Abstract: Energy conservation apparatus for internal combustion engines in which a restriction device is used with a fluid supply associated with a heat exchanger which converts the fluid into a vapor prior to being drawn into the fuel-air mixing carburetor or injector, the apparatus utilizing the negative pressure from the engine for moving the fluid through the system at a rate related to the engine displacement. In the apparatus, the restriction device effects control over the fluid quantity being moved into the engine in a vapor state, and operates through the presence of a fine-bore tube of a fixed diameter in the passage between the fluid supply and the engine.