Vacuum assembly of engines.

[Greg Locock]

A vacuum sticks things together because the external atmosphere pushes them together. Therefore the maximum pressure a vacuum assembly can exert is atmospheric pressure, no matter how good a vacuum you have.

Various approaches have been taken to getting rid of the cylinder head gasket, of which opposed piston designs are probably the most cumbersome. An all in one wet liner approach seems to me that it might have some merit, maybe.

[roon]

A vacuum sticks things together because the external atmosphere pushes them together. Therefore the maximum pressure a vacuum assembly can exert is atmospheric pressure, no matter how good a vacuum you have.

Various approaches have been taken to getting rid of the cylinder head gasket, of which opposed piston designs are probably the most cumbersome. An all in one wet liner approach seems to me that it might have some merit, maybe.

The liner being capped on one end? To provide the combustion chamber ceiling.

[Big Tea]

A vacuum sticks things together because the external atmosphere pushes them together. Therefore the maximum pressure a vacuum assembly can exert is atmospheric pressure, no matter how good a vacuum you have.

Various approaches have been taken to getting rid of the cylinder head gasket, of which opposed piston designs are probably the most cumbersome. An all in one wet liner approach seems to me that it might have some merit, maybe.

But do you not still need a head gasket for coolant, and then run into problems with differing expansion rates?

[Just_a_fan]

From my reading and understanding, the pulling force of a vacuum has surface area interaction.

Vacuum doesn't pull, pressure pushes.

[Greg Locock]

"The liner being capped on one end? To provide the combustion chamber ceiling." Yes, just a bucket basically with holes for the valves and plug.
"But do you not still need a head gasket for coolant, and then run into problems with differing expansion rates?" O rings.

I'm not saying it is a brilliantly thought out plan, just a feasible approach to getting rid of the hot high pressure part of the cylinder head gasket, and the head bolts.

Ha, I just remembered the Austin Rover K series engine design started life as a design with the crankcase split in two vertically. The liners and piston and crank assemblies were just dropped into one half then the other half was clamped to it. That's probably where I got it from. That idea survived less than 6 months.

[Rodak]

Hey Zynerji, since you're going to take your engine into space, why not take it to Jupiter? At the 'surface' of Jupiter your 1/2" stud would have a clamping force of about 115,000 lbs!!!

[Zynerji]

So, if I get this right...

Having an enormously low pressure vacuum like I've stated will only ever need 15lbs of pressure to separate the heads?

Doesn't it need to fill the vacuum during separation? Wouldn't the increased volume of the chamber necessitate a further lowering of the vacuum that is already present? Since the deepest man made vacuum is like 10^-14, couldn't this be used to easily break the world record? I mean, you would only need 15lbs of pressure to expand the chamber, regardless of current vacuum strength, according to you guys.

I wonder if NASA knows how easy this is? I'm sure the billions that they have spent trying to reach the 10^-22 Vacuum level of space could have easily made a few of these engines to break the record...

[NathanE]

I think it's great you are exploring this. It sounds like you are also getting into making stuff in a pretty serious way, I don't know of any folk who either personally or in a maker space have high vacuum metalic 3d printing capability so you're obviously putting lots of effort into tooling up.

It might be a good idea if you are designing parts that will have any kind of failure criticality to go back and redo (or depending on your age/background do first time round) at high school physics, a decent mechanics maths course, some materials science and maybe in time some kind of engineering study.

This will help you learn some principles of understanding how forces act on bodies, how those bodies respond to the application of forces statically and dynamically and how design principles are used to ensure that the things we build are capable of working without failure within the operating envelope they are likely to be exposed to.

It will also help you understand the difference between force and pressure.

[Just_a_fan]

So, if I get this right...

Having an enormously low pressure vacuum like I've stated will only ever need 15lbs of pressure to separate the heads?

Doesn't it need to fill the vacuum during separation? Wouldn't the increased volume of the chamber necessitate a further lowering of the vacuum that is already present? Since the deepest man made vacuum is like 10^-14, couldn't this be used to easily break the world record? I mean, you would only need 15lbs of pressure to expand the chamber, regardless of current vacuum strength, according to you guys.

I wonder if NASA knows how easy this is? I'm sure the billions that they have spent trying to reach the 10^-22 Vacuum level of space could have easily made a few of these engines to break the record...

There is no such thing as "enormously low pressure vacuum". Atmospheric pressure is approximately 14.7lb/sq in. Perfect vacuum is zero lb/sq in. Perfect vacuum doesn't really exist in nature unless you look at very small volumes -
even interstellar space has a few hydrogen atoms wandering about in it.

Therefore a square inch of vacuum stud can only generate a maximum clamping force of 14.7lb. This is why we have been saying that you'd need huge vacuum studs to make it hold together an engine.

Think about it: Your engine cylinder starts at a pressure of 14.7 lb/sq in (atmospheric pressure). The outside of the engine is at 14.7lb/sq in too so there no net force acting. You then compress that cylinder's contents by some ratio (the compression ratio of the engine) of, say, 10:1. The pressure in the cylinder is now 147 lb/sq in. That pressure is acting over the whole face of cylinder head that forms the top of the combustion chamber. If the cylinder has an area of 10 sq in, you now have a total force of 1470 lb trying to lift the cylinder head off the block (minus the 147lb from atmospheric pressure pushing the other way, of course). Your vacuum stud generates a clamping force of 14.7lb/sq in so you'd need 100sq in of vacuum stud to withstand the pressure. And that's before you add the pressure increase from burning the fuel to generate power.

As for your NASA reference: Think about how thin and light the construction of space craft is. That's because although they're in near vacuum conditions with lots of air inside, the pressure difference is still no more than 14.7lb/sq in. The ISS has standard atmospheric pressure inside, by the way, so the hull deals with 14.7lb/sq in.

[marmer]

Just to add the complexity of seals regarding pressure make it much more likely that a submarine could survive in space but the space station would struggle quite quickly in water submarines use lots of big bolts to help seal them not vacuums
Also the vibrations of an engine would surely make it very difficult to maintain a seal to keep a perfect vacuum

[Zynerji]

If a lapped stud is in a lapped hole, and the space left at the bottom of the hole is 1mm deep, and 12mm across and housing a high vacuum.

By pulling the stud out by 1 more mm, leaving a 2mm space, doesn't the vacuum pressure double?

Obviously, 1mm is far too much movement for a head gasket, and I appreciate the education of these latest posts over the mocking of some earlier ones.

A bit of math for syringe vacuum (which is almost identical to this use case)

More important than the force in this case is the change in volume inside the syringe. If we assume that we have an isothermal process then we can say that PV=P'V'. Since in a syringe it is only the length(l) of the cavity is changing we can rewrite this thus; P k l = P ′ k l ′ where k = π r 2 Pkl=P′kl′where k=πr2 P l = P ′ l ′ ⇒ Δ P = Δ l Pl=P′l′⇒ΔP=Δl So the change in pressure is dependant on how far back you pull the plunger, but you probably knew that anyway. However, we can also show that the amount of work you do pulling the plunger back is; w = c ln ( V V ′ ) w=cln⁡(VV′) Again, since only the legnth of the cavity is changing; w = c ln ( k l k l ′ ) w=cln⁡(klkl′) w = c ln ( l l ′ ) w=cln⁡(ll′) This shows that the amount of work you do in changing the pressure only depends on the distance you pull back the plunger and is independant of cross sectional area of the plunger.

Reference https://www.physicsforums.com/threads/a ... ge.169182/

Seems to me like the work necessary to separate is greater than applying 14.7 lbs of pressure.

[Rodak]

It's not mockery Zynerji, it's attempts to get you to understand what force you are dealing with. Vacuum is not a force, the pressure of the atmosphere pushing down is. If you remove all the gas under your stud, creating a perfect vacuum with a Torr of 10^-∞, the maximum force on the stud is atmospheric pressure. Put your stud setup in a vacuum chamber and pull a vacuum; there is now no force acting on the stud and it will just pull out. Where did the force holding it in come from? The weight of a column of air called the atmosphere, approximately 14.7 lb/in², 101.325 kPa at sea level. Vacuum is NOT a force, pressure of the air exerted on something under vacuum is. Torr is simply a measure of how much pressure there is while drawing a vacuum, so a very low Torr value is simply saying that your vacuum is approaching a limiting value, zero, which it will never achieve. Sure, set a 'world record' for Torr, but there still will be some gas left, and the pressure of the atmosphere pushing on your chamber or whatever will never exceed the pressure of the atmosphere. Please stop being obtuse.

[Just_a_fan]

By pulling the stud out by 1 more mm, leaving a 2mm space, doesn't the vacuum pressure double?

Seems to me like the work necessary to separate is greater than applying 14.7 lbs of pressure.

What? Double vacuum pressure? What does that mean?

Vacuum is zero. Zero pressure. You can't make a vacuum stronger by making it more "vacummy". Vacuum is the same as dark and silence. Dark is the total lack of light. Silence is the total lack of noise. You can't make dark darker or silence quieter and you can't make vacuum "vacuumier".

If I have a sealed container at atmospheric pressure then the container walls have zero force applied across them. If I double the volume of the container, the pressure inside (ignoring some errors caused by thermal input from the work done etc.) halves - this is Boyle's Law in practice. 14.7psi outside, 7.35psi inside. The walls are now subjected to 7.35psi inwards. If I keep doubling the volume, I keep halving the pressure inside. Eventually, I will get a container that is very much bigger than it was and has, approximately, a vacuum inside. The pressure inside is 0psi. The pressure outside is still 14.7psi. So the container walls are subjected to a pressure of 14.7psi. If I continue to increase the container's volume, the pressure inside remains at 0psi and the walls still feel 14.7psi across them. I can keep making the container bigger and this pressure will not change. This is what you are doing with your studs in holes. You are increasing the container's volume but you aren't increasing the pressure applied to it.

Of course, in my container example, at some point the container will buckle inwards because it will be so big that I can never make it stiff enough to resist the pressure over such a large area. And that's the point. To give a high clamping force, you either apply a large force over a small area (a cylinder head bolt does this) or a smaller force over a much larger area. The maximum force you can apply with vacuum studs is that provided by atmosphere at sea level. Thus you need a large surface area to give the required clamping force.

[Rodak]

Zynerji, there's force and there's work. Now you're doing work.

[Zynerji]

By pulling the stud out by 1 more mm, leaving a 2mm space, doesn't the vacuum pressure double?

Seems to me like the work necessary to separate is greater than applying 14.7 lbs of pressure.

What? Double vacuum pressure? What does that mean?

Vacuum is zero. Zero pressure. You can't make a vacuum stronger by making it more "vacummy". Vacuum is the same as dark and silence. Dark is the total lack of light. Silence is the total lack of noise. You can't make dark darker or silence quieter and you can't make vacuum "vacuumier".

If I have a sealed container at atmospheric pressure then the container walls have zero force applied across them. If I double the volume of the container, the pressure inside (ignoring some errors caused by thermal input from the work done etc.) halves - this is Boyle's Law in practice. 14.7psi outside, 7.35psi inside. The walls are now subjected to 7.35psi inwards. If I keep doubling the volume, I keep halving the pressure inside. Eventually, I will get a container that is very much bigger than it was and has, approximately, a vacuum inside. The pressure inside is 0psi. The pressure outside is still 14.7psi. So the container walls are subjected to a pressure of 14.7psi. If I continue to increase the container's volume, the pressure inside remains at 0psi and the walls still feel 14.7psi across them. I can keep making the container bigger and this pressure will not change. This is what you are doing with your studs in holes. You are increasing the container's volume but you aren't increasing the pressure applied to it.

Of course, in my container example, at some point the container will buckle inwards because it will be so big that I can never make it stiff enough to resist the pressure over such a large area. And that's the point. To give a high clamping force, you either apply a large force over a small area (a cylinder head bolt does this) or a smaller force over a much larger area. The maximum force you can apply with vacuum studs is that provided by atmosphere at sea level. Thus you need a large surface area to give the required clamping force.

So, you are saying that the pressure differential is never greater than atmospheric pressure?

Why does a vacuum have a strength measurement if it can't get "vacuumier"? What is the difference between 10^-6mbar vacuum and 10^-22mbar? I would expect increasing the volume of a container after it contains a vacuum of this magnitude would increase the negative exponent even more, requiring more work to overcome the resistance of increasing the volume. And I would expect that starting with an incredibly small volume would directly influence how quickly the rate of that exponent would change, as a 1/1000th of a mm gap would double in size very quickly as opposed to a 1mm gap.

You can convince me, and I appreciate your efforts in that direction. Thank you!