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Tutorial 6

This document discusses carburetion and calculations of air-fuel ratio for an internal combustion engine. It provides calculations for [1] suction at the throat of a carburetor venturi for a 4-cylinder engine, and [2] the venturi depression, throat diameter, and air velocity required for a carburetor to deliver a specified fuel flow rate and air-fuel ratio. The document is from BITS Pilani Hyderabad Campus and contains calculations, problems, and solutions related to carburetor venturi design and air-fuel ratio.

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Harrshit Limbodia
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88 views14 pages

Tutorial 6

This document discusses carburetion and calculations of air-fuel ratio for an internal combustion engine. It provides calculations for [1] suction at the throat of a carburetor venturi for a 4-cylinder engine, and [2] the venturi depression, throat diameter, and air velocity required for a carburetor to deliver a specified fuel flow rate and air-fuel ratio. The document is from BITS Pilani Hyderabad Campus and contains calculations, problems, and solutions related to carburetor venturi design and air-fuel ratio.

Uploaded by

Harrshit Limbodia
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PPTX, PDF, TXT or read online on Scribd
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BITS Pilani

Hyderabad Campus

Carburettors
Principle of Carburetion

BITS Pilani, Hyderabad Campus


Calculation of the A/F Ratio
Point 1 – inlet to the carburetor ( section A-A)
Point 2 – At the Venturi throat (section B-B)
Assume an adiabatic flow

Velocity at the throat (C2)

√ [()]
ϒ −1
p2 ϒ
C 2= 2 C p T 1 1 −
p1
Theoretical mass flow rate of air

√ [( ) ( ) ]
2 ϒ +1
𝐴2 𝑝1 p2 ϒ p2 ϒ
𝑚˙ 𝑎= 2𝐶 𝑝 −
𝑅√𝑇 1 p1 p1

BITS Pilani, Hyderabad Campus


Calculation of the A/F Ratio

Substitute Cp=1005 J/kg ; ϒ=1.4 ; R=287J/kg K

Actual Mass flow rate of air


𝑚
˙ 𝑎 ,𝑎𝑐𝑡𝑢𝑎𝑙 =𝐶 𝑑, 𝑎 ∗ 𝑚˙ 𝑎

√[( ) ( ) ]
1.43 1.71
𝐴2 𝑝 1 p2 p2
˙ 𝑎 ,𝑎𝑐𝑡𝑢𝑎𝑙 =0.1562
𝑚 𝐶 𝑑 ,𝑎 −
√𝑇 1 p1 p1

Actual mass flow rate of fuel

𝑚˙ 𝑓 ,𝑎𝑐𝑡𝑢𝑎𝑙 =𝐶 𝑑,𝑓 𝐴𝑓 √ 2𝜌 𝑓 ( p1 − p 2 −𝑔𝑧 𝜌 𝑓 )

A/F ratio =

BITS Pilani, Hyderabad Campus


Calculation of the A/F Ratio

√ [( ) ( ) ]
1.43 1.71
p2 p2
−
𝐴 𝐶 𝐴2 p1 p1
= 0.1562∗ 𝑝 1 ∗ 𝑑 , 𝑎
𝐹 𝐶𝑑 , 𝑓 𝐴 𝑓 2 𝑇 1 𝜌 𝑓 ( p 1 − p 2 − 𝑔𝑧 𝜌 𝑓 )

If compressibility of air neglected

˙ a , 𝑎𝑐𝑡𝑢𝑎𝑙 =𝐶 𝑑, 𝑎 𝐴2 √ 2 𝜌 𝑎 ( p1 − p2 )
𝑚

Cd - Coefficient if discharge; P –Pressure; A-cross sectional area; T- Temperature


z – Height of the nozzle exit above the level of fuel in the float bowl; - Density;

Subscripts : a- air ; f- fuel

BITS Pilani, Hyderabad Campus


Problem -1

1. A four cylinder four-stroke engine having diameter


and length of stroke as 100mm and 120mm
respectively is running at 1800 r.p.m. Its carburettor
venturi has a 28mm throat. Assuming co-efficient of
air flow 0.8, density of air 1.2kg/ and volumetric
efficiency of the engine as 75%,determine the
suction at the throat.

BITS Pilani, Hyderabad Campus


Solution-1

Suction at throat ():

Stroke volume =
=
= 0.00377
Actual volume per stroke =
= 0.75 0.00377
= 0.00283

BITS Pilani, Hyderabad Campus


Solution-1

Actual volume suction per second ():

= 0.00283
= 0.0424
=
= 0.05094

BITS Pilani, Hyderabad Campus


Solution-1

Initial T,P are not given, neglecting the compressibility of air

=
0.05094 = 0.8()
= 0.04457 bar.

BITS Pilani, Hyderabad Campus


Problem -2

2. A simple carburettor under a certain condition delivers


5.45kg/h of petrol with an air-fuel ratio of 15. The fuel jet
area is 2 with a co-efficient of discharge of 0.75. If the tip of
the fuel jet is 0.635cm above the level of petrol in the float
chamber and the venture throat coefficient of discharge is
assumed to be 0.8. calculate
(i) The venturi depression necessary to cause air and fuel
flow at the desired rate.
(ii) Venturi throat diameter
(iii) Velocity of air across the venture throat.
Take density of air=1.29kg/ and specific gravity=0.72

BITS Pilani, Hyderabad Campus


Solution -2

1. The Venturi throat depression

BITS Pilani, Hyderabad Campus


Solution -2

2. Venturi throat diameter,

Air flow rate = kg/s

BITS Pilani, Hyderabad Campus


Solution -2

3. Velocity of air across the venture throat

BITS Pilani, Hyderabad Campus


Thank you

BITS Pilani, Hyderabad Campus

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