I am learning about car basics. I found that engine is nothing but a system where heat energy is converted into up/down motion of the piston in the cylinder. Finally, this up/down motion is converted into rotary motion by locking the gear teethes against each other.

So, my understanding is that all that motion is available on the flywheel, when we shift gears, the rod connected from flywheel to the wheel changes the distance from the flywheel. This changes displacement and hence torque ( force * displacement) is changed. So, basically, piston is moving at the same speed only, when we accelerate the differential gears changes the speed. Is it correct? Or is it that when we accelerate even the piston starts moving faster? If yes, how is it possible, since the temperature or heat energy is same, we can't increase the temperature inside the cylinder suddenly on pressing the accelerator. Is my understanding correct here? Also, I see that in my car - the engine rpm doesn't keep changing. It changes a bit but not too much like odometer.


No. To increase the speed of the car, you do are 2 things: you press the accelerator, and you change gears.

Pressing the accelerator makes the engine turn faster, even when the car is out of gear and stationary. You can see the engine speed on the tachometer, which shows the Revolutions per Minute (RPM) of the engine. This is the number of times the crankshaft rotates per second. Typical figures are between 500 rpm and 7000 rpm. As every cylinder has to go up and down once for each revolution, they obviously move faster as you press the accelerator pedal further down.

You also need gears because an internal combustion engine has very little torque and power at low rpm, while it also cannot turn faster than some maximum speed without self-destructing. Hence, you use low gear at low speed (to have the engine run fast enough so it has enough power at low speed) and you use high gear at high speed (so the engine won't have to turn at excessive rpm). A good driver will choose the correct gear, depending on what they want to do. To accelerate fast, use a lower gear; to use less petrol, use a higher gear.

If the tacho in your car does not change much, that is because you have an automatic gearbox (or even a CVT gearbox), which changes gears to keep the engine running at optimum speed. If you manually select a gear (or if you drive really fast) you'll see the tacho change.

Note that electric cars do not have this problem. Electric motors have maximum torque at zero rpm. Hence, all you need to do is to set the overall gearing so that the car will go at maximum speed at the recommended maximum rpm of the motor. You do not need to worry about having the motor run at speeds that are too low: it can "run" at 0 rpm; this is unlike a petrol engine which would stall.


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