Viability of a liquid cooled cylinder via fuel?

Interesting idea. I think the fuel draw to the engine might be too small to allow a sufficient flow for cooling function (might actually boil the fuel). Also the fuel temperature change will change its density, with hotter fuel the mixture gets leaner. These are just guesses, you,ll never know until you try.

Some big diesels (like the ones used in tractors) use extra fuel flow to cool down parts of the cylinder head and fuel system, but that is achieved with a large capacity pump that circulates a vast amount of fuel from the tank to the system and then back to tank again. The fuel system takes the fuel to the engine from that flow. Back in my test engineer days I measured the fuel temp rise as the part of approval tests of many installations.

Idea: Water jacket could be used in a car with water too, if you can find a small enough radiator. Maybe just a small water tank made of brass, with a computer processor heat sink mounted on the side? No need for water pump if you build the system so that the entire radiator is above the cylinder water jacket, with the hoses going up all the way. The concept is known as thermosiphon, used in many stationary engines and some cars even, back in the day.