Lecture-8-9-10 (ME103).pptxhgjkhgkjhsjlknoih

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1. Thermodynamics (DT125 and FT123 ) Date: 7 th September 2022 Gas Power Cycles-1 Dr. Akhilendra Pratap Singh Department of Mechanical Engineering IIT (BHU), Varanasi, India Email: [email protected]

2. Objectives Role of thermodynamic cycles in power systems Types of gas power cycles and associated systems Assumptions applicable to gas power cycles Operation of reciprocating engines Performance of gas power cycles Analysis of gas power cycles Numerical problems

3. Basic Considerations in the Analysis of Power Cycles Most power-producing devices operate on cycles . Ideal cycle: A cycle that resembles the actual cycle closely but is made up totally of internally reversible processes . Reversible cycles such as Carnot cycle have the highest thermal efficiency of all heat engines operating between two temperature levels . Like ideal cycles, they are totally reversible , and unsuitable as a realistic model . Thermal efficiency of heat engines The analysis of many complex processes can be reduced to a manageable level by utilizing some idealizations . Modeling is a powerful engineering tool that provides great insight and simplicity at the expense of some loss in accuracy

4. The idealizations and simplifications in the analysis of power cycles: The cycle does not involve any friction . Therefore, the working fluid does not experience any pressure drop as it flows in pipes or devices such as heat exchangers. All expansion and compression processes take place in a quasi-equilibrium manner . The pipes connecting the various components of a system are well insulated , and heat transfer through them is negligible . On a T - s diagram , the ratio of the area enclosed by the cyclic curve to the area under the heat-addition process curve represents the thermal efficiency of the cycle. Any modification that increases the ratio of these two areas will also increase the thermal efficiency of the cycle.

5. T S Ƞ= Q in Q out W Q in W 1 2 3 4

6. The Carnot Cycle and Its Value in Engineering P-v and T - s diagrams of a Carnot cycle The Carnot cycle is composed of four totally reversible processes : isothermal heat addition , isentropic expansion , isothermal heat rejection , and isentropic compression. For both ideal and actual cycles: Thermal efficiency increases with an increase in the average temperature at which heat is supplied to the system or with a decrease in the average temperature at which heat is rejected from the system. A steady-flow Carnot engine

7. Air-standard Assumptions The combustion process is replaced by a heat-addition process in ideal cycles. The working fluid is air , which continuously circulates in a closed loop and always behaves as an ideal gas . All the processes that make up the cycle are internally reversible . The combustion process is replaced by a heat-addition process from an external source. The exhaust process is replaced by a heat-rejection process that restores the working fluid to its initial state. Cold-air-standard assumptions : The working fluid is considered to be air with constant specific heats at room temperature (25°C). Air-standard cycle: A cycle for which the air-standard assumptions are applicable.

8. Introduction of Engines The purpose of internal combustion (IC) engines is the production of mechanical power from the chemical energy contained in the fuel. In IC engines, energy is released by burning or oxidizing the fuel inside the engine. There are two types of engines: Internal combustion -- Combustion occurs in the working fluid -- Open cycle – the working fluid is replenished in each cycle -- Exhaust gas is dumped into the atmosphere External combustion -- Use of heat exchanger to transfer energy to the working fluid -- Example: Steam engine Main two types of IC engines are : -- Spark ignition (SI) engine (also called Otto engine or gasoline or petrol engines) -- Compression ignition (CI) engine (also called Diesel engine) These two types of engine have found wide application in transportation (land, sea and air) and power generation .

9. Engine Terminology Different Engine Parts

10. Basic Working Principle A reciprocating engine contains following main parts; Cylinder, Piston , Connecting rod , Crank The piston is pushed to right in the cylinder. The connecting rod is then pushed and in turn it causes the crank to rotate about its centre O. The engine shaft at (perpendicular to plane of paper) rotates and provide power. The piston reciprocates between two extreme points C1 and C2, called dead center. C1 is top dead center (TDC or TC) and C2 is bottom dead center (BDC or BC). Different Engine Parts

11. Engine Terminology When piston is at TDC, there is clearance between the piston and head of the cylinder. The volume of this space is called clearance volume ( V c ) . The volume between TDC and BDC is called swept volume or stroke volume ( V d ). The linear distance between TDC and BDC is known as stroke and apparently stroke is two times the radius of the crank. The ratio of maximum volume to minimum volume is the compression ratio ( r c ). Typical values of r c , are 8 to 12 for SI engines and 12 to 24 for CI engines. TDC Positions Which engine (diesel or patrol) is more efficient? Why diesel engines are more efficient compared to patrol engines?

12. Working of Spark Ignition Engine (Otto Cycle)

13. Working of Compression Ignition Engine (Diesel Cycle)

14. Overview of Reciprocating Engines Nomenclature for reciprocating engines Compression ratio Mean effective pressure

15. Thanks

16. Date: 14 th September 2022 Gas Power Cycles-2 Dr. Akhilendra Pratap Singh Department of Mechanical Engineering IIT (BHU), Varanasi, India Email: [email protected] Thermodynamics (DT125 and FT123 )

17. Introduction of Power Cycle A cycle during which a net amount of work is produced is called a power cycle , and a power cycle during which the working fluid remains a gas throughout the cycle, is called a gas power cycle . The most efficient cycle operating between a heat source at temperature T H and a sink at temperature T L is the Carnot cycle , and its thermal efficiency is given by, The actual gas cycles are rather complex. The approximations used to simplify the analysis are known as the air-standard assumptions . Under these assumptions, all the processes are assumed to be internally reversible ; the working fluid is assumed to be air , which behaves as an ideal gas ; and the combustion and exhaust processes are replaced by heat-addition and heat-rejection processes, respectively. Cold-air-standard assumptions???

18. Otto Cycle: The Ideal Cycle for Spark-ignition Engines The actual cycle does not have the sharp transitions between the different processes that the ideal cycle has

19. Otto Cycle P V T

20. Analysis of Otto Cycle Thermal efficiency of the ideal Otto cycle as a function of compression ratio ( k = 1.4 ) In SI engines, the compression ratio is limited by auto ignition or engine knock. The thermal efficiency of the Otto cycle increases with the specific heat ratio k of the working fluid

21. Differences Between Otto and Carnot cycles 2 5 1 3 Ƞ Otto = 5/(5+1+3)= 5/9 Ƞ Carnot = (2+5+1)/(2+5+1+3 )= 8/11 q out q in

22. Factors Affecting Work per Cycle The net cycle work of an engine can be increased by either : Increasing the r (1’ 2) Increase Q in (2 3 ”) P V 2 V 1 Q in W cycle 1 2 3 4 1’ 4’ 4’’ 3’’ 2’ 3’ Or (2 ’3’) r= V 1 / V 2

23. Numerical Problem An Otto cycle having a compression ratio of 9:1 uses air as the working fluid. Initially P 1 = 95 kPa , T 1 = 17 o C, and V 1 = 3.8 liters. During the heat addition process, 7.5 kJ of heat are added. Determine all T 's, P 's,  th , and the mean effective pressure. Assume constant specific heats with C v = 0.718 kJ/kg  K, k = 1.4. Process 1-2 is isentropic; therefore, recalling that r = V 1 / V 2 = 9, The first law closed system for process 2-3, Let q in = Q in / m and m = V 1 / v 1

24. Using the combined gas law (V 3 = V 2 ), Process 3-4 is isentropic; therefore,

25. Process 4-1 is constant volume. So the first law for the closed system gives, on a mass basis, The first law applied to the cycle gives (Recall  u cycle = 0) The thermal efficiency is

26. The mean effective pressure is

27. Thanks

28. Gas Power Cycles-3 Dr. Akhilendra Pratap Singh Department of Mechanical Engineering IIT (BHU), Varanasi, India Email: [email protected] Date: 15 th September 2022 Thermodynamics (DT125 and FT123 )

29. Diesel Cycle: The Ideal Cycle for CI Engines In diesel engines, only air is compressed during the compression stroke , eliminating the possibility of auto-ignition (engine knock). D iesel engines can be designed to operate at much higher compression ratios than SI engines, typically between 12 and 24 . In diesel engines, the spark plug is replaced by a fuel injector.

30. Working of Compression Ignition Engine (Diesel Cycle)

31. Early CI Engine Cycle vs. Diesel Cycle A I R Combustion Products Fuel injected at TC Intake Stroke FUEL Fuel/Air Mixture Air TC BC Compression Stroke Power Stroke Exhaust Stroke Q in Q out Compression Process Const pressure heat addition Process Expansion Process Const volume heat rejection Process Actual Cycle Diesel Cycle

32. Compression Ignition Engine (Diesel Cycle) 1-2 : isentropic compression 2-3: constant-volume heat addition 3-4: isentropic expansion 4-1: constant-volume heat rejection.

33. Analysis of Diesel Cycle Thermal efficiency of the ideal Diesel cycle as a function of compression and cutoff ratios ( k= 1.4). Cut-off ratio for the same compression ratio We know that V 1 = V 4 r c is called the cutoff ratio, defined as V 3 /V 2 , and is a measure of the duration of the heat addition at constant pressure. For higher efficiency of diesel engine, cutoff ratio should be low.

34. Practice Problem Calculate the efficiency of a diesel cycle for which compression ratio is 14 and cutoff ratio is 2. What will be the efficiency if cut off ratio is increased to 3 . Given k = 1.4.

35. Dual Cycle Dual cycle: A more realistic ideal cycle model for modern, high-speed compression ignition engine. Pilot Injection Main Injection Conventional Diesel Engines with Main Injection Modern Diesel Engines with Pilot and Main Injections Pilot Injection Main Injection

36. Dual Cycle If ( r c =1) If ( α =1 )

37. For the same inlet conditions P 1 , V 1 and the same compression ratio P 2 /P 1 For the same inlet conditions P 1 , V 1 and the same peak pressure P 3

38. Thanks

39. Gas Power Cycles-4 Dr. Akhilendra Pratap Singh Department of Mechanical Engineering IIT (BHU), Varanasi, India Email: [email protected] Date: 16 th September 2022 Thermodynamics (DT125 and FT123 )

40. Operation of an Open Cycle Gas Turbine Engine

41. Brayton Cycle: The Ideal Cycle for Gas-turbine Engines The combustion process is replaced by a constant-pressure heat-addition process from an external source, and the exhaust process is replaced by a constant-pressure heat-rejection process to the ambient air. 1-2 Isentropic compression (in a compressor) 2-3 Constant-pressure heat addition 3-4 Isentropic expansion (in a turbine) 4-1 Constant-pressure heat rejection An open-cycle gas-turbine engine A closed-cycle gas-turbine engine

42. Analysis of Brayton Cycle T - s and P-v diagrams for the ideal Brayton cycle Pressure ratio Thermal efficiency of the ideal Brayton cycle as a function of the r p

43. The fraction of the turbine work used to drive the compressor is called the back work ratio . The two major application areas of gas-turbine engines are aircraft propulsion and electric power generation . The highest temperature in the cycle is limited by the maximum temperature that the turbine blades can withstand . This also limits the pressure ratios that can be used in the cycle. The air in gas turbines supplies the necessary oxidant for the combustion of the fuel, and it serves as a coolant to keep the temperature of various components within safe limits. An air–fuel ratio of 50 or above is not uncommon.

44. The deviation of an actual gas-turbine cycle from the ideal Brayton cycle as a result of irreversibilities . Deviation of Actual Gas-Turbine Cycles from Idealized Ones Reasons: Irreversibilities in turbine and compressors, pressure drops, heat losses Isentropic efficiencies of the compressor and turbine Development of Gas Turbines Increasing the turbine inlet (or firing) temperatures Increasing the efficiencies of turbo-machinery components (turbines, compressors): Adding modifications to the basic cycle (intercooling, regeneration or recuperation, and reheating ).

45. The Brayton Cycle with Regeneration In gas-turbine engines, the temperature of the exhaust gas leaving the turbine is often considerably higher than the temperature of the air leaving the compressor. Therefore, the high-pressure air leaving the compressor can be heated by the hot exhaust gases in a counter-flow heat exchanger (a regenerator or a recuperator ). The thermal efficiency of the Brayton cycle increases as a result of regeneration since less fuel is used for the same work output . A gas-turbine engine with regenerator. T - s diagram of a Brayton cycle with regeneration.

46. The Brayton Cycle with Intercooling, Reheating, and Regeneration A gas-turbine engine with two-stage compression with intercooling, two-stage expansion with reheating, and regeneration and its T-s diagram.

47. Comparison of work inputs to a single-stage compressor (1 AC ) and a two-stage compressor with intercooling (1 ABD ). Multistage compression with intercooling: The work required to compress a gas between two specified pressures can be decreased by carrying out the compression process in stages and cooling the gas in between. This keeps the specific volume as low as possible. Multistage expansion with reheating keeps the specific volume of the working fluid as high as possible during an expansion process, thus maximizing work output. Intercooling and reheating always decreases the thermal efficiency unless they are accompanied by regeneration. As the number of compression and expansion stages increases, the gas-turbine cycle with intercooling, reheating, and regeneration approaches the Ericsson cycle.

48. Thanks