Air, Combustion, and Carburetor Function Explained

Posted on Apr 21, 2025 in Technology

Air Composition and Combustion Basics

Air Composition

Air is a mixture of various gases. Its approximate volumetric composition is 21% oxygen, 78% nitrogen, and 1% argon and other trace gases. For combustion analysis, this 1% portion (mainly argon) is often treated as nitrogen because it behaves similarly as an inert gas during the process.

The Combustion Process

The basic chemical reaction for the complete combustion of a hydrocarbon (HC) fuel with oxygen (O2) is:

HC + O2 → H2O + CO2

Different air-fuel mixture ratios affect combustion:

  • Stoichiometric Mixture: A mixture where there is exactly enough oxygen to completely burn all the hydrocarbon fuel.
  • Rich Mixture: A mixture where there is insufficient air (oxygen) to completely burn all the hydrocarbon fuel. This results in incomplete combustion, producing carbon monoxide (CO) in addition to water (H2O) and carbon dioxide (CO2): HC + O2 → H2O + CO2 + CO.
  • Lean Mixture: A mixture where there is excess air (oxygen) relative to the amount of fuel available for combustion.

Lambda Factor (λ)

A common way to specify the mixture richness is using the lambda factor (λ):

λ = (Actual Air Mass) / (Theoretical Stoichiometric Air Mass)

  • λ = 1: Stoichiometric mixture
  • λ < 1: Rich mixture
  • λ > 1: Lean mixture

Combustion Gases

  • Non-toxic Gases: Nitrogen (N2), Oxygen (O2 – if lean), Water Vapor (H2O), Carbon Dioxide (CO2).
  • Toxic Gases: Carbon Monoxide (CO), Unburned Hydrocarbons (HC), Oxides of Nitrogen (NOx), Solid Particulate Matter.

Carburetor Fundamentals and Operation

Carburetor Fuel Feed Circuit Schematic

A typical carburetor fuel supply system includes:

  1. Air Filter
  2. Carburetor
  3. Carburetor Throttle Valve (Butterfly)
  4. Intake Manifold
  5. Gasoline Tank (Fuel Tank)
  6. Fuel Tank Filter (Sock)
  7. Camshaft Lobe (for mechanical pump)
  8. Mechanical Fuel Pump

Mechanical Diaphragm Fuel Pump Operation

This type of pump uses engine rotation to supply fuel:

  • When an eccentric lobe on the camshaft actuates the pump lever (1), it pulls down the diaphragm (2).
  • This downward movement creates suction, opening the inlet valve (3) and drawing fuel from the tank through a filter (4).
  • When the camshaft lobe rotates away, the lever is released, and a spring (5) pushes the diaphragm upwards.
  • This upward movement pressurizes the fuel, closing the inlet valve (3) and opening the outlet valve (6), forcing fuel towards the carburetor.

Elementary Carburetor Components

A basic carburetor consists of essential parts:

  • Float Chamber (Bowl): Maintains a constant fuel level, typically slightly below the height of the main nozzle outlet in the venturi, ensuring proper fuel delivery without flooding.
  • Venturi (Diffuser): A narrowed section in the carburetor throat. As air flows through it, its speed increases, and pressure decreases (Bernoulli’s principle). This low pressure draws fuel from the float chamber through the main jet.
  • Fuel Jet (Main Jet/Surtidor): A calibrated orifice that meters the amount of fuel drawn into the airflow passing through the venturi.
  • Throttle Valve (Butterfly Valve): A plate located after the venturi that pivots to control the volume of the air-fuel mixture entering the engine’s intake manifold, thus regulating engine speed and power.

Advanced Carburetor Systems

Enrichment Systems (Accelerator Pump)

This system provides extra fuel during rapid acceleration.

The device often consists of:

  • An injector nozzle (1) aimed into the carburetor throat.
  • A passage connecting the nozzle to the float chamber (2).
  • A small pump (3), often diaphragm or piston type, linked to the accelerator pedal/throttle linkage.
  • Check valves controlling fuel flow into (4, suction) and out of (5, discharge) the pump chamber.

Operation: When the accelerator pedal is pressed quickly, the linkage (6, 8) moves the pump diaphragm (7), forcing a squirt of fuel through the injector nozzle (1) directly into the incoming air. This temporarily enriches the mixture to prevent hesitation during sudden throttle opening.

Cold Start System (Choke)

During a cold start, fuel vaporizes poorly and condenses on cold intake manifold walls and cylinder surfaces. To compensate, the air-fuel mixture must be significantly enriched.

A common cold start device is a choke valve:

  • A butterfly valve (1, choke plate) placed above the venturi/main nozzle (3).
  • When closed (or partially closed), it restricts airflow, creating a much higher vacuum at the main nozzle and idle/transfer ports.
  • This strong vacuum draws significantly more fuel, creating the rich mixture needed for starting a cold engine.
  • A spring (2) or linkage mechanism controls its position. It may also incorporate a separate valve (4) or mechanism to allow slight opening under high vacuum once the engine starts.

Double-Body (Dual) Carburetors

Dual-barrel or double-body carburetors are often used in racing or high-performance engines to provide good fuel control across a wide range of engine speeds.

Internal Schematic Elements:

  1. Common Float Chamber (supplying both barrels)
  2. Primary Throttle Valve
  3. Common Throttle Shaft (or linked shafts)
  4. Secondary Throttle Valve

Structure and Operation:

  • Formed by two carburetor barrels integrated into a single body, sharing a common float chamber but often having independent fuel circuits (jets, venturis).
  • One barrel, the primary, typically has a smaller diameter venturi and handles engine operation at idle and low-to-mid speeds, providing good fuel atomization and throttle response.
  • The second barrel, the secondary, usually has a larger diameter venturi. It begins to open (often via mechanical or vacuum linkage) at higher engine speeds/loads when more air-fuel mixture is required, maximizing peak power.