Powerband

The power band of an internal combustion engine or electric motor is the range of operating speeds under which the engine or motor is able to operate most efficiently. While engines and motors have a large range of operating speeds, the power band is usually a much smaller range of engine speed, only half or less of the total engine speed range (electric motors are an exception—see the section on electric motors below).

Specifically, power band is the range of RPM around peak power output. The power band of an internal combustion gasoline automobile engine typically starts at midrange engine speeds (around 4,000 RPM) where maximum torque is produced, and ends close to the redline after reaching maximum power between 5,000 and 6,500 RPM. Diesel engines in cars and small trucks may develop maximum torque below 2,000 RPM with the power peak below 5,000 RPM.

A mechanical transmission with a selection of different gear ratios is designed to make satisfactory power available over the full range of vehicle speeds. The goal of the selection of gear ratios is to keep the engine operating in its power band. The narrower the band, the more gears are needed, closer together in ratio. By careful gear selection, an engine can be operated in its power band, throughout all vehicle speeds. Such use prevents the engine from labouring at low speeds, or exceeding recommended operating speeds.

A narrow power band is often compensated for by a power-splitting device such as a clutch or torque converter to efficiently achieve a wide range of speeds. A continuously variable transmission can also avoid the issues of a narrow power band by keeping the engine running at an optimal speed.

In typical combustion engines found in vehicles, the torque is low at idling speed, reaches a maximal value between 1,500 and 6,500 RPM, and then falls more or less sharply toward the redline. Below the RPM of maximal torque, the compression is not ideal. Above this speed several factors start to limit the torque, such as growing friction, the required time for closing the valves and combustion, and insufficient intake flow. Due to increasing vibration and heat, an external RPM limitation may also be installed. Power is the product of torque multiplied by speed of rotation (analogous to force times speed), so peak power is produced in the upper speed range where there's both high torque and high RPM.

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Wikipedia