Earth sleeve

[Nozzle]

Differences in temperature = work done! I've come into the debate a bit late I admit that. There were some interesting points earlier about charging, intercoolers, and CR etc. Something to consider is that if one takes an engine and wants to make it super or turbo charged, then a "decompression" plate is generally used. AKA a really thick head gasket, or dished pistons. I refer to work on the Old skool A series though, not modern engines. Said reduction in CR is to avoid pinking while still using the same octane fuel.

Nozzle

 

[JohnW2]

Is that what you are not grasping? Are you forgetting the pressure?

Of course not. If you cool a gas at constant volume the pressure is the thing (the only thing left ) that will change as a consequence. The mass or density of a 'captive' amount of a gas cannot possibly change if the volume doesn't change (in view of your next comment, I suppose I have to add ... "assuming all of the contents remains in the gas phase"!).

If you had the gas cylinder I mentioned earlier - or transparent cigarette lighter - full of liquid gas. Use half of it so that it is half full of liquid. Is the liquid not denser than the gas which fills the top half?

A situation in which the vessel contains two different phases of the substance, in equilibrium with each other, is totally different from anything we're talking about.

Kind Regards, John

 

[JohnW2]

...but John is maintaining you cannot have different densities - due to different temperatures - in the same system.

No I'm not. I'm saying that if you have a gas (and only gas) maintained in a fixed volume container, changing the temperature cannot possibly alter the (average) density of the gas.

If you heat just part of the gas then, yes, there will be a temporary situation (until all of the gas comes to be at the same temperature) in which the 'local' density will be different within different parts of the container, but the average density of the entire contents can never change.

As I wrote in my last post, introducing the concept of 2-phase systems can do nothing but totally confuse the issue!

Kind Regards, John

 

[EFLImpudence]

No I'm not. I'm saying that if you have a gas (and only gas) maintained in a fixed volume container, changing the temperature cannot possibly alter the (average) density of the gas.

Well, now you have introduced "average".

As you have done that, you must therefore agree that it need not be constant throughout the container, consequently some of the gas, perhaps because of a cooler temperature in one part of the container, must have a higher density than the rest.
So - might this higher density gas be, for example, after an intercooler?

 

[JohnW2]

Well, now you have introduced "average".

I did, but only because you introduced the concept of different parts within the 'container' being at different temperatures. I had previously been talking about cooling all of the gas in a the container.

As you have done that, you must therefore agree that it need not be constant throughout the container, consequently some of the gas, perhaps because of a cooler temperature in one part of the container, must have a higher density than the rest.

If there are different temperatures within the 'container' then, yes, of course, there will be different densities within the container, so long as the temperature differences persist.

So - might this higher density gas be, for example, after an intercooler?

You seem to be talking as if I'm denying that the density of the air will probably be higher after the cooler. I'm not, but I am saying that I very much doubt that it is the "main reason" for the cooling, since, if that were the case, one could achieve the same thing, and avoid the need for the cooler, by compressing (hence heating) the air less in the first place.

In any event, as I keep saying, neither you nor I know enough about what goes on within the system to really be able to predict exactly what will happen. Before one can talk about density changes, one needs to understand what happens to volume and pressure during the cooling, which will be dependent upon the design and characteristics of the cooler.

One also needs to know about the pressure changes/gradients which would exist throughout the system even if the cooler was not cooling. You seem to be assuming that pressure will be constant throughout the system, but, even without cooling, that's no more possible than that the voltage (relative to some reference) will be the same at every point in a cable carrying current - just as we have voltage drops and voltage gradients when current is flowing through a conductor which has resistance, so we have pressure drops and pressure gradients when a gas or liquid flows through something which has resistance. If, as will presumably be the case with the cooler, there is a temperature gradient along the air path, that would be analogous to a cable that had a 'CSA gradient' along it's length - which is an interesting additional complication!

Kind Regards, John

 

[Mikefromlondon]

I thought that we understood from the links I quoted in my previous posts, (Boyle's Law) that in a closed system, the number of molecules or atoms of a gas cannot increase or decrease as none can escape, and none can enter the closed container, therefore volume of the container remains constant, so no matter at what temperature the gas is heated or cooled, its density cannot change, only the pressure can, and pressure is created by freely bouncing molecules or atoms randomly striking each other and the containers walls , and is the result of kinetic energy being released that results in heat being produced, so that clearly means that density within that closed system cannot change irrespective of temperature, only the pressure can change to compensate change in temperature. So i am afraid I cannot now buy that cooling from an intercooler increases density of air in a turbo charged engine, it just can't do that as this is a kind of closed system like a container as the pressure fluctuates but air density does not, of course if you had a leak in the system then it is a whole lot different scenario, then the density may change.

I gave two examples of compression (Pressure) that can change density ( Increase with pressure and temperature) .

1. When you keep stuffing more and more air in a given constant volume of space
like when you are filling a gas canister. you are effectively increasing Density, which results in pressure going up, this creates heat, so temperature goes up.

If you Increase Density, pressure and temperature will go up volume remains constant

2. When you reduce the volume of the space in which fixed amount of gas is trapped so it gets compressed that way and density becomes high and also temperature increases.
If you decrease Volume, Density, pressure and temperature goes up and volume goes down.

Cooler air is denser, yes that is correct but it only applies under certain conditions like being in an open system, open to atmospheric pressure, and this is usually at ground level (sea level) higher up colder air is less denser.

 

[EFLImpudence]

I did, but only because you introduced the concept of different parts within the 'container' being at different temperatures. I had previously been talking about cooling all of the gas in a the container.

Why? That's not related to an intercooled engine.

I wrote on page 4:

Image a gas cylinder (tall thin type) containing compressed air at, say, 4 bar.
Now - significantly cool the bottom half.
The overall pressure would reduce slightly but still be constant throughout the cylinder.
However, the bottom half air would be denser and therefore contain more oxygen per cu.cm. than the top half.
Agreed?

So, who were you arguing with?

 

[Mikefromlondon]

Why? That's not related to an intercooled engine.

I wrote on page 4:


So, who were you arguing with?

Efl, your example is only valid under those conditions you described, where in your example the air is not flowing through, but in a turbo engine the air being kept under pressure (boost) is also flowing as the engine requires for its need once every half turn it needs to fill one of the 4 cylinders.

 

[Nozzle]

That's right, a dynamic system

I thought that we understood from the links I quoted in my previous posts, (Boyle's Law) that in a closed system, the numb...... <snip> for brevity

Some of the assumptions made in this post and in previous contributions to the thread are assuming that the fluids in the system are static, and read an equilibrium.. however in the induction system of our example engine the fluids are moving, work being done on them (compressing), further work being done on the water in the intercooler (it is heated by the hot air), then more work being done on it (compressing in the cylinder from the energy from an adjactent cylinder) and then expanding - giving work back to the system. Through all this process the fluids are on the move and as such things like brownian motion have to be considered to be dynamic and not actually reaching a steady state.

Nozzle

 

[davelx]

Did it really use after-burners to take off? I must say that I thought it only used them to accelerate to supersonic speed.

Kind Regards, John

Yes, according to this (very interesting) site: http://www.concordesst.com/powerplant.html

 

[JohnW2]

I thought that we understood from the links I quoted in my previous posts, (Boyle's Law) that in a closed system, the number of molecules or atoms of a gas cannot increase or decrease as none can escape, and none can enter the closed container, therefore volume of the container remains constant, so no matter at what temperature the gas is heated or cooled, its density cannot change, only the pressure can....

Indeed. That is a point I have repeated made to EFLI. However, as you say, it relates only to a simple 'closed system' (e.g. a sealed container), whereas we are talking about something much more complicated than that. We are talking about a system which has several interconnected components, each of which will have pressure and temperature gradients within then, and of a system in which flow and pressure (and hence, to some extent temperature) will be varying cyclically.

The cooler cannot be considered to be 'closed', since molecules can (and constantly do) move from it into the manifold, and also potentially backwards into the compressor under some circumstances. ... and also, of course, the volume of the entire system changes every time the engine's inlet valves open or close. Furthermore, as EFLI has pointed out, even when one does have a closed (constant volume) system, whilst the mass of the contents (i.e. number of molecules) cannot change, it is possible for the density to be different in different parts of the system, even though the average density of the entire contents obviously cannot change. It really is a very complex situation to analyse, particularly when there is a continuous (but not constant) flow of air through the entire system. The concept of a 'closed' system therefore does not really exist - it is more a question of pressures, flows and resistances.

To give a simple illustration of the absence of a 'closed' system, if, with the engine not running, one heated the air in the intercooler, the pressure rise of air in the cooler would be less than you would expect, and the density of air in the cooler would decrease, since some molecules of air would move from the cooler into the manifold and some would move backwards into the compressor. Of course, as above, when the compressor is running, there is a constant ('pulsatile') flow of air through the entire system, which further complicates analysis of the system.

However, none of this alters my serious doubts about the suggestion that increasing density is the "main reason" for having an intercooler.

Kind Regards, John

 

[JohnW2]

Why? That's not related to an intercooled engine.

I think I have to disagree with that. There will inevitably be a temperature gradient from input to output of an intercooler (otherwise it wouldn't be cooling!), so every point in the air path through the cooler will be at a different temperature.

Also, as I've said, since the elements of the cooler will present a finite resistance to the flow of air, there will also be a pressure gradient across the cooler (i.e. every point in the air path through the cooler will be at different pressures) - which further complicates things.

Kind Regards, John

 

[Nozzle]

However, none of this alters my serious doubts about the suggestion that increasing density is the "main reason" for having an intercooler.

The point of an intercooler is to combat the problem of having a turbo, that is to say have a charge temperature too hot. Your thinking is correct. The point of a turbo-charger or super charger is to increase the density, one wouldn't add a different gadget to do more-of-the-same.

Nozzle

 

[kentishman]

I never realised earth sleeving was so complex...

 

[JohnW2]

The point of an intercooler is to combat the problem of having a turbo, that is to say have a charge temperature too hot. Your thinking is correct. The point of a turbo-charger or super charger is to increase the density, one wouldn't add a different gadget to do more-of-the-same.

Thanks for confirming - that is, of course, what I've been suggesting all along.

The one issue that is interesting me, and with which I'm struggling a bit, relates to the conditions under which cooling in the intercooler takes place. If that cooling happened under the dynamic equivalent of 'constant volume' conditions then, as far as I can see (and as was suggested by Mike in his initial post on the subject), the reductions in both temperature and pressure resulting from that cooling would simply get one back to where one would have been if one had compressed the air less in the first place (which would make no sense).

I therefore can but presume that the actual conditions of the cooling are such that, for a given degree of cooling, the reduction in pressure is less than one would see under "the dynamic equivalent of constant volume conditions" - presumably specifically that there must be a 'reduction in effective volume'. Is that the case?

Kind Regards, John