Carburetor

Carburetion:

The process by which combustible fuel-air mixture is made by mixing the proper amount of fuel is called Carburetion.

• The device which does this job is called Carburetor.
• Air speed > fuel speed at fuel nozzle to atomize fuel.
• Amount of fuel drawn into the engine depends upon the air velocity in the venturi.

Factors Affecting Carburetion:

1. Engine Speed:

The greater the speed of engine, the lesser the time for carburetion.

2. Vaporization Characteristic of Fuel:

If distillation temperature is lower, the vapor content is higher which results in good vaporization characteristics which make carburetion better.

3. Air Inlet Temperature:

If incoming air has high temperature then the volatility increases which result in good carburetion.

4. Design:

The intake system & Combustion chamber design has a significant impact on carburetion.

Air-Fuel Mixture & Their Requirements:

There are three types of air-fuel mixture which are as follows:

1. Chemical or Stoichiometric Mixture  ↔  which can burn completely.
2. Rich Mixture  ↔  Less air than stoichiometric mixture.
3. Lean Mixture  ↔  More air than stoichiometric mixture.

At different conditions of engine, different air-fuel mixture is required.

• Idling or Starting Condition: Require rich mixture which produce max. power to overcome friction.
• Cruising or Normal Condition: At constant load, engine requires lean mixture.
• Maximum Power or Acceleration: Engine require rich mixture because of high power and to prevent overheating of valves (thereby reduces flame temperature).

Carburetor:

Following are the components of Elementary Carburetor:

1. Fuel Strainer: Used to filter the fuel and prevent blockage of nozzle.
2. Float Chamber: Used to store, supply and adjust fuel level with a float.
3. Venturi Section: A round cylinder which is used to create pressure difference and atomization of fuel.
4. Fuel Discharge Nozzle: Used to atomize and produce spray of fuel.
5. Choke Valve: Used to control air supply.
6. Throttle Valve: Used to regulate the output by varying quantity of mixtures at different load conditions.

Principle Of Carburetion:

• The air comes in the intake sections of the carburetor from the air cleaner which removes dust particles.
• When air flow through the convergence section of venturi section, it creates pressure difference called Carburetor Depression which forces fuel to flow through fuel metering orifice (main jet) into the venturi.
• As fuel flow, the air flowing through venturi atomize fuel and make air-fuel mixture.
• The flow then passes the throttle valve and enters the intake manifold.

 Why do we need modifications in Elementary Carburetor:

Modifications are required in Elementary Carburetor because of following deficiencies:

1. At low load mixture becomes leaner, the engine requires mixture to be enriched at low loads.
2. At starting condition, the pressure difference is not significant so no fuel flow through main jet into venturi.
3. At high altitudes, density of air is changed, so flow is changed which effect the fuel flow.

Modern Carburetor Design:

Carburetor should be modified to get the best equivalence ratio v/s air flow distribution:

1. Main Metering System: Used to provide essentially constant lean or stoichiometric mixtures over the 20 to 80% of air flow.
2. Idle System: Should be use to meter the fuel flow at idle and low loads.
3. Enrichment System: Used to provide max. power through wide-open throttle.
4. Accelerator Pump: Which injects additional fuel when throttle is open rapidly.
5. Choke: Used to enrich the mixture and to ensure a combustible mixture within each cylinder at the ignition time.
6. Altitude Compensation: Used to adjust fuel flow when air density is changed.

Types Of Carburetor Based on Air-Flow Direction:

1. Up-Draught Carburetor:

Carburetor in which discharge of air is in the direction opposite to the direction gravity is called Up-Draught Carburetor.

• Operate at low speeds.
• Small mixing tube or throat.

2. Down-Draught Carburetor:

Carburetor in which discharge of air is in the same direction as that of gravity is called Down-Draught Carburetor.

• Ensure large fuel flow and used for high speeds.
• Large diameter of tube or throat.

2. Cross-Draught Carburetor:

Carburetor in which discharge of air is in the direction orthogonally to that of gravity is called Down-Draught Carburetor.

• Less number of joints so less pressure drop decreases.

Calculation Of Air-Fuel Ratio:

Air-Fuel Ratio Neglecting Compressibility of Air:

The above fuel derivation is same but derivation of air is changed due to neglecting air compressibility.

References:

• University notes + my knowledge.
• Videos from YouTube.
• Images from Google Images.
• Book: IC Engines by V Ganesan (4th Edition).
• Class Lectures.