Interesting project!
This engine is not a Diesel engine, it works like a steam engine: gas is supplied under pressure and expands in the cylinder. The compressed air from the reservoir is heated and greatly increased in volume by burning some fuel in it (at an intermediate pressure) in a combustor. This hot gas is what this motor is running on.
The inlet valve has the very thick stem because the inlet gas is at quite some pressure. This pressure produces a force acting to push the valve open that is proportional to the area of the back of the valve minus the cross sectional area of the valve stem so a fat stem much reduces the required valve spring strength.
Have a google of air motor and hydraulic motor images for some vaguely similar mechanics.
