1. When I start the car, why do I hear the engine working when I'm not pressing the gas pedal?

  2. How exactly does the torque and power of the gears work?
    Torque = Radius * Force
    Power = Torque * Speed
    Do the different gears have different torque? They should, because they have difference radiuses, right? I'm a bit confused on how torque and power relates to the torque and what happens when we switch gears.

  3. How automatic transmission cars detect what torque to put into the wheels and how do they put it?

  4. How electric cars detect what torque to put into the wheels and how do they put it?

  • $\begingroup$ "When I start the car, why do I hear the engine working when I'm not pressing the gas pedal?" Really? $\endgroup$
    – Transistor
    Sep 14, 2019 at 8:49
  • $\begingroup$ Yes, Transistor. $\endgroup$ Sep 14, 2019 at 11:14
  • $\begingroup$ I'm 17, I'mma drive a car soon so I want to know what's going on inside $\endgroup$ Sep 14, 2019 at 11:14

2 Answers 2


The noise you hear is the engine running at the idle speed. To provide minumu energy for lubrication, charging the battery, air conditiong, water pump etc. In newer cars it has been eliminated for duration of intermittent stops to save fuel and pollute less. The computer will start the engine the moment you release the brakes.

The gear box, manual and automatic, function is to extend the range of engine torque. Because gasoline engines normal RPM is not spread enough to cover all the torque demand of the car.

The gears torque has to do with their arrangement in the gear box and as you said to the ratio of their RPM to that of the engine crankshaft. A gear turning 1/2 times faster provides 2 times more torque.

The automatic transmissions used to be controlled mechanically by the engine RPM, carburetor vacuum and rear axel RPM which has to do with the speed of the car.

But modern transmissions past 90s are controlled by a computer with many sensors, such as the air and engine temperatures, road condition, humidity, altitude, etc.

Electric cars, Tesla namely, don't have a transmission. They have all the torque range needed because they have wide range of RPM up to 20kRPM.

They have a computer which converts the direct current of the battery to alternative 3 phase current whit variable frequency, read variable RPM, via an inverter. Completely controlled by the master computer and in many cases even override the driver to correct his or her errors.

  • $\begingroup$ Thank you. What exactly is RPM and how does it relate to torque and power? $\endgroup$ Sep 14, 2019 at 11:09
  • $\begingroup$ In Tesla, the torque is controlled by how much you've pressed the pedal? $\endgroup$ Sep 14, 2019 at 11:11
  • $\begingroup$ So the torque on every gear in the gearbox is the same, but the force and radius may differ? $\endgroup$ Sep 14, 2019 at 11:12
  • $\begingroup$ When the engine starts to work faster, how does the igniter know when to ignite? $\endgroup$ Sep 14, 2019 at 11:22
  • $\begingroup$ @NovaliumCompany, RPM is short for Revolution per minute, talking about a 4 stroke engine, it means engine rotation at out put shaft. each engine has a torque curve which shows what is the maximum torque versus RPM. -2) no Torque on each gear is different, usually the top gear torque and RPM are the same as the engine's the rest increase the torque by reducing the speed via engaging a larger gear to engine shaft.-3 Tesla increases the torque by increasing the amps to the engine as per pedal angle -4) If you mean the transient brake stop, the moment you let go of the brake the engine reignites. $\endgroup$
    – kamran
    Sep 14, 2019 at 16:56

Thankfully, I once took an Automotive engineering course. It was two years ago, but I still remember something... Of course, my answer is open for improvement, so if there is any constructive criticism, please go ahead

  1. The gas pedal is the controller for the throttle (think how much air is going into the engine), and this, in turn, controls the fuel injection. More pressing on the pedal means the throttle opens more, which means more air going in, and then, more fuel is input to use that excess amount of air. That means, even without pedal pressing, the engine is still running (albeit with not as much as air as when you are pressing it). This is done by design, because the convential engines lack "instant torque" ability like electric vehicles (as in, you press the pedal, the engine will start and the car can move immediately)

  2. The gear system in a car is used to "decrease" the rotational speed of the engine (say 2000 rounds per minute to about 70 rounds per minute), and with the acknowledgement that the power is (mostly) conserved, the torque value would increase between the output shaft of the engine and the wheel

  3. I don't remember precise details on the automatic transmission, but this guy has made a video on Youtube. However, you ask about how to detect "torque at the wheel". I'm afraid that I don't know the answer. Heck, I'm not even sure if we can actually measure it directly (as in what is the net torque applied at the wheel, from both the engine and the environment at a given time). I only remember we calculate it, deriving it from the formula to gain the requirements for the driveline.


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