In an internal combustion engine fuel system having a membrane separator for separating a primary fuel into a high octane fuel and a low octane fuel, and wherein the primary fuel is heated for separation in the membrane separator, the improvement comprising a heat pipe having an evaporator section positioned to be in the heat exchange relationship with exhaust gas from the internal combustion engine under conditions of use; and a heat output area in heat exchange relationship with a primary fuel as it is fed into the separator whereby the primary fuel is heated. In one embodiment the heat pipe is a variable conductance heat pipe having a top operating temperature not greater than about 160° C.