Home / Cars & Autos / Guide to Gearhead: Carburetor/Direct Injection



Gasoline only burns when it is mixed with oxygen.  A bucket of gasoline lit on fire would be unlikely to explode, but instead the fire would sit on top of the liquid as the vapors burned.  If the bucket was left in the room for a period of time, the room would fill with gasoline vapors and the room may explode… So, don’t try it at home.

If we want to run a combustion engine using gasoline, we’ve established that we need to mix the fuel with air.  This is why we need a carburetor.  A carburetor is actually a very simple piece of equipment that mixes fuel with air.  It does this by using the Venturi Effect.  What Giovanni Venturi discovered in the 18th century was, if you take a fluid and pass it through a pipe, it will move at a relatively consistent speed and maintain a relatively consistent pressure.  If you pinch that pipe in the center, the fluid will be forced to pass through the pinched part faster than the rest of the pipe, thus creating an area of low pressure downstream from the pinch.

In the case of the carburetor, the “fluid” is air.  So, you have the pinched tube with air flowing through it, now all you have to do is drill a hole in the low pressure part of the tube and connect it to a source of fuel (gasoline) and you have built a carburetor.  The air is forced through the tube, speeds up at the pinch, and creates a pocket of low pressure.  The low pressure sucks fuel through the hole, atomizing it into a fine mist.  Theoretically, when this mixture reaches the combustion chamber, it is the perfect ratio of air to fuel.  If you want to go faster, you need more of the air/fuel mixture.  All you have to do is move more air through the carburetor, this increases the speed of the air, which lowers the pressure even more, which pulls more fuel; the air to fuel ratio stays the same, though.

Why don’t we have these any more?  Well, today we have Electronic Fuel Injection (EFI).  The carburetor is passive – using suction to create a static air/fuel ratio.  EFI is active – EFI uses computers and sensors to vary the pressure that gasoline sprays through a small atomizing nozzle.  The air/fuel ratio is variable with EFI.  This means that you can increase power when you need it, and increase fuel efficiency when you don’t.

BONUS: The “Choke” is something used in carburetors to increase the amount of fuel in the air/fuel ratio.  This was often necessary to facilitate a cold start.

BONUS 2: Ever heard of the Vinturi®?  Wine is usually considered better when it’s allowed to “breathe.”  The Vinturi® is basically a carburetor for wine.  If you replace “air” with “wine” and “fuel” with “air,” you have a general idea of how it works.  As the wine flows through, the air is sucked in and mixes with the wine.

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