This invention relates to systems for evaporation cooling of internal combustion engines and operation of a heater by the coolant and, more particularly, to a new and improved system of this type. The principle of evaporation cooling of internal combustion engines, which is known in various embodiments described in the patent literature, offers the advantage of considerably greater efficiency than conventional liquid or convection cooling in that the heat transfer is determined by the heat of evaporation and, correspondingly, the heat of condensation.
Heating systems for vehicles usually utilize the waste heat of the internal combustion engine, which is carried away from the engine by the cooling water. In a typical vehicle heating system, the cooling water passes through one or more heat exchangers from which heat is removed by means of an air current generated by a fan. In some cases the internal combustion engine is cooled by evaporation cooling and the heat of the coolant is utilized to operate the vehicle heating system. For example, Published European Application No. 0 189 881 and German Offenlegungschrift No. 36 03 897 describe cooling systems operating according to the evaporation principle, wherein the coolant circulation chambers and conduits of the internal combustion engine are connected to a heating circuit containing a pump and a heat exchanger for heating the passenger compartment. The connection of this heating circuit, however, is arranged so that only coolant in the liquid state flows through the heat exchanger. Consequently the principal advantage of evaporation cooling, namely, the increased heat transfer due to the two-fold change of the physical condition of the coolant, is not utilized in the heating system. On the other hand, the conventional systems offer the advantage that, because the liquid coolant is supplied to the heating system from a location close to the point of heat generation, i.e., at the combustion chambers of the engine, heat for heating the automobile is available very soon after the engine has been started.
A further advantage of the conventional systems results from the fact that the heating circuit carries only a relatively small quantity of the cooling medium so that the removal of heat from the medium in the automobile heater delays the warming-up of the internal combustion engine to operating temperature only to a limited extent. In the conventional systems, the main coolant condenser becomes operative as soon as the engine temperature is high enough to produce evaporation of the cooling medium in the coolant conduits within the engine.
U.S. Pat. No. 4,367,699 also describes an internal combustion engine cooling system operating according to the evaporation principle in which the heat exchanger for the automobile heater is connected to a tank for separating vaporized and liquid coolant so that the heat exchanger for the heater receives coolant only in vapor form and is permanently connected in series with the main coolant condenser of the cooling system. In addition, the system described in that patent contains an especially "small" cooling circuit with a circulating pump which assures uniform warming-up of the internal combustion engine after it is started. When the coolant temperature is close to the boiling temperature, the "small" cooling circuit is interrupted by disconnection of the pump, and the evaporation cooling of the engine started.
Therefore, with respect to the operation of the heating heat exchanger itself, the arrangement described in U.S. Pat. No. 4,367,699 suffers from the fundamental disadvantage that the heating heat exchanger only receives coolant a relatively long time after the engine is started, i.e., after evaporation cooling has commenced. However, it is known that a fundamental necessity for the operation of automobiles is defogging the windshield and side windows by directing a heated air flow toward them immediately after the engine has been started, i.e, before driving has begun, and keeping them fog free. This basic requirement cannot be satisfied by the system described in U.S. Pat. No. 4,367,699.