Why is Engine RPM not directly proportional to Vehicle Speed, even when gears are not changed?
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Gear rations. Smaller gears turning bigger gears. Rpm of a small gear is way higher than a bigger gear cuz less distance to travel so all the things spinning in the engine are spinning at different speeds to make the drive shaft go a certain rpm
This can only happen in normal operation when you’ve got an automatic transmission.
There’s a component in an automatic transmission called a “torque converter”. This is a fluid coupling…a “propeller” and a turbine right next to each other with oil in between. It provides two important functions…when the car is stopped, it lets the engine keep turning without needing a clutch to disconnect the engine from the wheels (the coupling just slips). And it allows some mismatch between the engine speed and transmision speed…through some messy fluid mechanics (which we can get into if you want) it acts like a reduction gear, allowing the wheels to spin more slowly and increasing the torque.
This can happen in manual or DSG transmissions if the clutch slips, but then it’s a bad thing.
If you have an automatic gearbox it probably has a torque converter, basically an impeller pushing fluid, driven by the engine, and then a turbine being pushed by that fluid, driving the wheels. It’s a little less efficient but it means the engine and transmission speeds don’t have to be perfectly synchronized, which allows for much smoother shifting than you could otherwise have.
The reason engine RPM is not directly proportional to vehicle speed, even when gears are not changed, is because there is always some resistance to the engine’s rotational speed. This resistance is due to things like friction, wind resistance, and the weight of the engine’s parts. When the engine is running at a higher speed, there is more resistance to its rotation, and thus the engine will slow down more quickly. When the engine is running at a lower speed, there is less resistance to its rotation, and thus the engine will slow down more slowly.
It is if you are using a manual transmission, or an automatic transmission that has locked the torque converter.
The reason it is not in an automatic transmission is that the torque converter intentionally allows the engine and wheels to not turn at the same speed. This allows, for example, being able to stop at a stop sign or traffic light without having to shift to neutral, as well as allowing the automatic transmission to shift gears without having to disengage and engage a clutch. (There are “automated” transmissions that automatically shift but also have an automatically-controlled clutch, but those are usually found in semi trucks and need additional training [a CDL] to drive correctly)
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