The invention relates to an expansion machine having a shaft sealing ring and a valve. The expansion machine can be used for example for the waste heat utilization of an internal combustion engine.
Expansion machines having a shaft sealing ring for sealing a working medium which flows in the expansion machine are known from the prior art, for example from unexamined German application DE 10 2012 222 010 A1. The expansion machine according to the invention comprises a gear with an output shaft and a shaft sealing ring which interacts with the output shaft. The expansion machine has an inflow region and an outflow region and during operation is exposed to a throughflow of a working medium, wherein compressed working medium flows into the inflow region and expanded working medium flows out of the outflow region. The shaft sealing ring separates a gear space or valve space filled with working medium from an ambient space or from an additional machine, for example from a generator.
Due to the principle of operation of a shaft sealing ring, it is always advantageous if a greater pressure prevails on one side of the shaft sealing ring than on the opposite side. This pressure difference presses a sealing lip of the shaft sealing ring onto the output shaft and therefore seals the two sides against each other. Specifically, the discharge of working medium from the expansion machine is prevented as a consequence.
At many operating points, the expansion machine is operated at positive pressure, that is to say the expanded working medium has a pressure which lies above atmospheric pressure. The sealing lip is customarily then arranged in relation to the output shaft in such a way that a positive pressure in the expansion machine in relation to the ambient space, which is under atmospheric pressure, is reliably sealed.
If the expansion machine should now to be operated at negative pressure with this shaft sealing ring arranged in such a way that the expanded working medium would have a lower pressure compared with atmospheric pressure, then the higher pressure level of the ambient space would lift the sealing lip from the output shaft, and the sealing effect of the shaft sealing ring would be lost. Therefore, leakage of working medium into the ambient space would occur.