What is Heat engines: Definition and 53 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.

View More On Wikipedia.org
  1. L

    Engineering Reviewing Efficiency of Otto & Diesel Cycle Heat Engines

    Hi, i have this question and don't understand it. can somebody explain what i have to do. i know a ideal engine is a engine running on carnot cycle i what the cycles are i don't know what relative efficiency is and what there looking for (Q) Review the relative efficiency of ideal heat engines...
  2. E

    Thermodynamic sign convention for heat (i.e. in heat engines)

    Just to clarify, I'm aware of the two equivalent expressions of the first law ##\Delta U = Q + W## and ##\Delta U = Q - W## when applied to a certain system, though my question is primarily about ##Q## - for which, as far as I am aware, the convention is almost universally that ##Q > 0## if heat...
  3. E

    Engineering Efficiency of Heat Engines & Refrigerators: Is Impossible Possible?

    For the heat engine: First I converted all the temperatures to Kelvin, ηmax=1-(333)/(1000)=0.667 ηclaim=(1*10^3)/(1.75*10^3)=0.5714 So the heat engine seems to be less efficient than a Carnot heat engine which means it can exist. For the refrigerator: COPmax=(253)/(363-253)=2.3...
  4. M

    Engineering Thermodynamics - Second Law: 2 Heat Engines Connected Between 3 Metal Blocks

    Hi, I posted a similar question recently and gained some insight on these types of problems. However, I am slightly stumped on how to approach this variation of the problem. So I know that: - there is no net change in enthalpy of the blocks and the engine as the processes are reversible -...
  5. F

    How many reversible thermodynamic cycles are there between two heat reservoirs?

    Hi, I was revisiting my (high school level) understanding of thermodynamic cycles and I think I still have some doubts. Last year and more recently I posted a few questions which surely helped me, but I think I need more clarifications. In a nutshell, what I'd like to know is the following...
  6. M

    Heat engines working over time

    Homework Statement An office with a volume of 52.5 m^3 uses a heat engine that outputs 1200 W. Suppose that, initially, the office has a pressure of 1 atm and 20 degrees Celsius before the heater is turned on. After 10 minutes (6000 seconds), what is the final air temperature? Homework...
  7. F

    Heat engines can organize thermal energy

    Hello, I am reviewing some thermodynamics and heat engines. Heat engines are cyclical machines able to convert a portion (just a portion) of the thermal energy extracted from a hot reservoir into mechanical work. The residual thermal energy must dump into a lower temperature reservoir...
  8. G

    Heat injection and work during Rankine cycle

    I have been researching the Rankine cycle. I know that it consists of two adiabatic and two isobaric processes. However, I'm not entirely sure why we ignore the work done during the isobaric processes (this seems to be the case in the analysis I'm looking at). Is that because the work done there...
  9. danyull

    Efficient Heat Engine and Final Temperature Calculation

    Homework Statement Two identical bodies of constant heat capacity ##C_p## at temperatures ##T_1## and ##T_2## respectively are used as reservoirs for a heat engine. If the bodies remain at constant pressure, show that the amount of work obtainable is ##W = C_p (T_1 + T_2 − 2T_f)##, where...
  10. Dr_Jekyll

    I Trying to understand the drinking bird toy better

    I bought the drinking bird toy, and I want to get a better understanding of it, (https://en.wikipedia.org/wiki/Drinking_bird) I understand the basic way it works and that it is a simple heat engine, but I can't seem to find any information that helps me answer the following - I notice all...
  11. Tulio Cesar

    How Do Carnot's Efficiency Formulas for Heat Engines Align?

    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...
  12. S

    How Does a Heat Engine's Efficiency Affect Final Temperature and Work Output?

    Homework Statement Two identical tanks of water are at absolute temperatures ##T_A## and ##T_B## respectively, where ##T_{A} > T_{B}##. The tanks each have a heat capacity ##C##, and they are thermally isolated from their environment. Suppose that a heat engine is installed in contact with the...
  13. S

    Question about Reversible Engines and Carnot Efficiency

    Homework Statement I have a question regarding heat engines that cropped up whilst I was doing a practice question. I will summarise the results I obtained for the previous parts of the question so as to save your time. The highlighted parts of the image are where I am having some issues. I...
  14. M

    How to Calculate Energy and Work in a Stirling Cycle?

    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...
  15. shanepitts

    Exploring the Limits of Heat Engines with Ice Expansion

    Noting that ice expands by about 9%, why isn't it possible ot build a heat engine from this natural process?
  16. N

    Regarding efficiency of heat engines

    I tried to calculate the efficiency of a engine using the formula : Efficiency = (Net Work Done) / (Heat absorbed) And then I tried to calculate the efficiency of respective Carnot engine by two ways : 1) Efficiency = (Net Work Done) / (Heat absorbed) 2) Efficiency = 1 - (Temperature Lowest /...
  17. T

    How Does the Universe Use Temperature Differences to Create Structures?

    The main result of thermodynamics is that if you have a place that is hot and another place that is colder, you can operate a heat engine by absorbing heat from the hot place and dumping it in the cold place, extracting some useful work in the process. It gives you a way to calculate the...
  18. A

    Why the Carnot Cycle? | Understanding Efficiency in Heat Engines

    1-Is the efficiency of heat engines working in cycles other than Carnot independent of the nature of substance used? Can we still claim that maximum efficiency in converting heat to work is attained during reversible processes for such cycles? For which engines/cycles can we do this? 2- Why is...
  19. V

    What is the Ideal System for Running a Nuclear-Powered Heat Engine in Space?

    (I'm not sure I put this thread in the right sub-forum) If we need to operate a nuclear-powered heat engine in space to run a generator, meaning it can only be cooled by radiation, what kind of system should we go for, if we want the power per unit mass ratio to be as high as possible? I've...
  20. M

    Efficiency of Stirling heat engines

    Not sure if this should be posted in the general physics forums or in the hw/coursework section. I'm in my second semester of general physics and have to do a class project. For my project I built an alpha type Stirling engine. In the paper I need to write I need to explain how it works using...
  21. C

    Can Heat Engines Be Built? An Exploration

    Homework Statement a heat engine draws 200J of heat from the hot reservoir at 400K produces 110J of useful work and dumps 90J into the cold reservoir at 200K could this engine be built why or why not? Homework Equations PV=NkbT The Attempt at a Solution Not Sure on how I would set...
  22. B

    Heat engines and finding efficiency

    Hello, Homework Statement Engine A receives four times more input heat, produces six times more work, and rejects three times more heat than engine B. Find the efficiency of (a) engine A and (b) engine B. Homework Equations e = |W|/|QH| e = 1 - |Qc|/|QH| The Attempt at a...
  23. H

    Heat Engines and Work: Combining Two Engines

    Homework Statement There is a heat engine with Th on top at 100 J pointing down, Tc on bottom pointing down with 50 J and the work output is 50 J pointing right out of the engine. (ENGINE 1) a) Draw the diagram for a carnot refrigerator which draws 100 J of heat from the hot reservoir (Th...
  24. I

    Heat engines: how can we yield work?

    So, I'm studying for the MCAT. There..I've said it. :) But, it's not a homework question: I just don't understand the concept. My study book's thermodynamics section includes a very rough sketch of a heat engine to describe the 2nd Law of Thermodynamics. A side note in the text says that if...
  25. P

    ATH 101: Efficiency of Isobaric Expansion for Monatomic and Diatomic Gases

    In a heat engine we ultimately care about the efficiency of the entire cycle. That is, what percentage of the heat which flows into your system is converted to work. However, you could certainly ask that same question for an individual step in a cycle. Specifically, determine the efficiency of an...
  26. C

    Calculating Gasoline Needed for 50.0hp Heat Engine Output

    Homework Statement When gasoline(density=.7297 g/cm^3) is burned, it gives off 5.00 times 10^4 J/g(its heat of combustion). If a car's engine is 25% efficient, how much gasoline per hour must it burn in order to develop an output of 50.0hp. 1hp=746 W Homework Equations P=W/t e=W/Qh...
  27. M

    PV diagrams for heat engines are legitimate ?

    PV diagrams for heat engines are "legitimate"? Hello forum Since my student days i was wondering about how is it possible to represent the cycle of the gas in a heat engine (e.g. otto cycle) in a PV or whatever diagram when it is obvious that the gas is not in equilibrium during the process...
  28. G

    Why is cooling the gas necessary for efficient heat engines?

    By heating a gas inside a cylinder, you allow it to expand against a piston while maintaining a constant temperature. The energy that you add as heat goes into the PV work of the gas against the piston. To start again, you have to push the piston back to the start point, compress the gas...
  29. A

    Heat Engines, Entropy and Counter-Intuitiveness

    I am not a physicist and have no mathematical understanding, but I am very interested in the fact that heat engines need a cold sink, simply by virtue of the Second Law of Thermodynamics. As I understand it, as the temperature of the cold sink rises, its entropy increases at a faster rate than...
  30. R

    Why is gasoline preferred over diesel for cars?

    even though the thermal efficiency of diesel engine is more than that of gasoline engine , why is it that gasoline is preferred over diesel for cars?
  31. Redbelly98

    Phase change in heat engines & heat pumps

    This question has been in the back of my mind for a while. In short, is having a phase change between liquid and vapor more desirable for a heat engine or for a heat pump? Clearly, having a large area enclosed by the loop in a P-V diagram increased the amount of work in the cycle, so...
  32. G

    Why is 100% efficiency impossible for heat engines?

    My question involves heat engines. I understand that a heat engine typically uses energy provided in the form of heat to do work. According to the 2nd law of thermodynamics, however, not all heat energy can be converted into work energy, meaning that heat engines are not perfect. At least some...
  33. C

    Need help with a question involving heat engines, the carnot cycle and entropy.

    The Attempt at a Solution For a) I imagine it is easier to control the temperature of the hot reservoir because you could use a constant heat source e.g. bunsen burner. c) melting/freezing water. d) Entropy of the environment increases and the entropy of the universe always increases...
  34. M

    How Does a Heat Engine's Efficiency Impact Its Exhaust Heat Rate?

    Homework Statement A heat engine operates between 40°C and 380°C. Being a real engine, its efficiency is only 60% of that theoretically possible for a Carnot engine at these temperatures. If it absorbs heat at a rate of 60 kW, at what rate does it exhaust heat? Homework...
  35. B

    Thermo - Heat Engines, Heat Pumps

    So I wrote my Thermodynamics final today and one of the questions were as follows: A reversible heat engine supplies a reversible heat pump. Both draw off of a temperature reservoir T (can't remember the exact value, but there was one). The heat engine discharges heat into TL, and the heat...
  36. R

    Thermal Efficiency and Heat Extraction in Heat Engines: Figure Analysis

    1. What is the thermal efficiency for the heat engine shown in the figure? What is the heat extracted from the hot reservoir for the heat engine shown in the figure?t Figure shown here: 2.Wout = 1/2 B*H Also, Qc=Qh-Wout, efficiency: Wout/Qh 3. Wout = .5 * 200KPA*...
  37. S

    Thermal Radiation and Heat Engines

    Hey all, This is a simple question: Lets say we have a solid hot-body in outer-space that's emitting thermal radiation. Assuming that it doesn't sublimate, will it simply radiate black-body radiation until it reaches absolute zero? This doesn't sound right. Thanks
  38. J

    Calculating Work and Efficiency of an Aircraft Engine | Heat Engines Homework

    Homework Statement An aircraft engine akes in 9000 Joules of heat and discards 6400 J each cycles a) What is the mechanical work output of the engine during one cycle? b) What is the thermal efficiency of the engine? Homework Equations e=W/Qh The Attempt at a Solution A) Qh =...
  39. M

    Thermo reversible heat engines physics question

    I am having trouble with this question...It is from "applied thermodynamics for engineering technologists" by Eastop (Q5.2 if anyone has it)... The Question: Two reversible heat engines operate in series between a source at 527°C and a sink at 17°C. If the engines have equal efficiencies and...
  40. W

    How does entropy affect heat engines?

    I'm having trouble trying to get my head around the role entropy plays in a heat engine. When the energy from a hot reservoir enters the engine, it brings a certain amount of entropy with it. Then, the "waste heat" (which is energy) gets rid of the entropy. But, why does it take less energy to...
  41. H

    Heat Engines and the Carnot Cycle

    [SOLVED] Heat Engines and the Carnot Cycle Homework Statement The operating temperatures for a Carnot engine are Tc and Th= Tc + 55K. The efficiency of the engine is 15%. How do i find Tc?
  42. A

    How Much Work Can a Reversible Heat Engine Generate?

    [/B]Homework Statement What is the maximum amount of mechanical work that can be got by a reversible engine working from a finite heat source of thermal capacity, C=1000J/K, if it is initially at 373K and the engine discharges to a bath of virtually infinite heat capacity at 273K? [You may...
  43. B

    Actual vs MTE Efficiencies of Heat Engines

    The problem asks: A heat engine takes in 1000J of energy at 1000K and exhausts 600J at 500K. What are the actual and maximum theoretical efffiencies of this heat engine? I know the MTE is 1-Tc/Th = 1-500/1000 = .50 = 50% I'm not sure if I calculate the actual the same way with the Joules...
  44. B

    How Does Torque Relate to Heat Engines and the Second Law of Thermodynamics?

    we are learning about heat engines and the 2nd law of thermodynamics...and i don't know how to relate torque to these concepts... the question is... a multicylinder gasoline engine in an airplane, operating at 2500rev/min, takes in energy at 7.89E3 J and exhausts 4.58E3 J for each revolution...
  45. E

    How do you calculate the net efficiency of a series of heat engines?

    Homework Statement Suppose that two heat engines are connected in a series, such that the heat exhaust of the first engine is used as the heat input of the second (attached diagram below). The efficiencies of the engines are e1 and e2, respectively. Show that the net efficiency of the...
  46. M

    Kitty's Guide to Heat Engines, Refrigerators, & Air Conditioners

    Could someone explain to me how heat engines and refrigerators and air conditioners work. I'm very confused about the process and if they are isothermic, isovolumetric, or adiabatic. ~Kitty
  47. N

    Diesel Cycle - Heat Engines - Thermo.

    Hello, If anybody could answer this question for me that would be great. I have been wondering about it all morning. In the Diesel cycle for heat engines, on the second stage of operation, the Volume increases while pressure stays the same. I understand how this can be accomplished by raising...
  48. N

    Diesel Cycle - Heat Engines - Thermo.

    Hello, If anybody could answer this question for me that would be great. I have been wondering about it all morning. In the Diesel cycle for heat engines, on the second stage of operation, the Volume increases while pressure stays the same. I understand how this can be accomplished by raising...
  49. Reshma

    Heat Engines: An Undergraduate Guide to Books

    Could anyone suggest a book on heat engines at undergraduate level?
  50. Clausius2

    Uncovering the True Nature of NOx Production in Heat Engines

    As a future engineer, my proffessors teach us to be concerned about pollutant emissions. I always hear about NOx production inside heat engines and turbomachinery. But little is explained about the true nature of this problem. Usually, we are told that NOx is produced at high temperatures...