In this article we will discuss about Carburettors:- 1. Introduction to Carburettor 2. Factors Affecting Carburettors 3. Carburettor Types 4. Simple Carburettor 5. Solex Carburettor 6. Carter Carburettor 7. Carburettor Calculations.
Contents:
- Introduction to Carburettor
- Factors Affecting Carburettors
- Carburettor Types
- Simple Carburettor
- Solex Carburettor
- Carter Carburettor
- Carburettor Calculations
1. Introduction to Carburettor:
In SI engines, highly volatile fuel like petrol, alcohol, benzol are used. In this case air-fuel mixture is produced outside the engine cylinder. This air-fuel mixture is supplied to the engine cylinder during suction through the engine intake manifold. Then this air-fuel mixture is compressed by the upward moving piston and just before the end of compression, the mixture is ignited by means of spark, produced by spark plug.
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Carburettor is a device, which atomises and vapourises the fuel and mixes it with air in varying proportions so as to suit the changing conditions (or changing loads) of SI engines.
The process of automisation-vapourisation and mixing with air is known as carburetion. The air-fuel mixture so produced is called Combustible mixture.
This combustible mixture (air-fuel mixture) is carried to the engine cylinder through intake manifold and quantity of air-fuel mixture is controlled by throttle valve.
Now it is essential to know the correct meaning of following terms:
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1. Vaporisation:
Vaporisation means change of phase from liquid phase to vapour phase.
2. Automisation:
It is the mechanical breaking up of the liquid fuel into small particles i.e., when the liquid fuel is sprayed into air, the fuel particles absorb heat of air and get vaporized instantly.
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1. Stoichiometric Air-Fuel Mixture or Chemically Correct Air-Fuel Mixture:
(a) This mixture contains just sufficient air for the complete combustion of fuel.
(b) Note after combustion all the fuel is burnt and all the air is consumed.
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(c) For 1 kg of Octane (C8H18) fuel, 15.14 kg of air is needed for complete combustion.
Therefore, 15.14 : 1 is called chemically correct air-fuel ratio
2. Rich Mix:
More fuel-less air
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Therefore, 14, 13, 12, 10 or less: 1 is called Rich Mix.
3. Lean Mix:
Less fuel-more air
Therefore, 17, 18, 20, 22, or more : 1 is called Lean Mix.
The range of homogenous mixture which can be ignited in SI engines- 7:1 Air-fuel by mass on Rich Side and @ 20:1 on Lean Side.
1. For Power and Economy:
It is important to know the value of air-fuel at which max power and minimum fuel consumption is obtained.
Maximum power is obtained at a considerably rich mixture with s air-fuel ratio around 12:1. (This mixture is the best power mixture)
The minimum brake specific fuel consumption (bsfc) is obtained — at a slightly leaner mixture with air-fuel ratio around 16:1. (This mixture is the best economy mixture).
2. Mix Requirements for Automotive Engines:
For the start mixture required is very rich, because the engine is cold-fuel does not vaporise properly.
Idling Range (A-B):
There is no external load on the engine—has to produce frictional horse power (fhp).
Since the engine is not running hot a rich mixture is required. For idling air-fuel required is @ 12:1.
Normal or Crushing Range (B-C):
Throttle valve is gradually opened and load increases above 20%.
i. Fuel economy is prime consideration.
ii. Air-fuel ratio required @16 or 17:1.
Maximum Power or Acceleration Range (C-D):
For max power rich mixture is required
i. Nearly full throttle.
ii. Air-fuel ratio required 12 to 14:1.
2. Factors Affecting Carburettors
:
Following factors affect carburettion process:
1. Engine Speed:
Directly affects the time available for preparation of mixture by the carburettor. When the engine speed increases, velocity of air must be increased by providing venturi in the carburettor.
2. Temperature of Air at the Inlet:
When the temperature of atmosphere air increases, vaporisation increases, and decreases the density of air. Ultimately it results in decrease of efficiency and output power.
3. The Vaporisation Characteristics of Fuel:
It is always desirable to use more volatile fuels otherwise it is required to heat the fuels in the intake manifold to produce vaporisation. However this is expensive and reduces power output.
4. Design of Intake Manifold:
In case of multi-cylinder engines, proper design of intake manifold is a must to ensure proper distribution of fuel to each cylinder.
3. Carburettor Types
:
1. Open Choke Carburettors:
Venturi is of fixed dimensions. Metering is affected by varying the pressure drop across it. Example- Solex, Zenith, Carter carburettors. (Venturi-fixed dimensions; pressure drop varied).
2. Constant Vacuum Carburettors:
Area of air passage is varied, while pressure drop is kept constant. Example- S.U. carburettor.
(Venturi dimensions varied; pressure drop constant.)
4. Simple Carburettor:
A simple carburettor mainly consists of:
(a) A float chamber with hollow float, needle valve, air bleed hole.
(b) A cylindrical pipe called air-horn. Narrowest c/s of which is called venturi.
(c) A throttle valve.
(d) A choke valve.
(e) A main petrol jet.
Float with needle valve maintains constant level of petrol in the float chamber. As the petrol level goes down—float move down—and needle valve opens—then petrol flows into the float chamber—level rises and comes to pre-decided level. Through the air-bleed hole atmospheric pressure will be acting on the petrol surface.
During suction stroke, filtered air is drawn into the cylinder. This air passes through the Venturi. Venturi has a varying c/s, which first decreases to a minimum and then increases. Its narrowest c/s is called as the throat. As the air flows through the venturi, its velocity increases and pressure falls to a minimum at throat.
The pressure differences between throat and float chamber exists and is called as Intake Depression or (Carburettor depression). Because of this intake depression, petrol will be sprayed through the nozzle into the throat. It is to be noted that the float chamber is connected to the venturi through nozzle as shown.
When the petrol is sprayed in the high velocity air, petrol particles will absorb heat of air gets vaporised instantly. Then the combustible charge is produced and this charge admitted to engine cylinder. The amount of mixture is controlled by the throttle valve, which can be opened (or operated) by means of acceleration lever (or pedal) depending upon power requirements.
A choke valve is also provided, to start the engine from cold. When the choke lever is operated, it almost closes the air horn. The suction pressure will act on nozzle only. And more fuel less air (i.e., rich mixture) will go to engine and engine starts.
Note:
Nozzle opening in the throat will be 1.5 to 2 mm above the petrol surface level. This is provided in order to avoid evaporation losses of petrol when the engine is not running.
Limitations of Simple Carburettor:
1. It provides lean mixture at the starting warming up, idling range. Actually rich mixture is required.
2. At erasing it provides increasingly rich mixture.
3. At acceleration it provides lean mixture.
5. Solex Carburettor:
This carburettor has all the four basic circuits viz., Normal running; cold starting and warming up; Idling and slow running; Acceleration. It was used in Fiat cars, Ambassador cars, Gazal cars, Willys Jeeps etc. It is the basic carburettor—it is important to learn this carburettor so as to understand the working of any other newly developed carburettors.
1. Normal Running:
Figure 28.8 shows the schematic diagram of Solex Carburettor. It consists of a float chamber. On the top of float needle valve is provided. Float chamber is connected to the air horn through main jet and emulsion tube. Emulsion tube has air jets, jets and orifices as shown and is provided in the venturi of air horn.
Similar to the working of simple carburettor, during suction stroke, filtered air enters the air horn and in the venturi mixing of high velocity air and fuel takes place and the charge produced goes to the engine for normal running.
2. Cold Starting and Warming Up:
For cold starting—turn the starter lever to start position. When the starter flat disc with holes rotates and orifices (A) comes in contact with the dotted passage. Because of the engine suction which acts through the passage shown by (—>..—>) and air comes through the passage—mixing of air and fuel takes place — the charge goes to the engine cylinder and engine starts—then simultaneously acceleration is given to warm-up the engine. (It is to be noted that some operations are performed simultaneously).
3. Idling and Slow Running:
Once the engine starts and warm up—then for idle running the throttle valve will be almost closed (Not completely)—engine suction is applied through the passage shown by arrows (—>)—Petrol from the main jet is sucked and air will enter from top at (14)— mixing of air and fuel takes place and —charge goes to engine and engine will be just running at no load conditions.
4. Acceleration Circuit:
For acceleration— i.e., when more power is required—then acceleration lever/foot pedal in pressed as shown. Because of lever and fulcrum—diaphragm of acceleration pump is pressed towards LHS direction and extra petrol is supplied through acceleration jet—through injector into venturi. This fuel will be in addition to the rich mixture which is created because of more opening of throttle valve. It is also to be noted that throttle valve is also connected through the links to acceleration pedal.
6. Carter Carburettor
:
It is an American carburettor, used in jeeps.
It has three-venturies 3, 5, and 6.
At low speed-petrol is sprayed by nozzle 4, in the venturi 3.
Then the mixture flows through venturies 5 and 6, where homogenous mixture produced goes to engine.
1. Metering:
It has mechanical metering system. The metering rod (7) has 2-3 steps. At full throttle the rod 7 moves up.
2. Starting:
When choke is operated, more petrol-less air (rich mixture) goes to engine, and engine starts.
3. Idle and Low Speed:
Throttle valve almost closed, air from air horn, petrol from idle jet (10) produce mixture goes to engine.
4. Acceleration Circuit:
Acceleration pump provided to supply extra fuel.
7. Carburettor Calculations
:
P1 – Pressure of air at sec. 1-1 in N/m2
A1 – Air flow area at sec. 1-1 in m2
C1 – Velocity of air at sec. 1-1 m/sec
v1 – Sp. vol air sec. 1-1 m3/kg
T1 – Absolute temperature of air sec. 1-1K
