Turbine Engines

[Brigandier]

I know this may be a simple question, but pardon my ignorance.

Are turbines, by the laws of physics high hp low torque ways of producing energy?

Any supporting equations would greatly help, I'm fluent up to dif equations, so please don't hold back.

The reason I ask this is because I want to start doing some simulations of high torque vs raw horsepower in different scenarios.

Disclaimer: I am using this for personal knowledge, not personal gain, I will hold what you tell me in strictest academic honor, and if I figure something out, cite you accordingly.

Thanks, any help would be appreciated.

 

[ElFenix]

any device running at a higher rpm to develop the same horsepower will be making less torque. but turbines are not necessarily high rpm devices. to generate 60Hz electricity they run at 3600 rpm. anything in the megawatt generation range is going to have big torque. a wind turbine generates massive torque, turning at a just a handful rpm.

and that's not even taking into account power density. a diesel generating a megawatt is going to be much bigger than a turbine generating a megawatt.

 

[Puppies04]

any device running at a higher rpm to develop the same horsepower will be making less torque. but turbines are not necessarily high rpm devices. to generate 60Hz electricity they run at 3600 rpm. anything in the megawatt generation range is going to have big torque. a wind turbine generates massive torque, turning at a just a handful rpm.

and that's not even taking into account power density. a diesel generating a megawatt is going to be much bigger than a turbine generating a megawatt.

From the OP's original question I assumed he was talking about turbine engines (as in http://en.wikipedia.org/wiki/Turbine_engines ) Does a wind turbine still count as a turbine engine in the same way as say a steam or gas turbine?

 

[Evadman]

From the OP's original question I assumed he was talking about turbine engines (as in http://en.wikipedia.org/wiki/Turbine_engines ) Does a wind turbine still count as a turbine engine in the same way as say a steam or gas turbine?

A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.

A wind turbine extracts energy (torque) from fluid (air) and converts it into useful work (electricity). So, yes, it is the same thing. I can't tell what the OP is asking on: turbines that burn fuel to convert into work, or turbines that extract power from other sources like wind or water. The basic mechanics are the same, though the source energy requires different delivery or harvest methods.

 

[PandaBear]

Turbine is efficient if it is a large system. For small system it would be inefficient compare to piston engine due to heat loss. It is also very inefficient if the power output is not steady (i.e. constant speed). So it is never a good idea for a small automobile which needs a lot of power accelerating for a short period of time and a small amount of power cruising at constant speed, and changes between these 2 conditions all the time.

 

[yottabit]

The reason I ask this is because I want to start doing some simulations of high torque vs raw horsepower in different scenarios.

Gearing can change all that though. I mean if you have a turbine that lacks torque and produces 400 hp at 100,000 RPM, by the time that is geared down to a useful RPM you'll be making lots of torque with the same power (minus some minor losses)

The issue is not so much lack of torque as the necessity to run at a constant RPM/load like someone else mentioned. That's one of the reasons they are great for helicopters, and generators.

The reason they *lack torque* is because they can generate so much power for a given size by producing it at such a high rpm. Reciprocating mass holds back the power potential of piston engines because they can only spin up to some critical speed before the connecting rod breaks from all those forces. In a perfectly balanced turbine there are no reciprocating forces and they can spin as fast as they can manage so long as the turbine holds up to the radial forces on it.

Think of a hypothetical 1L engine with 50 hp @ 5000 RPM and 50 ft lb of torque peak. Now compare that to a 1L hypothetical engine with 300 hp @ 30,000 RPM and 50 ft lb of torque peak. Does the latter engine "lack" torque? Suppose even the latter engine only has 20 ft lb of torque- by the time you geared it down to the same RPM as the more "torquey" engine, it would have over twice as much torque!

 

[CurseTheSky]

So it sounds like the biggest draw-back of a turbine engine in an automobile is the fact that you only need peak output, not a continuous steady stream of power.

What about using a turbine (rather than an ICE) to charge a battery? Would it be efficient enough if you kicked it on when the battery drops below a threshold (say, 50&#37 and then back off when the battery reaches near full?

 

[exdeath]

Turbine >>>> piston engine when you need LOTS of power CONTINUOUSLY, then the turbine is tons more efficient and powerful than what a required equivalent piston engine, if possible to even produce, could provide.

It's an "economy of scale" problem manifesting at a technological and mechanical level. At the low and variable power needs that something small like a car has, a turbine is wasteful and inefficient. A 20,000 HP turbine engine is much MUCH more efficient, lighter, and more compact and reliable than a 20,000 HP piston engine in an application that requires it. While it's more efficient on it's own scale, it's still a 20,000 HP engine and isn't going to be more efficient than a 150 HP piston engine. Scaling down a turbine engine to 150 HP loses vs piston just as scaling up a piston engine to 20,000 HP loses vs turbine. Just different power delivery properties for different purposes.

There is that initial hurdle where you need a minimum sustained power to begin with. When scaled to small sizes, they are inefficient and lack the power compared to a piston engine. At larger sizes where you need that power all the time, the power to weight and fuel efficiency amortize the entry fee, but it would give extremely poor mileage in something like a car running around generating 50,000 lbs of trust sitting at a red light.

Similarly a bus is more efficient than a car when transporting lots of people all the time at the macro scale, but compared to a Prius only gets 6 mpg and costs thousands in fuel and would not work very well as a personal vehicle at the micro scale.

But yeah I've wondered why we don't have something like a cruise missile size turbine generator with constant output in an electric drive hybrid vehicle. While larger than a car, gas turbine-electric hybrid locomotives are extremely powerful and efficient.

 

[yottabit]

So it sounds like the biggest draw-back of a turbine engine in an automobile is the fact that you only need peak output, not a continuous steady stream of power.

What about using a turbine (rather than an ICE) to charge a battery? Would it be efficient enough if you kicked it on when the battery drops below a threshold (say, 50%) and then back off when the battery reaches near full?

There are hybrid designs based exactly on that concept! You are ahead of your time
I'm honestly not sure whether or not one has ever been made though

Looks like Jaguar is working on one:
http://www.wired.com/autopia/2010/02/jaguar-developing-jet-powered-hybrid/

 

[drinkmorejava]

Is there a specific application your interested in. A few people have mentioned cars, but gas turbines generally have significant limitations there

-Noise
-Poor Acceleration (relative to combustion engines)
-High cycle fatigue due to many accelerations and decelerations. In aircraft this is normally avoided by reaching a certain power level and sitting there for most of the flight (cruise).

Newer cars (hybrids) can avoid the last two problems by using electric motors as the drive source and charging the batteries with the turbine. However, at that point, torque and hp from the turbine aren't applicable to what I think you're interested in.

 

[Plugers]

What about a high constant rpm turbine with an infinitely variable transmission? Just change the ratio for acceleration.

 

[sm625]

Big airplanes like a 747 use an APU specifically designed to generate electricity. If they were efficient at all they would be used to power electric motors in buses and maybe even long haul trucks. At this point, diesel is still cheaper.

 

[sao123]

im pretty sure HP Torque and RPMs are all related:

HP = Torque * (RPM / 5252)

 

[yottabit]

im pretty sure HP Torque and RPMs are all related:

HP = Torque * (RPM / 5252)

yes if torque is in ft-lbf

One horsepower is 550 ft-lbf/s

1 RPM = 2*pi radians/minute = 6.2831 radians/minute = .1047 rad/second

550 ft-lbf/s / .1047 rad/s ~= 5253

 

[Zorba]

The mid fan shaft on the GE90 transfers load from the LP Turbine to the Fan/Booster. In rough numbers, it transfers around ~50,000 HP at ~2500 RPM, so that is a torque of 105 kip-ft. Not exactly low torque.

The thing is, the smaller a turbine the faster it wants to spin, the bigger it is the slower it wants to spin. The bigger it is the more power/torque.

Another problem with small turbines is the gaps and leakages become very big players in the efficiency equation. For example, a modern commercial can run with steady-state turbine blade tip clearances of less the 0.005." The running gap can't really be made smaller for small engines, so the percentage of flowpath area represented by this gap increases significantly as the engine radius shrinks. Other leakage paths also do not scale as you move smaller, reducing the efficiency further.

One problem I see with using gas turbine engines in buses, cars, etc. is the amount of regulations required to keep them safe WRT the high speed rotating parts. Aircraft engines have very specific life limits on all high speed rotating parts (rotors). These same regulations would have to be applied to any light weight gas turbine engine. Land and Marine engines get away from some of these issues because they come up to speed and stay there for months, so they have very low cycles. They also have much heavier casings. Even still, land and marine engines at least have recommended life limits (not sure if they are enforced by law). Not to mention the very exotic and costly materials required to make a gas turbine efficient.