Questions about turbojet engines.

[PoDuck]

I have come up with a new design for a turbojet engine that should consume less fuel, be sturdy enough to withstand almost any pressure and temperature you can throw at it as long as the melting point of the metal is avoided, and those temperatures can stay away from the bearings, you can damage every component in the compressor and turbine and as long as it still spins should stay running to some degree, and be more simple to build. If it is damaged, it seems to slow itself down to a speed that prevents it from cavitating apart. I have no experience building them to such a small scale as for model aircraft though.

Since I'm starting from scratch, I'm curious what weak points and limitations current model jet engines have, or even full size engines for that matter. Are there rpm limits purely because of the design of the blades?

Does anyone know how to calculate the speed at which the rotational inertia of a cylinder of a certain metal and size will cause it to explode?

Are limitations on pressures and speeds a problem relating to the compressor and turbine, or are they related to something else such as bearings.

I currently have been experimenting with gas turbine engines using the principles I intend to use, and as soon as I figure out how to scale it down it should be an extremely unique design. The cool thing about these turbines is they are uniquely suited to small applications since their efficiency increases as they are reduced in size.

Is there a way to calculate the best pressure difference without tons of experimentation? What should the pressure differences be without burning fuel?

I seem to be getting between 70-80% thermal efficiency out of my gas turbines. Once I was sure that it jumped to 88%, but I couldn't reproduce that. That's so high that I'm convinced that I'm not calculating it right. I guess I should ask what the normal way of calculating thermal efficiency is in order to see if I'm doing it right. I assumed I was doing it right since I was calculating about a 40-50% efficiency from a turbocharger based design that I started with. I don't think I could put my temperature gauges any closer to the intake or exhause of my turbine. I would also like to know how to calculate the mechanical efficiency.

Any help would be appreciated.

Thanks

[Sail 'n Soar]

I seem to be getting between 70-80% thermal efficiency out of my gas turbines. Once I was sure that it jumped to 88%, but I couldn't reproduce that. That's so high that I'm convinced that I'm not calculating it right.

Thanks

You're right. You cannot be calculating thermal efficiency correctly. Even central powerplants can't even approach those numbers. In terms of actual numbers, the following site indicates that today's jet engines' thermal efficiency is APPROACHING 50%. http://www.aviationexperts.org/AIAApresentation.html

Thermal efficiency is work out divided by heat in, or e = W/Q. Each engine cycle has a different way to calculate the ideal efficiency for that cycle, but NONE are more efficient than what is known as the Carnot cycle efficiency. The answer to your question is beyond what could be presented adequately here. Suggest you do a web search for a tutorial at whatever your level of technical maturity is. My quick search found none I could recommend.

[Sail 'n Soar]

This site is a good first tutorial.

http://www.turbinetechnologies.com/minilab/Technical%20Papers/Univ%20of%20Toledo.pdf

[PoDuck]

I appreciate the replies. I'm still having difficulty figuring thermal efficiency out on my own. After reading the link Sail 'n Soar gave, I have realized that much more goes into calculating thermal efficiency than I thought. I just am not understanding that. It's not that I'm a mathematical idiot, I just never took physics and that paper assumes I have I guess. I just don't understand what measurements it wants where in those equations.

In the long run, this motor might be better suited for use as a motor in a turboprop or helecoptor since it produces much more horsepower at lower rpm's than conventional turbines. Since it takes less volume of air to reach the same rpm's and horsepower, it might be less suited for producing thrust than it is for producing mechanical power.

Truthfully though, the efficiencies of the motors don't matter to me as much as pressures. In other words, what would the ideal combustion chamber pressure be? Right now, I'm working with about 30 psi, but I'm wondering if I should go higher. Is there such a thing as too much pressure, assuming that the rest of the engine can handle the pressure itself?

I know I'm asking some questions that are difficult to explain to someone with no background in the subject, but I'm convinced that I have found a better way to do the same job, and I have no idea what I'm doing when it comes to these types of engines. Also I haven't decided what the best use for these would be yet. That will have to wait until I have perfected a model. I hope that doesn't make things more difficult.

[banktoturn]

PoDuck,

Full scale turbojet engines have the same issue of begin better at generating mechanical power than thrust, directly. They solve the problem by ingesting more air than is needed for combustion, and passing a fraction of it through the outer portion of the first stage's radius and bypassing the combustion chamber. The first stage is used as a fan, and as a result these are called turbofan engines. Generally, this is how the more efficient engines work.

You will get the best thermodynamic efficiency by making the pressure in the combustion chamber as high as you can.

I have no way to give any guidance (and I'm not an expert anyway) from the information you have given. I suspect that you will need to hook up with someone "in the business" to make any progress with your idea.

Good luck,

banktoturn

[PoDuck]

Well, what happens if I generate more pressure at a given rpm than the turbine wants to release at the same rpm? Would there not be a point that such a discrepancy causes major problems? Possibly the motor would refuse to idle and continue increasing in speed until it either blows up or the fuel is shut off?

I would love to hook up with someone that has experience in building similar motors, and the physics behind them. I'm not sure where to look though.

The turbine I created isn't completely new, I've just added some modifications that cause it to speed up much faster and reach higher much higher rpm's than the original with the same input volumes. The compressor I'm using is similar design, but highly modified in order to maximize the volume of air that can be compressed, and the pressures they can reach at any given rpm. To my knowlege, nobody but me has created a working gas turbine engine with these types of turbines or compressors, although a couple have spoken of it.

[Sail 'n Soar]

PoDuck,

Full scale turbojet engines have the same issue of begin better at generating mechanical power than thrust, directly. They solve the problem by ingesting more air than is needed for combustion, and passing a fraction of it through the outer portion of the first stage's radius and bypassing the combustion chamber. The first stage is used as a fan, and as a result these are called turbofan engines. Generally, this is how the more efficient engines work.

You will get the best thermodynamic efficiency by making the pressure in the combustion chamber as high as you can.

I have no way to give any guidance (and I'm not an expert anyway) from the information you have given. I suspect that you will need to hook up with someone "in the business" to make any progress with your idea.

Good luck,

banktoturn

PoDuck,

You are confusing propulsive efficiency with thermal efficiency. Both pressure and absolute temperature ratios play into thermal efficiency.

Gerry

[PoDuck]

I don't believe I was. I thought that thermal efficiency was basically a calculation of the amount of heat lost to the turbine prior to exiting as exhaust.

The later statement I made was because of an observation I made about the amount of horsepower I show on my dyno as compared to the horsepower I get from my conventional turbine at the same RPM. In other words, my turbine produces much more torque throughout the RPM range, especially at lower RPM than my conventional turbine of similar size. I was making a separate observation there stating that with an output shaft, it would be much more efficient as a means for powering mechanical equipment than a conventional turbine of similar size.

[John Lueke]

PoDuck

You aren't providing a lot of info in an effort to protect your idea so it is difficult to provide advice. It sounds like your main figure of merit is a horspower measurement on a dyno. One thing you could do is find the energy content of the fuel you are using in Btu's per gallon, measure the fuel flow and calculate how much energy you would get if the thermal efficiency is 100% and then divide your dyno measurment by that number(after getting both into the same units) to find thermal efficiency. If it turns out that your measurements of 70-80% are correct, then you have all the answers and people will be knocking on your door!

John Lueke

[Sail 'n Soar]

I don't believe I was. I thought that thermal efficiency was basically a calculation of the amount of heat lost to the turbine prior to exiting as exhaust.

The later statement I made was because of an observation I made about the amount of horsepower I show on my dyno as compared to the horsepower I get from my conventional turbine at the same RPM. In other words, my turbine produces much more torque throughout the RPM range, especially at lower RPM than my conventional turbine of similar size. I was making a separate observation there stating that with an output shaft, it would be much more efficient as a means for powering mechanical equipment than a conventional turbine of similar size.

Actually, it was my mystake. :o I was really commenting on banktoturn's advice relative to bypass/fan air and max pressures possible. First, increaswing bypass ratio increases propulsive efficiency, not thermal efficiency. Second, increasing combustion chamber pressures as much as possible without considering the temperature increases through fuel burn just dissipates energy with no return. Additionally, at some point the mechanical and aero losses with increased chamber pressures will more than offset any marginal thermal efficiency gain.

If it turns out that your measurements of 70-80% are correct, then you have all the answers and people will be knocking on your door!

Such performance claims for an engine operating on a thermal cycle using other than unobtainium fuels and materials designed to run at real world inlet temperatures will be right up there with perpetual motion and anti gravity machines. People may be knocking on your door, but they'll either be from the National Enquirer or wearing white coats and carrying a straight jacket! :D

[banktoturn]

PoDuck,

You are confusing propulsive efficiency with thermal efficiency. Both pressure and absolute temperature ratios play into thermal efficiency.

Gerry

Sail 'n Soar,

I didn't realize this was a response to me. I must disagree; I don't think that I am confusing propulsive efficiency with thermodynamic efficiency (which is NOT the same as thermal efficiency, btw). I did not claim that increasing the bypass ratio would improve thermodynamic efficiency. I only made that claim about increasing the pressure in the combustion chamber, and that claim is correct. It is possible to encounter diminishing returns by pursuing higher pressures there, but that was not the question at hand.

banktoturn

[Sail 'n Soar]

They solve the problem by ingesting more air than is needed for combustion, and passing a fraction of it through the outer portion of the first stage's radius and bypassing the combustion chamber.

Peace

[Sail 'n Soar]

Full scale turbojet engines have the same issue of begin better at generating mechanical power than thrust, directly. They solve the problem by ingesting more air than is needed for combustion, and passing a fraction of it through the outer portion of the first stage's radius and bypassing the combustion chamber. The first stage is used as a fan, and as a result these are called turbofan engines. Generally, this is how the more efficient engines work.


Thought my prior post might have been a little too brief. banktoturn's description is as good a description as any of bypass air. The more air you have go around the combustion chamber,along with the turbomachinery associated with it, aka, the core, the higher the bypass ratio and the higher the propulsive efficiency. Propulsive efficiency increases as the exhaust velocity approaches the free stream felocity. Thrust increases at the same time as the engine acts on a larger mass of air. So thrust increases and propusive efficiency increases, but the thermodynamic efficiency has not necessarily changed. In fact, with the additional loss elements, it may even have dropped. Thus my prio statement.

[Sail 'n Soar]

Clarification: I was referring to both the fan and engine core exhausts when I was describing the difference between the engine exhaust and the free stream velocities.

[John Lueke]

PoDuck

What happens if you run your engine with no load on the dyno? Does it stabilize at a safe rpm? You said that it provides more torque through the rpm range than other turbine engines, so it sounds like you have a load on the shaft during most of the running. Tell us more about how you do the test.

[Sail 'n Soar]

I seem to be getting between 70-80% thermal efficiency out of my gas turbines. Once I was sure that it jumped to 88%, but I couldn't reproduce that. That's so high that I'm convinced that I'm not calculating it right. I guess I should ask what the normal way of calculating thermal efficiency is in order to see if I'm doing it right. I assumed I was doing it right since I was calculating about a 40-50% efficiency from a turbocharger based design that I started with. I don't think I could put my temperature gauges any closer to the intake or exhause of my turbine. I would also like to know how to calculate the mechanical efficiency.

Any help would be appreciated.

Thanks

Looking back at your original post it suddenly came to me that the argument over your engine's efficiency is a red herring. In any case, you certainly aren't going to reach agreement here. An easier to measure universal measure is specific fuel consumption. SFC is applicable whether you are speaking of jet engines or power extraction engines. Rather than enter another open ended debate over definitions I refer you to the following site. http://142.26.194.131/aerodynamics1/Performance/Page4.html

Ultimately, this directly measurable figure captures the impact of both thermodynamic and propulsive efficiency, and fuel energy value. In any case, SFC will capture the real benefit of your engine compared to other approaches.

I have no clue what the SFC range of model turbines is, but ceck out this NASA simulator for representative values for full scale engines. http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html There you can vary bypass ratio, flight speed, altitude, throttle position, etc.

Ideally you should be able to measure the thrust vs. flight speed vs throttle position test data for your engine and compare it with others of the same size/weight/operating envelope class. Nevertheless, assuming your exhaust velocities are significantly higher than anticipated max flight velocity, determinig SFC based on static thrust is still a good comparison metric.