Unfortunately '8 Stroke' cannot be used on 4 or 6 cylinder engines. An odd number of cylinders is essential to give even firing impulses in both 4 stroke and 8 stroke modes. Then the fuel injector for one of the inlet valves can be switched off for alternate 'air only' inductions and the 8 Stroke cycle. This same injector is then switched on for 4 stroke and higher power with no need to change the valve opening sequence. The advantage of this method is that fuel can be pre-mixed with the inlet air.
Fuel injection can occur in the inlet manifold.
This gives more time and turbulence to achieve better mixing of the fuel and air allowing more complete and faster combustion than the simple version above. Again, an odd number of cylinders is needed. The scale of the fuel savings At full power there is no fuel saving but as the load reduces, the savings increase dramatically. Stroke' combustion engines may be more fuel efficient, cheaper, and more practical than fuel cells, if hydrogen becomes available as a fuel. Both CO and HC are the results of incomplete combustion and are greatly reduced by more complete '8 Stroke' combustion.