When the driver applies pressure to the gas pedal, the rate of the RPM's increases. Why?

I can infer that this has something to do with more fuel/air in the cylinder and/or an earlier spark but through all the engine tech videos and blogs, I have never seen this question asked (or answered).

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1 Answer 1


The simple answer is the throttle blades open and allow more air in, but there's a lot more to it than that.

You have to realize up front that an internal combustion engine (ICE) is nothing more than a big air pump. Air goes in, mixes with fuel, gets compressed by the piston, the spark plug lights the mixture forcing the piston down, which rotates the crank, which creates power. The burnt air/fuel (now exhaust) is routed out of the engine and through the exhaust. This is called the Otto Cycle and is the basis of all four stroke engines.

When the foot steps on the pedal, one of three things can happen, depending on what type of engine management system is in place.

  1. If drive by cable (DBC) with fuel injection, the foot steps on the pedal and the pedal moves a cable, which directly moves the throttle plates inside the throttle body. A mechanism called a throttle position sensor (TPS), which then tells the computer how far the throttle plates are open, which it then commands the injectors to fire a predetermined amount of fuel into the engine, which mixes with the incoming air. Then the spark plug goes off at the right time and things start to happen.

  2. If drive by cable (DBC) with a carburetor, the cable opens the throttle plates, which allows more air to enter the engine. The incoming air which rushes through the venturis of the carburetor which allows air/fuel to mix, which enters into the engine. Then the spark plug goes off at the right time and things start to happen.

  3. If drive by wire (DBW), the foot steps on the pedal and the pedal reacts with a rheostat, which tells the computer the amount of movement of your foot. Then the computer commands the throttle body to open the amount required to match the command. A motor in the throttle body then rotates the throttle plates which allows the amount of air into the engine to do what is required. The computer also commands the injectors to fire a certain amount of fuel into the cylinders, which mixes with the air. Then the spark plug goes off at the right time and things start to happen. (NOTE: There is no carbureted version of DBW. This is fuel injected only.)

  • So is it an earlier spark or more air/fuel or both that makes the RPM's climb? Mar 31, 2019 at 3:41
  • @Tripster202 - Some of both, but air/fuel is the main part. You have to advance the spark in order to get ahead of the rotation and to speed things up. Mar 31, 2019 at 4:05
  • Just wanted to add a clarification (for future readers who find this question), in EFI systems (1 and 3) the amount of fuel injected is not determined by the throttle position, it depends on the amount + temperature/density of air metered from sensors in the intake. This is primarily controlled by the throttle position (which is the main take-away of this question), but can also be affected by other factors such as intake restrictions/design and ambient temperature. Might be a good idea to edit the answer so we don't cause misunderstandings. Apr 1, 2019 at 15:27
  • @Kitsunemimi - Agree with your assessment, however the purpose of the writ was to be very basic. The question has to do with what causes the engine to go faster, which is primarily due to the throttle plates (whether in FI or carb'd solutions) opening. The amount of injected/ported fuel going into the engine is an after effect of the plates opening and the amount of air (which density is affected by temperature) coming into the engine. Apr 1, 2019 at 15:41

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