Power band

This article is about the measure of engine efficiency. For the fashion accessory, see Power Balance. For the method of communication, see Broadband over power lines.

A diagram comparing the power and torque of a "torquey" engine versus a "peaky" one

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.

Applications

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.

Gasoline Engines

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 6000 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.

Diesel Engines

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. 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.

Electric Motors

Electric motors are unique in many ways, especially when it comes to the power band. The maximum torque of an electric motor is reached instantly and stays at that level for the majority of its operating speed range. The exact characteristics vary greatly with the type of electric motor. 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

Gas turbines operate at extremely high rpms by comparison, and exhibit narrow powerbands, and poor throttleability and throttle response.

See also

• Engine tuning
• Overdrive
• Shift kit
• Shift time
• Continuously variable transmission
• Combined diesel or gas
• Combined diesel-electric and gas
• Combined gas or gas