Heat engine Definition and 21 Discussions

In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state temperature. A heat source generates thermal energy that brings the working substance to the high temperature state. The working substance generates work in the working body of the engine while transferring heat to the colder sink until it reaches a low temperature state. During this process some of the thermal energy is converted into work by exploiting the properties of the working substance. The working substance can be any system with a non-zero heat capacity, but it usually is a gas or liquid. During this process, some heat is normally lost to the surroundings and is not converted to work. Also, some energy is unusable because of friction and drag.
In general, an engine converts energy to mechanical work. Heat engines distinguish themselves from other types of engines by the fact that their efficiency is fundamentally limited by Carnot's theorem. Although this efficiency limitation can be a drawback, an advantage of heat engines is that most forms of energy can be easily converted to heat by processes like exothermic reactions (such as combustion), nuclear fission, absorption of light or energetic particles, friction, dissipation and resistance. Since the heat source that supplies thermal energy to the engine can thus be powered by virtually any kind of energy, heat engines cover a wide range of applications.
Heat engines are often confused with the cycles they attempt to implement. Typically, the term "engine" is used for a physical device and "cycle" for the models.

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  1. F

    Calculating pressure from a known equation of energy

    My attempted solution is as follows: Obviously the heat transfer happens during transitions 1->2 and 3->1. It's also clear that P1 = P3 V1 = V2 E2 - E1 = Integral[T dQ , from state 1 to state 2] E3 - E2 = - Integral[P dV , from state 2 to state 3] E1 - E3 = Integral[T dQ , from state 3 to...
  2. Poorneshwar 2

    I Difference between the thermal and mechanical efficiency

    Hello, I'm confused between the difference between the thermal and mechanical efficiency of a Sterling heat engines or heat engines in general. I hope you could be able to guide and help me. Thank you
  3. Poorneshwar 2

    I Work done by a gas in a heat engine which compresses a spring

    Please, i need help
  4. V

    Energy transformations in an IC engine cylinder

    A drop of fuel is ignited in an engine cylinder, that produces heat, light and sound energies from the chemical energy stored in the drop of oil. What I am not clear about is how heat energy gets transformed into mechanical work? I think the heat energy produced from ignition flows from burnt...
  5. T

    Efficiency of a heat engine

    Homework Statement I am having an issue trying to decipher this question, as I am not sure if it a lack of knowledge on my half or there is an assumption I have to make. Homework Equations ##\epsilon=\frac{W_{total}}{Q_{in}}## The Attempt at a Solution My issue is calculating the heat...
  6. UMath1

    Heat Engine Efficiency and Entropy

    In deriving the Carnot Efficiency, the assumption is made that theoretically most efficient engine will generate no net entropy, meaning that the entropy that enters the system during heat absorption must equal the entropy that leaves the engine during heat rejection. Why is the case? Why would...
  7. patrickmoloney

    Why can the efficiency of a refrigerator (heat pump) be greater than one?

    In a heat engine thermal energy is converted into mechanic energy. In a heat pump or refrigeration cycle - fluid is being circulated between hot and cold wells. But how can the efficiency be greater than 1? Is it because COP_{\text{heating}}>COP_{\text{cooling}}
  8. Toby_phys

    Simple 3 stage engine

    A possible ideal-gas cycle operates as follows: 1. From an initial state (##p_1##, ##V_1##) the gas is cooled at constant pressure to (##p_1##, ##V_2##); Let's call the start and end temperature ##T_1## and ##T_2## 2.The gas is heated at constant volume to (##p_2##, ##V_2##);Lets call the...
  9. M

    2 body engine, final equilibrium temperate and work produced

    Homework Statement 2 bodies with contant heat capacity C and 2C, are initially at temp T and 2T. if a heat engine executing a reversible carnot cycle operates between the two bodies until their temperatures are equal, what is the final temp of the bodies and how much work is preformed by the...
  10. Waggles

    Renewable Energy Concept - Stirling Engine Composting

    So I've got this crazy idea. To put it simply, to use the heat from anaerobic composting to power hot-air engine, specifically a Stirling Engine. Based solely on my own reading, the center of a closed composting heap can reach anywhere from 120-160 degrees without killing the microorganisms...
  11. E

    Temperature in a Carnot heat engine

    Homework Statement A Carnot heat engine takes 95 cycles to lift a 10 kg. mass a height of 11 m . The engine exhausts 14 J of heat per cycle to a cold reservoir at 0∘C. What is the temperature of the hot reservoir? Homework Equations η=1-(Tc/Th)=W/Qh The Attempt at a Solution I've tried...
  12. R

    Heat Engine problem in Thermophysics

    Homework Statement Im working on the following problem and could need some help in answering them: Work is being produced from a cycle. In order to produce this work, energy is being taking from a high temperature sources at a ratio of 1000 kJ/kg and the extra energy is being deliver to a...
  13. L

    How to derive this heat input equation?

    There's one equation that I've seen being used already, which by itself is quite simple, but I can't understand where it comes from. The context is as follows: suppose we have one heat engine operating between systems A and B whose temperatures are T_A>T_B. Let's suppose further that the...
  14. Tulio Cesar

    Thermodynamics - Heat engines

    My question is: according to Carnot cycle, the maximum efficiency of a heat engine is given by 1 - T2/T1, where T2 is the temperature of the cold source and T1 the temperature of the hot source. So, accordingly, as higher T2 is for a same T1, lowest is the efficiency of the engine. But, the...
  15. Willfrid Somogyi

    Heat Extracted and Delivered to Reservoirs in a Heat Engine

    NO TEMPLATE---MISPLACED HOMEWORK So it seems like a pretty simple question, and in all likelihood it is, but my lecturer somehow managed to miss this bit in his lecture notes. A heat engine operates between 500K and 300K with 20% of the efficiency of Carnot engine operating between the same...
  16. L

    Why does heat engine need to do negative work to surround?

    Every thermodynamics cycle needs to do negative work to the environment, which lower its total positive work. For example, in Carnot cycle, the most efficiency possible: 1/ Engine receives heat from hot reservoir, expands and do positive work to surround 2/ Surround does work to engine...
  17. M

    Stirling Cycle Problem

    Homework Statement Suppose that 282 moles of a monatomic ideal gas is initially contained in a piston with a volume of 0.81 m^3 at a temperature of 574 K. The piston is connected to a hot reservoir with a temperature of 1365 K and a cold reservoir with a temperature of 574 K. The gas undergoes...
  18. vetgirl1990

    Efficiency of a Carnot Engine

    The efficiency of a Carnot Engine is described by the relationship: Tc/Th = Qc/Qh, so that e(Carnot) = 1 - Tc/Th For heat engines, can their efficiency also be related to temperature as well? Or is the description of their efficiency just: e(heat engine) = W / Qh = 1 - Qc/Qh I am inclined to...
  19. A

    Thermodynamics - Carnot Cycle-esque question?

    Homework Statement A reversible heat engine produces work from the temperature difference that exists between a mass of m = 9 kg of an ideal gas (cv = 716 J/kgK, R = 287 J/kgK) in a rigid container and a heat reservoir at THR = 285 K. The only heat transfer interaction experienced by the...
  20. S

    Heat Engine Problem

    Homework Statement You have an infinite heat reservoir with temperature Th. But you’ve only got a finite cool reservoir, with initial temperature Tc0 and heat capacity C. Find an expression for the maximum work you can extract if you operate an engine between these two reservoirs. Homework...
  21. D

    Can a thermodynamic heat engine include all these processes?

    Homework Statement Hi, I'm a new user and got this problem. Sorry to bother, but I couldn't find any relevant information from my Physics books.. The problem states: Build a heat engine using these processes: adiabatic, isothermal, isochoric and isothermal. The work-cycle of the engine must...