The carburettor: What is it and what does it do?

author

Joel Ilton

Tuesday, 9 July 2019

Long before the invention of modern day fuel injection, manufacturers came up with an interesting way of delivering the fuel and air mixture into the engine - a device called the carburettor.

Basing it’s operation on science, the carburettor works based on the Bernoulli Principle that, in layman’s terms, means the speed of the airflow into the engine controls the amount of fuel that is drawn into the mixture.

Confused? Let’s try and simplify it more.

 

The basic operation of the carburettor utilises an open pipe which allows the air to pass through the air cleaner into the inlet manifold.

This pipe is designed to increase air speed using the venturi method - it is narrower in the middle and wider at each end with air speed increasing in the narrow section.

Next, the air passes over the throttle valve, which controls how much airflow can pass through into the inlet manifold.

This is controlled by the throttle pedal. Pushing the pedal down opens the valve and increases the amount of air flowing into the engine.

Lifting off the pedal closes it again.

 

You’re probably wondering when the fuel makes an appearance.

This happens when the air moves past the venturi.

There are a number of small holes located in this narrow part of the pipe and as the air speeds up the fuel is ‘sucked’ into the airstream to create the air/fuel mixture ready for combustion inside the engine.

The amount of fuel is determined by the size of the holes, which are commonly known as jets. These can be adjusted to increase or decrease the amount of fuel mixing with the air.

 

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This setup is the very basic single barrel (meaning one venturi tube) carburettor.

As more performance was needed, manufacturers came up with multi-barrel carburettors that have two or four venturi tubes in one larger carburettor.

While these setups increase performance, they also increase fuel consumption.

Multi-barrel carburettors are also more complex as more moving parts are required, and are harder to fine tune with jet sizes to keep the engine happy.

 

You may remember jumping in your older car when it was cold and having real difficulty starting the engine.

This is where the choke would come in handy.

When the engine is cold, the normal fuel mixture isn't enough to fire up the engine, so the choke is used to close off the venturi, creating a vacuum inside the barrel, which draws in more fuel through the fuel jets.

This extra fuel is easier to ignite when the engine is cold, which makes starting the engine on those chilly mornings simple.

The first choke mechanisms were controlled from inside the car with a lever, but as carburettors became more technically advanced, automatic chokes were fitted to better control start up.

 

Still confused? Let’s try this - air comes in, sucks in fuel, mixes together and flows into the engine. Mixture goes boom! Car moves. Easy, right?

author

WRITTEN BY

Joel Ilton

Finding a passion for cars from a young age, Joel carried out work experience as a mechanic whilst at school before starting an apprenticeship after finishing year 12.

After almost 10 years on the tools and in customer service, he moved into the IT realm as a Data Analyst and In-House mechanic at AutoGuru.

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