Reversed Brayton Cycle - LPF( ) Compressor

Part – 4: Thermodynamics

LPF( ) Compressor

4.7.3 Reversed Brayton Cycle

Practically, the reversed Carnot cycle cannot be used for refrigeration purpose as the isentropic process requires very high speed operation, whereas the isothermal process requires very low speed operation.

4.7.3 Reversed Brayton Cycle

(a) Air refrigeration system

(b) Air refrigeration system

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(c) Air refrigeration system

The working of air-refrigeration cycle is represented on p-v and T-s diagrams in Fig. (b) and (c).

Process 1-2 represents the suction of air into the compressor. Process 2-3 represents the isentropic compression of air by the compressor. Process 3-5 represents the discharge of high pressure air from the compressor into the heat exchanger. The reduction in volume of air from v to v is due to the cooling of air in the heat exchanger.

Process 5-6 represents the isentropic expansion of air in the expander. Process 6-2 represents the absorption of heat from the evaporator at constant pressure.

Part 4.8: I.C. Engines 4.8.1 Basics of I.C. Engine

Engine Components: The I.C. Engine Figure below showing its various components.

Name of the part Material used

Cylinder Cast iron

Cylinder head Cast iron, aluminum alloy

Piston Cast iron, aluminum alloy

Piston rings Silicon cast iron

Connecting rods Steel

Crank shaft Alloy steel

Bearing White metal

Cylinder liner Nickel alloy steel

Engine’s erminology

 Piston Swept Volume (Vs): The nominal volume generated by the piston when travelling from one dead centre to the next one.

Vs = A L

 Clearance Volume (Vc): The nominal volume of the space on the combustion side of the piston at top dead centre

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V = Vc + Vs

 Compression Ratio (r): It is ratio of cylinder volume to clearance volume.

r = V/Vc

I.C. Engine classification:

 On the basis of the number of stroke engine can be four-stroke engine or can be two – stroke engine.

 On the basis of the working cycle it can be spark ignition (otto cycle) engine or it can be compression ignition engine (diesel cycle).

Four – stroke Engine:

Stroke Valve position

Suction stroke. Suction valve open

Exhaust valve closed

Compression stroke Both valves closed

Expansion stroke Both valves closed

Exhaust stroke Exhaust valve open

Suction valve closed.

Valve timing diagrams:

 For four-stroke S.I. engine

 For two-stroke engine

Four-stroke cycle Two-stroke cycle

 The cycle is completed The cycle is completed in in four strokes of the piston. two strokes of the piston.

 It has only one power stroke It has one power stroke in in two revolutions of crank each revolution of crank

Shaft shaft

 Turning moment is not More uniform turning uniform hence heavier moment hence lighter flywheel is needed flywheel is needed.

 More volumetric efficiency Less volumetric efficiency

 Higher thermal efficiency Lower thermal efficiency

 It contains valves It contains only ports not valves

 Better part load efficiency Poor part load efficiency

S.I. Engines C. I. Engines

 Based on otto cycle Based on diesel cycle

 Fuel has high self Fuel has low self ignition temperature ignition temperature

 Compression ratio Compression ratio is between 6 to 10.5 is between 14 to 22

 Lower max. efficiency Higher max. efficiency

 Lighter Heavier

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For a muticylinder engine a smaller flywheel is required Performance parameter:

 Indicated thermal efficiency ( ): It is a ratio of energy in the indicated horse power to the fuel energy.

I. P. = Indicated power mf = Mass of fuel

QLHV = Lower Heat Calorific Value

 Brake thermal efficiency ( ): Brake thermal efficiency is the ratio of energy in the brake power to the fuel energy.

= ^{. .}

b. p. = break power.

 Mechanical efficiency ( ): It is a ratio of brake power to the indicated horse power.

= ^{. .}

. .

= f. p. = i. p. – b. p.

f. p. = friction power

f.p. is usually assumed constant. At part loads b.p. is changed, thus from b.p. & f.p., ip. can be calculated.

 Volumentric efficiency ( ): It is defined as the ratio of the air actually induced at ambient conditions to the swept volume of engine.

 Relative efficiency or efficiency ratio: It is defined as ratio of thermal efficiency of the actual cycle to that of the ideal cycle.

 Specific fuel consumption (sfc): It is expressed in grams per horsepower-hour or per kWh.

bsfc =

. . kg/kWh isfc =

. . kg/kWh

 Fuel-Air Ratio: It is relative proportion of the fuel and air in the engine.

 Mean Piston speed = 2LN

‘L’ – length of cylinder

‘ ’ – r.p.m.

Mean effective pressure,

= ^{. .}

n = for 4s n = N for 2s

k = No. of cylinders

 Equivalent ratio:

= 1 chemically correct < 1 lean mixture > 1 rich mixture NOTE:

 In line engines : all cylinders are arranged linearly and transmit power to a single crankshaft

 Radial engines: air cooled aircraft engines, odd cylinders are employed for balancing, pistons of all cylinders are coupled to same crankshaft.

4.8.2 AirStandardCycles

Assumptions in ideal or air standard cycle

 The working medium is a perfect gas throughout, i.e., it follows the law pV= mRT.

 The working medium has constant specific heats.

 The working medium does not undergo any chemical change throughout the cycle.

Quick Refresher Guide Thermodynamics

 The kinetic and potential energies of the working fluid are neglected.

 The operation of the engine is frictionless.

 All the process are reversible.

The constant volume or Otto cycle:

Process Remark

1 – 2 Adiabatic and reversible compression

2 – 3 Combustion

3 – 4 Adiabatic and reversible expansion

4 – 1 Exhaust stroke 47.5% and at compression ratio 10 is 60.2%

Mean effective pressure (mep)

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The efficiency of the diesel cycle is different from that of the Otto cycle only by the bracketed term, which is always greater that unity.

Mean Effective Pressure (mep) mep =

= ^[ ^]

The efficiency decreases as cut off ratio increases. If cut off ratio is greater than 10% of stroke, smoking occurs in an actual engine because there is no sufficient time for the combustion process to be completed before the exhaust valve opens.

The dual combustion or mixed or limited pressure cycle

 The name dual combustion is derived from the fact that it incorporates the features of both otto and diesel cycles.

 High speed diesel engine is based on this.

= 1 * + = , =

 If = 1 in above equation it becomes otto cycle and when = 1, it becomes diesel cycle.

V 3 4

5 1 P

f g s

5 T

v = constant v = constant (otto )

P = constant (Diesel)

mep [ ]

Comparison of Otto, Diesel, and dual Combustion (Limited - pressure) Cycles:

 For same compression ratio and same heat input: The heat rejected in the Otto cycle is less than that in the diesel cycle and dual combustion cycle thus the efficiency of the Otto cycle is more than the diesel and the dual combustion cycle for same compression ratio and same heat input.

otto dual diesel

 For constant maximum pressure and same heat input.

S T

1 2

4 3

5 6

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 For same maximum pressure and temperature

> >

S 2

5 6

Constant volume T

Constant pressure Constant pressure

1 2

6 T

Constant volume

In document QRG_ME.pdf (Page 153-165)