1. Dr Roger Bennett R.A.Bennett@Reading.ac.uk Rm. 23 Xtn. 8559 Directed Reading

2. Summary – Indicator diagrams and real engines Carnot cycle viewed from the Temperature- Entropy (T-S) indicator diagram and its comparison to the P-V diagram. Efficiency of the Otto cycle (Petrol engine). Comparison of compression ratio’s of petrol and diesel engines.

3. Summary – Enthalpy and flow processes Joule –Thompson throttling Derivation of the Joule coefficient. Properties of real gases.

4. Carnot Cycle P-V diagrams familiar. 2 adiabats 2 isotherms T-S diagram Carnot cycle closed rectangle. Reversible adaibats are isentropic. đQ R =0 Area enclosed = work done per cycle. V P a b 1 2 3 4 c d S T a b 1 2 3 4 c d

5. Otto Cycle P-V diagram. 2 adiabats 2 isochores r is the compression ratio. Fuel and air is compressed on a  b. Taken from University Physics 11 th Ed., Young and Freedman.

6. Diesel Cycle Diesel engine slightly different to other cycles. 2 adiabats 1 isochore 1 isobar r is again the compression ratio. Only air is compressed on a  b.

7. Efficiencies of real engines Thermal efficiency of Otto cycle (petrol engine)  =1-1/(r (  -1) ) Typically r=8 and  =1.4 so efficiencies ~55% A real engine is typically ~35% Diesel cycle injects fuel at start of power stroke with fuel igniting spontaneously. Typically r is higher~15-20. Efficiency then ~65%. Diesels much more complex engines due to injecting fuels at high pressure.

8. Joule Thompson-Throttling Typical model is pushing gas through a porous plug in an adiabatic container. Enthalpy U+PV is conserved Various examples, turbines and nozzles – see C.B.P. Finn Thermal physics. PiTiPiTi PfTfPfTf Porous plug PiTiViPiTiVi Porous plug P i +dPPfPf

9. Joule Coefficient Joule Thompson effect is the change of temperature of a gas as it expands through a throttle from high pressure to lower. For an ideal gas  JT =0 This effect is used for the liquefaction of gases.