Dr Roger Bennett R A BennettReading ac uk

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

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.

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

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

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.

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.

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.

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. Pi+d. P Pi T i Porous plug Pi T i Vi Porous plug P f. T f Pf

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.