Jet Engines

[bishop99]

Hi, I hope ive posted this in the correct section. I have to write an A-level physics research project and i cant get my head around why a jet engine is more economical at high altitude, i understand that the air density is much lower but....:confused:

Could someone please explain this?

Thanks

[chornedsnorkack]

Could it actually have to so with a high true air speed at high altitudes?

[bishop99]

:confused: Could be? im not sure. I thought that a jet engine worked by pressurising the air intake thn igniting it, but if the pressure outside is lower then surely the pressure is lower in the engine so more fuel has to be added to keep up the forward thrust? :confused:

[flyer_spotter]

Well, jet engine performance is proportional to the density of the air, so when the air density decreases the performance will also be weaken.
That means if your plane is flying at very high altitude, there's actually decrease in drag so the aircraft actually travels faster although with lower engine performance.

hope this helps, not an expert myself...if needed, please correct me someone.:O

[HCB]

I haven't looked at this for a while, but my understanding is that a jet engine is not actually more efficient at high altitude: in fact the opposite applies. Gas turbines work best with dense air. In addition to the higher specific fuel consumption at high altitude, jet engines are also dramatically less powerful at high altitude. However, these effects are more than offset by the massively reduced drag of the aeroplane in thin air, thus aeroplanes as a whole are more efficient at high altitude.

[Gary Lager]

Reducing temperature improves the thermodynamic efficiency of the engine, and combined with a higher TAS for the same IAS as at lower altitude makes it a better bet to fly higher.

The density argument is self-cancelling - yes, the mass of air through the engine is reduced as a result of increasing altitude, but so is the drag on the airframe, so thrust required is less.

Even at different altitudes, at the same IAS the dynamic pressure on the aircraft is the same; which is equivalent to saying that for a fixed geometry intake mass flow of air is the same. Therefore if IAS is constant, reducing density doesn't come into it. What does improve with altitude is TAS and thermodynamic efficiency.

[chornedsnorkack]

The IAS logic says that if you take an airplane of given weight and wing area and, say, double its true air speed at a given density, the aerodynamic forces, both drag and lift, quadruple (and lift no longer equals weight). If you decrease density of air, both drag and lift decrease proportional to density. Thus, if you were to, say, double the speed of aircraft and simultaneously decrease the air density four times (typical to the cruise altitudes!), the drag and lift would be both unchanged.

This logic holds true at small Mach numbers, but breaks down on approaching the speed of sound.

Now, if you climb to cruise at constant IAS, the density of air, in that example, decreasing 4 times and the true airspeed only doubling, while the cross-section of engine inlet is unchanged - so the mass of air scooped up per unit time is decreased by half.

[HCB]

Gary Lager: surely the efficiency of an engine is not solely determined by its thermodynamic efficiency? As I said, it's a while since I looked into this, but a quick Google threw up the following links:

A Wikibook (I know, I know!) about jet propulsion (http://en.wikibooks.org/wiki/Jet_Propulsion/Mechanics#Specific_fuel_consumption) containing the following phrase: "A typical high bypass engine will consume about 8mg/Ns at maximum takeoff and 15mg/Ns at maximum cruise thrust."

A report (http://www.anirudh.net/seminar/html) about the GE90 where SFC is given as 15.600 mg/N-s in the cruise and 7.910 mg/N-s at TO. The engine would therefore appear to be almost twice as efficient at sea level compared to cruise altitude.

Clearly temperature is an important factor, but I don't think it offsets the density loss at altitude. Mach number would also play a role I think.

[G SXTY]

It’s been a while since my ATPL performance course (and it hurt my brain at the time), but in very simple layman’s terms:

Jet engines are most efficient at high power settings (say around 90-95% of maximum fan speed, or N1). For the best specific fuel consumption (i.e. miles per gallon) you would want to operate at an altitude where the aircraft maintains unaccelerated straight and level flight with the engines running at 90-95% N1.

Setting this sort of power at low altitudes (and therefore higher air density), the engines would produce so much thrust that the aircraft would accelerate way past its maximum operating speed (Vmo). As density reduces with altitude, so does thrust, so by climbing to high altitude (typically 33 – 39,000 feet depending on aircraft type, performance and weight) you reach a point where the thrust delivered at your most efficient engine speed is insufficient to climb or accelerate any further, but will just maintain unaccelerated straight and level flight at your optimum cruise speed or mach no. Of course, in the real world, weather, ATC restrictions and a whole host of other factors interfere to make the selection of cruise speed and altitude a bit of a compromise.

Hope that makes sense, but if not don’t worry - this being Pprune, someone much more knowledgeable than me will be along in a minute . . . ;)

[chornedsnorkack]

I think efficiency of an engine is defined by fuel spent for unit work - in case of an aircraft that faces roughly constant drag, by fuel spent per distance covered. Not by fuel spent per unit time!

This means that at slow speed, like on takeoff or hover, the efficiency of an engine is zero. It is not getting anywhere!

Now, a jet engine works by burning fuel and accelerating the fuel and a small amount of air to a great speed.

This means that jet engines are efficient at great true airspeeds - at airspeeds comparable but lower than those of jet blast. At lower speeds, much the energy of the jet is wasted.

Incidentally, it follows that jet engines are inefficient at all subsonic speeds. However, wings are much less efficient at supersonic than subsonic speeds... so the engine/wing combination at supersonic speeds is not very much better than at high subsonic speeds.

[Gary Lager]

There are many measures of a jet engine's 'efficiency' - thermal efficiency just relates to the ability of the engine to extract thermal energy from the fuel and minimise heat losses. This efficiency improves with decreasing ambient temperature. Consequence - fly higher where the air is colder.

Froude efficiency is related to the increase in velocity imparted to the air mass flowing through the engine, this increases with increasing aircraft speed (since the thrust is equal to the rate of change of momentum of the air mass flowing through the engine, and at higher air flow speeds you need less increase in velocity to produce the same change in momentum, since the mass flow is higher). Consequence - flying faster improves efficiency (to the point where compressiblity starts to create a disproportionate rise in drag)

However, at constant IAS (ignoring compressibility at the intake), the mass flow will be the same, since:

air mass flow rate = Intake area x TAS x actual density = Intake area x IAS x sea level density

...therefore air mass flow rate at constant IAS is the same, regardless of actual air density, but drag is also constant at constant IAS (ignoring compressibility, again), so density plays no part.

Although compressibility plays a part, the general trend in improving efficiency is still valid - one could compare engine fuel flow rates at 210KIAS at low and high altitudes, at which speed high Mach no. effects would not really be playing a huge part; high altitude fuel flow rate would still be less than at low altitude, because the engine is more thermally efficient.

[chornedsnorkack]

However, at constant IAS (ignoring compressibility at the intake), the mass flow will be the same, since:

air mass flow rate = Intake area x TAS x actual density = Intake area x IAS x sea level density

...therefore air mass flow rate at constant IAS is the same, regardless of actual air density, but drag is also constant at constant IAS (ignoring compressibility, again), so density plays no part.


Er, is it so?

The mass flow rate is proportional to density and TAS.

The aerodynamic forces, drag and lift are proportional to density and, at low Mach, square of TAS.

So, climbing at constant IAS, the density should fall faster than the TAS increases. The mass flow should decrease.

[enicalyth]

Read Peterson and Hill. But in the mean time...

You are playing all the right notes but not necessarily in the right order. Also what you think are constants aren't and what you think are variables are but not necessarily in the way you assume. You also think the physical dimensions of the engine define everything.. er not quite.

All, engines work best if the input is cold and the output to the "flywheel" for the want of a better word is hot.

For the moment forget density. An air breathing engine wants oxygen. It just so happens there is relatively speaking more of it at sea level where air is most dense. If an engine needs oxygen and the designer can help it find oxygen in dense and thin air, believe me the oxygen goes in all right and the effective mouth size gulping it is bigger or smaller than the hole at the front. A carefully designed inducer and diffuser does this in a jet but is dam' more difficult to persuade an unducted fan or prop.

A prop just wants a working fluid and it could be five-fifths nitrogen for all it cares. But it is a profligate user of air, neither inducing it neatly through the disk nor giving two figs where it goes afterwards.

A jet operates by thrust created by the compressor and partially used by the turbine that turns the compressor but if the compressor overpowers the turbine so to speak we have thrust for flight. Otherwise it chokes and stalls.

A choked airflow can always be unchoked without loss of pressure by heating it further. Up goes the temperature, up goes the speed of sound, away goes the choking. If a prop ever, god forbid, got choked there is very little recoverable work because temperature in and out are much the same and I forbid you to throw petrol or kerosine out of the window and set fire to it to heat up the air and unchoke the flow.

On the other hand if you can gently induce and contain the airflow into a tin can I don't mind the compressor compressing the air so that by the fourth or fifth stage it chokes. Firstly it has got a lot hotter so it can do work... heat and work are interchangeable. Secondly I can unchoke it at will by a controlled burn of kerosine in an enclosed space. Heat the air, increase the speed of sound and choking is gone. So the burner really unchokes the choked compressor to give the turbine its best fighting chance to turn the compressor and some extra fuel is chucked in to cover the losses. Thirdly, choked the compressor might be but that only sets a limit on the amount of air the engine can ingest which is determined by the effective intake area which is not the same by any means as the physical dimensions. Come with me and I will show you that a running engine ingests air not from dead ahead only but from all over the shop. True in some flight regimes the effective area is smaller than the physical size by tape measure but the overwhelming majority of subsonic and trans-sonic machinery sucks in more than outward appearances suggest.

So the jet engine we know and love so well really hands the designer several tools to take it past the obstacles provided by nature. Now... TSFC.

TSFC? It will be a lower value at sea level than at altitude. But don't be fooled. Fuel consumption is always better where oxygen is aplenty but you are missing the point that it is thrust specific. How much thrust you need muchly depends on lift/drag ratio as well as weight of course. Poor L/D occurs down low and slow and completely swamps the oxygen rich fuel consumption benefits displayed on the data sheet. Come on! You know a 747 burns fuel at a rate of thirty tons an hour on take-off but half an hour later she's guzzling at a rate of ten tons an hour. All things equal lift is proportional to air density but the square of velocity. Retract those flaps and wheels, get her up where the air is cold and speed her along. Wonderful L/D ratio. Pity the TSFC doubles but thin or not the air will be drawn in because the inlet designer did a good job.

So TSFC might look good low and slow but my god you need lots of thrust to stay in the air. High on the fly TSFC may look twice as "bad" but hey L/D is not say 7:1 or worse but 17:1 and even better.

Oh I'm rambling b*gger it.

[AIRWAY]

G'day,

If further info is needed i believe Rolls Royce have availabe online for download a book called Jet Engine, if you can't find it let me know and i can always send you the file (quite a big file).

:ok:

[Gary Lager]

Entire post edited because, after 20minutes lying in bed, I realised I had been typing complete b*lls.

chorned - you're right. What I'd got wrong was that sea level density x IAS SQUARED = actaul density x TAS SQUARED. Air mass flow through the engine does decrease with altitude, even at constant IAS.

But I've thought of a simpler example.

My B737, at sea level, has an IAS of 250kts, giving a fuel flow rate of (a guess) 1,750 kg/hr per engine. At 35000' and 250 kts IAS, the drag is constant (I'm sure that bit's right!), so thrust is the same, but fuel flow per engine is more like 1,100 kg/hr per engine. The engine speed is higher, so closer to the design point (as enicalyth mentions), and thermal efficiency is better (as I have), so the same thrust is produced even though the air flow (and thereby the fuel flow, keeping the fuel/air ratio the same) is slightly less.

Now, the REAL efficiency lies in the fact that at 35000 and 250 kts, my TAS (and therefore GS, still air) is nearer 450kts. So for our (slightly) reduced fuel flow/thrust, we've flown nearly twice as far - accountants happy, esso not so.

Please tell me this isn't completely wrong, I'm on earlies tomorrow & need to be able to sleep!

[G SXTY]

Oh God, we're discussing L/D ratios and compressibility now. I'm having flashbacks about my ATPLs, I can remember the fear, the confusion, the sheer boredom of it all.

Losing . . .

will . .

to .

live.

:p

(By the way bishop99, you've probably got enough information here to go and build a jet engine, never mind an 'A' level project).

[Capt Claret]

bishop99

To offer another perspective.


A Jet engine is an internal combustion engine. An internal combustion engine burns a ratio of fuel against the mass of air (O2) used in combustion.
A jet engine is most efficient at high RPM (>90%).
At sea level and lower levels the engine produces power in excess of that needed to achieve VMO/MMO in level flight.
As the aircraft climbs, the mass airflow reduces due to the air density decreasing, ergo, the amount of fuel burned for a given engine speed decreases.
Thrust also decreases with altitude for a given engine speed, conveniently so too does drag.

[vapilot2004]

I may be reiterating already clear (as Claret) :) responses, but I thought I would have a go at this...........

The power curve of a turbine engine ensures that despite the power loss at altitude due to the lower air density, said power loss is substantially less than the dramatic drag reduction of the airframe at altitude - also due to the lower air density.

More available thrust VS less airframe drag = mucho go-o :)

It's almost like getting a free lunch.

[bishop99]

You Guys are great, i do believe i have a nearly sound understanding of the jet engine and the variables with height etc...etc. I would just like to thank you all for your time and help,it is very much appreciated:ok:

Im now off to write the badboy up, oh the joys of education.

Thanks again

[chornedsnorkack]

TSFC? It will be a lower value at sea level than at altitude. But don't be fooled. Fuel consumption is always better where oxygen is aplenty but you are missing the point that it is thrust specific. How much thrust you need muchly depends on lift/drag ratio as well as weight of course. Poor L/D occurs down low and slow and completely swamps the oxygen rich fuel consumption benefits displayed on the data sheet. Come on! You know a 747 burns fuel at a rate of thirty tons an hour on take-off but half an hour later she's guzzling at a rate of ten tons an hour. All things equal lift is proportional to air density but the square of velocity. Retract those flaps and wheels, get her up where the air is cold and speed her along. Wonderful L/D ratio. Pity the TSFC doubles but thin or not the air will be drawn in because the inlet designer did a good job.
So TSFC might look good low and slow but my god you need lots of thrust to stay in the air. High on the fly TSFC may look twice as "bad" but hey L/D is not say 7:1 or worse but 17:1 and even better.
Oh I'm rambling b*gger it.

The poor L/D has little to do with altitude!

Airplanes have poor L/D immediately after takeoff because they have a lot of drag-inducing things like landing gear out, and also have high-lift devices that allow them to fly at low IAS but at a cost of high drag, plus they may be flying at a high angle of attitude which has same effect (low IAS but high drag).

Accelerate your 747 to a safe IAS speed for clean configuration - gear retracted, flaps retracted - and I suppose that the L/D would reach the values close to the cruise ones rather soon.

But the engines would still be inefficient - prolonged flight at low level is a waste of fuel even in clear configuration.

Also do not confuse the termodynamic effects of cold air with effects of altitude through density and TAS!

In midwinter, I presume you might fly your 747 to Yellowknife or Klondike or Fairbanks or Yakutsk - all of which places are located less than 300 metres above sea level - and find -50 degrees Celsius on ground level. This usually happens in anticyclonal weather, so even the pressure altitude could be negative - and the density altitude would be very negative.

The plane would generate a lot of lift, and drag, at very low true air speeds. And the jet engines would have huge amounts of air, and oxygen, available to suck in. Nevertheless, I presume that the true efficiency of engines would be low because the TAS is low. It would take a lot of fuel to fly low and clean from, say, Winnipeg to Aklavik or Yakutsk to Norilsk in midwinter, if your powerplant is a jet engine...

You would get much further by climbing to greater altitudes, where the air is thinner and TAS is faster. Quite independent of any changes of L/D.

[barit1]

...
Jet engines are most efficient at high power settings (say around 90-95% of maximum fan speed, or N1). For the best specific fuel consumption (i.e. miles per gallon) you would want to operate at an altitude where the aircraft maintains unaccelerated straight and level flight with the engines running at 90-95% N1...

G SXTY's got it. It's not that the engine is more efficient at altitude, but that the engine is best matched to the aircraft at altitude. The machine is bloody overpowered at takeoff, is working hardest at top of climb, and happiest in cruise. :ok:

[chornedsnorkack]

G SXTY's got it. It's not that the engine is more efficient at altitude, but that the engine is best matched to the aircraft at altitude. The machine is bloody overpowered at takeoff, is working hardest at top of climb, and happiest in cruise. :ok:
Then consider an underpowered aircraft - say a twin with one engine out or a trijet with one or two engines out, or a quadjet with two engines out. Assuming that the inoperative engines are properly shut down, they burn no fuel and no fuel is leaking.

After the plane has descended to its ceiling and continues on a cruise... the remaining engines are working harder than in normal cruise, therefore burning more fuel each, but the inoperative engines burn none at all.

Does the aircraft then lose efficiency and therefore range? Does a plane cover longer distance with all engines operative cruise or some engines out cruise, given the same fuel load?

[barit1]

The operating engines may actually be at a more optimum operating point (better SFC), although that's a big MAYBE.

But with less total thrust available, a lower cruise altitude is forced on the machine, which ruins any advantage. The higher air density means more thrust available, but more fuel burned too.

[chornedsnorkack]

But with less total thrust available, a lower cruise altitude is forced on the machine, which ruins any advantage. The higher air density means more thrust available, but more fuel burned too.

Which is probably because the jet engines are less efficient at lower TAS and therefore generally less efficient at lower altitude. If a plane descends below the optimum cruise altitude for a jet then I suspect that whatever the plane does - running all engines at less than optimum thrust, or shutting some down and running the others at the optimum thrust - the plane would be less efficient in burning more fuel per distance, and therefore lose range for a given fuel reserve.

[barit1]

Bear in mind too that at lower altitude with 1 or more shut down, the fuel burn PER HOUR may be lower, which is advantageous when you're killing time in a loiter. Patrol aircraft often use this to extend their loiter duration.

However, this means relatively less distance traveled, which is bad news if your job is to get from point A to point B in good time.

[chornedsnorkack]

Bear in mind too that at lower altitude with 1 or more shut down, the fuel burn PER HOUR may be lower, which is advantageous when you're killing time in a loiter. Patrol aircraft often use this to extend their loiter duration.
However, this means relatively less distance traveled, which is bad news if your job is to get from point A to point B in good time.
Indeed. Back at the efficiency. The total time that a plane stays airborne with a given amount of fuel may indeed be longer if the plane flies low and slow. But the total distance covered is probably less.

Which is not just a matter of "good time". It also is bad news if the job is to get to point B or another runway at all, any time, rather than ditch or crash off-airport after fuel exhaustion short of diversion.

How much is the range of a typical airliner diminished if it has to stay below 3000 metres or so because it cannot be pressurized?