To compare both types of engine and to simplify the calculations, each engine has square cylinders 12 inches by 12 inches with 12 inches of stroke. This equal 1 cubic foot of area for the air.

Internal Combustion The internal combustion engine at the bottom of the intake stroke and beginning the compression stroke There is 1 cubic foot or 1728 cubic inches of air inside the square cylinder. Which has a content of approximately 362.5 cubic inches of oxygen (1728 X 20.98%) and 117.5 cubic inches of gasoline vapour( Stoich A/F Ratio 14.7-1). Total Volume = 1845.5 cubic inches (air, oxygen and gasoline) Air Temperature = engine cylinder wall temperature Air Pressure = 1 Standard Atmosphere (14.696 PSI).
At the top of the compression stroke, the piston has moved up 11 inches compressing the cylinder contents and now is ready for ignition. There has been no change in the volume of the cylinder. The air pressure has increased by 11 times and temperature accordingly . Total Volume = 1845.5 cubic inches (air, oxygen and gasoline) Air Temperature = has increased considerably from the air compression Air Pressure = 11 Standard Atmosphere (14.696 PSI x 11 = 161 PSI).
External Combustion Using the same dimension as the internal combustion engine cylinder, a 12 inch by 12 inch by 12 inch combustion chamber is used. The piston free wheels to the top of the stroke. Where the combustion chamber is sitting above the top of the stroke, loaded ready for ignition. Because the air, oxygen and gasoline is loaded in the combustion chamber at 3 Standard Atmospheres or more, (14.698 PSI X 3 = 44.0 PSI) the volume also increases by 3. Example 1728 X 3 = 5184 cubic inches X 20.98% of oxygen = 1087.6 cubic inches of oxygen. Also the gasoline volume changes, 5184 cubic inches of air and oxygen X 14.7 Ratio = 352.6 cubic inches of gasoline vapour. Total Volume = 5536.6 cubic inches (air, oxygen and gasoline) Air Temperature = temperature of the combustion chamber Air Pressure = 3 Standard Atmosphere (14.696 PSI x 3 = 44.0 PSI).

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