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Thread: How much power can an exhaust turbine generate?

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  1. #1
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    How much power can an exhaust turbine generate?

    Anyone know how much power an exhaust mounted turbine (turbocharger minus the compressor) can generate at the shaft? Say the amount necessary to generate 1 bar of boost with a compressor?

    I've had one suggestion that around 2lbft at 80,000rpm (around 30bhp) would be a reasonable expectation? Does this sound about right?

    If the engine is designed to run NA and the output of the turbine is used for other purposes, does the turbine itself become a big parasitic loss to engine output or is it still essentially recovering energy from spent exhaust gases that would otherwise be dissipated to the atmosphere?

    TIA for any input!

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    It depends on.......an heavy load of other things not mentioned.
    If the turbine isn't used to turbocharge the engine, yess, it's a loss.
    If the gained power achieved trough the turbine is used to power other auxiliaries that would have used exactly that power...why to use the turbine in first place?
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  3. #3
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    It would be a big paracitic loss but 30hp doesn't sound too far off. Of course it would depend on just what turbine/turbo charger half you were running and what engine you attached it to. What you are asking about is a bottoming cycle. An alternative bottoming cycle would be some type of heat engine (steam turbine, sterling engine, etc)

  4. #4
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    would it be a loss if you were to look at it on a system level? If say, you can use the energy recovered from the exhaust to run aux. system and not take it off the accessory drive of the engine?

    Mind you I think that the efficiency involved might make this not desirable(and probably better spent to make more power on the engine side instead)...
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    A little bit over 9000 hp.
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  6. #6
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    You also have to factor in all the heat that thing generates and cooling it. It'll be less efficient by that point.

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    Not necessarily. The power loss due to a turbo has to do with restricting exhaust gas flow. But most engines are heavily restricted already due to the design of exhaust manifolds. Most engines could be redesigned with an exhaust turbine in place that would make auxiliary power and still be less restrictive than stock exhaust manifolds.

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    Quote Originally Posted by Bob View Post
    Not necessarily. The power loss due to a turbo has to do with restricting exhaust gas flow. But most engines are heavily restricted already due to the design of exhaust manifolds. Most engines could be redesigned with an exhaust turbine in place that would make auxiliary power and still be less restrictive than stock exhaust manifolds.
    Eh, no. The manifold itself can be more efficient, but putting a turbine in there is always going to cause more restriction no matter what. It *has* to in order to turn the turbine. There's no function of the turbine that doubles as the function of the exhaust manifold.

    Really, besides turbocharging the only others people seem to use are regenerative breaking, and I guess suspension flywheels? Basically when you can do the work of another system (resistance) but recover energy.
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    Quote Originally Posted by digitalcraft View Post
    Eh, no. The manifold itself can be more efficient, but putting a turbine in there is always going to cause more restriction no matter what. It *has* to in order to turn the turbine. There's no function of the turbine that doubles as the function of the exhaust manifold.

    Really, besides turbocharging the only others people seem to use are regenerative breaking, and I guess suspension flywheels? Basically when you can do the work of another system (resistance) but recover energy.
    Orly? And how's your mech. engineering degree going? Yes, other things equal, a turbine will cause greater restriction. But I was NOT stipulating that other things be equal. Did you bother to read the rest of the post?

    For example, consider the chevy small block. Case A, stock, it's highly restricted by factory manifolds. Case B, manifolds are replaced by free flowing headers, and some exhaust gases are diverted through a small turbine. It is possible for Case B to be less restrictive overall than A, while the turbine captures additional power for other uses. This does not violate thermodynamics- it simply is more efficient in that more heat energy is captured.

    Also, you should be very very careful making ANY broad statement about fluid dynamics. Anyone who studies them knows the rules and and also knows that there are hundreds of things that we still don't understand.

  10. #10
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    Quote Originally Posted by Kozy View Post
    Anyone know how much power an exhaust mounted turbine (turbocharger minus the compressor) can generate at the shaft? Say the amount necessary to generate 1 bar of boost with a compressor?

    I've had one suggestion that around 2lbft at 80,000rpm (around 30bhp) would be a reasonable expectation? Does this sound about right?

    If the engine is designed to run NA and the output of the turbine is used for other purposes, does the turbine itself become a big parasitic loss to engine output or is it still essentially recovering energy from spent exhaust gases that would otherwise be dissipated to the atmosphere?

    TIA for any input!
    Can you find some exhaust gas temperature and pressure data for a turbocharged engine? I haven't studied gas turbines for a while but isn't it just a matter of figuring out what the temperature loss and pressure rise is across the turbine to determine how much power is being generated?

    Quote Originally Posted by RacingManiac
    would it be a loss if you were to look at it on a system level? If say, you can use the energy recovered from the exhaust to run aux. system and not take it off the accessory drive of the engine?
    I think it would depend on if it is more efficient to extract the energy from the exhaust gas using the turbine, or pulling the energy from the crank (i.e. thru a belt and pulley). Any idea how efficient a typical automotive belt and pulley are? If I had to guess I would say the mechanical system is more efficient than the fluid system.

    EDIT: The above isn’t quite right. I implied that the turbine efficiency needed to be greater than the belt and pulley efficiency and this not true. As you alluded to, the turbine efficiency is irrelevant since the exhaust gas energy is ‘lost’ so it doesn’t matter if you recover it efficiently or not, it is still recovered (but not free).

    Really what I should have said is does the impact of the turbine (i.e. increase in backpressure) create more power loss than that which is consumed by the belt and pulley. That is what will determine which solution is better.

    So for example if you have an ancillary device that needs 50 HP and you can get this from a belt and pulley with 90% efficiency, the engine needs to provide 55.5 HP (5.5 HP are lost). To power the same device with the turbine, lets assume it’s efficiency is 50%, you need to extract 100 HP from the exhaust gases (50 HP is lost). Then the questions becomes can the turbine extract 100 HP from the exhaust gases without reducing the engine crank output by more than 55.5 HP.

    It is an interesting question.
    Last edited by Alastor; 07-07-2009 at 07:48 PM.
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  11. #11
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    Re: Alastor

    That is what I was implying, since on an engine without a turbo, the exhaust energy is for all intents and purpose, lost, a turbo is essentially a way to recover that energy by making the motor itself more powerful by using exhaust energy to improve combustion process thus recovering that energy. As I've said I don't know why you might do this because the efficiency of an exhaust driven accessory charger is probably questionable at best, but I can't help but wonder if it is a noted improvement over say, a non-turbo engine with crank driven accessories drive.

    And alternatively, I suppose you can also use some form of steam turbine to recover the exhaust energy via a heat exchanging process thus perhaps with less impact to flow, though again, this type of arrangement seems less feasible on automotive and more suitable for industrial type operation...
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  12. #12
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    Quote Originally Posted by RacingManiac View Post
    Re: Alastor

    That is what I was implying, since on an engine without a turbo, the exhaust energy is for all intents and purpose, lost, a turbo is essentially a way to recover that energy by making the motor itself more powerful by using exhaust energy to improve combustion process thus recovering that energy. As I've said I don't know why you might do this because the efficiency of an exhaust driven accessory charger is probably questionable at best, but I can't help but wonder if it is a noted improvement over say, a non-turbo engine with crank driven accessories drive.

    And alternatively, I suppose you can also use some form of steam turbine to recover the exhaust energy via a heat exchanging process thus perhaps with less impact to flow, though again, this type of arrangement seems less feasible on automotive and more suitable for industrial type operation...
    I recall one manufacturer experimenting with exactly that system not long ago (heat exchange driving a steam turbine) I think it was BMW but I can't remember.

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    Welcome to UCP.

  15. #15
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    Load is main thing to which it depends, the figure you have given might be average power.

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