# General issues on cycles

This session introduces the concept of cycle and the corresponding notations which will be used in the analysis of systems.

## Course reference:

• “Cycles / energy and exergy balances”

To follow the presentation, go to the next step

(Session realized on 06/16/11 by Renaud Gicquel)

# General issues on cycles

• Closed cycles
• External combustion engines
• Refrigeration cycles
• Open cycles
• Internal combustion engines

# General issues on cycles

• Cycles running forward
• compressible fluid engines
• condensable fluid engines
• internal combustion engines
• external combustion engines

# General issues on cycles

• Cycles running in reverse
• refrigeration cycles
• heat pump cycles
• mechanical vapor compression cycles

# Efficiency (or effectiveness)

• heat engine
$\eta =\frac{\tau }{Q}$

# Heat provided

• Heat released: Q0
• Losses by convection, conduction and radiation from the boiler or the combustion chamber
• Losses from flue gases
• Heat supplied to the fluid: Q1

# Power produced

• Net power produced by the machine τ
• Thermodynamic efficiency, or internal efficiency ηi, ratio of power produced τ to Q1
• Auxiliary equipment, friction…

# Definition of efficiency

• Ratio of the useful energy effect to the purchased energy put in

# Carnot effectiveness

• Purely reversible cycle:
• without friction
• heat exchange between the fluid and the external sources is reversible (with no temperature difference)
• intermediate phases: adiabatic and reversible, i.e. isentropic

# Carnot effectiveness

• Engine cycle

$\eta =\frac{{T}_{1}-{T}_{2}}{{T}_{1}}$
• Regeneration cycle

$\eta =\frac{{T}_{2}}{{T}_{1}-{T}_{2}}$

$\eta =\frac{{T}_{1}}{{T}_{1}-{T}_{2}}$

# Differences with Carnot cycle

• difference in temperature between the machine and the hot and cold sources
• hot and cold sources non isothermal
• mechanical irreversibilities
• temperatures reached at the outlet of compression and expansion not equal to those of the hot or cold sources

# Theoretical and real cycles

• Technological difficulties
• Theoretical cycles:
• thermodynamic reference
• determine the limits in terms of efficiency
• guide the engineer in his approach to improving engines

# General issues on cycles

This session allowed us to define the concept of cycle efficiency and to make some recalls on the theoretical Carnot cycle.