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 ?



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

Low rpm means that an engine can burn less fuel per second, which results in less power (for a given displacement (size)). So operating an engine at low rpm means you need a big, heavy engine which is fine for a ship but annoying for a car.

At very high rpm the amount of time for the fuel to burn in the cylinder actually starts to become a problem. You can think of the flame expanding outwards from the sparkplug. If you don’t burn all the fuel before the piston has moved down and the exhaust valve opened then you are wasting fuel and so lose efficiency. You also need very high flow rates of air through your valves which leads to drag and so a loss of efficiency. Formula 1 engines that operate at high rpm are very clever and quite different from production vehicle engines. And of course high engine speeds mean higher accelerations and so forces on the components, coupled with increased temperatures, leading to reduced life.

By having the optimal level lower down the rpm range the engine can run quieter and will also put less stress and wear on the parts than they would at higher rpms.

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.

As rpms go up, torq will drop off but hp will go up longer. Torq gets you moving, hp keeps you moving. But in stop and go traffic youre constantly shifting. So you go past the most powerful rpm before shifting so when the next gear comes in youre close to that point and the engine doesnt lug and blow a valve seal but maintains a higher level of efficiency in fuel use and wear as it travels through the gears.