Ch19 - Chapter 19: The Second Law of Thermodynamics...

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Chapter 19: The Second Law of Thermodynamics Directions of thermodynamic processes ± Irreversible and reversible processes • Thermodynamic processes that occur in nature are all irreversible processes which proceed spontaneously in one direction but not the other. • In a reversible process the system must be capable of being returned to its original state with no other change in the surroundings. • A reversible process proceeds slowly through equilibrium states.
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Heat engines ± Heat engine • Any device that transforms heat partly into work or mechanical energy is called a heat engine. • A quantity of matter inside the engine undergoes inflow and outflow of heat, expansion and compression, and sometimes change of phase. • This matter inside the engine is called the working substance of the engine. • The simplest kind of engine to analyze is one in which the working substance undergoes a cyclic process. • All heat engines absorb heat from a source at a relatively high temperature, perform some mechanical work, and discard or reject some heat at a lower temperature. W Q W Q U U U = = = = 0 1 2 For a cyclic process
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Heat engines ± Energy flow diagram for a heat engine H T 0 > H Q C T 0 < C Q Temperature of hot reservoir Temperature of cold reservoir Heat flow from hot reservoir per cycle Heat flow from cold reservoir per cycle Net heat Q absorbed per cycle: C H C H Q Q Q Q Q = + = Net work done W per cycle: C H C H Q Q Q Q Q W = + = = waste!
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Heat engines (cont’d) ± Energy flow diagram for a heat engine (cont’d) Net heat Q absorbed per cycle: C H C H Q Q Q Q Q = + = Net work done W per cycle: C H C H Q Q Q Q Q W = + = = Thermal efficiency ε: H C H C H Q Q Q Q Q W = + = = 1 1 ε
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Heat engines (cont’d) ± Steam engines (external combustion)
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Internal-combustion engines ± Gasoline engine (a heat engine) • First a mixture of air and gasoline vapor flows into a cylinder through an open intake valve while the piston descends, increasing the volume of the cylinder from a minimum of V to a maximum of rV (r: compression ratio 8-10). • At the end of intake stroke, the intake valve closes and the mixture is compressed, approximately adiabatically, back to volume V during the compression stroke. • Then the mixture is ignited by the spark plug, and the heated gas expands, approximately adiabatically, back to volume rV, pushing on the piston-power stroke.
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Internal-combustion engines (cont’d) ± The Otto cycle : b a : c b : d c : a d compression stroke (adiabatic compression) ignite fuel (heating at constant volume) power stroke (adiabatic expansion) Reject heat to environment (cooling at constant volume) 0 ) ( : > = b c V H T T nC Q c b 0 ) ( : < = d a V C T T nC Q a d b c d a b c H C H T T T T T T Q Q Q + = + = ε 1 1 ) ( : = γ V T rV T b a b a 1 1 ) ( : = V T rV T d c c d 1 1 1 1 1 1 ) ( ) 1 )( ( = + = r T T r T T r T r T T T r T r T a d a d a d d a a d 1 1 1 1 1 1 = = r r r thermal efficiency in Otto cycle r=8, γ =1.4-> ε =56%
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Internal-combustion engines (cont’d) ± The Otto cycle
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Ch19 - Chapter 19: The Second Law of Thermodynamics...

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