its my understanding that piston rings create the biggest drag
inside an engine,
so to reduce this friction,I was wondering if it
would be possible to make pistons without oil rings?

what type of material would be required for such pistons and block to make it tight enough to prevent oil blow by?

Pistons are made without rings.

The rings are a seperate item fitted to the piston before assembly.

For an engine to run without rings, the piston would need to be virtually size for size with the bore and remain that way during operation.

If the fit was good enough to have the appropriate level of seal for oil and combustion gasses, the friction would be at least as high as the seperate devices (rings)/ball valve designed to specifically to optimise this seal.

Even with an identical co-efficient of expansion, the piston temperature will vary at a different rate to the block, and different parts of the piston and block will have different relative temperatures, so they will expand differently and lose the seal or seize.

The internal combustion engine as we know it today is highly developed and manufactures are under constant pressure to reduce costs while improving fuel efficiency, emission performance, power, drive ability and durability.

If the cost of fitting rings could be left out without decreasing some other aspects, it would be done.

Well....

The piston would need to have a coefficent of thermal expansion which was in keeping of that of the bore liners. If not massive friction forces or blow-by/lack of compression would ensue as the engine warmed up.

However, if the above could be controlled the piston material would need also need to have a very low coeeficient of friction to render the use of rings obsolete.

On the whole it is, at present, an unfeasible idea.

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