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: combustion engines typically generate maximum torque at perhaps 2500 RPM. The peak horsepower might be 5000 RPM. Such an engine would have a power band of 2500 to 5000 RPM, in which the engine would be very efficient.
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.
Internal combustion engines
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.
In more common applications, a modern, well designed and engineered fuel injected, computer controlled, multi-valve and optionally variable-valve timing equipped gasoline engine using good fuel can achieve remarkable flexibility in automobile applications, with a near-flat torque output from 1500 to 6700 RPM, allowing easy cruising and forgiving low-speed behaviour. However, achieving maximum horsepower for strong acceleration or high road speed still requires high RPM, as power is the product of torque multiplied by speed of rotation (analogous to force times speed). Though the literal power band covers most of the operating RPM range, particularly in first gear (as there is no lower gear to shift down to, and no "flat spot" in which the engine does not produce any power), the effective band changes in each gear, becoming the range limited at the upper end by either the limiter, or a point roughly located between peak power and the redline where power drops off, and at the lower end the engine's idling speed.
A typical roadgoing ("high-speed") diesel has a narrower band, generating peak torque at lower rpm (often 1500–2000 RPM) but also with a sharper fall-off below this, and reaching peak power around 3500-4500 RPM, again rapidly losing strength above this speed. If using a turbo-charger, the maximal torque is reached at lower rpm and limited to a constant value in a large range (1500-3000 rpm). Turbo diesel engines can display this characteristic very markedly as it combines with the turbo lag (narrowed, exaggerated power band) intrinsic to most turbocharged engines. Therefore, the manufacturer's (or purchaser's/modifier's) choice of gearing, and appropriate use of the available ratios, is even more crucial to make best use of the available power and avoid being "bogged down" in flat spots.
Larger diesel engines in locomotives and some watercraft use diesel-electric drive. This eliminates the complexities of extremely low gearing, as described below.
The largest ("low-speed") diesels- large generators on land, and marine diesels at sea- may turn at only hundreds of rpm or even below, with idling speeds of 20-30 rpm. These engines are usually two-stroke diesel engines.
Electric motors are unique in many ways, especially when it comes to the power band. The exact characteristics vary greatly with the type of electric motor. The maximum torque of a universal motor (vacuum cleaner, small machines, drills, starter motors) is reached instantly and falls for higher rpm. For induction motors connected to a fixed frequency AC source (most common in large applications) the maximum torque is usually just below the synchronous rpm, sinks to zero for this rpm and becomes negative above (induction generator); at low rpm the torque is usually slightly lower. Synchronous motors can be used only at one to the AC source synchronous velocity. In modern applications, synchronous and induction motors with electronic control of the frequency are used, e.g. Brushless DC electric motors. In this case, unless external limitations are applied, the maximum torque is achieved at low rpm.
For example, the AC motor found in the Tesla Roadster produces near constant maximum torque from 0 to about 6000 rpm, while maximum power occurs at about 10000 rpm, long after torque begins to drop off. The Roadster's redline is 14000 rpm. Other electric motors may in fact produce maximum torque throughout their entire operating range, although their maximum operating speed may be limited for improved reliability.
Gas turbines operate at extremely high rpms by comparison, and exhibit narrow powerbands, and poor throttleability and throttle response.