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Old 05-07-2012, 09:12 PM
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Default What's more efficient, a piston or a turbine?

I've entertained the thought of building a radial piston steam engine for a few years. I just happened to acquire 5 pneumatic actuators that are pretty much what I envisioned making the thing with. So the idea is rekindled and I've been devoting more than average thought to it. A few questions have come up - why are steam piston engines no longer used? All the nuke stuff that I'm aware of, that runs on steam, uses turbines. Are turbines more efficient than pistons? Seems to me like pistons would be more efficient since they capture the expanding gas and use it over a period of time instead of just letting it blow straight through; pneumatic power tools seem awfully wasteful to me.
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Old 05-07-2012, 09:34 PM
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My understanding (that and 5˘ will get a cup of coffee) is turbines are much more efficient.

They are much more precision machines too, much higher tolerances and requiring much more exotic materials, though they can be amazingly small. I remember a prototype that weighed only a few pounds several years ago, for micro drones. It produced several pounds of thrust.
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Old 05-07-2012, 11:03 PM
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I get a feeling that this is one of those questions where if I were told X and I asked why, I would be ill equipped to understand the answer.
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Old 05-08-2012, 02:43 AM
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It's not a simple answer to a simple question. However, to oversimplify the answer, the thermal efficiency of an Otto cycle piston engine is a function of the compression ratio. The thermal efficiency of a Brayton cycle turbine engine is a function of the pressure ratio. Generally speaking, piston engines are more efficient, but turbines are lighter and smoother running because of the rotation vs. piston movement.
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Old 05-08-2012, 03:03 AM
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Quote:
Originally Posted by mlog View Post
Generally speaking, piston engines are more efficient, but turbines are lighter and smoother running because of the rotation vs. piston movement.
do you think the same would be true of a radial piston engine? Imagine, all 5 pistons are acting on the crank simultaneously from different angles. I suspect it would be smooth, but I have no evidence or experience behind that suspicion.
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Old 05-08-2012, 03:08 PM
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A piston steam engine is a "low" pressure,slow speed machine. A turbine is a "high" pressure, high speed machine. Thats the simple answer.

For work to be done like in a generator, helicopter, or wheeled vehicle you need to add into the equation the gearing needed to take the high speed of the turbine down to a lower "usable" speed. A jet airplane is using the high speed of the turbine to create thrust so no gearing needed.

The two different types of engines fill two different type of applications.
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Old 05-08-2012, 04:57 PM
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Quote:
Originally Posted by strantor View Post
do you think the same would be true of a radial piston engine? Imagine, all 5 pistons are acting on the crank simultaneously from different angles. I suspect it would be smooth, but I have no evidence or experience behind that suspicion.
Ok I'll bite. I don't think your efficiency will be significantly better from multi cylinder as for the same displacement engine the multicyl has more seal (piston ring) area and more drag. I would consider a single cyl engine (or maybe a 2 cyl?) and large flywheel.

In large scale steam the turbines are very efficient but that drops a lot as the size comes down. Large turbines have many blade sets, maybe as many as 20 and each set harnesses some of the steam energy.

Small turbines get inefficient as the leakage gaps etc are much larger compared to the blade diameter, and you get less blade sets, then you lose efficiency as the RPM goes right up and you get higher bearing losses and add in reduction gearing losses etc before you can get power out.

I would think on a home made setup you could get the most efficient conversion of your steam resource by a piston engine if carefully designed and careful heat insulation.

And really for efficiency you need cold water for cooling to get that max temp differential, which is why they build power stations next to deep lakes and oceans.
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Old 05-08-2012, 05:16 PM
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Classic jet turbines have no cooling, it is fundamental to the design. It can be a weakness for things like cars, but they also can not burn up. While their materials tech tends to be much more advanced, they are much simpler mechanically.

I don't know much about power stations, but being an air force brat taught me the basics of jet engines for aircraft.

You want some interesting reading, google

tiny jet engines

It appears there is a large tech culture making jet engines for small applications, some of them will surprise you.
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Old 05-09-2012, 12:12 AM
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If you think you might want to experiment with a turbin engine, the WWW if full of video of projects that use a trubo-charger to make an engine. It's a pretty remarkable thing to see how these things are repurposed.

Sorry this is a little O/T. I thought it might be of interest.
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Old 05-09-2012, 09:17 AM
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Navigation in a vessel turbine propelled is remarkably smooth. And much less noisy!

Usually, only when giving half or full astern you can say what the engine is doing.

Good for sleeping.
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