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Gas Power Systems -1

Gas Power Systems -1. Air Standard Cycles The Spark Ignition Engine. Overview. Air standard cycles for reciprocating engines General operational features Indicator diagrams Spark ignition engines The Otto cycle. General classes of engines. Reciprocating internal combustion engines.

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Gas Power Systems -1

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  1. Gas Power Systems -1 Air Standard Cycles The Spark Ignition Engine

  2. Overview • Air standard cycles for reciprocating engines • General operational features • Indicator diagrams • Spark ignition engines • The Otto cycle

  3. General classes of engines • Reciprocating internal combustion engines. • Reciprocating compressors • Reciprocating steam engines

  4. Single action reciprocating engine. Single vs. double action engines Double action reciprocating engine.

  5. An analog instrument that measures pressure vs. Displacement in a reciprocating engine. The graph is in terms of P and V. Area is proportional to the work done per cycle p V Indicator diagrams

  6. Work per cycle is represented in terms of a mean effective pressure and the displacement. p MEP = Mean effective pressure V X = Displacement Indicator Diagrams

  7. To compute the work per cycle, use the MEP and the displacement. Work = MEP x A x Displacement. MEP = kiY, where ki is a constant Y = the average ordinate on the indicator diagram. X = displacement A = area of cylinder heat. Indicator Diagrams

  8. Processes: a-b Intake b-c Compression c-d Combustion (spark ignited) d-e Power Stroke e-f Gas Exhaust b-a Exhaust Stroke p d c e a b V Displacement Clearance volume Ideal indicator diagramSpark ignition engine - The Otto cycle

  9. The spark ignition engine - The Otto cycle

  10. The air standard Otto cycle approximates the automotive internal combustion engine and aircraft engines. Assume a quasistatic process, isentropic compression, and isentropic exhaust. 4 p s = Constant 3 5 1 6 2 V The Otto cycle Displacement

  11. s = Constant 4 T 4 p 3 3 5 5 1 6 2 6 2 s V V = Constant P-V diagram for the actual variable mass system. This is the T-s diagram for the fixed mass system. The Otto cycle

  12. Real Process 1-2 Intake of fuel/air mixture at P1 2-3 Compression to P3 3-4 Combustion, V = cons. and increasing P 4-5 Expansion 5-6 Exhaust at constant V and falling pressure 6-1Exhaust at P1 Air Cycle Approximation (Constant mass, closed internally reversible, cycle) 2-3 Isentropic compression 3-4 Heat addition at constant volume 4-5 Isentropic expansion 5-6 Heat rejection at constant volume The air-standard Otto cycle

  13. The reversible Otto cycle • Internally reversible processes • Constant specific heats • k = Cp/Cv = Constant • Polytropic compression and expansion, PVk = Const. • Treat air as an ideal gas.

  14. 4 T QH 3 5 QC 2 6 s V = Constant Thermodynamic analysis for the reversible Otto cycle (1)

  15. Thermodynamic analysisfor the air-standard Otto Cycle (2)

  16. Thermodynamic Analysisfor the air-standard Otto Cycle

  17. Thermal efficiency

  18. Key assumptions: (1) Internally reversible processes (2) Constant specific heats Important consequence: (1) Efficiency is independent of working fluid (2) Efficiency independent of temperatures Thermal efficiency

  19. Efficiency of Air Standard Otto Cycle k = 1.3 = Cp/Cv T QH 4  3 WNET 5 QC 2,6 s rv

  20. k = 1.4 k = 1.3

  21. 4 T QH 3 5 QC 2,6 s V = Constant The Otto cycle with real gases • Variable specific heats. • Efficiency based on internal energies obtained from the gas tables which take into account variable specific heats.

  22. End of Gas Power Systems - 1

  23. Key terms and concepts Spark ignition engine Otto cycle Clearance volume Indicator diagram Indicator constant, ki Mean effective pressure Displacement

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