Power band

From Wikipedia, the free encyclopedia

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(US. Eng) or gearbox (Br. Eng) 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 essentially 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 RPMs or exceeding recommended operating speeds.

Gasoline Engines

Powerbands can surpass 14,000 rpm in motorcycles and some racing automobiles. Lightweight pistons and conrods and 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 rpms fall.

In more-pedestrian applications, a modern, well designed and engineered fuel injected, computer controlled, multivalve (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 road manners. 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 x speed). Though the literal power band covers most of the operating RPM range, particularly in 1st gear (as there is no lower gear to shift down to, and no powerless "flat spot" much above a medium jogging pace), the effective band whilst on the move changes in each gear, becoming the range spanned at the upper end by either the limiter, or a point roughly located between peak power and the redline/rev limiter where power drops off, and at the lower end whatever engine speed changing up from that point in the next lower gear lands you at.

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.

v t e Automotive engine

Part of the Automobile series

Basic terminology	Bore Compression ratio Crank Cylinder Dead centre Diesel engine Dry sump Engine balance Engine configuration Engine displacement Engine knocking Firing order Hydrolock Petrol engine Power band Redline Spark-ignition engine Stroke Stroke ratio Wet sump

Main components	Connecting rod Crankcase Crankpin Crankshaft Crossflow cylinder head Crossplane Cylinder bank Cylinder block Cylinder head Flywheel Head gasket Hypereutectic piston Main bearing Piston Piston ring Reverse-flow cylinder head Starter ring gear Sump

Valvetrain	Cam Cam follower Camshaft Desmodromic valve Hydraulic tappet Multi-valve Overhead camshaft Overhead valve Pneumatic valve springs Poppet valve Pushrod Rocker arm Sleeve valve Tappet Timing belt Timing mark Valve float Variable valve timing

Aspiration	Air filter Blowoff valve Boost controller Butterfly valve Centrifugal-type supercharger Cold air intake Dump valve Electronic throttle control Forced induction Inlet manifold Intake Intercooler Manifold vacuum Naturally aspirated engine Ram-air intake Scroll-type supercharger Short ram air intake Supercharger Throttle Throttle body Turbocharger Twin-turbo Variable-geometry turbocharger Variable-length intake manifold Warm air intake

Fuel system	Carburetor Common rail Direct injection Fuel filter Fuel injection Fuel pump Fuel tank Gasoline direct injection Indirect injection Injection pump Lean burn Unit injector

Ignition	Contact breaker Magneto Distributor Electrical ballast High tension leads Ignition coil Spark plug Wasted spark

Electrics and engine management	Air–fuel ratio meter Alternator Automatic Performance Control Car battery (lead–acid battery) Crankshaft position sensor Dynamo Drive by wire Electronic control unit Engine control unit Engine coolant temperature sensor Glow plug Idle air control actuator MAP sensor Mass flow sensor Oxygen sensor Starter motor Throttle position sensor

Exhaust system	Automobile emissions control Catalytic converter Diesel particulate filter Exhaust manifold Glasspack Muffler

Engine cooling	Air cooling Antifreeze (ethylene glycol) Core plug Electric fan Fan belt Radiator Thermostat Water cooling Viscous fan (fan clutch)

Other components	Balance shaft Block heater Combustion chamber Cylinder head porting Gasket Motor oil Oil filter Oil pump Oil sludge PCV valve Seal Synthetic oil Underdrive pulleys

Portal Category

This article is issued from Wikipedia. The text is available under the Creative Commons Attribution/Share Alike; additional terms may apply for the media files.