Chapter 9Chapter 9GAS POWER CYCLES

(Part 1)(Part 1)

Chapter 9Chapter 9GAS POWER CYCLES

(Part 1a)(Part 1a)

Objectivesj1. Evaluate the performance of gas power cycles.

2 Develop simplifying assumptions applicable to gas power cycles2. Develop simplifying assumptions applicable to gas power cycles.

3. Review the operation of reciprocating engines.

4 Analyze both closed and open gas power cycles4. Analyze both closed and open gas power cycles.

5. Solve problems based on the Otto and Diesel cycles.

6. Solve problems based on the Brayton cycle; Brayton cycle with regeneration;p y y ; y y g ;and Brayton cycle with intercooling, reheating, and regeneration.

7. Identify simplifying assumptions and perform second-law analysis on gaspower cyclespower cycles.

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Basic Considerations In Power Cycles AnalysisMost power-producing devices operate on cycles.

Ideal cycle: A cycle that resembles the actual cycleclosely but is made up totally of internally reversibleprocesses is called an ideal cycle.

Recall: Thermal efficiency of heat enginesy g

The analysis of many complexprocesses can be reduced to amanageable level by utilizing

Reversible cycles such as Carnot cycle have thehighest thermal efficiency of all heat engines operatingbetween the same temperature levels. manageable level by utilizing

some idealizations.between the same temperature levels.

Unlike ideal cycles, they are totally reversible, andunsuitable as a realistic model.

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Idealizations (simplifications) in the analysis of power cycles1. The cycle does not involve any friction. Therefore,

the working fluid does not experience any pressuredrop as it flows in pipes or heat exchangers.

On a T-s diagram, the ratio of the areaenclosed by the cyclic curve to the areaunder the heat-addition process curverepresents the thermal efficiency of the

2. All expansion and compression processes takeplace in a quasi-equilibrium manner.

3. The pipes connecting the various components of a

represents the thermal efficiency of thecycle.

system are well insulated, so heat transferthrough them is negligible.

Care should be exercised in the5

Care should be exercised in theinterpretation of the results fromideal cycles.

On both P-v and T-s diagrams, the area enclosed by the process curve represents the net work of the cycle.

Carnot Cycle - Its Value In EngineeringThe Carnot cycle is composed of 4 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 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.

6P-v and T-s diagrams of a Carnot cycle.Example: A steady-flow Carnot engine.

Air-standard Assumptions1 Th ki fl id i i hi h ti l 1. The working fluid is air, which continuously

circulates in a closed loop and always behaves as an ideal gas.

2 All th th t k th l 2. All the processes that make up the cycle are internally reversible.

3. The combustion process is replaced by a h t dditi f t l heat-addition process from an external source.

4. The exhaust process is replaced by a h t j ti th t t th

The combustion process is replaced by a heat-addition process in ideal cycles.

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 7Air-standard cycle: A cycle for which the air-standard assumptions are applicable.

Chapter 9Chapter 9GAS POWER CYCLES

(Part 1b)(Part 1b)

Overview of Reciprocating EnginesThe reciprocating engine (basically a piston–cylinder device) is an invention that has proved to be very versatile and has a wide range of applications.

Reciprocating engine is theh f th t j it fpowerhouse of the vast majority of

automobiles, trucks, light aircraft,ships, electric power generators,and many other devicesand many other devices.

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Basic ComponentsTh i t i t i th li d b t t fi d iti ll d th t d dThe piston reciprocates in the cylinder between two fixed positions called the top deadcentre (TDC) - the position that forms the smallest volume in the cylinder - and the bottomdead centre (BDC) - position that forms the largest volume in the cylinder.

The distance between TDC and BDC is called the stroke ofthe engine. The diameter of the piston is called the bore.

Compression ratio: 10Compression ratio:

Performance CharacteristicsNet work output per cycle:Net work output per cycle:

Mean effective pressure (MEP):A fictitious pressure that, if it is acted on the pistonduring the entire power stroke would produce theduring the entire power stroke, would produce thesame amount of net work as that produced during theactual cycle.

Classifications of IC Engines:Classifications of IC Engines:1. Spark-ignition (SI) or Petrol engines2. Compression-ignition (CI) or Diesel

11engines

Otto Cycle: Ideal Spark-Ignition Engines Cycle The piston executes four complete strokes within the cylinder The crankshaft The piston executes four complete strokes within the cylinder. The crankshaft completes two revolutions for each thermodynamic cycle.

These engines are called four-stroke IC engines.

12Actual and ideal cycles in spark-ignition engines on a P-v diagram.

T-s Diagram of Ideal Otto Cycle

IC Engines Classifications:

F t k lFour-stroke cycle1 cycle = 4 stroke = 2 revolutions of crankshaftTwo-stroke cycle1 cycle = 2 stroke = 1 revolution of crankshaft1 cycle = 2 stroke = 1 revolution of crankshaft

Sequence of processes:

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I t t k i ll f f ti d ib d li t d i tTwo-Stroke IC EnginesIn two-stroke engines, all four functions described earlier are executed in twostrokes: the power and compression stroke.Generally less efficient, but are relatively simple and inexpensive. They have high

t i ht d t l tipower-to-weight and power-to-volume ratios.

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Thermal Efficiency of Otto CycleThe heat supplied to the working fluid during constant-volume heating (combustion),

The heat rejected from the working fluid during constant-volume cooling (exhaust),

Temperature-volume relation,

Thermal efficiency,

p ,

Cold-air standard assumption.

15Compression ratio,

Engine Knock (Autoignition)Premature ignition of the fuel produces audible noise called engine knock. It hurtsperformance and causes engine damage.

Autoignition places upper limit on compression ratios that can be used in SI engines.S ifi h t ti k ff t th th l ffi i f th Ott lSpecific heat ratio, k affects the thermal efficiency of the Otto cycle.

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Chapter 9Chapter 9GAS POWER CYCLES

(Part 1c)(Part 1c)

Diesel Cycle: Ideal Cycle for CI EnginesIn diesel engines, only air is compressed during the compression stroke, eliminatingthe possibility of autoignition. These engines can be designed to operate at highercompression ratios, typically between 12 and 24.

Th b ti t k l

Fuels that are less refined (thus less expensive) can be used in diesel engines.

The combustion process takes place over alonger interval - fuel injection starts whenthe piston approaches TDC and continuesduring the first part of power stroke.g p pHence, combustion process in the idealDiesel cycle is approximated as a constant-pressure heat-addition process.

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Sequence of processes:1-2 Isentropic compression2-3 Constant-pressure heat addition3 4 I t i i

Seque ce o p ocesses

3-4 Isentropic expansion4-1 Constant-volume heat rejection.

Note: Petrol and diesel engines differ only in the manner the heat addition (or combustion)

t k lprocess takes place.It is approximated as a constant volume process in the petrol engine cycle and as a constant pressure process in the Diesel constant pressure process in the Diesel engine cycle.

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Thermal Efficiency of Diesel CycleHeat supplied to the working fluid during the constant-pressure heating (combustion),

Heat rejected from the working fluid during the t t l li ( h t)constant-volume cooling (exhaust),

Th l ffi i f Di l l ( l)Thermal efficiency of Diesel cycle (general),

- constant specific heats

20Cutoff ratio,

For the same compression ratio, thermal efficiency of Otto cycle is greater than thatof the Diesel cycleof the Diesel cycle.

As the cutoff ratio decreases, the thermal efficiency of the Diesel cycle increases

Thermal efficiencies of large diesel engines efficiency of the Diesel cycle increases. When rc =1, the efficiencies of the Otto and Diesel cycles are identical.

range from about 35 to 40 percent.Higher efficiency and lower fuel costsmake diesel engines attractive inapplications such as in locomotive enginesapplications such as in locomotive engines,emergency power generation units, largeships, and heavy trucks.

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Dual Cycle: Realistic Ideal Cycle for CI EnginesApproximating the combustion process asa constant-volume or a constant-pressureheat addition process is overly simplisticheat-addition process is overly simplisticand not quite realistic.A better approach would be to model thecombustion process in both SI and CIcombustion process in both SI and CIengines as a combination of two heat-transfer processes, one at constant volumeand the other at constant pressureand the other at constant pressure.The ideal cycle based on this concept iscalled the dual cycle.

Note: Both the Otto and the Diesel cycles can be obtained 22as special cases of the dual cycle.