How Cars Work · The Mix
What the gas pedal actually does
There is one more part of this story, and it explains what your mom is really doing when she presses the gas pedal.
She is not pumping fuel into the engine. The carburetor takes care of that on its own.
She is opening a flap.
Inside the carburetor, downstream of the narrow throat, there is a flat metal disc on a hinge. When the disc is sideways across the tube, almost no air can get past, and the engine runs slowly — barely getting any air, barely making any power, just enough to keep itself going. (This is what the engine is doing when the car is sitting at a red light.) When the disc is tilted edge-on to the airflow, the way is wide open, air rushes through, and the engine runs fast. The disc is called the throttle.
The gas pedal is connected by a metal cable to the throttle. Press the pedal, the cable pulls, the throttle swings open, more air rushes through the carburetor, and — this is the clever part — more fuel automatically comes with it. Because the carburetor uses fast air to suck up fuel, more air means more fuel, in roughly the right ratio. The mix takes care of itself. Press harder, more air, more fuel, more bang, faster wheels.
In other words, the gas pedal does not really control the gas. It controls the air. The gas just tags along.
This is also why an old car’s gas pedal is sometimes called the throttle pedal. That is what it actually is.
Did You Know? Until the 1980s, every car had a small knob on the dashboard called the choke. On a cold morning, before turning the key, a driver would pull the choke knob out, then push it back in slowly as the engine warmed up. The choke was a second flap inside the carburetor that, when closed, blocked some of the incoming air. Less air through the throat made the mixture richer (more fuel, less air), which a cold engine needed in order to start. As the engine warmed up, the driver would gradually open the choke until the engine was running on a normal mixture again. Modern cars handle all this automatically with computers and sensors. But for sixty years, “choking the engine” in cold weather was just part of starting a car.
Ready to burn
Now we know what is happening inside the cylinder during the SUCK stroke. The piston pulls down. The throttle is open by however much your mom is pressing the pedal. Air rushes through the carburetor. Fuel gets pulled into the airstream and torn into a mist. The cylinder fills up with a precise haze of gasoline-and-air vapor, ready to be squeezed and exploded.
The next thing the engine needs is the spark.
That is Chapter 3.
Questions to talk about
No answer key. These are for arguing over, with someone who read the same chapter.
- The chapter says the gas pedal “does not really control the gas — it controls the air.” Why does more air automatically bring more fuel with it? Try explaining the carburettor to someone using only the two-straw experiment.
- A carburettor hits roughly fourteen parts air to one part fuel with no sensors, no computer and no moving decisions — just a narrowed tube. Is that better engineering than a computer doing the same job, or worse? What would you trade for what?
- Drivers once had to pull a choke knob on cold mornings and ease it back in as the engine warmed. What did the driver have to understand about the engine to do that well? What do we lose when a machine takes that job over?