why a gas car engine’s optimal range is often between 2000 to 3000 RPM even though they can go lower and up to ~8000 RPM ?

Question

why a gas car engine’s optimal range is often between 2000 to 3000 RPM even though they can go lower and up to ~8000 RPM ?

In: Engineering

Answer 1

It is an engineering tradeoff. For a given vehicle, there is a “response” in terms of acceleration that is needed. This response is governed by the torque output (at the wheels) of the engine. Higher torque = better acceleration = car feels more responsive.

For a given engine RPM, it is generally possible to add more or larger cylinders to increase torque. But this makes engines large, heavy and expensive especially if a lot of torque needed at very low RPM. Making an engine output lots of torque at low RPMs usually makes the engine run badly at high RPMs. This refers mostly to gas/petrol engines. Diesel engines and electric motors naturally have high torque at low RPMs.

For a given engine configuration, torque generally increases as RPM increases until it hits a max then more or less remains constant until peak RPM. Running at higher RPM means the gearbox and engine has to withstand more stress, makes more noise and more gears are needed to keep the engine in that optimal RPM zone as the car accelerates. This is expensive and also requires the driver or gearbox to change a lot of gears as the car accelerates (think semi-trucks).

So the designers (for passenger vehicles) tradeoff on the above two – which is to have good enough torque band at a reasonable RPM and still be fairly quiet and responsive.