Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36SummaryCHAPTER 9 GAS POWER CYCLESLecture slides byMehmet KanogluCopyright © The McGraw-Hill Education. Permission required for reproduction or display.Thermodynamics: An Engineering Approach 8th EditionYunus A. Çengel, Michael A. BolesMcGraw-Hill, 20152Objectives•Evaluate the performance of gas power cycles for which the working fluid remains a gas throughout the entire cycle.•Develop simplifying assumptions applicable to gas power cycles.•Review the operation of reciprocating engines.•Analyze both closed and open gas power cycles.•Solve problems based on the Otto, Diesel, Stirling, and Ericsson cycles.•Solve problems based on the Brayton cycle; the Brayton cycle with regeneration; and the Brayton cycle with intercooling, reheating, and regeneration.•Analyze jet-propulsion cycles.•Identify simplifying assumptions for second-law analysis of gas power cycles.•Perform second-law analysis of gas power cycles.3BASIC CONSIDERATIONS IN THE ANALYSIS OF POWER CYCLESMost 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 is called an ideal cycle.Reversible cycles such as Carnot cycle have the highest thermal efficiency of all heat engines operating between the same temperature levels. Unlike ideal cycles, they are totally reversible, and unsuitable as a realistic model.Thermal efficiency of heat engines:4The ideal cycles are internally reversible, but, unlike the Carnot cycle, they are not necessarily externally reversible. Therefore, the thermal efficiency of an ideal cycle, in general, is less than that of a totally reversible cycle operating between the same temperature limits. However, it is still considerably higher than the thermal efficiency of an actual cycle because of the idealizations utilized.5The idealizations and simplifications in the analysis of power cycles:1. 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.2. All expansion and compression processes take place in a quasi-equilibrium manner.3. 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.6THE CARNOT CYCLE AND ITS VALUE IN ENGINEERINGThe 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.7Derivation of the Efficiency of the Carnot Cycle8AIR-STANDARD ASSUMPTIONSAir-standard assumptions:1. The working fluid is air, which continuously circulates in a closed loop and always behaves as an ideal gas.2. All the processes that make up the cycle are internally reversible.3. The combustion process is replaced by a heat-addition process from an external source.4. The exhaust process is replaced by a heat-rejection process that restores the working fluid to its initial state.Cold-air-standard assumptions: When 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.9AN OVERVIEW OF RECIPROCATING ENGINES•Spark-ignition (SI) engines•Compression-ignition (CI) enginesCompression ratio10Mean effective pressureThe mean effective pressure can be used as a parameter to compare the performances of reciprocating engines of equal size. The engine with a larger value of MEP delivers more net work per cycle and thus performs better.11OTTO CYCLE: THE IDEAL CYCLE FOR SPARK-IGNITION ENGINES12Schematic of a two-stroke reciprocating engine.The two-stroke engines are generally less efficient than their four-stroke counterparts but they are relatively simple and inexpensive, and they have high power-to-weight and power-to-volume ratios.Four-stroke cycle1 cycle = 4 stroke = 2 revolutionTwo-stroke cycle1 cycle = 2 stroke = 1 revolution1314Air enters the cylinder through the open intake valve at atmospheric pressure P0 during process 0-1 as the piston moves from TDC to BDC. The intake valve is closed at state 1 and air is compressed isentropically to state 2. Heat is transferred at constant volume (process 2-3); it is expanded isentropically to state 4; and heat is rejected at constant volume (process 4-1). Air is expelled through the open exhaust valve (process 1-0).Work interactions during intake and exhaust cancel each other, and thus inclusion of the intake and exhaust processes has no effect on the net work output from the cycle. However, when calculating power output from the cycle during an ideal Otto cycle analysis, we must consider the fact that the ideal Otto cycle has four strokes just like actual four-stroke spark-ignition engine.15In SI engines, the compression ratio is limited by autoignition or engine knock.16DIESEL CYCLE: THE IDEAL CYCLEFOR COMPRESSION-IGNITION ENGINESIn diesel engines, only air is compressed during the compression stroke, eliminating the possibility of autoignition (engine knock). Therefore, diesel engines can be designed to operate at much higher compression ratios than SI engines, typically between 12 and 24.1-2 isentropic compression 2-3 constant-volume heat addition 3-4 isentropic expansion 4-1 constant-volume heat rejection.17Thermal efficiency of the ideal Diesel cycle as a function of compression and cutoff ratios (k=1.4).Cutoff ratiofor the same compression ratio18Dual cycle: A more realistic ideal cycle model for modern, high-speed compression ignition engine.In modern high-speed compression ignition engines, fuel is injected into the combustion