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Internal combustion engine efficiency

  1. Feb 19, 2015 #1
    hello

    we know that there is 42% limitation in the efficiency of internal combustion engine, thermodynamically calculated by Carnot and we can never overcome this

    can someone explain me in few simple words why this is true?

    also, does this apply to the energy from internal combustion to generate electricity via a electrical generator?

    thanks!
     
  2. jcsd
  3. Feb 19, 2015 #2

    DrClaude

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    That number actually depends on the temperatures at which the engine is operating. There is also another thermodynamic cycle, the Otto cycle, which represents an ideal internal combustion engine and has an even lower upper limit on the efficiency.

    I can't give you a thermodynamics course online. The simplest answer is that a cyclic engine requires a hot source at Th and a cold sink at Tc, and the Carnot cycle is the most efficient thermodynamics engine working between these two temperatures and satisfying the 1st and 2nd laws of thermodynamics. Other cycles will lead to the production of additional entropy, meaning that more heat has to be dumped in the cold sink, leaving less heat to do actual work.

    You will have additional losses when converting the work into electricity, so the overall efficiency is going to be lower.
     
  4. Feb 19, 2015 #3
    I think you can separate your question in two parts:

    1) Why is the Carnot Efficiency the limit and can never be overcome?
    The Carnot heat engine is an idealization that does not lose heat to the surroundings or work due to friction. You get the top Carnot efficiency considering that all the heat available when burning a combustion product is transformed into work. The Carnot theorem is a relation between the temperature of the “hot source” (the flame of the combustion) against the temperature of the “cold sink” (the environment where you are dumping the residuals from the process). In ideal conditions, the temperature of combustion of a particular fuel is fixed. On the other hand, you can put your engine in the most cold environment possible, let’s say somewhere near the north pole where you have the lowest temperatures on earth. This would give the best Carnot efficiency possible. Actually, the colder the surrounding environment, the better the efficiency of a heat engine, but here on Earth that has a limit. (Note: remember that to calculate the Carnot Efficiency the temperature has to be set on Kelvin, so no such thing as a “negative temperature” is possible).
    In the real life however, not all the available temperature difference is transformed into energy. In a real heat engine many things happen to keep you away from the Carnot Efficiency, for example you cannot be sure of a perfect combustion; most of the heat released by the combustion is taken away in the refrigeration circuit; some work is lost in friction in the engine’s parts in movement, etc. So you can never beat the Carnot Efficiency which doesn’t take any of that into account.

    2) does this apply to the energy from internal combustion to generate electricity via a electrical generator?
    Here you are mixing to things. On one hand there is transformation of the chemical energy stored in the fuel into mechanical energy. For that, the engine releases the chemical energy in the form of heat, which then is used to generate work. As we have discussed the top efficiency for this process would be the Carnot Efficiency. On the other hand, you have the transformation of the already obtained mechanical energy into electrical energy through the generator. In this case the efficiencies are very high (up to 98% if I recall correctly).

    To sum it up, the most important looses of efficiency occur in the process of getting the available chemical energy in the fuel and transform it into mechanical energy. After that, the mechanical/electrical transformation is very efficient.
     
  5. Feb 19, 2015 #4
    thanks for your reply

    is there an zero-prerequisities tutorial that starts from zero knowledge and builds up to Carnot circle?

    I did that in school but I remember nothing

    wikipedia articles don't help much
     
  6. Feb 19, 2015 #5
    There is a "Thermodynamics for Dummies" book... not to call you a "Dummy" or anything like that hehe.I have a lot of "for Dummies" myself. I haven't check that out, but those but are usually very good for that kind of learning.
     
  7. Feb 19, 2015 #6

    DrClaude

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  8. Feb 19, 2015 #7
  9. Feb 19, 2015 #8

    DrClaude

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    I disagree here. The "cold sink" temperature in the internal combustion engine corresponds to the lower temperature of the burnt fuel mixture after expansion ("exhaust"). This temperature will be much higher than the ambiant temperature. Again, look at the Otto cycle for something closer to the actual workings of an internal combustion engine, while still being "ideal".
     
  10. Feb 19, 2015 #9

    SteamKing

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    It's not clear for which internal combustion cycle there is a 42% limitation on efficiency. You should cite some sources for this figure.

    There have been large slow-speed diesel engines constructed to power ships which exceed this 'limit', and there are also Brayton cycle engines (better known as gas turbines) which have also had better efficiency.

    http://en.wikipedia.org/wiki/Thermal_efficiency

    http://en.wikipedia.org/wiki/Diesel_engine

    http://www.power-technology.com/features/feature1084/
     
  11. Feb 19, 2015 #10
    no, no gas turbines, we are talking about internal combustion engines
     
  12. Feb 19, 2015 #11

    SteamKing

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    Technically, gas turbines are internal combustion engines. They don't use pistons in cylinders like SI or CI motors, though.
     
  13. Feb 19, 2015 #12
    Yes, you are right! It was a misconception from my side. However, I still think that efficiency of internal combustions engines increases with lower temperature of the environment. Is that wrong?
     
  14. Oct 5, 2015 #13
    Seems like people are talking efficiency based on heat only. The real bases for efficiency is brake specific fuel consumption. Horsepower per hour for a given quantity of fuel. Based on heat is very misleading. A standard gas engine rated 25% efficiency will have less than 1/8th the horsepower output per hour for the same quantity of fuel in a VRE engine. Currently a classified design. My point is heat and actual power output in horsepower can be missleading
     
  15. Oct 5, 2015 #14

    cjl

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    You'll have to provide some sources for your claims if you want anyone to take you seriously, especially the claim that a "classified VRE engine" (whatever that is) can make 8 times the horsepower on the same fuel flow rate as a 25% efficient gas engine (this is impossible, by the way).
     
  16. Oct 5, 2015 #15

    russ_watters

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    Horsepower per hour is a meaningless unit. The horsepower output at a given set of conditions does not vary over time.

    I share cjl's skepticism, but note that PF is not a place for discussion of personal theories, if that is what this is.
     
  17. Oct 5, 2015 #16
    Specific fuel consumption based on constant horsepower output.
     
  18. Oct 5, 2015 #17
    How it that impossible? My comparison is based on mechanical power output. You base your idea of impossible on the limited knowledge you possess. If you really think the standard crankshaft engine is the best idea then you are lost in the past.
     
  19. Oct 5, 2015 #18
    8 times the power is 8 times the useable mechanical power. Same quantity of fuel and pressure inside the combustion chamber. 8 times the torque. Torque and rotation equals horsepower something we all know.
     
  20. Oct 5, 2015 #19

    Nidum

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  21. Oct 5, 2015 #20

    cjl

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    Because the efficiency is the proportion of the chemical energy input that is transformed to usable mechanical output. 25% efficiency means that 25% of the input energy is turned into usable work at the output shaft. 8 times as much power with the same input fuel would mean that the output shaft was being driven with twice as much energy as the input fuel had, which is clearly impossible (and claiming it is possible constitutes a claim of a perpetual motion machine, which is against the rules of this forum). I'm not claiming modern engines are perfect - far from it in fact, but an engine's efficiency will always be <100%.
     
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