What's more efficient, a piston or a turbine?

Discussion in 'Physics' started by strantor, May 7, 2012.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    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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  2. Wendy


    Mar 24, 2008
    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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  3. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    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.
  4. mlog


    Feb 11, 2012
    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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  5. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    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.
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
    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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  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    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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  8. Wendy


    Mar 24, 2008
    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.
    Last edited: May 8, 2012
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  9. Brownout

    Well-Known Member

    Jan 10, 2012
    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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  10. atferrari

    AAC Fanatic!

    Jan 6, 2004
    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.
  11. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    Last edited: May 9, 2012
  12. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    I served on a nuclear submarine. Yes, it is smooth, but I don't have much other experience in the way of vessels to compare it to. When we would go all ahead emergency, the boat would do what I can only describe as "peeling out in the water" - it would shake and vibrate and groan and moan. It was fun. Super quiet too. over a billion dollars worth of technology, I am aiming my sights a little lower. I have a feeling that any type of steam turbine (other than possibilities with hobby jet engines and car turbos) is going to require significant mechanical engineering and access to high level tools. The piston engine however is doable with what I have in the garage,
  13. shortbus

    AAC Fanatic!

    Sep 30, 2009
    Have you looked into "Tesla turbines"?

    But the piston type engine is the way to go for a vehicle, no separate gear box/transmission required. A turbine needs to spin at much too high a RPM to be used without a high ratio gear box to slow it down to usable wheel speed. Even the ships use a final drive before the prop, right?

    The biggest reason gasoline engines survived and steam died in cars was the time it took to get a head of steam up to start to move. Gas was instant and took over the car market. With the new flash steam generators and electronic controls it could be a whole new ball game.
  14. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    On US Navy gas turbine ships a double reduction gear shifts it down from about 14000 at the turbine to about 700 rpm on the main shaft with controllable pitch propellers that can adjust from full forward to full backward in seconds.

  15. russ_hensel

    AAC Fanatic!

    Jan 11, 2009
    Efficiency in any heat engine is fundamentally limited by thermodynamics. For all cycles the higher the input T ( for a given output T ) the higher the efficiency. The Carnot cycle is the most efficient.

    In practical situations the turbine often wins in large part because it can with stand higher inlet temperature.
  16. Brownout

    Well-Known Member

    Jan 10, 2012
    Years ago, I read an article from a guy who built a steam trubo engine in his garage, using only good mechanical fabrication skills. He hammered out the "fins" -- for lack of knowledge of the proper term -- by hand. It was a single row of fins attached to a disk. He used it to power his DIY steam cleaner machine.

    Maybe if you poke around a little, you might find the article archive or something similar. I forget where I read it; might have been Popular Mechanics or something like that.
    Last edited: May 10, 2012
  17. xian555

    New Member

    Mar 9, 2017
    1) the assumption of the need for a gearbox is out of context:
    a) the end use of the motor's power is determining: as was indicated by yourself for a jet plane no gearing for a ship gearing
    b) the full power train must be considered not just the engine: an alternative to a gearbox is a gigh frequency generator (mass and volume of generators are inversely proportional to electric power frequency) e.g. 1000 Hp 400 Hz startrer motors for large aircraft jet engines are tiny in comparison with 1000 Hp 60 Hz electric motors
    c) the need for a gearbox is not necessarily an impairment. Overall objectives must be considered:
    i) capital/purchase cost ? turbines high
    ii) nature of load: lots of stop/go or variable powerloading, or mainly operating all the time at an optimal regime? tubines cost effective and optimal for constant regimes
    iii) maintenance costs . turbines are expensive and so is the tooling to do so, although few moving parts there are more partsthan in a piston engine
    iv) technology accessibility: a hobby project is one thing but a real world application is another: in a hobby application to get a turbine engine to work is one thing - run it a few hours, show your freinds, have fun; in a real application turbines run at very high pressures and temperatutes and require very special alloys for components and very expensive very precise machining of every blade - there was mention of how many turbine stages ? 20 ? and how many blades per stage?

    2) the comment that the gas piston engine overtook the steam turbine engine because of time to build upsteam head is covers several things. Several US and European car manufacturers designed and buildsome concept models using gas turbines. Power build up delay has several solutions that exist today and even back then - remember how old the first electric car is? Also the gas turbine engine car wastoo ahead of its time ( remember the Apple Lisa computer?), there was an issue of too hot tubine exhaust gases blowing at back of car, inertia of an industry already heavily committed to manufacturing if piston engines (casts, tooling, parts, know how), general human dislike of significant change, and of course politics.

    Also it's important to distinguish two VERY different engine types:
    I} Internal combustion engines vs external combustion engines
    II} if an engine uses steam or other fluid it is external combustion, otherwise it's an internal combustion engine; each has it's advantages and disadvantages
    III} turbines exist in both internal combustion, as in airplanes, and external as in power plants; this is also true of piston engines
    IV} It is important to also asses if the engine is open or closed cycle: car piston engines and aircraft jet engines are open with exhaust to atmosphere causing pollution and wasting significant energy. External combustion engines can be equipped with more pollution controls and thermal recovery units.
    V} Cooling, that Wendy brought up is an important aspect. However the use of rivers and body's of water for cooling residual steam out of a turbine (or piston) is a "dirty old practice", causing thermal pollution and initially greatly underestimating the impact on the eco system. Use of the large concrete (hyperbolic profile) towers is much less harmful, but much more expensive. Best is "district heating" where low grade steam is used to heat (or cool using ORC) buildings and homes around the plant. In large power plants with steam turbines to turn electric generators, cooling gets a bit exautic using hydrogen for some of the cooling aspects. Hydrogen is extremely explosive, and requires special alloys for circulating as it "eats" the carbon in steels (yes stainless is still basically iron carbon & other metals). Loss of carbon makes for harder more brittlemetal leading to microfractures and failure ( badabing badaboom).
    V} Lubrication when steam is used is a pain in the but. Open steam engines like old locomotives was not an issue as they were open circuit (and wasreful i.e. energy inefficient). The main source of maintenance was cleaning the boiler of scale from using ground water. Inclosed circuit steam systems all water is demineralized, remineralized to equilibrium levels (demineralized water sucks minerals from its container as demineralized water is a tyoe of vacuum) All municipal treated water, at leastfor towns of a minimal size do this as it is easier to completely deminealize and remineralize with only wanted minerals to wanted levels. Then the condensate is de-oxygenated by a physical and a chemical process. Then antioxidants (most are carcinogenic) are added so that ordinary steel pipies (so-caled "black steel" versus stainless steel, galvanized steel, or other material with temperature and pressure capability.) So now we have this perfect "clean" water for machine use (quite toxic to life). This water/steam is expensive. Use of hydrocarbon lubricants for moving parts creates acids that are corrosive. So 3 choices:
    a) use expensive lubricants that are compatible (but will have some entraiment in the condensate return)
    b) use lubricant-free seals that have significant frictiional lossesare expensive and a pain to replace (physically)
    c) do not recirculate the condensate from,say a small steam pump (commonly usedin explosive atmospheres) and pay a little for treating some make up water.
    For lubrication it's a case by case situation

    VI} Nature of fuel source:
    Here the term machine is used to mean motor or compresdor. When fluid or combustion power goes in and mechanical power comes out it's a motor(piston or turbine). The reverse is a compressor (piston or turbine).

    a) piston machines
    i) external combustion
    • can use superheated (dry) steam
    • can use saturated (wet) steam
    • refrigerant
    ii) internal combustion
    • gasoline, diesel
    • propane, butane, natural gas, LPG
    • wood gas, alcohol, biogas
    • Bunker C ( one grade thinner than tar) and MFO
    variable fuel designsare possible e.g. gasoline/propane/natural gas
    and variable quality fuel designs exist e.g for military apps

    b) turbine machines
    ○ forget any fuel thatcan't be easily gasified.
    ○ not usually tolerant to fuel quality.
    ○ any fuel that does not burn clean will result in frequent expensive maintenance (each turbine blade, for efficiency, gas a narrow lift operatng range and any fowling of lead edges quicly results in loss of performance, like ice on an airplane wing)
    ○ no wet steam only dry (super heated steam).
    ○. air intake sucsceptible to engine damage onopen turbine designs (birds air planes, lots on cars and locomotives

    VII} Cost and availability of fuels

    This thread hits one of my soft spots. Although 4 years old I hope my 2cents will generate more.

    Many technologyies of the past that did not take off need to be revisited with a whole new reality of materials (metals, lubricants, fuels, seals, variety of usesfrom markets 10 fold what they were), machining, simulation and modelling, aswell as concern fo environment, new instrumentation, electronics and programming possibilities.
  18. Janis59

    Active Member

    Aug 21, 2017
    RE:""Generally speaking, piston engines are more efficient, but turbines are lighter and smoother running""
    Nice idea, but completely wrong. There is best ever made vehicle engines with slightly over 30% effectivity coefficient and even if one may produce the `ideal` Carnot engine it would have only 49,999%. However the turbines, even most prost like at wind generators has Betz limit of 59% but if it are made to catch all the wind as turbines of electro-stations the typical coefficient is 98...99,9%. For piston engine it is theoretically impossible, but here it is just norm of technologies in the market.
  19. crutschow


    Mar 14, 2008
    Here's an interesting modern piston steam engine that claims high efficiency and is a sealed system.
    But it has yet to be proven.
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