Earth sleeve

[JohnW2]

I think you both are missing the point that, as well as the compression, what is important is the oxygen content.

I'm not missing anything - for a given composition of air, those two things are effectively the same, both conceptually and numerically.

If one compresses air X-fold (from atmospheric pressure), then a given volume (e.g. the volume of the cylinder of an engine) will contain X times the mass of air that it would at atmospheric pressure - so, for a given concentration of oxygen (say the usual ~21%), the amount of oxygen delivered to the cylinder will also increase X-fold.

Kind Regards, John

 

[Mikefromlondon]

I think you both are missing the point that, as well as the compression, what is important is the oxygen content.

Air is 29% oxygen at all temperatures but - if because of expansion due to the temperature - the air has expanded to, say, twice the volume then actual amount of oxygen, per unit of volume, will be halved.

Look at climbing a mountain - the air at the top is still 29% oxygen but you will find it difficult to breath as the air is a lot thinner (less pressure - it has expanded) and so contains an actual lesser amount of oxygen per lung-full than at ground level (more pressure - it is compacted).

That makes perfect sense, yes, as air molecules expand, they occupy more space, so in other words say you can pack 1000 air molecules in a 1 cubic cm, then as it heats up, it expands, so now it may only house 500 molecules, leaving us short changed. and because there are now only 500 molecules so that 1 cubic Cm cube now weighs half as much! Hot air balloon!

Indeed in a car engine if there is less oxygen atoms, it would restrict good combustion to yield maximum power from fuel which needs oxygen to burn thoroughly, or you would just be wasting unburnt fuel, and not getting the power you intended, so cooling it down makes it denser, and packs more oxygen atoms, so you get more lean burn and get max power out.

But, wait a minute, I thought the whole idea of a compressor (turbo) was to stuff more oxygen atoms in that 1 cm cube, so instead of packing 1000 at normal air pressure ground level, we can may be force 6000 oxygen atoms, irrespective of the temperature, so i am still confused and I am going round in loops!

let us assume that in a normal aspirated engine, at a certain RPM, it draws 10,000 atoms of oxygen in a single downward stroke, along with other atoms consisting of other gasses present in the air, it compresses that on its upward stroke, which basically means there are still only 10,000 oxygen atoms give and take 1 or two that may escape past piston rings and through dodgy valve seats, and a precise measured amount of fuel is also injected (stoichiometric )and a spark ignites the mixture . A combustion takes place, which burns the fuel releasing stored energy and as well as depleting oxygen, converting it to other unwanted components such as carbon dioxide, carbon monoxide, and other harmful gasses such as nitrogen and water vapors, plus the heat, so the amount of energy released depends on the amount of fuel, as well as the amount of oxygen atoms (air) so it still leaves me questions why intercooler is needed if turbo is indeed a device that forces air into a cylinder, thus packing hundreds of thousands of oxygen atoms by force, so why use an intercooler when air entering cylinders is going to get hot any way, for the fact engine would be hot in itself, as well as further compressing the mixture would heat that already compressed air even more.

 

[JohnW2]

That makes perfect sense, yes, as ...

Indeed it does - but as I just wrote to EFLI, the degree of compression and the degree of increase of the amount of oxygen in a given volume (like an engine cylinder) are exactly the same thing .... e.g. compress the air to a quarter of its volume at atmospheric pressure and the compressed air will then contain 4 times as much oxygen as it would at atmospheric pressure.

Kind Regards, John

 

[EFLImpudence]

Yes 21% - my mistake.

I'm not missing anything - for a given composition of air, those two things are effectively the same, both conceptually and numerically.

No, it depends on the temperature.

Hence the intercooler.

 

[JohnW2]

Indeed in a car engine if there is less oxygen atoms, it would restrict good combustion to yield maximum power from fuel which needs oxygen to burn thoroughly, or you would just be wasting unburnt fuel, and not getting the power you intended, so cooling it down makes it denser, and packs more oxygen atoms, so you get more lean burn and get max power out.

That's part of it but, of course, when you turbo-charge an engine you do not usually just increase the amount of oxygen going into the cylinder - you will usually also be increasing the amount of fuel being put into the cylinder.

Kind Regards, John

 

[JohnW2]

No, it depends on the temperature.

If you have, say, 1000 mL of air at atmospheric pressure which contains X grams of oxygen and compress it to a volume of 250 mL, you will still have X grams of oxygen, regardless of considerations of temperature. If you feed that air to a cylinder of, say, 250 mL capacity, with uncompressed gas you will be delivering X/4 g of oxygen to the cylinder, but with the compressed gas you would be delivering X g of oxygen (four times as much) - again, regardless of considerations of temperature.

Temperature will affect the pressure of the gas (for a given volume), but cannot alter the amount of oxygen in the gas. If you took a sealed container containing X g of oxygen and heated it up, you presumably wouldn't expect the amount of oxygen in the container to ever change from X g, would you?!!

Kind Regards, John

 

[JohnW2]

... it still leaves me questions why intercooler is needed if turbo is indeed a device that forces air into a cylinder, thus packing hundreds of thousands of oxygen atoms by force, so why use an intercooler when air entering cylinders is going to get hot any way, for the fact engine would be hot in itself, as well as further compressing the mixture would heat that already compressed air even more.

I think you may be under-estimating the extent of the temperature change during adiabatic compression (probably coupled with heating by the exhaust gases). The temperature can easily rise to fairly close to, or even above, the ignition point of petrol, so that, without the intercooler, there could be ignition/'detonation' of the fuel (either 'immediately' or during the temp rise occurring during the compression stroke), leading at best to 'knocking', and at worst to damage to/destruction of the engine, with the petrol engine trying to be a diesel one! Similarly with diesel engines, with the risk of ignition/detonation long before the piston has completed it's compression stroke.

However, as I've said, in practice the intercooler would not be used to reduce the temp of the intake gases to anything like as low as ambient - possibly just low enough to avoid the problems described above. Having said that, I suspect, but do not know, that it would be difficult to optimise an engine for both 'cold' and 'very hot' input gases, so if one wanted it to be fairly efficient when not using the turbo (as well as when using it), maybe the gases are cooled more (when using turbo) than would be necessary simply to avoid premature ignition of the fuel.

Kind Regards, John

 

[EFLImpudence]

If you have, say, 1000 mL of air at atmospheric pressure which contains X grams of oxygen and compress it to a volume of 250 mL, you will still have X grams of oxygen, regardless of considerations of temperature.

Yes , you would, but the atmospheric pressure changes with temperature .
Normally aspirated engines go better when it's cold.

If you feed that air to a cylinder of, say, 250 mL capacity, with uncompressed gas you will be delivering X/4 g of oxygen to the cylinder, but with the compressed gas you would be delivering X g of oxygen (four times as much) - again, regardless of considerations of temperature.

BUT the actual amount of oxygen depends on the temperature.
If the air has expanded because of heat then its content of oxygen by volume will be less.
You cannot have differing pressures in the same system. If the turbo heats the air on compression then the 4x pressured side will not contain 4x the mass of air.

Temperature will affect the pressure of the gas (for a given volume), but cannot alter the amount of oxygen in the gas.

No, but if the volume increases because of heat then the amount of oxygen per cu.cm. will decrease.

If you took a sealed container containing X g of oxygen and heated it up, you presumably wouldn't expect the amount of oxygen in the container to ever change from X g, would you?!!

No, but that's not the same as a turbo and engine.

 

[Mikefromlondon]

John, Your last post makes more sense now. Thanks.

Volumetric efficiency the more dense the air is the more the cylinder can draw in.

However twin and earth with no id sleeving on the bare copper there is nothing to say that cable can't be used for other than earth bonding, but where it has the yellow/green covering then not permitted to over sleeve.

I think (as far as my understanding was conceived) volumeteric efficiency is to do with engine speed (RPM) since at higher RPM, piston going down can only draw a certain amount of air before it begins its upward journey, thereby not able to draw as much air in as it would when at lower RPM. This can be due to piston going down far more rapidly than air can enter through restricted ports, so some vacuum is also remains in a cylinder when it has reached BDC, or in other words a cylinder can pack 100% volume of air when engine is running at low RPM but at higher RPM only 80% as an example, this is where larger valves and polished ports helps to get more in to produce slightly higher power and torque at higher RPM, turbo boosting was thus introduced to overcome volumetric deficiency, so it pushes air into the cylinder rather than atmospheric pressure able to push air in through resricted ports.

I hope we are all learning something useful, by end of this thread we should be able to make our own gas boilers and service it ourselves! who needs an RGI! (LOL)

Oh and anyone who uses an earth wire in a T&E for carrying anything other than fault currents will not go down too nicely in my books!

 

[JohnW2]

Yes , you would, but the atmospheric pressure changes with temperature .

Did you really mean to type that? If so, what are you meaning? I don't think the relatively small variantions in atmospheric pressure and temperature are very relevant in comparison with the massive changes in pressure and temperature we are considering in relation to a turbo-charger.

If the air has expanded because of heat then its content of oxygen by volume will be less.

This is getting complicated. Needless to say, I don't disagree with what you've written, but what we are actually talking about is not "air has expanded because of heat" but (at least as the primary step) "temperature has increased because air has contracted (been compressed)". As far as I can see, it therefore all depends upon how the compressor actually works....

.... if we split the process into discrete 1-engine-cycle steps, then, for each engine cycle, we could put an appropriate volume of air into a sealed container whose volume could be changed (e.g. some sort of cylinder with a piston), compress it until its volume was equal to the volume required by the engine's cylinder(s) and then put that compressed (and hence heated) air into those cylinder(s). The amount of oxygen in that air fed into the engine would therefore be equal to the amount of oxygen which had been in the (much larger volume of) air prior to compression, regardless of any considerations of temperatures.

As with my previous example, if the air had been compressed by a factor of 4, then the amount of oxygen fed to the engine would be four times what it would have been if atmospheric pressure (and ambient temp) air had been used - again, regardless of considerations of temperatures. In the absence of any cooling, the pressure of air fed to the engine would be much more than 4 times ambient temp, because of the increase in temperature, but that doesn't alter what I've said - with the scenario I have described, the amount of oxygen fed to the engine would be the same regardless of how much or how little cooling one did.

What I don't know is how comparable the real-word 'continuous' compressor operation is with the 'discrete' process described above.

Kind Regards, John

 

[JohnW2]

Oh and anyone who uses an earth wire in a T&E for carrying anything other than fault currents will not go down too nicely in my books!

Nor mine - and, as I recently wrote, the regs would not be very impressed either - since it is not permissible to use a conductor which is, at best, 'single insulated' as a live conductor unless it is within conduit or suchlike.

Kind Regards, John

 

[EFLImpudence]

What, then, do you think the purpose of the intercooler is and what does it do?

 

[JohnW2]

What, then, do you think the purpose of the intercooler is and what does it do?

See what I wrote in my post #52 - as I understand it, its purpose is primarily to reduce the risk of premature ignition of fuel - either 'spontaneously' (if very hot) or (if not quite that hot) during the heating which occurs during the compression stroke of the engine.

Kind Regards, John

 

[EFLImpudence]

See what I wrote in my post #52 - as I understand it, its purpose is primarily to reduce the risk of premature ignition of fuel - either 'spontaneously' (if very hot) or (if not quite that hot) during the heating which occurs during the compression stroke of the engine.

No, it is not that. It is to increase the density of the air - to increase the amount of oxygen per volume unit.


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?

 

[Mikefromlondon]

As John said, intercooler cools down exteremly hot air being pushed out of the turbo (since it sits next to exhaust turbine as well as the action of compressing air also yields heat) therefore before it is forced into the cylinders at extremely higher temperature it may pre-ignite fuel air mixture that can cause detonation and knocks, which obviously is detrimental to engine components, so yes even if the turbo compresses enough amount of oxygen, but at afar greater temperature, it is desirable to lower its temperature so that pre-ignition is avoided, so going back to my original question why use an intercooler when cooling may lower its compression (pressure) but as we all discussed various aspects of it, I think John's explanation seems more like it. For now I think we have gained good bit of knowledge on engines and turbos. So having that bare earth wire explains it all.