In this post, I’ll be detailing the carburetor in the Cessna 172 and talking about what it does and how it works.
To start, one should know that an engine works by burning a mixture of fuel and air in a specific proportion. This miniature explosion drives a cylinder down, and then back up again, providing power. This power is used to turn the wheels on your car or the propeller on your airplane.
So what proportion is best when mixing fuel and air together? Approximately 1 unit of fuel to 12 units of air is an ideal ratio. So why aren’t planes manufactured so that there is always this 1:12 ratio of fuel to air? The answer has to do with altitude. As you fly higher, air becomes less dense, and due to this decrease in density, the 1:12 ratio on the ground may actually be closer to 1:10 or 1:8 at altitude. In order to balance this out, we need to modify the fuel-air mixture so that the ratio returns to an optimal level. This process is called “leaning” and simply decreases the amount of fuel mixed with the incoming air – this returns the ratio to normal.
All of this happens in the carburetor. In the C172, the carburetor is described as an up-draft, float-type carburetor mounted on the bottom of the engine. “Up-draft” simply means that air enters the bottom of the carburetor, then flows upwards through the system, and out the top to the intake manifold tubes. “Float-type” refers to the fact that a float system regulates the amount of fuel that sits in the float chamber of the carburetor. The float chamber is a small chamber of readily available fuel that is waiting to be mixed with incoming air.
The diagram below may provide a better visual of the process inside of a carburetor. Essentially, air flows up through a tube, called a Venturi, and is mixed with fuel coming from the float chamber. A pin inside the float chamber controls how much fuel is sent to the Venturi, and thus controls the mixture, either leaning or enriching it as necessary. When the mixture knob is moved to the idle-cutoff position, this pin fully blocks any fuel flow to the engine, which in turn starves the engine and shuts it down.
Last but not least, a butterfly valve inside the carburetor controls the amount of this fuel-air mixture that is allowed to go to the engine. This is your throttle control.
Diagram: From The Ground Up
If you have any questions, feel free to post in the comments. I hope I’ve done an adequate job explaining this system, but if there’s something I’ve left out or you feel is important, let me know and I’ll add it in.
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