Stacked Bathtubs

There is friction when piping any fluid through passages. If everything else is equal, then the length of the path is a factor. May be minor, but still a factor. In the case of the bathtubs, the zig-zag path would be longer. It may amount to about a 0.01% increase in pressure drop in that case. For the piston rings, the passage length would be a vastly bigger factor.

As far as the rings rotating, there may be a wear pattern that holds the rings at a prefered orientation. Also, the end effect is probably insignificant compared to other factors that affect engine performance. Out of my knowledge level past that point.

 

I was taught, in a three ring piston, to position the ring gaps 120* apart, not 180. That way all three gaps would be equidistant apart.

As for the bathtub analogy, the stacked bath tub would act as one large tank, with (with the exception of a small degree of pipe friction) the pressure being equal in the tank if it were pressurized from an outside source.

If un-pressurized, the pressure in the bottom tub would be higher by the factor of the height of the top of the water column in the top tub. (Head)

Icarus

 

What's a ring gap? I thought the rings were supposed to make a seal so the gas in the cylinder would not leak out.

 

Gravity works whether the water is pressurized or not, so the lower tub has a greater head either way.

Offsetting this is the flow loss, which is dependent on the geometry of the tubs, the flow rate, and the viscosity of the liquid. It is impossible to answer this question without knowing the details.

What we do know is that separating the points of leakage will be no worse than having them aligned, and the greater the distance between leakage points the greater the possible resistance to flow. In other words, putting the gaps farther apart generally helps and it can't hurt.

As for 180° vs 120°, 180° would produce less leakage if rings were non-deformable, but of course they aren't. The 120° spacing keeps the weak spots from stacking up one on top of the other, so there is less deformation under load.

Tom

 

They are, but unfortunately physics gets in the way. Ideally the rings would just be part of the piston, but in practice that would be too difficult to produce. So instead rings are snapped into groves around the piston. The rings need to expand in order to snap them around the piston, and they need some play in order to stay in contact with the cylinder wall. This is done by making rings discontinuous: they are cut so that they can expand a bit. The cut is called a "ring gap".

To keep the ring gaps from leaking a lot, several rings are stacked together with unaligned gaps. Under pressure, the rings compress together and form a seal, but they are still free to slide about a bit and adjust for expansion and wear.

Tom

 

As the engine heats up, the rings expand reducing the gap. If you start with too small a gap, the gap goes to zero and as the rings continue to expand, the ring pressure on the cylinder wall gets to be too high and things start wearing faster, then fail completely.

There are "gapless" piston rings.
http://www.racetep.com/totalsealframe.html
but they aren't widely used

 

Rings are not usually stacked together, they sit in individual groves within the piston the exception is modern oil control rings that are built of three or four separate pieces for greater flexibility.

 

As a Private Pilot and an Airframe & Powerplant Mechanic - each certified by the FAA - my first concern is the aircraft and the precious souls on board (sob's) Still, we can use the stacked bathtubs in this discussion. How do I begin? Let’s number the bathtubs 1 thru 4, from top to bottom.

Flow between the levels is directly dependant on the difference of the pressures between the levels (delta P). Also, in the case of water in a bathtub, gravity plays a most significant roll in causing flow. If you prefer to do this experiment using the effects of pressurized water in a closed system and ignoring gravity, the system could be oriented in any direction with no regard to “up & down”. Let’s stick with gravity for a minute.

If you put a stopper in the bottom tub and begin to fill the system from the top, flow will cease (or can cease) when all of the tubs are full, and there will be no delta P between the levels (ignoring head pressure).

When you pull the stopper from the bottom tub, water will begin to accelerate and flow out of the hole in tub #4. Water leaving the lower tub then begins to affect a pressure decrease in the lower tub. Only when the pressure has lowered significantly in the top of tub #4, will water begin to flow into it from tub #3. Only then is when static pressure in tub #3 will begin to decrease. Equally, the pressure decrease at the top of tub #3 begins to allow flow from tub #2. Following this thru, it can be said that the pressure decrease will “travel” from where the stopper was pulled, toward the top-most tub, and when there is a pressure decrease below the hole in tub #1, water will begin to flow from tub #1.

My point is that none of this is instantaneous and that flow rates and pressure changes have a nature of cause and effect over time. When there is no flow, resistance to flow is at maximum (the stopper), and all pressures will equalize even through the smallest orifices. When the stopper is removed, resistance is lowered at that specific location, and the water begins to accelerate toward that location. As flow increases, resistance through an orifice increases to a point that flow stabilizes based on Pressure/Resistance.

In the piston ring scenario, there are also chambers involved (or volumes). Tub #1 can be the volume of the cylinder above the piston – including down to the top of the upper compression ring. #2 is the volume between the top and the second ring, and etc., ending with a numbered volume for the crankcase.

In an internal combustion engine (ICE), these volumes are much smaller than bathtubs, and the time issues are very much shorter. Also, the pressures and temperatures involved are very much higher, and the effects of gravity are insignificant. (Temperatures affect viscosities of the gasses, among other things)

In an ICE turning 3600 rpm, the rise time of a piston from bottom-dead-center to top-dead-center is 0.00833 seconds (8-1/3 milliseconds). In addition, compression pressures can be in the neighborhood of 200 psi. After ignition, pressure in the cylinder on the power stroke can exceed 1000psi. As the flow of the hot gas is slowed by, but begins to leak past the top piston ring, the pressure in chamber #2 below the top ring begins to rise. Only when the pressure in volume #2 rises significantly, gases will begin to leak past ring #2, which then begins to pressurize volume #3. Now I can say etc., etc., etc.

The speed of the flow between volumes is bases on many factors. Important to this discussion however, is the location of the leaks (ring gaps). In its most simple form, before flow can begin through a leak, the “pressure front” (if you will) must travel from the location of the high pressure to the location of the low pressure (drain hole or ring gap). Based of this, I suggest that location of the flow into a volume and the location of the flow out of a volume to be as separated as possible. Another important factor of staggered ring gaps is the more even distribution of heat (you DON'T want to burn a piston in flight!).

As for your expensive Continental or Lycoming power plant, I would overhaul it exactly as documented in the overhaul manual (for many reasons!). I find that when speculating about improvements conjured up in my spare time, it is far too easy to overlook important, significant factors and data. In this case, the consequences could be fatal.

My three cents worth!

Bill

 

A side note to the above.

In electricity 101, we see that the flow of elctrons is directly related to the pressure upon them and inversely related to the resistance of the system, where Volts = Pressure, Loads = Resistance, and Aperage = Flow.

 

As a Private Pilot and an Airframe & Powerplant Mechanic - each certified by the FAA - my first concern is the aircraft and the precious souls on board (sob's) Still, we can use the stacked bathtubs in this discussion. How do I begin? Let’s number the bathtubs 1 thru 4, from top to bottom.

Flow between the levels is directly dependant on the difference of the pressures between the levels (delta P). Also, in the case of water in a bathtub, gravity plays a most significant roll in causing flow. If you prefer to do this experiment using the effects of pressurized water in a closed system and ignoring gravity, the system could be oriented in any direction with no regard to “up & downâ€. Let’s stick with gravity for a minute.

If you put a stopper in the bottom tub and begin to fill the system from the top, flow will cease (or can cease) when all of the tubs are full, and there will be no delta P between the levels (ignoring head pressure).

When you pull the stopper from the bottom tub, water will begin to accelerate and flow out of the hole in tub #4. Water leaving the lower tub then begins to affect a pressure decrease in the lower tub. Only when the pressure has lowered significantly in the top of tub #4, will water begin to flow into it from tub #3. Only then is when static pressure in tub #3 will begin to decrease. Equally, the pressure decrease at the top of tub #3 begins to allow flow from tub #2. Following this thru, it can be said that the pressure decrease will “travel†from where the stopper was pulled, toward the top-most tub, and when there is a pressure decrease below the hole in tub #1, water will begin to flow from tub #1.

My point is that none of this is instantaneous and that flow rates and pressure changes have a nature of cause and effect over time. When there is no flow, resistance to flow is at maximum (the stopper), and all pressures will equalize even through the smallest orifices. When the stopper is removed, resistance is lowered at that specific location, and the water begins to accelerate toward that location. As flow increases, resistance through an orifice increases to a point that flow stabilizes based on Pressure/Resistance.

In the piston ring scenario, there are also chambers involved (or volumes). Tub #1 can be the volume of the cylinder above the piston – including down to the top of the upper compression ring. #2 is the volume between the top and the second ring, and etc., ending with a numbered volume for the crankcase.

In an internal combustion engine (ICE), these volumes are much smaller than bathtubs, and the time issues are very much shorter. Also, the pressures and temperatures involved are very much higher, and the effects of gravity are insignificant. (Temperatures affect viscosities of the gasses, among other things)

In an ICE turning 3600 rpm, the rise time of a piston from bottom-dead-center to top-dead-center is 0.00833 seconds (8-1/3 milliseconds). In addition, compression pressures can be in the neighborhood of 200 psi. After ignition, pressure in the cylinder on the power stroke can exceed 1000psi. As the flow of the hot gas is slowed by, but begins to leak past the top piston ring, the pressure in chamber #2 below the top ring begins to rise. Only when the pressure in volume #2 rises significantly, gases will begin to leak past ring #2, which then begins to pressurize volume #3. Now I can say etc., etc., etc.

The speed of the flow between volumes is bases on many factors. Important to this discussion however, is the location of the leaks (ring gaps). In its most simple form, before flow can begin through a leak, the “pressure front†(if you will) must travel from the location of the high pressure to the location of the low pressure (drain hole or ring gap). Based of this, I suggest that location of the flow into a volume and the location of the flow out of a volume to be as separated as possible. Another important factor of staggered ring gaps is the more even distribution of heat (you DON'T want to burn a piston in flight!).

As for your expensive Continental or Lycoming power plant, I would overhaul it exactly as documented in the overhaul manual (for many reasons!). I find that when speculating about improvements conjured up in my spare time, it is far too easy to overlook important, significant factors and data. In this case, the consequences could be fatal.

My three cents worth!

Bill

 

"""""""Someday when I have a few hours with nothing better to do I'll oil up an old piston and rings and an old cylinder, bolt the cylinder to a junk crankcase with the gaps set at various orientations (and block the piston at its TDC location), and do differential compression checks at each orientation, to see just how much gap alignment DOES affect the leakage rate. My guess is it'll be so small I won't be able to read it."""""""


If I may, Mr Airportkid, these are not the conditions that exist during takeoff or cruise. Measuring leak rates above a static (not moving) piston is very unlike the pressure losses that occur in mere milliseconds in an ICE at RPM. As I stated, the pressure front takes time to propagate - the shorter the time, the shorter the distance of propagation, etc.

Furthermore, things happen in an operating ICE that do not occur on the bench; for example – ring slap, where the rings tend to move up and down in their piston slot, and rings experiencing a changing cylinder diameter AND/OR radius.

In addition, I suggest doing your experiments, testing, and data collection using quality engine parts that replicate the aircraft quality and tolerance standards. “ ….an old piston and rings and an old cylinder, bolt the cylinder to a junk crankcase, etc., and “(and block the piston at its TDC location)†does not provide the test data that you desire. In that case, certainly it will leak, and certainly the rings will migrate (would migrate in an operating ICE). They also will not be well seated or operated at temperature, and they may even break when you move them.

Do your testing with new parts in a freshly prepared cylinder. Otherwise, your data will be meaningless.

I could go on & on but again I say – overhaul the engine as stated in the manual.

 

Mr. Bill, your description is generally correct, but you error when you say that "only when the pressure has lowered significantly...will the water begin to flow". The water will begin to flow from tub 3 to tub 4 as soon as the pressure drops in tub 4. The modifier "significantly" is incorrect.

If we take into account the viscosity of water there will be a very small delta P before the water starts to flow, but it won't be significant, and certainly it is less than the effects of the head that we are ignoring in this example.

It's not the proximity, it's the difficulty involved in getting to the outlet hole. Using your marble example, if the two holes are directly connected, marbles flow freely. If the two holes are connected by a soda straw, marbles flow only if they are smaller than the diameter of the straw. Note that proximity is not a factor.

Those are the two extremes. Now let's assume that the two holes are connected by a rubber tube. The marbles can squeeze down the tube, but it takes a lot of effort. The marbles will not freely flow out of the exit hole, but some can be squeezed out with effort. The longer the rubber tube, the more effort required to squeeze down it. It's not the proximity of the holes, but the effort required to make the trip.

If you stack two rings with the gaps apposed, it takes a lot of force to push oil molecules through the small contact space between the stacked rings. The longer the distance, the more effort.

If you separate the two rings by a significant space, the orientation of the gaps will make no difference. The oil will freely flow through the open space.

Tom

 

Originally Posted by MrBillTulsa
<snip>

When you pull the stopper from the bottom tub, water will begin to accelerate and flow out of the hole in tub #4. Water leaving the lower tub then begins to affect a pressure decrease in the lower tub. Only when the pressure has lowered significantly in the top of tub #4, will water begin to flow into it from tub #3.

<snip>


Mr. Bill, your description is generally correct, but you error when you say that "only when the pressure has lowered significantly...will the water begin to flow". The water will begin to flow from tub 3 to tub 4 as soon as the pressure drops in tub 4. The modifier "significantly" is incorrect.

Qbee42

For the most part, I agree. The word “significantly” is probably misleading due to the density of the water medium. Possibly the word “sufficiently” is more appropriate. I also agree that the delta P required to cause flow is not very much (a change of pressure in water equalizes quickly to all areas, due to the density of the water @ STP).

I contend that the “delta P front” travels distance over time. Even if it travels quite fast, it’s not instantaneous. Also, the propagation of the dP front is directly related to the density of the medium.

 

The bathtubs are a poor analogy, for the reason that the drains are large enough that drag due to friction upon the water flowing through the system is minimal. All that really matters is the pressure and the smallest aperture in the system. Inside the piston, on the other hand, the gaps are presumably so small that fluid drag is the primary factor.

My suggestion is that you replace the ICE in your airplane with a much more reliable turbine engine, or better yet, an electric motor. Either way, you won't have to worry about piston rings and the positions of their gaps.

Caveat: I know nothing about engines except that they stink and pollute and they're dirty and greasy and complicated and they break down and leave you stranded on the railroad tracks or thousands of feet up in the air, unlike electric motors, which are reliable and they purr like kittens, and the FSM wants everybody to switch from gas to electric.

 

In fresh water at 25°C it travels at 1497 m/s. Nothing is instantaneous, short of quantum effects, but some things are fairly fast.

Tom

 

Oddball thought. Could lining up all the gaps on the piston rings start a scoring ridge on the side of the cylinder? There would be rubbing everywhere on the cylinder except where the ring gaps are aligned. (Makes the 120 degree answer make more sense.)