Page 130 - Mechanical Engineer's Data Handbook

THERMODYNAMICS AND HEAT TRANSFER 119

Work done (per kg) W= (T, - T2)(s1 -s4) 3.9.3 Otto cycle (constant-volume
Heat supplied (per kg) Q = T, (s, - s4) cycle)

This is the basic cycle for the petrol engine, the gas
engine and the high-speed oil engine. Heat is supplied
and rejected at constant volume, and expansion and
3.9.2 Constant pressure cycle compression take place isentropically. The thermal
efficiency depends only on the compression ratio.
In this cycle, heat is supplied and rejected at constant
pressure; expansion and compression are assumed to 1
take place at constant entropy. The cycle was once Efficiency q= 1 -- rY-l
known as the Joule or Brayton cycle and used for
hot-air engines. It is now the ideal cycle for the closed
gas turbine unit.

3

I
V

1 P V
Efficiency '1 = 1 - -, where r = 2
( p 1 PI
3.9.4 Diesel cycle (constant-pressure
w= Cp(T3 - T4) - cp( r2 - T,) combustion)
Ti
r
Work ratio = 1 - - Although this is called the 'diesel cycle', practical diesel
T3 engines do not follow it very closely. In this case heat is
added at constant pressure; otherwise the cycle is the
same as the Otto cycle.

P

I
V