Forgive me if this is a newbie question.  "Piston valves" were the preferred type in steam engines.  Why were they unsuitable for internal combustion?  Can modern materials make them usable?

I can think of several advantages of sliding piston valves:

1.  The valve can remain fully closed while the slider accelerates, allowing it to go from fully closed to fully open very quickly.  Same for the reverse.

2.  They should be much gentler on the cams and associated parts.

3.  One valve can be made to do both intake and exhaust.

Any comments?  Or direct me to an article, perhaps?* ball vlave

I believe the biggest problem would be lubrication, or the lack of it. Thermal expansion wouldn't help and it would be sad if the "valve" got hot quicker than its cylinder. I have an interesting four stroke model airplane engine that uses a rotary valve for both intake and exhaust, but the oil is in the fuel. And the oil carries away a lot of heat, and lubricates the valve shaft.

Well it would seem to me the problem with alternative valve trains is that you gain a significant amount of fictional area, with little corresponding gains in gas flow.

The drawback to piston-valve type systems would be complexity. For a piston-valve to flow adequate amounts of gas, we would have to assume a piston diameter of at least the width of the intake (or exhaust) port the valve was used to meter. This can be a very substantial size. Even if the piston were hollow, it would seem that some weight penalty could be expected. Furthermore, each piston-valve would have to ride within a cylinder that was provided with sufficient lubricating oil. So now we need to add oil squirters to the mix. Also the piston-valve would have to seal its own cylinder, so we need to add some form of oil control ring.

The total package would seem to have a total mass that exceeds a poppet type valve. The increased mass would limit the rate at which the proposed piston-valve could reciprocate. Although it was mentioned that the same valve could be used to meter both an intake and an exhaust port. The reduction in valve train speed this could afford may negate the negative effects of increased mass.

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