Combustion CIET LAM DEPARTMENT OF MECHANICAL ENGINEERING 1

Combustion CIET, LAM, DEPARTMENT OF MECHANICAL ENGINEERING 1

Introduction l Definition of combustion • Combustion is a chemical reaction in which certain elements • l l of the fuel like hydrogen and carbon combine with oxygen liberating heat energy and causing an increase in temperature of the gases Presence of combustible mixture and means of initiating the process are necessary Homogeneous mixture Heterogeneous mixture 2

Combustion in S. I. Engines Homogeneous mixture from carburetor Piston at the end of compression stroke Combustion initiated by spark plug Burnt mixture Un burnt mixture Flame front spreading over a combustible mixture with certain velocity Flame propagation 3

Combustion in S. I. Engines Burnt mixture Un burnt mixture Flame front is a narrow zone separating the fresh mixture from the combustion products Flame propagation is caused by heat transfer and diffusion of burning fuel molecules from the combustion zone to the adjacent layers of un burnt mixture. Flame speed ≈ 40 cm/s, for =1 The velocity with which the flame front moves w. r. t. the unburned mixture in a direction normal to its surface is called the NORMAL FLAME VELOCITY Max. Flame speed occurs at =1. 1 to 1. 2 ( slightly rich mixture) For Richer mixtures Flame extinguishes as with the speed drops. Flame propagation Flame speed can be increased by introducing turbulence and proper air movement Qloss from combustion = Qdue to combustion The rate of chemical reaction determines the combustion characteristics 4

Combustion in C. I. Engines Piston at the end of compression Fuel injection The rate of combustion is determined by the velocity of mutual diffusion of fuel vapors and air and the rate of chemical reaction is of minor importance Combustion starts in the zones where =1. 1 to 1. 2 corresponding to maximum rate of chemical reaction Self-ignition or spontaneous ignition of F-A mixture at high temperature developed due to higher compression ratios, is of primary importance in determining the combustion characteristics. 5

Stages of Combustion in S. I. Engines Pressure TDC BDC sion Compression res Comp a 0 180 360 Crank angle (deg) Theoretical pressure crank angle (p- ) diagram 6

Stages of Combustion in S. I. Engines Pressure TDC BDC sion Compression res Comp a 0 180 360 Crank angle (deg) Theoretical pressure crank angle (p- ) diagram 7

Stages of Combustion in S. I. Engines Pressure TDC BDC sion Compression res Comp a 0 180 360 Crank angle (deg) Theoretical pressure crank angle (p- ) diagram 8

Stages of Combustion in S. I. Engines Pressure TDC BDC sion Compression res Comp b a 0 180 360 Crank angle (deg) Theoretical pressure crank angle (p- ) diagram 9

Stages of Combustion in S. I. Engines Pressure sion res Comp Combustion c b spark a 0 180 360 Crank angle (deg) Theoretical pressure crank angle (p- ) diagram 10

Stages of Combustion in S. I. Engines Pressure sion res Comp Combustion c Ex pa ns ion b spark a 0 180 360 Crank angle (deg) Theoretical pressure crank angle (p- ) diagram 11

Stages of Combustion in S. I. Engines Pressure sion res Comp Combustion c Ex pa ns ion b spark a 0 180 360 Crank angle (deg) Theoretical pressure crank angle (p- ) diagram 12

Stages of Combustion in S. I. Engines Pressure sion res Comp Combustion c Ex pa ns ion b spark d a 0 180 360 Crank angle (deg) Theoretical pressure crank angle (p- ) diagram 13

Stages of Combustion in S. I. Engines 30 Pressure (bar) TDC BDC 20 10 Compression 0 100 80 60 40 20 8 0 20 40 Crank angle (deg) Actual pressure crank angle (p- ) diagram 14 60 80

Stages of Combustion in S. I. Engines 30 Pressure (bar) TDC BDC 20 10 sion Compression res mp Co 0 100 80 60 40 20 8 0 20 40 Crank angle (deg) Actual pressure crank angle (p- ) diagram 15 60 80

Stages of Combustion in S. I. Engines 20 Spark Pressure (bar) 30 10 n and combustion sio res p m A Co 0 100 80 60 40 20 8 0 20 40 Crank angle (deg) Actual pressure crank angle (p- ) diagram 16 60 80

Stages of Combustion in S. I. Engines 20 Spark Pressure (bar) 30 10 Combustion n and combustion sio res p m A Co 0 100 80 60 40 B Moto ring TDC 20 8 0 20 40 Crank angle (deg) Actual pressure crank angle (p- ) diagram 17 60 80

Stages of Combustion in S. I. Engines C 20 Ex pa ns ion Spark Pressure (bar) 30 10 Combustion n sio res p m A Co 0 100 80 60 40 B Moto ring TDC 20 8 0 20 40 Crank angle (deg) Actual pressure crank angle (p- ) diagram 18 60 80

Stages of Combustion in S. I. Engines C 20 Ex pa 10 Combustion n sio res p m 0 100 80 60 40 B A Co ns ion D Spark Pressure (bar) 30 Moto ring TDC 20 8 0 20 40 Crank angle (deg) Actual pressure crank angle (p- ) diagram 19 60 80

Stages of Combustion in S. I. Engines C 20 Ex pa 10 Combustion n sio res p m 0 100 80 60 40 B A Co ns ion D Spark Pressure (bar) 30 Moto ring TDC 20 8 0 20 40 Crank angle (deg) Actual pressure crank angle (p- ) diagram 20 60 80

Motoring curve 30 Pressure (bar) TDC BDC 20 10 Compression 0 100 80 60 40 20 8 0 20 40 Crank angle (deg) pressure crank angle (p- ) diagram 21 60 80

Motoring curve 30 Pressure (bar) TDC BDC 20 10 on si s e pr m Co Compression 0 100 80 60 40 20 8 0 20 40 Crank angle (deg) pressure crank angle (p- ) diagram 22 60 80

Motoring curve 30 Pressure (bar) TDC BDC 20 10 on si s e pr m Co Compression 0 100 80 60 40 20 8 0 20 40 Crank angle (deg) pressure crank angle (p- ) diagram 23 60 80

Motoring curve 30 Pressure (bar) TDC BDC 20 10 on si s e pr m Co Compression 0 100 80 60 40 20 8 0 20 40 Crank angle (deg) pressure crank angle (p- ) diagram 24 60 80

Motoring curve 30 Pressure (bar) TDC BDC 20 10 on si s e pr Ex pa ns m Co Expansion 0 100 80 60 40 20 8 0 20 40 Crank angle (deg) pressure crank angle (p- ) diagram 25 60 80

Motoring curve 30 Pressure (bar) TDC BDC 20 10 Ex pa sion Expansion ns res mp ion Co 0 100 80 60 40 20 8 0 20 40 Crank angle (deg) pressure crank angle (p- ) diagram 26 60 80

Motoring curve 30 Pressure (bar) TDC BDC 20 10 Ex pa sion Expansion ns res mp ion Co 0 100 80 60 40 20 8 0 20 40 Crank angle (deg) pressure crank angle (p- ) diagram 27 60 80

Stages of Combustion in S. I. Engines 30 I II Spark D 10 B Combustion n ssio e r p A Moto ring Com 0 100 80 60 40 n Pressure (bar) io ns II Propagation of flame III After burning 20 III pa Ex I Ignition lag C TDC 20 8 0 20 40 60 80 Crank angle (deg) Actual pressure crank angle (p- ) diagram 28

Stages l l l A to B B to C C to D 29

Flame front propagation The factors which influence the flame front are l Reaction rate • l Transposition rate • • l The rate at which the flame eats its way into the unburned charge Due to the physical movement of the flame front relative to the cylinder wall The result of the pressure differential between the burning gases and the unburnt gases in the combustion chamber Turbulence 30

Flame front propagation Distance of flame travel across the chamber (%) 100 80 60 40 20 B Area I A 0 Low transposition rate Low Turbulence Low reaction rate 0 20 40 60 80 100 Time of flame travel across the chamber (%) 31

Flame front propagation Distance of flame travel across the chamber (%) 100 C 80 Area II 60 High transposition rate High Turbulence High reaction rate 40 20 B Area I A 0 Low transposition rate Low Turbulence Low reaction rate 0 20 40 60 80 100 Time of flame travel across the chamber (%) 32

Flame front propagation Low transposition rate Distance of flame travel across the chamber (%) 100 Low Turbulence Low reaction rate D Area III C 80 Area II 60 High transposition rate High Turbulence High reaction rate 40 20 B Area I A 0 Low transposition rate Low Turbulence Low reaction rate 0 20 40 60 80 100 Time of flame travel across the chamber (%) 33

Factors influencing the flame speed l l l 0. 006 Stoichiometric mixture l Turbulence Fuel-air ratio Temperature and pressure Compression ratio Engine output Engine speed Engine size Time in seconds l 0. 004 0. 002 A 0. 000 60 100 Lean 140 180 Rich Equivalence ratio 34 220

Rate of Pressure rise 30 Pressure (bar) Peak pressure Start of pressure rise 20 Rate of pressure rise Indicated by the slope of the curves between start of pressure rise and the peak pressure 10 Mot orin g cu rve Compression 0 120 80 40 Power TDC 40 80 120 Crank angle (deg) Illustrations of various Combustion rates 35

Rate of Pressure rise 30 Pressure (bar) Peak pressure Start of pressure rise 20 Rate of pressure rise Indicated by the slope of the curves between start of pressure rise and the peak pressure 10 Mot orin g cu rve Compression 0 120 80 40 Power TDC 40 80 120 Crank angle (deg) Illustrations of various Combustion rates 36

Rate of Pressure rise 30 Pressure (bar) Peak pressure Start of pressure rise 20 Rate of pressure rise Indicated by the slope of the curves between start of pressure rise and the peak pressure 10 Mot orin g cu rve Compression 0 120 80 40 Power TDC 40 80 120 Crank angle (deg) Illustrations of various Combustion rates 37

Rate of Pressure rise 30 High rate-I Pressure (bar) Peak pressure Start of pressure rise Normal rate-II 20 Rate of pressure rise Low rate-III Indicated by the slope of the curves between start of pressure rise and the peak pressure 10 Mot orin g cu rve Compression 0 120 80 40 Power TDC 40 80 120 Crank angle (deg) Illustrations of various Combustion rates 38