A standard fuel-injected engine system has a four-cycle internal-combustion engine with a fuel-supply manifold connected via injectors to the individual combustion chambers, a fuel tank holding a supply of gasoline, a fuel-supply line leading from the tank to the fuel-supply manifold, and a pump in the line. The tank is normally closed but vented for pressure-equalization purposes into the air-intake manifold of the engine. A return line runs from the fuel-supply manifold back to the tank and an overpressure valve in this return line prevents excess pressure from building up in the fuel manifold. The tank is normally at atmospheric pressure, although it may be somewhat pressurized.
The disadvantage of this system is that the gasoline is moved about and heated considerably. The tank itself is normally juxtaposed with components of the exhaust system, and both the fuel-supply and return lines run along the muffler and other exhaust-system parts so that the fuel in them is heated. In addition the pump itself generates heat and is itself often mounted on or in the fuel tank so that it also heats the fuel. This heat can break down the fuel by vaporizing off the more highly volatile components of the gasoline.