Power band

The power band of an engine or electric motor refers to the range of operating speeds under which the engine or motor is able to operate 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 Electric Motors below).

Specifically, power band is defined by the range from peak torque to peak horsepower (or sometimes to redline). For example: internal combustion gasoline/petrol automobile engines generate maximum torque typically between 3,500 and 4,500 RPM. The peak horsepower might be 5000 to 6,500 RPM. Diesel engines in cars and small trucks may develop maximum torque below 2,000 RPM with horsepower peak below 5,000 RPM.

Since some engines produce their highest power over a relatively narrow range of speeds, a power-splitting device such as a clutch or torque converter is often used to efficiently achieve a wide range of speeds.

A mechanical transmission with a selection of different gear ratios is designed to make satisfactory power and torque 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.

In typical combustion engines found in vehicles, the torque is low at idling speed, reaches a maximal value between 1500 and 6000 rpm, and then falls more or less sharply toward the redline. Below the rpm of maximal torque the compression is not ideal. Above this rpm several factors limit the torque, among others the growing friction, the closing time of the valves, the combustion time and the insufficient cross section of the intake. Due to the higher vibrations and overheating, an external rpm limitation may be installed.

Engines with turbo-charger usually have higher torque starting at lower rpm.

Powerbands can surpass 14,000 rpm in motorcycles and some racing automobiles. Lightweight pistons and connecting rods with short strokes are used to reduce inertia, and thus stresses on parts. Advances in valve technology similarly reduce valve float at such speeds. As engines grow larger (in particular, their strokes), powerband revolutions fall.

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Wikipedia