Rev limiter
Encyclopedia
A rev limiter is a device fitted to an internal combustion engine
to restrict its maximum rotational speed
. This is usually carried out to prevent damage to the engine, however sometimes these devices are fitted to prevent an engine reaching the point at which it develops maximum power. This may be useful in a driving school car or similar, or utilised in systems preventing Valet parking
operatives from joy-riding in high-performance vehicles placed in their charge. When fitted to racing or modified street cars these adjustable devices provide more precise control than the original components built in by the manufacturer. Limiters can be used in conjunction with a shift light
, which illuminates when the desired RPM is reached, indicating to the driver that a gear change is required. This allows the driver to keep his eye on the road and not have to glance repeatedly at the tachometer
to monitor the engine speed.
. The RPM level that results with the spark being arrested can be a constant level, or, with the proper ignition control modules, variable. Variable rate ignition modules can be adjusted quickly and easily to achieve the appropriate RPM limit for different situations, such as street racing, drag racing, road course racing, highway driving, and lawn turfing.
Multiple stage ignition modules offer greater RPM limit control. The first stage can be used to limit RPM levels when launching a vehicle from a stationary position, providing maximum power and traction. The second stage is activated after launch to set a higher RPM limit for wide-open-throttle acceleration.
Engines with hydraulic tappet
s (such as the Buick/Rover V8) often have what is in effect a rev limiter by virtue of their design. The tappet clearances are maintained by the flow of the engine's lubricating oil. At high engine speeds, the oil pressure rises to such an extent that the tappets 'pump up', closing the valve clearance and preventing the valves fully closing. This sharply reduces engine power, causing speed to drop.
. Electronic adjustments require special diagnostic equipment to access control settings, and are limited to authorized personnel, such as a fleet manager or mechanic.
Due to their electrical nature, however, it is still possible to drive an engine beyond the maximum RPM level set by the limiter. This most commonly occurs in the case of selecting the incorrect gear with a manual transmission - for example, selecting second gear instead of fourth after shifting out of third. The speed of the vehicle and the drive wheels may drive the engine far beyond its intended range in this situation, likely resulting in rapid engine wear, and in extreme cases possible outright engine failure.
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
to restrict its maximum rotational speed
Revolutions per minute
Revolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...
. This is usually carried out to prevent damage to the engine, however sometimes these devices are fitted to prevent an engine reaching the point at which it develops maximum power. This may be useful in a driving school car or similar, or utilised in systems preventing Valet parking
Valet parking
Valet parking is a parking service offered by some restaurants, stores, and other businesses, particularly in North America. In contrast to "self-parking", where customers find a parking space on their own, customers' vehicles are parked for them by a person called a valet...
operatives from joy-riding in high-performance vehicles placed in their charge. When fitted to racing or modified street cars these adjustable devices provide more precise control than the original components built in by the manufacturer. Limiters can be used in conjunction with a shift light
Shift light
thumb|right|A digital dash and data acquisition system for car and motorcycle racing incorporating [[LED]] shift lightsA shift light is a warning lamp fitted to vehicles in order to indicate to the driver that maximum RPM has almost been reached...
, which illuminates when the desired RPM is reached, indicating to the driver that a gear change is required. This allows the driver to keep his eye on the road and not have to glance repeatedly at the tachometer
Tachometer
A tachometer is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute on a calibrated analogue dial, but digital displays are increasingly common...
to monitor the engine speed.
Usage
Limiters prevent damage to an engine by interrupting the power that is distributed to the spark plugs, this prevents the engine from operating above a pre-determined RPM level known as the redlineRedline
Redline refers to the maximum engine speed at which an internal combustion engine or traction motor and its components are designed to operate without causing damage to the components themselves or other parts of the engine...
. The RPM level that results with the spark being arrested can be a constant level, or, with the proper ignition control modules, variable. Variable rate ignition modules can be adjusted quickly and easily to achieve the appropriate RPM limit for different situations, such as street racing, drag racing, road course racing, highway driving, and lawn turfing.
Multiple stage ignition modules offer greater RPM limit control. The first stage can be used to limit RPM levels when launching a vehicle from a stationary position, providing maximum power and traction. The second stage is activated after launch to set a higher RPM limit for wide-open-throttle acceleration.
Engines with hydraulic tappet
Tappet
A tappet is the noise made by a worn cam follower, however the term is widely used to represent the cam follower itself. In mechanical engineering it is a projection which imparts a linear motion to some other component within an assembly...
s (such as the Buick/Rover V8) often have what is in effect a rev limiter by virtue of their design. The tappet clearances are maintained by the flow of the engine's lubricating oil. At high engine speeds, the oil pressure rises to such an extent that the tappets 'pump up', closing the valve clearance and preventing the valves fully closing. This sharply reduces engine power, causing speed to drop.
Fuel efficiency
Limiters are used as a means of increasing the fuel efficiency of commercial motor vehicles. Limiters can be individually adjusted by fleet owners to meet the performance requirements of many different driving conditions. A limiter used for this purpose, however, especially if it is mechanical in nature, is more commonly categorized as a governorGovernor (device)
A governor, or speed limiter, is a device used to measure and regulate the speed of a machine, such as an engine. A classic example is the centrifugal governor, also known as the Watt or fly-ball governor, which uses a rotating assembly of weights mounted on arms to determine how fast the engine...
. Electronic adjustments require special diagnostic equipment to access control settings, and are limited to authorized personnel, such as a fleet manager or mechanic.
Due to their electrical nature, however, it is still possible to drive an engine beyond the maximum RPM level set by the limiter. This most commonly occurs in the case of selecting the incorrect gear with a manual transmission - for example, selecting second gear instead of fourth after shifting out of third. The speed of the vehicle and the drive wheels may drive the engine far beyond its intended range in this situation, likely resulting in rapid engine wear, and in extreme cases possible outright engine failure.