Page 104 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

Example 3.3

o
The combustion gases in a petrol engine cylinder are at 30 bar and 800 C before

V 5 . 8
expansion. The gases expand through a volume ratio ( 2 ) of ( ) and occupy 510
V 1 1
3
cm after expansion. When the engine is air cooled the polytropic expansion index n
= 1.15. What is the temperature and pressure of the gas after expansion, and what is

the work output?

Solution to Example 3.1

Data: p1 = 30 bar; T1 = 800 + 273 = 1073 K; n = 1.15

V
3
2 = 8.5; V2 = 510 cm ;
V 1
t2 = ? p2 = ? W = ?

Considering air as a perfect gas, for the polytropic process, the property relation is given

by equation 3.16 as:

V  n 1 −
T 2 = T 1 1 
 V 2 

 1  . 1 15− 1
= 1073x   5 . 8 


= 778.4 K
o
= 505.4 C
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