A carburetor is a mechanical device that mixes air and gasoline before the mixture enters an engine cylinder. It was used in many older cars, motorcycles, lawn mowers, and small engines before electronic fuel injection became common. Its job matters because an engine needs the right air fuel mixture to burn smoothly, make power, and avoid wasting fuel.
The carburetor does this without a computer by using airflow, pressure differences, and carefully sized passages.
Key Facts
- A carburetor mixes air and fuel before the intake valve sends the mixture into the cylinder.
- The venturi narrows the air passage, increasing air speed and lowering pressure.
- Bernoulli idea: faster airflow in the venturi produces lower static pressure.
- Fuel is pulled from the float bowl through a jet when pressure in the venturi is lower than fuel bowl pressure.
- Stoichiometric gasoline mixture is about 14.7 parts air to 1 part fuel by mass.
- Engine speed depends on throttle opening: more open throttle allows more air flow and more fuel flow.
Vocabulary
- Carburetor
- A mechanical fuel metering device that blends air and gasoline into a combustible mixture for an engine.
- Venturi
- A narrowed section of the carburetor throat where air speeds up and pressure drops.
- Jet
- A small calibrated opening that controls how much fuel can flow into the air stream.
- Float bowl
- A small fuel reservoir in the carburetor that keeps fuel at a nearly constant level.
- Throttle plate
- A rotating valve that controls how much air and fuel mixture can enter the engine.
Common Mistakes to Avoid
- Thinking the carburetor sprays fuel because of an electric pump. In a basic carburetor, fuel is mainly drawn into the air stream by the pressure drop created in the venturi.
- Confusing the choke with the throttle. The choke restricts incoming air to make a richer mixture for cold starting, while the throttle controls engine power by changing total airflow.
- Assuming more fuel always means more power. Too much fuel makes the mixture rich, which can burn poorly, waste fuel, foul spark plugs, and reduce power.
- Ignoring air density changes. Cold air, hot air, and high altitude change how much oxygen enters the engine, so a fixed carburetor setting may not produce the same mixture everywhere.
Practice Questions
- 1 A gasoline engine is running at the ideal air fuel ratio of 14.7:1 by mass. If 29.4 g of air enters the engine, how many grams of gasoline are needed?
- 2 A carburetor throat narrows from a cross-sectional area of 12 cm2 to 4 cm2. If air speed before the narrow section is 6 m/s, estimate the air speed in the venturi using continuity, assuming constant air density.
- 3 Explain why partially closing the choke helps a cold engine start, and why leaving it closed after the engine warms up can cause poor running.