cairo 2nd petrol lecture 7

2nd year Petrol dept.2011-2012

Mechanical EngineeringThermodynamics

Lecture (7) Gas Power Cycles

Lecturer : Dr. Esmail Bialy

Background

1- Ideal Gas Relation

2- 1st Law of Thermodynamics (Closed Systems)

Ideal Gas Relation

Forms

1- PV=mRT

Pressure Volume mass Gas Const. Temperature

(kPa) (m3) (kg) (kJ/kg.K) (K)

Forms

2- Pv=RT

Pressure Specific Volume Gas Const. Temperature

(kPa) (m3/kg) (kJ/kg.K) (K)

Forms

3- PV=nRuT

Pressure Volume no. of Universal Temperature

kmoles Gas Const. (kPa) (m3) (kmole) (kJ/kmole.K)

(K)تاني عايزينها مش

Specific Heats

Specific Heats

Specific Heats

γγ

Specific HeatsFrom the last two relations, we can drive another two

relations

Cp=γR/(γ-1) For Air= 1.0035 kJ/kg.K

Cv=R/(γ-1) For Air= 0.716 kJ/kg.K

Note: 1-For Ideal GasesΔ h= CpΔT

Δu = Cv ΔT

2- For SteamΔ h= h2-h1

Δu = u2-u1 from steam tables

1st Law of Thermodynamics(Closed Systems)

(Revision)

Note!!!rp, r, rc

rp: Pressure ratio (Brayton Cycle)r: Compression ratio (Otto & Diesel Cycles)rc: Cutoff ratio ( Diesel Cycle)

Example (5-10):

A Diesel engine has a compression ratio of 20:1 with an inlet

state of 95 kPa, 290 K, and volume 0.5 L. The maximum cycle

temperature is 1800 K. find the maximum pressure, the net

specific work, and the thermal efficiency.