Non-Contacting Finger Seal Piston for Oil less Engines 2020-01-1096
The current quest to reduce CO2 emissions combined to new technologies has sparked an interest in revisiting radically different engine configuration concepts, such as adiabatic and split-cycle engines. For both of these concepts to achieve their full potential, the combustion chamber must then be sealed without lubricating oil. A promising approach, that has yet remained elusive, is to lubricate the piston-liner interface with gases. This paper explores the concept of using non-contacting finger seals, a sealing technology developed for gas turbines, to seal piston engines. The finger seals, made of a gas-lubricated pad at the end of a flexible beam, are fixed on a rotating piston that uses the centrifugal force to close the piston-liner gap. A physics-based fluid-structure model is developed to predict the position of the finger elements and sealing performances. The model shows that the radial displacement of the fingers naturally creates a convergent profile with the liner that generates sufficient aerodynamic pressure to prevent the piston to contact the liner. The results also show that the achievable leaking area could be as small than current ring gaps and friction losses would be negligible compared to that of conventional rings. On the other hand, the study also reveals challenges that will need to be addressed in order for finger seals to be practical in engines, such as the sensitivity to the initial gap and bore distortion as well as the manufacturing tolerances required to ensure contact-free operation.