What is Heat engine: Definition and 162 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.
The efficiency of a heat engine is calculated as ##\eta = |W|/|Q_h| = 1- |Q_c|/|Q_h|##. If this engine operates between the temperatures ##T_c## and ##T_h##, then Carnot's theorem states that ##\eta<\eta_C = 1-T_c/T_h##. This means ##T_c/T_h < |Q_c|/|Q_h|##.
Now assume that the heat engine is...
I didn't have much trouble with part a but I'm struggling with b,c, and d. I considered the efficiency formula for a heat engine e = work done by engine/ qh
but i am unsure of how to approach it.
for part c) not sure how i can get to Tc without knowing Th
for d) my gut is telling me 5/2 but i...
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...
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
https://news.mit.edu/2022/thermal-heat-engine-0413
This team that developed this device claims it is capable of generating electrical current from a 1900C heat source via the photovoltaic effect.
I was wondering if the generated electricity was put through an initially 1901C electrical...
Consider mixing ice water with the hot water and vice versa so that the two reservoirs are closer to the same temperature. What will happen to the mechanical/thermodynamic efficiency and ideal Carnot efficiency of a heat engine?
My book states the answer to this problem is .
I have gotten very close to the answer. My problem is my Tc/th and tc/tm are flipped compared to the solution.
I feel like I am missing something in my algebra but can't see where I am going wrong. Could I get some help identifying where my...
Through an intriguing fictitious dialog between Sadi Carnot and Robert Sterling, Prof. Israel Urieli of the Ohio University shows that it is not required to invoke entropy, the second law of thermodynamics, and the Carnot cycle with the [ideal] adiabatic processes in order to find out the...
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...
So efficiency is W/Qin.
W= 0 for isochoric processes and for the isobaric, P(change in V). So W=Pi(Vi-Vf)+Pf(Vi-Vf)
Qin is negative Qs.This would happen at step 2 and 3. For the isobaric, Q=ncv(change in T) and for isochoric, Q=ncp(change in T).
Now if I put everything in the equation I get...
I have a question about building efficient heat engines in outer space. In theory you could have a hot reservoir heated by the sun that was several hundred degrees C, and a cold reservoir that was very cold - maybe 50K - 100K or even colder. Thus, theoretically at least, a heat engine could be...
Nota that the path a to b is isothermal since J is proportional to L, then we can find the value of T_a and T_b using the equation of state and the figure. We have,
\begin{equation}
J_0=\alpha L_0T_b
\end{equation}
or
\begin{equation}
T_b=T_a=\frac{J_0}{\alpha L_0}=T_0
\end{equation}
Also, by...
If a heat engine converts heat into "work", will ice used to run a Stirling heat engine last longer than ice allowed to melt by itself?
To try and answer this, I obtained a Stirling engine and ran this experiment:
With the engine running:
And not running:
Without the engine running the...
A Rankine Cycle heat engine uses ammonia as a working fluid, turbine entry temperature is 25 Bar at 60 degrees Celsius.
The turbine outlet pressure is 4 Bar, the question is: what is the outlet temperature? and if the output power is 6 Gigawatt (6x10^9 watt) what would the mass flow rate of the...
Hi,
I am quite confused about how to approach this problem. I have seen variations of this problem where there is a heat engine between two blocks, but in this case the surroundings are massless, so I don't believe that approach will work here.
Method:
I have first started with the case that...
My attempt:
I though :
ΔQ_w= 1*4200 * (-100) J=-420000J
Q_ice=334000*m_ice = ΔQ_w
But it was totaly wrong!
The solution showed :
Because the heat engine is reversible the efficiency η = 1- (T_cold / T)
T_cold is always 273 K while the hot temperature changes from 373 K to 273 K during this...
Here's my attempt for the first part:
For the first body, the work obtained is
##W_1 = C_P (T_1 - T_f)##
while for the second body, it is
##W_2 = C_P(T_2 - T_f).##
So the net work obtained is the sum of these two:
##W = W_1 + W_2 = C_P (T_1 + T_2 - 2 T_f)##
and that proves the first part...
The first picture was provided along the problem statement. The second has my annotations.
I initially began by calculating the ratio of efficiencies, since the work done is obviously the same and cancels out, but after failing and having seen the form of the solution I saw that that cannot...
Hi,
I was just wondering about the efficiency of a cycle that is not Carnot cycle.
In that case one should use \eta = 1-\left|\frac{Q_{\rm out}}{Q_{\rm in}}\right|, where Q_{\rm in} and Q_{\rm out} are the amounts of heat absorbed and released during the cycle.
For instance, I guess that in...
An ideal diatomic gas undergoes an elliptic cyclic process characterized by the following points in a ##PV## diagram:
$$(3/2P_1, V1)$$
$$(2P_1, (V1+V2)/2)$$
$$(3/2P_1, V2)$$
$$(P_1, (V1+V2)/2)$$This system is used as a heat engine (converting the added heat into mechanical work).
Evaluate the...
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...
Hi all,
the efficiency ##\eta## of a generic heat engine working between two temperatures is bound from above by the efficiency ##\eta_{\rm C}## of a Carnot machine working between the same temperatures.
That is, if the temperatures are the same, a (ideal) Carnot machine is better than any...
We all know how a thermostat works 2 different type of metal are attach together, when temperature changes 1 metal strap expands more than the other strap causing the straight metal strap to bend.
45 years ago when I was in college the professor brought a desk top heat engine to class. He...
Homework Statement
In a reversed heat engine, the work done on the engine is 85 kJ and the heat transfer to the engine from the low temperature region is 260 kJ. Determine:
1. the heat transfer to the high temperature region
2. the coefficient of performance as a refrigerator
Homework...
If this is a valid enough definition of a heat engine:
All heat engines transfer heat energy from higher to lower temperatures, and do work in the process.
... then is the human body a heat engine?
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...
Homework Statement
The emission of radiation from the Sun’s disc is observed to peak at 0.5 μm wave- length and that from the Moon’s disc at 10.0μm. A heat engine to power a Moon base is to be constructed using radiation collected from the Sun. What is the maximum theoretical efficiency of such...
Greetings!
I did the famous "Mass Lifter Heat Engine" experiment in which a mass is put at the top of a piston enclosed within a cylinder. The cylinder is connected to an aluminium canister of air and the air inside this canister expands and contracts the piston with thermal contact. We had 2...
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}}
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...
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...
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...
Homework Statement
I am trying to calculate the efficiency of this heat engine that has two step. an adiabatic compression, followed by a linear expansion back to the original point. I keep getting an efficiency of 1, which I know can't be right...
Homework Equations
##e = \frac{W}{Q_H}##
The...
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...
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...
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...
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...
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...
Hi everyone,
I have been looking at Carnot heat engines in a bid to better understand entropy, and I can't figure out how it actually does work. Why does the piston move?
In some diagrams I have seen weights being removed from the piston, reducing the pressure at constant temperature and...
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...
I had an exam last week and I just got it back today. On the exam was a question that I got wrong even though his wording was terrible (he's from India) and I feel that it was not clearly expressed what he was saying. The question is: "A heat engine is 20% efficient. If it absorbs 500 J of heat...
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...
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...
Homework Statement
A hot gas stream at 600K and 200 kPa is cooled at constant pressure to 300K in a pipe by direct thermal contact with the atmosphere. The mass flow rate of the stream is 0,1 kg/s and the atmospheric temperature and pressure are 300K and 100 kPa. Modeling the gas as an ideal...
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...
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...