TITLE.PM5

498 ENGINEERING THERMODYNAMICS

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∆U 0 is a negative quantity since the internal energy of the reactants includes the potential
chemical energy and heat is transferred from the system.
It may be noted that in case of real constant volume combustion processes the initial and
final temperatures will not be same as T 0 (reference temperature). The change in internal energy,
for analytical purposes, between reactants at state 1 to products at state 2 can be considered in the
following three steps (stages) :
(i) The change for the reactants from state 1 to T 0.
(ii) The constant volume combustion process from reactants to products at T 0.
(iii) The change for the products from T 0 to state 2.

Fig. 11.2 Fig. 11.3
The entire process can be thought of as taking place in piston-cylinder device as shown in
Fig. 11.4.

(i) (ii) (iii)

UR 1 UR 0 UP 0 UP 2

Reactants (i) (ii) (iii)

Internal

energy

Reactants Products Products

Piston

Cylinder

V,T 11 V,T 00 V,T 00 V,T 11

Combus-

tion

Q

Fig. 11.4
Thus U 2 – U 1 , the change in internal energy between states 1 and 2, can be written as
(UUPR 21 − ) to show the chemical changes involved and this can be further expanded for analytical
purposes as follows :

UUPR 21 − = (UUPP 20 − ) + (UUPR 00 − ) + (UURR 01 − )