TITLE.PM5

632 ENGINEERING THERMODYNAMICS

dharm

\M-therm\Th13-3.pm5

Mean effective pressure pm is given by :

pm =

pvr r

v r

r

11

11

1

11

1 1

γγγρ ργ

γ

−−−− −

− F −

HG

I

KJ

[( ) ( )]

()

or pm =

pr r

r

1 111

11

γγγρ ργ

γ

[( ) ( )]

()()

−− −

−−

−

. ...(13.9)

Example 13.17. A diesel engine has a compression ratio of 15 and heat addition at con-
stant pressure takes place at 6% of stroke. Find the air standard efficiency of the engine.
Take γ for air as 1.4.
Solution. Refer Fig. 13.16.

V

p

V 3

2 3

4

1

Vs

V 1

V 2

Fig. 13.16

Compression ratio, r =

F

HG

I

KJ

V

V

1

2

= 15

γ for air = 1.4

Air standard efficiency of diesel cycle is given by

ηdiesel = 1 –

11

γ^11

ρ

γ ρ

γ

()r −

−

−

L

N

M

M

O

Q

P

P

...(i)

where ρ = cut-off ratio =

V

V

3

2

But V 3 – V 2 =

6

100

Vs (Vs = stroke volume)