Like any internal combustion engine, the Grail Engine burns air and fuel and uses the resulting explosion to push a piston up and down to drive motion. But it's the way that the Grail engine does it that makes it so unique.
As the piston travels upward, a vacuum is created underneath it, which sucks fresh air in through a one-way reed valve. Air then fills three chambers -- the pre-compression chamber, the vent-to-piston ports and piston-intake-ports.
When the piston reaches the top of its travel path, ignition occurs thanks to the three spark plugs. This forces the piston downward, which compresses the air trapped in the crankcase up to this point.
As the piston reaches bottom dead center, the exhaust valve opens and exhaust evacuates cylinder as compressed air enters the combustion chamber via the vent-to-piston and piston-intake-ports as noted above through a one-way valve located within the piston.
Compressed fresh air swirls upward in a vortex-like fashion, evacuating the cylinder of exhaust gas and replenishing the cylinder with fresh air. Once the exhaust valve closes, fuel is direct injected and mixed in the chamber. The mixture of air and fuel are compressed and ignited by the three spark plugs, creating an explosion that pushes the piston down and creates motion for the next cycle.
Seems pretty simple, right? Overall, the Grail Engine appears very similar to an ordinary two-stroke engine, except compressed air plays a much bigger role here. But there are some unique factors at work here, too. For one, Riley says the swirl mixing of compressed air and fuel is the most efficient air/fuel atomization mixture that can be obtained for optimum combustion. In addition, he says no other engine has a design with forced induction of compressed air entering the combustion chamber while at the same time air-cooling the top of the piston to lower emissions.
Finally, the three spark plugs create a rapid bloom of explosions, which help generate "forced semi-homogeneous charged compression ignition," or FS-HCCI. This process compresses air and fuel to the point where they nearly combust on their own (without spark). And when they do combust, all the remaining fuel in the chamber combusts at once -- simultaneously -- delivering much better efficiency. This results in more power and better fuel economy [sources: Green Car Congress, Wojdyla].