Final control questions on the subject “heat engineering” The purpose and function of the subject. Working parameter. Status parameters. Base words and phrases

4. Cycles of heat machines 
In order to get useful work from the heat engine, the following conditions 
must be met: 
1. There must be a working body. Heat and work are exchanged through this 
body. 
2. There should be at least two sources with different temperatures - a heater 
with a high temperature and a cooler with a low temperature. 
3. The work of the heat engine should be cyclic, that is, the working body 
should return to its initial state after several processes. 
As a result, the useful work of the cycle becomes zero. 
Thus, by successively repeating the 1-a-2-v-1 cycle in which heat is supplied and 
removed, a periodic heat engine can be obtained. We introduce a new concept of the 
coefficient called the thermal efficiency of the cycle (f.i.c.). The ratio of the work of 
the cycle to the amount of heat given to the working body in the cycle is the thermal 
f.i.k. of the cycle. is called Determining the thermal efficiency of the cycle and 
according to the above definition, we get the following: 
1
2
1
2
1
1
q
q
q
q
q
q
ц
t







(5.4) 
Thermal f.i.c. of the cycle describes the degree of improvement of any cycle: thermal 
f.i.c. the larger, the more refined the cycle. 
More work is done in a cycle with a large si when the work body is given exactly 
the same amount of heat q1 in the cycle. 
Thermal f.i.k. of heat machines is always less than 1 (or 100%), because not all of 
the heat q1 delivered to the working body is converted into useful work. Part of this 
heat (q2) is given to the coolant (environment). 
Taking into account the above, another interpretation of the second law of 
thermodynamics can be given: "It is impossible to fully convert heat into work 
through a heat engine." 

---

The second type of heat engine, which fully converts the supplied heat into 
work, is called a perpetual motion engine. 
As mentioned above, the existence of the second type of perpetual motion machine 
does not contradict the first law of thermodynamics. However, the existence of such 
a machine contradicts the second law of thermodynamics, because part of the heat 
produced must be transferred to the cooler. 
If the cycle is performed so that the line of compression is located above the line of 
expansion, in this case, since the work of compression is greater than the work of 
expansion, it is necessary to provide work from some external source to implement 
such a cycle. 
As a result of the implementation of the reverse cycle, heat is taken from a cold 
source and given to a hot source; if, as in the case of a straight cycle, we define the 
heat taken from the cold source by q2, and the heat supplied to the hot source by q1, 
then it is inevitable that q1=q2+l. In the reverse cycle, heat q1 is supplied to the hot 
source equal to the sum of the heat q2 taken from the cold source and the heat 
equivalent to the work λ in the cycle. Thus, as a result of the implementation of the 
reverse cycle, the cooling of the cold source occurs. The reverse cycle consists of a 
chiller cycle. 
The degree of improvement of the reverse cycle is determined by the cooling 
coefficient of the cycle. 
2
l
q


(5.5) 
Since a loop consists of feedback processes, the loop itself is also feedback. 
The working body expands from state 1 according to adiabata 1-4, and its 
temperature decreases from T1 to T2. After that, the working body continues to 
expand according to the isotherm 4-3 and receives heat q2 from the cooler with 
temperature T2. The working body is then compressed by 3-2 adiabata and its 
temperature rises from T2 to T1. In the last process, the working body is isothermally 
compressed according to 2-1 and heat q1 is transferred to the heater. Thus, to 
perform the reverse cycle, expending l work on the heater: 
q
1
=q
2
+l 
(5.17) 
heat is transferred.