Jet engines run better in cold environments?

[Megagoth1702]

Hey folks.

 

Simple and quick:

Why do jet engines run better in a cold environment? That seems to be the common opinion and I can't quickly find an answer on google.

 

Thanks in advance.

[sobek]

Why do jet engines run better in a cold environment? That seems to be the common opinion and I can't quickly find an answer on google.

 

Simple, air density is higher, which automatically increases the mass flow without the engine having to work harder. In some instances, it might also influence EGT, thus enabling to burn more fuel without reaching the design temp limits, but i think that is an exception.

Edited December 31, 2011 by sobek

[Devrim]

I say yes. But I don't know about technical details.

 

Engines run better in cold weather. They have more performance in cold enviroment than hot one. I know that from my car's diesel engine.

May be you can search for car engines.

[AlphaOneSix]

sobek has hit the nail on the head.

 

All engines compress the incoming air prior to mixing it with fuel. As air gets colder, it becomes more dense, and therefore is sort of "precompressed", so the engine's compressor (or compression stroke on a reciprocating engine) doesn't have to work as hard for the same amount of compression.

[Krebs20]

I will take a leap of faith with this statement as I think it is true.

 

All aircraft and aircraft engines perform better at lower tempertures. Provided that they are already running. It may not want to start in the 1st place.

 

I'm sure someone is about to pull out an aircraft than will make this false.

[sobek]

For those who don't run in fear from (relatively simple) formulas, there's a nice nasa page on the jet thrust equation, it's relatively easy to see how the density plays into thrust.

[KLR Rico]

Simple, air density is higher, which automatically increases the mass flow without the engine having to work harder. In some instances, it might also influence EGT, thus enabling to burn more fuel without reaching the design temp limits, but i think that is an exception.

 

Exactly what Sobek said... Temperature has a huge impact on an engine's performance. We've had engines that were hitting their EGT limits on take-off during the day, so we tried to schedule them to fly at night to squeeze out a few more hours. :D

[effte]

Rico hints at another important point. You get as much thrust as you can burn fuel. The more fuel you burn, the more you heat the air and the more power is produced. For maximum power, you run the engine to the structural limit (max RPM) or the limit when you melt engine parts (usually turbine blades), whichever comes first. Ir you are temperature limited, lower ambient temp means you can burn more fuel and reach a higher power setting.

[Megagoth1702]

Awesome explanations folks, thank you a lot! :)

[Shein]

Rico hints at another important point. You get as much thrust as you can burn fuel. The more fuel you burn, the more you heat the air and the more power is produced. For maximum power, you run the engine to the structural limit (max RPM) or the limit when you melt engine parts (usually turbine blades), whichever comes first. Ir you are temperature limited, lower ambient temp means you can burn more fuel and reach a higher power setting.

 

It is the turbine blades. Turbine engines are only 11-14% efficient. If someone could make a turbine that melted at even 200 or 300 degrees hotter, we could drastically improve the performance and efficiency of these engines. They've done experiments with coating the turbine blades in ceramics but they're just too damn fragile and brittle.

 

Also, +1 to what everyone said about air density being greater at colder temperatures :)

[Inseckt]

14% seems too me a bit low? I'm no expert, but some modern combustion engines (diesel for instance) have an efficiency number of about 40-50%. Thus it seem a bit weird that turbines are only 14%, meaning they would need to carry a LOT of fuel for the power they put out.... I could be all wrong of course, just tell me why?

[KLR Rico]

About ceramic blades, the T56-A-15 (upgraded engines for the C-130E/H) has them on the first stage. The ceramic in addition to other improvements in cooling technology lead to an TIT (turbine inlet temp) limit increase of over 100C compared to the older -7's (1083C vs 971C). As you said, it doesn't take a whole lot of increase in EGT to produce a whole lot more power. The -15's would hit the aircraft's structural power limit every time, but the -7's would never almost never see it unless it was really cold (there's that temperature thing again! ;)) You're right about them being fragile though! It's not that all uncommon to borescope a -15 and find a whole bunch of 1st stage blades missing! It'd pull power just fine, but the tailpipes would crack from the vibration.

[KLR Rico]

I've heard of thermal efficiencies of upwards of 40% for turbofans. The real benefit over say a diesel is the power to weight. Imagine the weight of a diesel that produces 30,000 HP.:)

[Shein]

Turbofans can get up there, but turbojets not so much... maybe I'm thinking of thrust specific fuel consumption? its been a year or so now since I took turbines... I do know for a fact that turbine engines as a whole are the least efficient engine. Guess what the most efficient is?

 

Electric. They're in the 85-90% range.

[Frostiken]

I will take a leap of faith with this statement as I think it is true.

 

All aircraft and aircraft engines perform better at lower tempertures. Provided that they are already running. It may not want to start in the 1st place.

 

I'm sure someone is about to pull out an aircraft than will make this false.

 

Icing aside, the only effects I've seen cold temperatures have on a jet aircraft are problems with flight controls on startup due to cold actuators and hydro.

[effte]

Frostiken,

you need to hang out at more northern (or southern) latitudes. :)

 

Oil temperatures become a problem eventually, with aux gearboxes requiring warmup time and what not.

 

Cheers,

Fred

[sobek]

Electric. They're in the 85-90% range.

 

Yes, but this is only the conversion from electric current to torque. The problem there is that you can't store energy with electricity very well (energy density is a lot less than if stored chemically). The conversion from chemically stored energy into electric current is what ruins the efficiency.

[macedk]

maybe answered :)

 

is it modelled in the sim ?

[AlphaOneSix]

Then for your sake I hope the A-29 never enters production. :D

[GGTharos]

... and that will be the end of the US's ability to keep any sort of air superiority as well.

 

Since they know this, I would presume your prediction is quite wrong :D

 

Electric would be sweet were it not for the weakest link: power source. Wish as I might there is no Mr. Fusion a la Back to the Future. Then the only issue would be crew endurance...but then again that's one reason why the F-35 is the last manned tactical aircraft the U.S. will ever produce :)

[Moa]

Yeah, especially with that latest incident losing a stealth UAV over Iran I would say whatever manned-fighter factions are at the Pentagon will be milking it for what it is worth.

 

There will probably always be manned combat aircraft until Artificial Integillence *and* sensors exceed human capabilities *and* are a lot cheaper than training some young, keen fellow.

 

The most dangerous and predictable tasks will probably become more and more UAV based (eg. taking out fixed air defences).

[Moa]

While the qualitative 'rules of thumb' are good you can get a good quantative understanding (how great is each effect?) using the NASA engine model applet:

http://www.grc.nasa.gov/WWW/k-12/airplane/ngnsim.html

 

ps: Of course you have the Java Runtime Environment (JRE) already installed for applets, as you need this for the 104th pilot statistics system :)

[Cougar12dk]

*Disclaimer* I've only just started reading this thread, so this will most likely already have been written. *Disclaimer*

 

Also, +1 to what everyone said about air density being greater at colder temperatures :)

 

Which is wrong. It gets colder the further apart the air molecules are (the higher you get). Think of it this way:

 

The closer the molecules (higher air density), the more friction is needed, thus you need more engine power to sustain a given airspeed.

 

Which is why airliners travel at high altitudes. To save fuel, and the passangers' money, and to maximize distance travelled.

 

This was in the first couple of days of commercial flight school.

[effte]

Reading the whole thread probably would have been a good idea. At a given pressure, colder means denser.

 

Ideal gas law:

 

pV = nRT

 

pressure, Volume, number of particles, R is a constant, T is the temperature (in Kelvin, of course). Only fully applicable to ideal gases, which the atmosphere isn't, but still tells you the relationships involved.

 

n = pV/(RT), or "the amount of air molecules (i e density) in a given volume is proportional to the inverse of the temperature (1/T)". Double the temperature, half the density.

 

The rest, well, don't think you know anything just since it was taught in ground school. I'm afraid they teach just enough to let people sit up front and drive without screwing up too badly, and simplify to make it understandable enough after a very short time in the classrom and without a background in physics, maths or engineering. Unfortunately, "enough" in this case is often far from "correct". I'm often horrified by just how little you really are required to know and understand to hold an ATPL, leave alone a CPL. Fortunately, most pilots know more... or we'd have an AF447 every few weeks.

[Bucic]

The rest, well, don't think you know anything just since it was taught in ground school. I'm afraid they teach just enough to let people sit up front and drive without screwing up too badly, and simplify to make it understandable enough after a very short time in the classrom and without a background in physics, maths or engineering. Unfortunately, "enough" in this case is often far from "correct". I'm often horrified by just how little you really are required to know and understand to hold an ATPL, leave alone a CPL. Fortunately, most pilots know more... or we'd have an AF447 every few weeks.

Not long ago I met an engineer... who actually was wondering... whether it's harder to run up stationary stairs or moving stairs (given the geometry of the stairs is the same). Cracked me up. Should I put a real and practical understanding of Newtons Laws in my resume? :D