1

Internal Combustion Engines

Lecture-29

Ujjwal K Saha, Ph.D.Department of Mechanical Engineering

Indian Institute of Technology Guwahati

Prepared underQIP-CD Cell Project

2

Introduction

• Combination of a reciprocating engine and a rotary turbine (an attempt to combine high thermal efficiency of the reciprocating engine with high power/weight ratio of a rotary turbine).

• Uses its own exhaust into a gas turbine to produce power.

• Output of the engine is used for compressing the air for its own supercharge.

• Diesel cylinder is called a gasifier.

3

Types of Free Piston Engine

Inward compression typeInward compression typeOutward compression typeOutward compression type

4

Components of a free piston engine

5

• It consists of a gasifier and a turbine.• Compressor pistons are directly attached

to diesel pistons.• Motion of pistons is controlled by cushion

volume, compressor space and main diesel combustion chamber space, and not by the mechanical linkages. Hence, it is called a free piston engine.

Components of a free piston engine

6

• The centre pivot of mechanism oscillates, so it drives the fuel pump and other auxiliaries.

• The exhaust from the diesel cylinder is fed to a axial flow or impulse turbine.

• The stroke of pistons in a gasifier is variable and depends upon the amount of fuel injected.

Components of a free piston engine

7

Operation of a free piston engine• During the inward stroke, the air in the

compressor cylinder is compressed and is delivered to a scavenging box.

• In the power cylinder, the air compresses to 35 bar approx.

• Fuel is injected at a pressure of 400 to 700 bar, and combustion starts which moves the piston outwards.

8

Operation of a free piston engine

• During the outward stroke intake valve opens and air is inducted in the compressor cylinder.

• Air in the bounce cylinder is compressed, which is expanded in returning the piston back to firing position for next inward stroke.

• At the end of outward stroke the exhaust ports are uncovered. Hence, exhaust gases flow to the gas turbine.

9

Operation of a free piston engine

• At the end of outward stroke the exhaust ports are uncovered. Hence, exhaust gases flow to the gas turbine.

• Simultaneously, the scavenging ports are also opened, and the air from scavenged box enters the power cylinder.

• Excess air also passes to turbine.

10

Thermodynamics of a free piston engine

COMPRESSOR

ENGINE

TURBINE

1 2 3

4

11

T-s diagram

T

S

1

2 4

3

5

a

b

12

Stability of Piston Movements

• OUTWARD STROKE:A1 = A2e + A3e• INWARD STROKE:A1 = A2c + A2c

Where A1 is work done in bounce cylinder and A2 is work done in power cylinder and A3 is work done in compression cylinder.

e- expansion, c- compression

13

Stability of Piston Movements

It leads to fundamental condition of stability i.e. net work done by the power cylinder must be equal to the net work absorbed by the compressor cylinder, the bounce volume remains constant.

14

Effect of fuel rack position

• Increase in fuel supply leads to increase in stroke of pistons.

• Each fuel rack position corresponds to a particular stroke value.

• The outer movement of pistons will be more than the movement towards the inner dead centre.

15

Effect of bounce cylinder pressure

• Increasing bounce cylinder pressure leads to increase in A1

• Since A2 and A3 remains same i.e. pressure in compression cylinder increases.

• Position of the stroke is determined by the pressure level in the bounce cylinder.

16

Effect of working pressure

• Results in a change in stroke length.• And change in position of stroke.• Maximum position of IDC: when

pressure after compression < pressure required for combustion

• Increase in working pressure reduces the stroke length but ID point is increased.

17

OBSERVATIONS• By increase in load and increase in bounce

level the stroke and stroke position can be kept same for increased fuel supply.

• Upper limit of load: time available for combustion is not adequate.

• Position of IDC and ODC is limited to a narrow range.

• Self ignition requires that IDC should be nearest to central point of power cylinder.

• Adequate scavenging requires that ODC should be nearest to centre of power cylinder.

• The maximum position of ODC is limited by design.

18

Performance of free piston engine

Details GS-34 CS-751. Output

(i) Continuous maximum rating shaft, KW(ii) Equivalent, shaft kW

932745

313261

2. Gas(i) Delivery, kg/sec(ii) Pressure, bar gauge(iii) Temperature, °C

43

437

2.83.2470

3. Engine Cylinder diameter, cm 34 19

4. Compressor cylinder diameter, cm 90 53

5. Stroke Piston, cm 44 24

6. Mean piston speed 500 480

7. No. Of Oscillations, cycles/min. 570 1000

19

Performance of free piston engine

Details GS-34 CS-75

7. No. Of Oscillations, cycles/min. 570 1000

8. (i) Specific fuel consumption, kg per gas kW(ii) Specific fuel consumption, kg per shaft kW

0.196-

0.1990.244

9. Efficiency(i) Thermal efficiency based on gas hp(ii) Overall thermal efficiency

43%-

42%34%

10. Size of gas colllecting tank 10*swept volume

7*swept volume

11. Weight, Ton 8 2.25

12. Overall length, m 4.2 2.3

20

Advantages of free piston engine

• Simple• Power to weight ratio• Lower turbine operating temperatures• Multifuel capability• Flexibility and reliability• Easy starting and control• Waste heat recovery• less noise and vibrations

21

• Poor fuel economy• Stability• Part load efficiency• High combustion rates• Reduction gearing is required

Disadvantages of free piston engine

22

• Submarine air compressor units• Suitable for power generation in

medium power range• Used in ship propulsion• For pumping oil• Mixed gas-steam cycle

Applications

23

1.1. Crouse WH, and Anglin DLCrouse WH, and Anglin DL, (1985), Automotive Engines, Tata McGraw Hill.2.2. Eastop TD, and McConkey A,Eastop TD, and McConkey A, (1993), Applied Thermodynamics for Engg.

Technologists, Addison Wisley.3.3. Fergusan CR, and Kirkpatrick ATFergusan CR, and Kirkpatrick AT,, (2001), Internal Combustion Engines, John

Wiley & Sons.4.4. Gill PW, Smith JH, and Ziurys EJGill PW, Smith JH, and Ziurys EJ,, (1959), Fundamentals of I. C. Engines, Oxford

and IBH Pub Ltd. 5.5. Heisler H,Heisler H, (1999), Vehicle and Engine Technology, Arnold Publishers.6.6. Heywood JB,Heywood JB, (1989), Internal Combustion Engine Fundamentals, McGraw Hill.7.7. Heywood JB, and Sher E,Heywood JB, and Sher E, (1999), The Two-Stroke Cycle Engine, Taylor & Francis.8.8. MathurMathur ML, and Sharma RP,ML, and Sharma RP, (1994), A Course in Internal Combustion Engines,

Dhanpat Rai & Sons, New Delhi.9.9. Pulkrabek WW,Pulkrabek WW, (1997), Engineering Fundamentals of the I. C. Engine, Prentice Hall.10.10. Rogers GFC, and Mayhew YRRogers GFC, and Mayhew YR, (1992), Engineering Thermodynamics, Addison

Wisley. 11.11. Stone R,Stone R, (1992), Internal Combustion Engines, The Macmillan Press Limited,

London.12.12. Taylor CF,Taylor CF, (1985), The Internal-Combustion Engine in Theory and Practice, Vol. 1 & 2,

The MIT Press, Cambridge, Massachusetts.

References

24

1. http://www.mne.psu.edu/simpson/courses2. http://me.queensu.ca/courses 3. http://www.eng.fsu.edu4. http://www.personal.utulsa.edu5. http://www.glenroseffa.org/6. http://www.howstuffworks.com7. http://www.me.psu.edu 8. http://www.uic.edu/classes/me/ me429/lecture-air-cyc-web%5B1%5D.ppt9. http://www.osti.gov/fcvt/HETE2004/Stable.pdf10. http://www.rmi.org/sitepages/pid457.php11. http://www.tpub.com/content/engine/14081/css12. http://webpages.csus.edu13. http://www.nebo.edu/misc/learning_resources/ ppt/6-1214. http://netlogo.modelingcomplexity.org/Small_engines.ppt15. http://www.ku.edu/~kunrotc/academics/180/Lesson%2008%20Diesel.ppt16. http://navsci.berkeley.edu/NS10/PPT/ 17. http://www.career-center.org/ secondary/powerpoint/sge-parts.ppt18. http://mcdetflw.tecom.usmc.mil19. http://ferl.becta.org.uk/display.cfm20. http://www.eng.fsu.edu/ME_senior_design/2002/folder14/ccd/Combustion21. http://www.me.udel.edu22. http://online.physics.uiuc.edu/courses/phys14023. http://widget.ecn.purdue.edu/~yanchen/ME200/ME200-8.ppt -

Web Resources