Basics of energy conversation cycles
Heat Engines and Efficiencies
The objective is to build devices which receive heat and produce work (like an aircraft engine or a car engine) or receive work and produce heat (like an air conditioner) in a sustained mannersustained manner. All operations need to be cyclic. The cycle comprises of a set of processes during which one of the properties is kept constant (V,p,T etc.) Heat Engines (contd…)
A minimum of 3 such processes are required to construct a cycle. All processes need not have work interactions (eg: isochoric) All processes need not involve heat interactions either (eg: adiabatic process). Heat Engines (Contd…)
A cycle will consist of processes: involving some positive work interactions and some negative. If sum of +ve interactions is > -ve interactions the cycle will produce work If it is the other way, it will need work to operate.
On the same lines some processes may have +ve and some -ve heat interactions. Commonsense tells us that to return to the same point
after going round we need at one path of opposite direction. I law does not forbid all heat interactions being +ve nor
all work interactions being -ve.
But, we know that you can’t construct a cycle with all
All -ve Q’s nor with all +ve or all -ve W’s
Any cycle you can construct will have some processes
Q +ve some with -ve.
Heat Engines (Contd…)
Let Q1,Q3,Q5…. be +ve heat interactions (Heat supplied)Q2,Q4,Q6…. be -ve heat interactions (heat rejected)From the first law we haveQ1+Q3+Q5..-Q2-Q4-Q6-... = Net work delivered (Wnet)ΣQ+ve-ΣQ-ve= WnetThe efficiency of the cycle is defined as η= Wnet/ΣQ+vePhilosophy →What we have achieved ÷what we have spent to achieve itHeat Engines (Contd…) Heat Engines (Contd…)
Consider the OTTO Cycle(on which your car engine works)It consists of two isochores and two adiabatics •There is no heat interaction during 1-2 and 3-4
•Heat is added during constant volume heating (2-3) Q2-3=...
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