# Power required to compress air in a gas turbine

#### Jeremy Ardley

I'm looking for advice on how much power is required to compress air in a gas turbine engine - parameterised by degree of compression and mass flow.

Normal gas turbine engines have exhaust turbine(s) on the same shaft as the air compressors. They bleed some of the exhaust energy to drive the compressor. It's obviously the most efficient way.

The downside of this approach is that the exhaust turbines(s) and bearings need to be very precise and made of exotic materials to withstand the exhaust heat.

For reference there was a Russian aircraft in WW-II that flew using a conventional engine, but for emergencies had a method of compressing air using a piston compressor (maybe from the main engine?) and used that to drive a type of after-burner. It only lasted a few minutes and was very wasteful of fuel.

Ultimately I'm looking to see if a battery-electric axial or radial air compressor is viable in a small scale gas turbine for model aircraft use - thus avoiding the need for high temperature turbines in the exhaust train.

Just to kick the ball off, set compressor intake diameter to 100mm, inlet pressure 100 kPa and outlet pressure 500 kPa or 1000 kPa.

Related Aerospace and Astronautics Engineering News on Phys.org

#### 256bits

Gold Member
thus avoiding the need for high temperature turbines in the exhaust train
How will the energy be extracted from the burnt fuel with no turbine?

#### Jeremy Ardley

How will the energy be extracted from the burnt fuel with no turbine?
The turbine only extracts a fraction of the hot gas energy - essentially by slowing it down. This is used to power the compressor.

The thrust of a gas turbine is created by the mass flow of hot gas out of the rear nozzle. In optimum designs this is at local atmospheric pressure but with high mass flow rate and high velocity. Ignoring reactions with ambient air (c.f. water hose reaction) the thrust is directly related to the mass of the output hot gas and its velocity relative to the engine.

#### anorlunda

Mentor
Gold Member
Aircraft engine design is complex, even at model airplane level.

Consider turboprop, turbojet, and turbofan engines. Each of those is optimum in specific speed ranges. The primary difference between them is the differing ratios of compressor/turbine powers. (The propeller is part of the compressor in a turboprop. Turbofan engines bypass a lot of the air around the turbine.)

Jet engines are started with electric start motors, so it does not sound entirely impossible. But I question the weight of the batteries needed to provide that power during flight.

The calculations needed to answer your question can be very difficult. A bit of research may find some data on the compressor/turbine performance rations of existing aircraft engines, but the data may not be applicable to model size engines. What is your engineering background? Could you do those design calculations?

#### russ_watters

Mentor
Ultimately I'm looking to see if a battery-electric axial or radial air compressor is viable in a small scale gas turbine for model aircraft use - thus avoiding the need for high temperature turbines in the exhaust train.
It couldn't possibly be better than using the turbine. In addition to the weight of the electric motor, batteries have a vastly lower energy density than jet fuel, so you are adding a huge amount of weight to an aircraft by doing this.
Just to kick the ball off, set compressor intake diameter to 100mm, inlet pressure 100 kPa and outlet pressure 500 kPa or 1000 kPa.
What flow rate? Are those numbers reasonable for a jet compressor? They seem awfully low to me...

You can probably look (google) up real jet engines and their thermodynamics to find what the compressor power requirement is. It's not small.

Homework Helper
Gold Member

#### CWatters

Homework Helper
Gold Member
Bit more googling found this and around page 26 has some example calculations for the power needed to drive the compressor and the overall power out. The ratio in the example is roughly 150/500 eg the power needed to drive the compressor is about 30% of the power providing thrust. They assumed both the compressor was 85% efficient.

Failing that try googling..

Turbocharger jet engine build and analysis - Semantic Scholar by HJ Lee. 2016

Edit : sorry link nonworking. No idea why. Copied and pasted from Google.

#### Jeremy Ardley

The comment from @CWatters on the power ratios is most useful. Roughly one third of the energy in the combustion chamber is used to drive the compressor and the remaining 2/3 to provide thrust.

If all things were equal - and they never are - then an electric powered compressor could be boosted by fuel injection in a combustor to get roughly 3 x thrust. I think this was the basis for the Russian aircraft design.

So a potential design is an electric compressor used alone for cruise flight but boosted for high thrust requirements - take-off - by burning fuel more or less as an afterburner.

Given compressing air wastes energy compared to simply accelerating it in a ducted fan design there would need to be a geometry change as well. I guess a simple sleeve valve might work for that.

P.S. I found a pretty comprehensive paper on the general gas turbine topic https://himech.files.wordpress.com/2010/02/dke672_ch7.pdf

#### russ_watters

Mentor
The comment from @CWatters on the power ratios is most useful. Roughly one third of the energy in the combustion chamber is used to drive the compressor and the remaining 2/3 to provide thrust.

If all things were equal - and they never are - then an electric powered compressor could be boosted by fuel injection in a combustor to get roughly 3 x thrust.
No, that should be 3/2=1.5x thrust.
Given compressing air wastes energy compared to simply accelerating it in a ducted fan design there would need to be a geometry change as well. I guess a simple sleeve valve might work for that.
Are you saying now that you want to do away with the compressor completely? You're going the wrong direction with your ideas. The compressor is not a waste of energy, it creates the high pressure required to generate substantial thrust!

#### Jeremy Ardley

@russ_watters No I'm not doing away with the compressor. It will still stay there - probably a radial compressor, though I'm looking at a design right now that uses an electric motor driving an axial compressor https://hackaday.io/project/21569-3d-printed-axial-compressor

The idea is that energy could be saved by venting the compressed air rearwards rather than pushing it through a combustion chamber. Modern gas turbine engines have bleed valves that do this mainly to keep the engine stable during acceleration stages.

#### CWatters

Homework Helper
Gold Member
@russ_watters No I'm not doing away with the compressor. It will still stay there - probably a radial compressor, though I'm looking at a design right now that uses an electric motor driving an axial compressor https://hackaday.io/project/21569-3d-printed-axial-compressor

The idea is that energy could be saved by venting the compressed air rearwards rather than pushing it through a combustion chamber. Modern gas turbine engines have bleed valves that do this mainly to keep the engine stable during acceleration stages.
Not quite sure I see the advantage of using an electric motor. Why not just make a regular high(er) bypass engine?

#### russ_watters

Mentor
@russ_watters No I'm not doing away with the compressor. It will still stay there - probably a radial compressor, though I'm looking at a design right now that uses an electric motor driving an axial compressor https://hackaday.io/project/21569-3d-printed-axial-compressor

The idea is that energy could be saved by venting the compressed air rearwards rather than pushing it through a combustion chamber.
...so then are you doing away with the combustion [chamber]?

#### Jeremy Ardley

@russ_watters - My error.

Pcombust = 3 x Pin

The power output from an electric powered compressor with combustion will be Pin + Pcombust = 4 x Pin.

@CWatters A ducted fan is a high(er) bypass engine. At most it will use the Electric Pin. This topic is about boosting the energy of the system above the electric power input

#### Jeremy Ardley

so then are you doing away with the combustion [chamber]?
No. I'm proposing switching it in and out depending on need. For takeoff it's switched in. For cruise it's switched out and thrust is by electric fan only.

#### CWatters

Homework Helper
Gold Member
Ok but I suspect that a fan design optimised to compress air for a turbine isn't the same as one optimised for thrust.

#### Jeremy Ardley

I suspect that a fan design optimised to compress air for a turbine isn't the same as one optimised for thrust.
You can see this in high bypass engines - essentially a ducted fan driven by a jet core. A significant portion of the inlet air is driven by the fan, bypasses the jet core, and is later joined with the jet core exhaust (and potentially gets extra energy from the core heat).

I think my best bet is to construct a radial compressor plus a ducted fan and experiment with them.

#### russ_watters

Mentor
You can see this in high bypass engines - essentially a ducted fan driven by a jet core. A significant portion of the inlet air is driven by the fan, bypasses the jet core, and is later joined with the jet core exhaust (and potentially gets extra energy from the core heat).
Perhaps decoupling the fan from the compressor...
I think my best bet is to construct a radial compressor plus a ducted fan and experiment with them.
That seems like an awful lot of effort to put into a vaguely formed idea, but ok, it's your time...

If at this point you are still talking about model airplanes, you should probably look at weight, power and performance for electric models and consider how they would be affected by adding fuel and a jet engine that aren't being used.

#### anorlunda

Mentor
Gold Member
I think the OP is neglecting the dominant factor that several others pointed out.

Energy Density of Jet A fuel 43 MJ/kg
Energy Density of Lithium battery 0.9 MJ/kg

No matter how complex you make your cycle, it is highly unlikely that you will overcome that disadvantage. In the OP, you gave your motivation as
avoiding the need for high temperature turbines in the exhaust train.
We are telling you that your efforts would be better spent in dealing with those high temperatures.

#### Jeremy Ardley

This is a practical example of what I was asking about. Regrettably I don't speak Russian but I get the impression the engine gave '3kg' thrust or around 30 Newtons

#### Ketch22

I would have to do a little more to show the actual calculations. But Russ Waters was almost exactly right on the 2/3 power finding. I know from working in industrial use of a LM 1500 which in Aero use is a J79. These old school engines produce approximately 45,000 hp at the combustors. The Turbine extracts 30,000 hp to drive the compressors. This leaves 15,000 hp of usable thrust. In terms of combustion engines this is pretty good for old school stuff. New high bypass engines are more efficient but I have no experience in calculating these out.

#### Harry Cott

Turbine Engines are very complex in its design. All versions of the engine includes of two main part that can be easily separated for maintenance: a gas generator supplies hot pressurized gas to a free power turbine.

#### anorlunda

Mentor
Gold Member
I believe that these efficiency versus speed relationships apply to model airplanes as well as full scale airplanes. For realistic speeds, an electric powered model plane is most efficient with a propeller.

Photo is the work of https://commons.wikimedia.org/wiki/User:Marc_Lacoste

"Power required to compress air in a gas turbine"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving