combination with reduced engine displacement and high supercharging pressure," Smith says, pointing out the three cornerstones of the system.
1) The engine displacement must be kept small, he says, since a conventional four-stroke gasoline engine is most efficient at maximizing the energy in the fuel when it is running at a high load."A small engine must work harder and run closer to full load if it is to perform the same work as a bigger engine," Smith says.
The reasons for this include the fact that pumping losses are lower in a small engine, the engine is lighter, has lighter internal reciprocating mass, and lower frictional losses.