A piston compression ring usually sits mid-piston, thusly:
_________ +-------- --+ _________
where it serves to make sure that the gap between piston and cylinder walls don't leak. The outward side of the ring is constantly forced into the cylinder wall from mechanical pressure, ie: the ring is actully too big for the cylinder. There's a gap in the ring to allow for heat expansion; through this some blowby escapes, partially stopped by the other rings which are similarly designed.
But a really really really tight seal is only needed during the combustion stroke.
Proposed is a compression seal which sits on top of the crown:
(______) ________
making it an open-topped design. Any time there's more pressure in the chamber than there is in the crankcase the sides are forced outwards. The greatest differential, thus the most effective seal, is during the combustion stroke. For the other strokes it takes a more laid-back approach: still sealing but not so ardently.
Other rings may still be extant but, since they don't have to assist in blowby mitigation, they don't need to be as tight.
Overall there's a friction reduction.-- FlyingToaster, Oct 04 2012 Won't the ring, as shown, tend to roll into the gap during the upstroke under pressure?-- MechE, Oct 04 2012 The graphics are for illustration. The ring is fixed to the piston, airtightly at the bottom of the ring, openly at the top.
The important bit is that the ring is fixed to the piston underneath where the ring contacts the cylinder.
It could be like this:
(| ¯ ¯ ¯ ¯ |) .| .|
with the top mostly filled in, as long as there's an air gap at the top of the ring but not at the bottom.-- FlyingToaster, Oct 04 2012 some quibbles:
The ring is now far tighter than it used to be for part of the cycle. benefits? not clear. But wear in that specific area will be dramatically increased and its already an area where wear is causes a problem with conventional rings. if we allow for a wider surface area maybe this isn't a problem.
Rings frequently shatter when exposed to detonation, whatever material you use must be able to resist this force.
Rings are insulated from the heat of combustion to preserve their metallurgical properties. In your idea the ring material is directly exposed to combustion. I'm pretty sure this is going to be a profound challenge.
the piston cannot expand as much as previous designs without allowing for a very large gap with the ring.
The combustion chamber clearance to the piston will have to increase or be radically modified reducing the desired "quench" or area of narrow clearance between piston and head.
The ring land and grove for this sort of ring looks like it would be hard to lubricate and might get very hot, a recipe for failure.
To the positive I think that this might have application in pistons of tremendous diameter, such as those found in boats, in these cases the thermal and lubrication issues may be moot.-- WcW, Oct 04 2012 Even if the ring is in a different location, you're going to be pushing the upper edge, hard, against the cylinder wall at the same time as you have an upstroke (during compression). That will tend to roll the lip of the ring. I'm not saying it's insoluble, but the tendency is there.-- MechE, Oct 04 2012 I've added an illustration to the previous anno: perhaps "cup on top of crown" was a poorly chosen example to use.
[WcW] The idea of the Idea is to have the ring forced into the wall by a high combustion chamber pressure, not by a mechanically fixed force that operates and fricts at all times.
Indeed the best place might be at the very bottom of the piston instead though that would complicate things immensely since you still need to get the pressure of the chamber inside the ring itself to force it outwards.
[MechE] there's an order-of-magnitude'ish difference in pressure between the compression stroke and the combustion stroke. Note that the "combustion stroke pressure" is what normal rings put out all the time, not just during the top half'ish of the combustion stroke when it's actually needed to mitigate blowby potential.-- FlyingToaster, Oct 04 2012 What [WcW] said about temperature and metallurgy.
What might be better is a two-part piston where the groove for the No.1 compression ring is a beveled slot between the upper and lower halves. When the piston crown is forced down on the power stroke, the ring is squeezed outward, improving the seal. Likewise on the compression stroke the downforce tightens the seal.
On the induction and exhaust strokes, friction is reduced as the piston crown moves away from the body, possibly by the action of a spring.-- 8th of 7, Oct 04 2012 I It seems to me that the ring would seal excessively well in the power stroke and poorly the rest of the time, unless it was under some tenstion, which imho would defeat the whole purpose.-- WcW, Oct 04 2012 ^ err, that actually is the purpose: sloppy, friction-free sealing when an extremely tight seal isn't required. The piston still fits within the cylinder with the smallest gap possible and lubricated, just that instead of a full-time compression ring there's a part-time one that only comes into play when needed.-- FlyingToaster, Oct 04 2012 If your seal is less effective during the compression stroke, is there a risk of unburnt fuel/air mixture leaking past the piston and accumulating in the crankcase?
Not that this would be any kind of problem, that is.-- Wrongfellow, Oct 05 2012 ^ in a carburetted engine perhaps ... I don't know how much an un-ringed piston/cylinder would leak normally at a maximum of say 25bar (CR of 10).-- FlyingToaster, Oct 05 2012 I thought this might be an idea for a mechanical device to enable The Queen to still wear her crown in the unfortunate event of a shrinkage of the head region.-- xenzag, Oct 05 2012 bit of natural selection that: the ears keep the crown from sliding downward.-- FlyingToaster, Oct 05 2012 I wonder whether anyone has put an ear excercising machine for royals with smaller heads than their predecessors on the HB? Unless [FT] is right that the royals are genetically predisposed to extremely large ear muscles...-- TomP, Oct 05 2012 Royalty naturally selects for thick neck muscles ("heavy is the head that . . ."), and either large consistently shaped heads, or protruding ears.
Sadly, the "pollex rex" mutation (2nd opposing thumb), that enabled a firm grasp on the scepter/mace, has been lost through the ages.-- FlyingToaster, Oct 10 2012 random, halfbakery