3. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Heat Engines Any type of engine or machine which derives Heat Energy from the combustion of the fuel or any other source and converts this energy into Mechanical Work is known as a Heat Engine . Classification : 1. External Combustion Engine (E. C. Engine) : Combustion of fuel takes place outside the cylinder. e.g. Steam Turbine, Gas Turbine Steam Engine, etc.

4. ME0223 SEM-IV Applied Thermodynamics & Heat Engines 2. Internal Combustion Engine (I.C. Engine) : Combustion of fuel occurs inside the cylinder. Heat Engines e.g. Automobiles, Marine, etc.

5. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Heat Engines Advantages of External Combustion Engines over Internal Combustion Engines : 1. Starting Torque is generally high . 2. Due to external combustion, cheaper fuels can be used ( even solid fuels ! ). 3. Due to external combustion, flexibility in arrangement is possible . 4. Self – Starting units. Internal Combustion Engines require additional unit for starting the engine ! Advantages of Internal Combustion Engines over External Combustion Engines : 1. Overall efficiency is high . 2. Greater mechanical simplicity. 3. Weight – to – Power ratio is low. 4. Easy Starting in cold conditions. 5. Compact and require less space.

12. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Application of I. C. Engines APPLICATIONS Road vehicles. Aircrafts. Locomotives. Construction Equipments Pumping Sets Generators for Hospitals, Cinema Hall, and Public Places.

14. Assumptions ME0223 SEM-IV Applied Thermodynamics & Heat Engines 1. The working medium is assumed to be a Perfect Gas and follows the relation PV = mRT 2. There is no change in the mass of the working medium . 3. All the processes that contribute the cycle are reversible . 4. Heat is assumed to be supplied from a constant high temperature source ; and not from chemical reactions during the cycle. 5. Some heat is assumed to be rejected to a constant low temperature sink during the cycle. 6. It is assumed that there are no heat losses from the system to the surrounding. 7. Working medium has constant specific heat throughout the cycle. 8. Physical constants viz. Cp, Cv, γ and M of working medium are same as those of air at standard atmospheric conditions . Cp = 1.005 kJ / kg.K Cv = 0.717 kJ / kg.K γ = 1.4 M = 29 kg / kmole

15. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Otto Cycle Basis of Spark – Ignition Engines . 0 -1 : Suction 1 -2 : Isentropic Compression 2 -3 : Constant Vol. Heat Addition 3 -4 : Isentropic Expansion 1 -0 : Exhaust 4 -1 : Constant Vol. Heat Rejection 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R Qs 1 2 Temperature, T Entropy, s 3 Isochoric 4 Q R

16. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Otto Cycle – Thermal Efficiency 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R

17. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Otto Cycle – Thermal Efficiency 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R

19. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Otto Cycle – Work Output 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R

20. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Otto Cycle – Mean Effective Pressure Work Output α Pr. Ratio, ( r p ) &, MEP α Internal Work Output 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R Pr. Ratio ↑ ≡ MEP ↑

21. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Diesel Cycle In S. I. Engines, max. compression ratio (r) is limited by self – ignition of the fuel . This can be released if air and fuel are compressed separately and brought together at the time of combustion. i.e. Fuel can be injected into the cylinder with compressed air at high temperature . i.e. Fuel ignites on its own and no special device for ignition is required. This is known as Compression Ignition (C. I.) Engine. Ideal Cycle corresponding to this process is known as Diesel Cycle . Main Difference : Otto Cycle ≡ Heat Addition at Constant Volume. Diesel Cycle ≡ Heat Addition at Constant Pressure.

22. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Diesel Cycle Basis of Compression – Ignition Engines . 0 -1 : Suction 1 -2 : Isentropic Compression 2 -3 : Constant Pr. Heat Addition 3 -4 : Isentropic Expansion 1 -0 : Exhaust 4 -1 : Constant Vol. Heat Rejection Isochoric Qs 1 2 Temperature, T Entropy, s 3 Isobaric 4 Q R 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R

23. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Diesel Cycle – Thermal Efficiency 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R

24. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Diesel Cycle – Thermal Efficiency AND 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R

25. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Diesel Cycle – Thermal Efficiency Efficiency of Diesel Cycle is different than that of the Otto Cycle by the bracketed factor . This factor is always more than unity. (> 1) In practice, however, operating Compression Ratio for Diesel Engines (16 – 24) are much higher than that for Otto Engines (6 – 10) . 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R Otto Cycle is more efficient than Diesel Cycle , for given Compression Ratio Efficiency of Diesel Engine is higher than that of Otto Engine

26. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Diesel Cycle – Work Output 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R

27. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Diesel Cycle – Mean Effective Pressure 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R

28. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Dual Cycle Combustion process is neither Constant Volume nor Constant Pressure Process. 2. Rapid uncontrolled combustion at the end. Hence, a blend / mixture of both the processes are proposed as a compromise. Real engine requires : 1. Finite time for chemical reaction during combustion process. Combustion can not take place at Constant Volume . Combustion can not take place at Constant Pressure .

29. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Dual Cycle 0 -1 : Suction 1 -2 : Isentropic Compression 2 -3 : Constant Vol. Heat Addition 3 -4 : Isentropic Expansion 1 -0 : Exhaust 4 -1 : Constant Vol. Heat Rejection 2 -3 : Constant Pr. Heat Addition 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R 5 Qs Qs 1 2 Temperature, T Entropy, s 3 Isobaric 4 Q R Isochoric Isochoric 5 Qs

30. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Dual Cycle – Thermal Efficiency 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R 5 Qs

31. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Dual Cycle – Thermal Efficiency 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R 5 Qs

32. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Dual Cycle – Thermal Efficiency For ( r p ) > 1; η Dual ↑ for given ( r c ) and ( γ ) With ( r c ) = 1 ≡ Otto Cycle With ( r p ) = 1 ≡ Diesel Cycle 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R 5 Qs Efficiency of Dual Cycle lies in between that of Otto Cycle and Diesel Cycle .

33. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Dual Cycle – Work Output 0 1 Pressure, P Volume, V Isentropic 2 Qs 3 4 Q R 5 Qs

34. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Dual Cycle – Mean Effective Pressure 0 1 Pressure, P Volume, V 2 Qs 3 4 Q R 5 Qs

35. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Four – Stroke / Compression Ignition (C.I.) Engine

36. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Four – Stroke / Compression Ignition (C.I.) Engine

37. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Four – Stroke Engine – Valve Timing Diagram

38. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Two – Stroke / Spark Ignition (S.I.) Engine

39. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Two – Stroke / Spark Ignition (S.I.) Engine

40. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Two – Stroke Engine – Valve Timing Diagram

41. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Comparison : Two – Stroke Vs. Four Stroke Sr. No. Particulars Four – Stroke Cycle Two – Stroke Cycle 1. Cycle Completion 4 strokes / 2 revolutions 2 strokes / 1 revolution 2. Power Strokes 1 in 2 revolutions 1 per revolution 3. Volumetric Efficiency High Low 4. Thermal and Part – Load Efficiency High Low 5. Power for same Engine Size Small; as 1 power stroke for 2 revolutions Large; as 1 power stroke per revolutions 6. Flywheel Heavier Lighter 7. Cooling / Lubrication Lesser Greater 8. Valve Mechanism Required Not Required 9. Initial Cost Higher Lower

42. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Comparison : S.I. Vs. C.I. Engines Sr. No. Particulars S. I. Engine C. I. Engine 1. Thermodynamic Cycle Otto Diesel 2. Fuel Used Gasoline Diesel 3. Air : Fuel Ratio 6 : 1 – 20 : 1 16 : 1 – 100 : 1 4. Compression Ratio Avg. 7 – 9 Avg. 15 – 18 5. Combustion Spark Ignition Compression Ignition 6. Fuel Supply Carburettor Fuel Injector 7. Operating Pressure 60 bar max. 120 bar max. 8. Operating Speed Up to 6000 RPM Up to 3500 RPM 9. Calorific Value 44 MJ/kg 42 MJ/kg 10. Running Cost High Low 11. Maintenance Cost Minor Major

43. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Comparison : Gasoline Vs. Diesel Engines Sr. No. Gasoline Engine Diesel Engine 1. Working : Otto Cycle Working : Diesel Cycle 2. Suction Stroke : Air / Fuel mixture is taken in Suction Stroke : only Air is taken in 3. Spark Plug Fuel Injector 4. Spark Ignition generates Power Compression Ignition generates Power 5. Thermal Efficiency – 35 % Thermal Efficiency – 40 % 6. Compact Bulky 7. Running Cost – High Running Cost – Low 8. Light – Weight Heavy – Weight 9. Fuel : Costly Fuel : Cheaper 10. Gasoline : Volatile and Danger Diesel : Non-volatile and Safe. 11. Less Dependable More Dependable

45. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Magneto – Ignition Systems

47. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Comparison : Battery Vs. Magneto Ignition Sr. No. Battery Ignition System Magneto – Ignition System 1. Current obtained from Battery Current generated from Magneto 2. Sparking is good even at low speeds Poor sparking at low speeds 3. Engine starting is easier Difficult starting 4. Engine can not be started, if battery is discharged No such difficulty, as battery is not needed 5. More space requirement Less space requirement 6. Complicated wiring Simple wiring 7. Cheaper Costly 8. Spark intensity falls as engine speed rises Spark intensity improves as engine speed rises 9. Used in cars, buses and trucks Used in motorcycles, scooters and racing cars

49. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Performance of I.C. Engines A. Power and Mechanical Efficiency : Indicated Power ≡ Total Power developed in the Combustion Chamber, due to the combustion of fuel. n = No. of Cylinders P mi = Indicated Mean Effective Pressure (bar) L = Length of Stroke (m) A = Area of Piston (m 2 ) k = ½ for 4 – Stroke Engine, = 1 for 2 – Stroke Engine N = Speed of Engine (RPM)

50. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Performance of I.C. Engines A. Power and Mechanical Efficiency : Brake Power ≡ Power developed by an engine at the output shaft. N = Speed of Engine (RPM) T = Torque (N – m) Frictional Power (F. P.) = I. P. – B. P.

51. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Performance of I.C. Engines B. Mean Effective Pressure : Mean Effective Pressure ≡ Hypothetical Pressure which is thought to be acting on the Piston throughout Power Stroke. F mep = I mep – B mep I mep ≡ MEP based on I.P. B mep ≡ MEP based on B.P. F mep ≡ MEP based on F.P. Power and Torque are dependent on Engine Size . Thermodynamically incorrect way to judge the performance w.r.t. Power / Torque . MEP is the correct way to compare the performance of various engines.

52. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Performance of I.C. Engines C. Specific Output : Specific Output ≡ Brake Output per unit Piston Displacement. D. Volumetric Efficiency : Volumetric Efficiency ≡ Ratio of Actual Vol. (reduced to N.T.P.) of the Charge drawn in during the suction stroke, to the Swept Vol. of the Piston. Avg. Vol. Efficiency = 70 – 80 % Supercharged Engine ≈ 100 %

53. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Performance of I.C. Engines E. Fuel : Air Ratio : Fuel : Air Ratio ≡ Ratio of Mass of Fuel to that of Air, in the mixture. Rel. Fuel : Air Ratio ≡ Ratio of Actual Fuel : Air Ratio to that of Schoichiometric Fuel : Air Ratio . F. Sp. Fuel Consumption : Sp. Fuel Consumption ≡ Mass of Fuel consumed per kW Power.

54. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Performance of I.C. Engines G. Thermal Efficiency : Thermal Efficiency ≡ Ratio of Indicated Work done, to the Energy Supplied by the fuel.

56. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Performance of I.C. Engines H. Heat Balance : m w = Mass of Cooling Water used (kg/min) Cw = Sp. Heat of Water (kJ/kg. ° C) T 1 = Initial Temp. of Cooling Water ( ° C) T 2 = Final Temp. of Cooling Water ( ° C) m e = Mass of Exhaust Gases (kg/min) C Pg = Sp. Heat of Exhaust Gases @ Const. Pr. (kJ/kg. ° C) T e = Temp. of Exhaust Gases ( ° C) T r = Room Temperature ( ° C) Heat Supplied by Fuel = Heat equivalent of I.P. = Heat taken away by Cooling Water = Heat taken away by Exhaust Gases =

57. ME0223 SEM-IV Applied Thermodynamics & Heat Engines Performance of I.C. Engines H. Heat Balance : Sr. No. Input Amount (kJ) Per cent (%) Output Amount (kJ) Per cent (%) 1. Heat Supplied by Fuel A 100 Heat equivalent to I.P. B α 2. Heat taken by Cooling Water C β 3. Heat taken by Exhaust Gases D γ 4. Heat Unaccounted E = A – (B+C+D) E δ Total A 100 Total A 100