Earth sleeve

[davelx]

... similar, I imagine, to the reason why supersonic fighter jets (and, I imagine, Concorde in its day!) have after-burners, rather than double-size engines and/or twice as many engines.

Kind Regards, John

No need to imagine any longer!

 

[John D v2.0]

Actual on the point of low speeds, if you want a really good power to weight ratio y just miss out the cylinders completely and just have the turbo and the fuel burning. Then it's only efficient at a certain high power level but a lot lighter. That's why planes have them as the weight is more important than efficiency at a range of power levels.

 

[EFLImpudence]

I'm not sure I have been proved totally wrong in that the article Mike linked says things like can lead to detonation and they are a huge aid in tuning out the detonation so, presumably the turbo can provide a higher boost than without an intercooler - as must have been the case before them.

Modern engines have knock sensors (as well as all the others) and adjust the timing accordingly and so all things are balanced to provide the optimum performance.

The main benefit of the intercooler is, as I have said, increasing the density and, hence, the oxygen content of a specific volume of air.
This will slightly reduce the compression but obviously this lower compression will have been designed to be the right amount.



With regard to the query about the engine performance at low loads and speeds, some larger engine cars do indeed cut off half the cylinders when this is the case - thus halving the size of the engine when not needed.

 

[John D v2.0]

The intercooler will provide a bigger margin for avoiding dentation but it depends how the rest of the engine is designed. It wouldn't be the same for engines designed not to have an intercooler.
The knock sensor is more about protecting the engine from damage, it wouldn't actually keep things optimal as the physical parameters of the engine don't change.
Likewise for cutting off half the cylinders, this would be good to save on moving masses inside the engine but you're still dragging the while thing up and down hills with you.

 

[Mikefromlondon]

(Efl)I don't think you need to worry too much if being proved right or wrong, it is good to discuss things, as it helps everyone learn more or expand further their knowledge in more depth, I am sure my aim was not to prove you or anyone wrong, I have learned a few things myself as now I have a good explanation as to why an intercooler is necessary, whereas before this I thought all the work done by a compressor was being undone by cooling off the forced air, but various articles have said what you have said that colder air is denser, but this statement is relative, as i explained colder air on mountain tops is far less denser than at ground level, in a turbo engine it is a whole lot different issue. there are variables.

As we have now landed on subject of concord, and jet engines, one thing I have found hard to believe is the fact how a turbine engine compresses air to something like 25 to 1, and it completely amazes me and defies logic since all the blades on a turbine engine spin at the same rate, so how does it logically compress air, when truly speaking when the first set of blades move the surrounding air they simply pass that to second set of blades, which in theory should just allow the air to move to the next set of blades, and since the rear of a turbine engine is open ended, how does it manage to compress so much air, I could understand if the second set of blades were spinning at different speed or in an opposite phase such that air is forced to compress. I have always felt the need to understand why air can simply get compressed merely by number of sets of blades spinning at the same speed and in the same direction. logically I would not even have thought that this could work, it is like a chain of people (representing sets of blades) the first person passes an object to second and second to third and so forth, in the end the last person will only be passing out a single object and not two or twenty objects. Unless the design of a turbine engine is such that the first person is able to pass more quickly than the last person in the chain is slower than all, so this causes a bottle neck , so could the first set push in more air than others sets not able to handle the incoming volume of air and eventually the last set of blades pass out limited amount of air under great compression, I would love to hear from anyone able to explain logically how air can be compressed by means of blades all revolving at the same speed on the same shaft..

 

[Nozzle]

As we have now landed on subject of concord, and jet engines, one thing I have found hard to believe is the fact how a turbine engine compresses air to something like 25 to 1, and it completely amazes me and defies logic since all the blades on a turbine engine spin at the same rate, so how does it logically compress air, when truly speaking when the first set of blades move the surrounding air they simply pass that to second set of blades, which in theory should just allow the air to move to the next set of blades, and since the rear of a turbine engine is open ended, how does it manage to compress so much air, I could understand if the second set of blades were spinning at different speed or in an opposite phase such that air is forced to compress. I have always felt the need to understand why air can simply get compressed merely by number of sets of blades spinning at the same speed and in the same direction. logically I would not even have thought that this could work, it is like a chain of people (representing sets of blades) the first person passes an object to second and second to third and so forth, in the end the last person will only be passing out a single object and not two or twenty objects. Unless the design of a turbine engine is such that the first person is able to pass more quickly than the last person in the chain is slower than all, so this causes a bottle neck , so could the first set push in more air than others sets not able to handle the incoming volume of air and eventually the last set of blades pass out limited amount of air under great compression, I would love to hear from anyone able to explain logically how air can be compressed by means of blades all revolving at the same speed on the same shaft..

Because the blades get smaller (shorter), and thus the casing gets smaller. The opposite happens in a steam turbine where the steam hirst gets shoved through the smallest turbine and then through increasingly larger blades to get more work out.

What will really blow your mind about turbine blade tech (in a jet engine) is how the melting point of the steel turbine blade is lower than the temperature of the combustion products... and yet they don't soften and stretch or come in contact with the outer casing at such high rotational velocity

Nozzle

 

[John D v2.0]

Mike like in your chain of people, it's still the same amount of air, just the pressure is higher. Just imagine your blokes are standing on a flight of stairs, so each one is increasing the height more and more but the same number of bags.

 

[Mikefromlondon]

Because the blades get smaller (shorter), and thus the casing gets smaller. The opposite happens in a steam turbine where the steam hirst gets shoved through the smallest turbine and then through increasingly larger blades to get more work out.

What will really blow your mind about turbine blade tech (in a jet engine) is how the melting point of the steel turbine blade is lower than the temperature of the combustion products... and yet they don't soften and stretch or come in contact with the outer casing at such high rotational velocity

Nozzle

Many thanks, I see the logic if the end gets tapered and other blades sets have different number of blades and angles etc, so this is where the trick lies.

 

[JohnW2]

I'm not sure I have been proved totally wrong in that the article Mike linked...

I've not necessarily suggested that you have!

.... says things like can lead to detonation and they are a huge aid in tuning out the detonation so, presumably the turbo can provide a higher boost than without an intercooler - as must have been the case before them. ... Modern engines have knock sensors (as well as all the others) and adjust the timing accordingly and so all things are balanced to provide the optimum performance.

The systems certainly do their best. However, if detonation occurs (due to temperature and pressure) significantly before TDC, then no amount of 'tuning' (whether of timing or whatever) can do much, if anything about. Indeed, ignition timing (and, indeed, the spark itself) is obviously totally irrelevant if detonation occurs before the spark.

The main benefit of the intercooler is, as I have said, increasing the density and, hence, the oxygen content of a specific volume of air. This will slightly reduce the compression but obviously this lower compression will have been designed to be the right amount.

I would certainly question whether that is the "main" benefit, but it may be a factor.

One of my difficulties is that your assertion that cooling the air will result in an increase in density (hence amount of oxygen in a given volume) is far from a universal truth. Rather, it depends upon the conditions. If one cools air at constant volume (i.e. if it is contained in a sealed container), the mass of air (or oxygen) within that fixed volume, hence the density, will obviously remain unchanged no matter what one does to the temperature. Only if volume is allowed to change can a change in density possibly occur. ... which brings me back to what I keep saying - that everything depends upon whether the conditions within an intercooler are closer to constant pressure, closer to constant volume, or what ... and I just don't know the answer to that.

With regard to the query about the engine performance at low loads and speeds, some larger engine cars do indeed cut off half the cylinders when this is the case - thus halving the size of the engine when not needed.

That makes sense, although it's the first time I've heard of it being done. However, given that this issue arose in relation to power-to-weight ratios, one thing those cars cannot do is 'switch off' any of the engine wight at low speeds/loads

Kind Regards, John

 

[JohnW2]

No need to imagine any longer! ... <Concorde take-off>

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

 

[John D v2.0]

John w2, I know some engines eg gm twinport use a solenoid to stop some of the valves opening at low power. This is supposed to improve the swirl in the cylinder with low air velocity.

 

[EFLImpudence]

If one cools air at constant volume (i.e. if it is contained in a sealed container), the mass of air (or oxygen) within that fixed volume, hence the density, will obviously remain unchanged no matter what one does to the temperature. Only if volume is allowed to change can a change in density possibly occur.

Is that what you are not grasping?

Are you forgetting the pressure?

In reverse:
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?

 

[securespark]

From earth sleeving to Concorde in 6 easy pages!

 

[Nozzle]

Introducing liquids to "gas equations problems" is not going to help the mental model! I think that issue is about vapour pressure, and the the butane flashes off to gas phase in doing so it increases the pressure in the closed space - the increase in pressure then stops further liquid flashing off to gas. If you drop the pressure, by letting some out, there is further flashing off of the liquid propane in opposition to the drop in pressure.

Nozzle

 

[EFLImpudence]

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