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  • pratap singh, upendra

    Member • Jun 5, 2012

    actually the term highest thermal efficiency is only relative.

    In this discussion we actually compare the thermal efficiencies of different engines under identical conditions of temperature difference etc. and infer from the mathematical formulations that Carnot engiine has got highest thermal efficiency(among all engines).

    also remember the assumptions that we make during the mathematical analysis of a carnot engine. Those assumptions actually do not apply under practical engines. That is why carnot engine is also termed 'ideal'
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  • mechky

    Member • Jun 5, 2012

    nibhani
    how the work done in carnot cycle is low and mean effective pressure is also low while it is highest thermal efficiency cycle...?
    The efficiency of Carnot cycle is high due to the assumption - isentropic compression and expansion, because of which there are no losses occurring in the cycle. One of the factors determining efficiency is how heat is added to the working fluid in the cycle, and how it is removed. The Carnot cycle achieves maximum efficiency because all the heat is added to the working fluid at the maximum temperature , and removed at the minimum temperature . In contrast, in an internal combustion engine, the temperature of the fuel-air mixture in the cylinder is nowhere near its peak temperature as the fuel starts to burn, and only reaches the peak temperature as all the fuel is consumed, so the average temperature at which heat is added is lower, reducing efficiency.

    The low work output is due to the very narrow strip we obtain in the p-v diagram.
    [​IMG]
    The net work output in area 12341 is small as compared to area 412BA4.
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  • zaveri

    Member • Jun 6, 2012

    i agree with mechky and proffy
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  • nibhani

    Member • Jun 6, 2012

    its an excellent answer...
    mechky
    The efficiency of Carnot cycle is high due to the assumption - isentropic compression and expansion, because of which there are no losses occurring in the cycle. One of the factors determining efficiency is how heat is added to the working fluid in the cycle, and how it is removed. The Carnot cycle achieves maximum efficiency because all the heat is added to the working fluid at the maximum temperature , and removed at the minimum temperature . In contrast, in an internal combustion engine, the temperature of the fuel-air mixture in the cylinder is nowhere near its peak temperature as the fuel starts to burn, and only reaches the peak temperature as all the fuel is consumed, so the average temperature at which heat is added is lower, reducing efficiency.

    The low work output is due to the very narrow strip we obtain in the p-v diagram.
    [​IMG]
    The net work output in area 12341 is small as compared to area 412BA4.
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  • nibhani

    Member • Jun 6, 2012

    nibhani
    its an excellent answer...
    thanks for answering.....
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  • nibhani

    Member • Jun 6, 2012

    proffy
    actually the term highest thermal efficiency is only relative.

    In this discussion we actually compare the thermal efficiencies of different engines under identical conditions of temperature difference etc. and infer from the mathematical formulations that Carnot engiine has got highest thermal efficiency(among all engines).

    also remember the assumptions that we make during the mathematical analysis of a carnot engine. Those assumptions actually do not apply under practical engines. That is why carnot engine is also termed 'ideal'
    my question was that how the work done in carnot cycle is low while it is an highest efficiency cycle...
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  • nibhani

    Member • Jun 6, 2012

    zaveri
    i agree with mechky and proffy
    i am also agree with both...
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  • vikaskumar11233

    Member • Jun 7, 2012

    According to me The Carnot cycle when acting as a heat engine consists of the following steps:

    1. Reversible <a href="https://en.wikipedia.org/wiki/Isothermal" target="_blank" rel="nofollow noopener noreferrer">Isothermal</a> expansion of the gas at the "hot" temperature, TH (isothermal heat addition or absorption). During this step (1 to 2 on Figure 1, A to B in Figure 2) the expanding gas makes the piston work on the surroundings. The gas expansion is propelled by absorption of quantity Q1 of heat from the high temperature reservoir.
    2. <a href="https://en.wikipedia.org/wiki/Isentropic_process" target="_blank" rel="nofollow noopener noreferrer">Isentropic Process</a> (<a href="https://en.wikipedia.org/wiki/Reversible_adiabatic_process" target="_blank" rel="nofollow noopener noreferrer">Reversible Adiabatic Process</a>) expansion of the gas (isentropic work output). For this step (2 to 3 on Figure 1, B to C in Figure 2) the piston and cylinder are assumed to be thermally insulated, thus they neither gain nor lose heat. The gas continues to expand, working on the surroundings. The gas expansion causes it to cool to the "cold" temperature, TC.
    3. Reversible isothermal compression of the gas at the "cold" temperature, TC. (isothermal heat rejection) (3 to 4 on Figure 1, C to D on Figure 2) Now the surroundings do work on the gas, causing quantity Q2 of heat to flow out of the gas to the low temperature reservoir.
    4. Isentropic compression of the gas (isentropic work input). (4 to 1 on Figure 1, D to A on Figure 2) Once again the piston and cylinder are assumed to be thermally insulated. During this step, the surroundings do work on the gas, compressing it and causing the temperature to rise to TH. At this point the gas is in the same state as at the start of step 1.
    5. your answer is related to the 4th point.
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  • agafi04

    Member • Nov 1, 2012

    Does carnot cyce have mean effective pressure formula
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  • VISHNUGOPAN

    Member • Nov 4, 2012

    nibhani
    how the work done in carnot cycle is low and mean effective pressure is also low while it is highest thermal efficiency cycle...?
    Simply saying In a Carnot cycle, we consider all the processes as reversible. Reversible adiabatic and reversible isothermal processes. And from carnot's theorem we know that "All irreversible heat engines between two heat reservoirs are less efficient than a carnot engine (reversible engine) operating between the same reservoirs"........
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