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Fuel systems(Carburetor)

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•To meter the liquid fuel as required by the engine

•Atomize the fuel and mix it with the air stream

•Enable the engine to run smoothly in all engine operating conditions.

Function of the carburetor 

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Pa = P

!P = P "P# P# 

Aair 

Afuel

$ %hanging

Aair  &Afuel ''

(imple %arburetor 

Throttle opening

!P

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(imple %arburetor problems

)dling circuitAcceleration *+esponse time,

%hoc- *%old starting,

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%arburetor )dle circuit and mix

adustment

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/ariable 0et %arburetor 

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)nta-e (ystem of a 1asoline )nternal

%ombustion Engine with a Two23arrel

%arburetor 

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# 4

Air 

# 4

Fuel

Assumptions5• )ncompressible flow• 6o 7eat transfer • Frictionless flow

• 6o external wor-• 8istance between 9enture inlet and

 throat is negligible

%alculation of F&A ratio

A. For air 

Apply 3ernoulli equation between and #

2

2

2

2

2

1

2

1

1

1

22 gZ 

V  P  gZ 

V  P ++=++

 ρ  ρ 

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# 4

Air 

# 4

Fuel

)1....(..........2

2

2

1

2

2

2

1

2

221

21

V V  P V V  P  P 

a

a

a

a

−=→−

=−

∴

==

 ρ  ρ 

 ρ  ρ  ρ 

Apply continuity equation

  factor approuch A

 AWhere

 A

 A

 P 

V 

 A

 AV 

 P 

 A

 AV V 

 P 

in From

 A

 AV V V  AV  A

a

a

a

a

a

a

2

1

2

2

1

2

2

2

1

22

2

2

1

22

2

2

2

1

221222111

1

1

2

122

12

)2....(..........

   

 

 

 −

   

  

 −

∆

=∴

   

  

 −=

∆→  

 

  

 −=

∆→

   

  

 =→=

 ρ 

 ρ  ρ 

 ρ  ρ 

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# 4

Air 

# 4

Fuel

→=

   

 

 

 −   11

2

1

2

 A

 A

)f A::: A#

a

a P 

V  ρ 

∆=

  2

2

Actual 9elocity5)f %a = flow coefficient

%a = /a,act & /a,th

%a ; < /#,act ; /#,th

ass flow rate5

aaaa

aaaa

a

a

aaaaa

 P C 

 P C 

 P C V  AC m

 ρ  ρ 

 ρ  ρ 

 ρ  ρ  ρ 

2

2

2

22

∆=

∆=

∆==

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  f  

a

  f  

a

  f  

a

  f  

a

  f  

a

 P 

 P 

 A

 A

C 

C 

m

m

 F 

 A

∆∆

   

  

    

  

    

  

 ==∴

 ρ 

 ρ 

# 4

Air 

# 4

Fuel

3. For Fuel

  f    f    f    f    f     P C m   ρ  ρ    2∆=

(imilarly as air