Can I use denatured alcohol to run a gas lawnmower without modifications or damage?
Just curious
Not a chance.
Not without some modifications.
You could have the jet in the carb drilled to the right diameter, and then it would work. The fuel consumption would go up by a third or thereabouts, however, and it would probably be harder to start when cold. A shot of starting fluid could overcome that. You might have to change a fuel hose, filter, carb gaskets, and maybe the tank. Denatured alcohol attacks some materials that handle a 10% ethanol / gas mix just fine.
Does denatured alcohol still cost more than gasoline?
Right, not a chance.
The correct air/fuel ratio for a gas engine is around 14.7 to 1 (Ideally, an engine pumps in about 14.7 pounds of air to burn each pound of gasoline.) Ethanol burns at a ratio more like 8 to 1 (Alcohol engines are more tolerant of changes in fuel ratio, but they usually run around here.) In other words, the engine’s carburetor would have to be modified to pass about twice as much ethanol to run well.
To give an example, an old friend used to run a 96 inch alcohol powered Harley (FWIW, Charlie had it set up for tractor pulls. The final drive included a jackshaft and the bike supposedly had a top speed of about 25mph.) The main jet (the main fuel passage) in most gas Harleys of that size is usually a little bit bigger than a pencil lead. The main jet in Charlie’s bike was the size of a small pencil.
In gallon containers yes it’s maybe $14/gal, though in bulk I don’t know.