Rev limiter

Last updated

A rev limiter is a device fitted in modern vehicles that have internal combustion engines. They are intended to protect an engine by restricting its maximum rotational speed, measured in revolutions per minute (RPM). Rev limiters are pre-set by the engine manufacturer. There are also aftermarket units where a separate controller is installed using a custom RPM setting. A limiter prevents a vehicle's engine from being pushed beyond the manufacturer's limit, known as the redline (literally the red line marked on the tachometer). At some point beyond the redline, engine damage may occur.

Contents

Operation

The typical layout of a manual transmission shifter Manual Layout.svg
The typical layout of a manual transmission shifter

Limiters usually work by shutting off a component necessary for the combustion processes to occur. Compression-ignition engines use mechanical governors or limiters to shut off electronic fuel injectors. A spark-ignition engine may also shut off fuel or stop the spark ignition and some just reduce the engine's power by changing the spark timing.

In the case of an automatic transmission in "drive" mode, the engine RPM stays safely within the range that the transmission chooses. Only when over revving the engine in "park", "neutral" or "manual" modes is there any need for a rev limiter. These vehicles often do not include a tachometer. Without this gauge, the redline cannot be seen but there is so little risk of excessive engine speed with fully automatic transmissions that engine RPM is not a concern.

However, with a manual transmission engine RPM can redline in "neutral", or by shifting to a higher gear too late, or by shifting to a lower gear too early. In the case of "neutral" or shifting up too late, a rev limiter can easily keep engine RPM below the redline.

If a manual transmission is shifted down too early, the speed of the vehicle will drive the engine over the redline. In this case, a rev limiter will cut engine power but it cannot prevent the engine's RPM from going beyond the redline.

Perhaps the worst situation occurs when a shift is "missed". In the diagram shown, it is possible to be at high RPM and "miss" shifting from 2nd to 3rd and get 1st gear instead. This will result in exceeding the redline and there is nothing to prevent an engine from being severely damaged in this way. Using the clutch as quickly as possible may avoid engine damage. [1]

Most small engines, such as on lawn mowers have a speed governor. As the RPM of the engine increases, the throttle plate in the carburetor is gradually closed, reducing the amount of fuel and air admitted to the engine, until the engine RPM is stable. If RPM drops below the desired value, the throttle plate will automatically open, admitting more fuel/air mix to the engine. Adjusting the throttle generally adjusts spring tension on the governor, which in turn allows the engine to run faster or slower, as desired. While the redline cannot be seen on most small engines, due to their lack of a tachometer, the risk of excessive engine speed is not generally a concern. [2]

Types of control

Fuel control

Fuel-cutting rev limiters are the most common because they wear less on exhaust components. These systems usually lean out the engine's overspeed by shutting off the fuel injectors, and are the only practical system on diesel engines. [3] This is less popular in high performance engines due to high temperatures in lean operation.

Spark control

Ignition control rev limiting systems work by shutting off the spark plugs once the engine overspeeds. [3] This is less common in production vehicles because the system still injects fuel into the cylinder and consequently releases unburned fuel which may ignite at a turbo charger or in the exhaust pipe. This can affect the temperatures in the exhaust, causing premature wear on the catalytic converter. [3]


Throttle control

Vehicles equipped with drive-by-wire allow the ECU to modulate throttle position to keep engine RPMs in a safe range. This is by far the safest method of limiting engine speed and is used on most modern production cars.

Hard-cut vs. soft-cut limiters

Hard-cut limiters

Hard-cut limiters completely cut fuel or spark to the engine. These types of limiters activate at the set RPM and "bounce" off of it if throttle is applied. The "bouncing" occurs because the limiter will cut off fuel or spark at the set RPM, which causes the RPM to drop. If the engine is in a state of open throttle when the RPM drops, the RPM will then raise back to the limit. This causes the engine to cycle its power on and off.

Soft-cut limiters

Soft-cut limiters are a type of rev limiter that partially cuts off fuel to the engine. These limiters may also retard the ignition timing. If using a soft-cut rev limiter, the engine will start to cut fuel or retard ignition timing before the set RPM until it slowly reaches it and remains there.

Physical limiters

The maximum RPM of an engine is limited to the airflow through the engine, the displacement of the engine, the mass and balance of the rotating parts, along with the bore and stroke of the pistons. [4] Formula One engines can rev up to 15,000 rpm as per Formula One rules [5] because of their smaller displacement, low mass, and short stroke.

Engines with hydraulic tappets (such as the Buick/Rover V8) often have 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', causing valve float. This sharply reduces engine power, causing speed to drop.

Racing uses

The RPM level that results with the spark timing 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 and highway driving.

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.

Engine damage beyond the redline

Connecting rod failure and subsequent damage by the crankshaft Biella rotta per fatica.jpg
Connecting rod failure and subsequent damage by the crankshaft

There is considerable variation between manufactures on where to have the redline for their engines: from 100 [6] to 12,000 RPM. [7] If an engine goes overspeed, commonly called "over-revving", damage to the piston and valvetrain may occur when a valve stays open longer than usual. Valve float can possibly result in loss of compression, misfire, or a valve and piston colliding with each other. [8] It's also possible the engine will throw a connecting rod between the crankshaft and piston. The engine will then need to be repaired or replaced entirely.

See also

Related Research Articles

<span class="mw-page-title-main">Four-stroke engine</span> Internal combustion engine type

A four-strokeengine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:

  1. Intake: Also known as induction or suction. This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing a partial vacuum in the cylinder through its downward motion.
  2. Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.
  3. Combustion: Also known as power or ignition. This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. the compressed air-fuel mixture is ignited by a spark plug or by heat generated by high compression, forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.
  4. Exhaust: Also known as outlet. During the exhaust stroke, the piston, once again, returns from B.D.C. to T.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust port.
<span class="mw-page-title-main">Engine tuning</span> Optimisation of engine performance

Engine tuning is the adjustment or modification of the internal combustion engine or Engine Control Unit (ECU) to yield optimal performance and increase the engine's power output, economy, or durability. These goals may be mutually exclusive; an engine may be de-tuned with respect to output power in exchange for better economy or longer engine life due to lessened stress on engine components.

Pre-ignition in a spark-ignition engine is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is initiated by an ignition source other than the spark, such as hot spots in the combustion chamber, a spark plug that runs too hot for the application, or carbonaceous deposits in the combustion chamber heated to incandescence by previous engine combustion events.

<span class="mw-page-title-main">Aircraft engine controls</span>

Aircraft engine controls provide a means for the pilot to control and monitor the operation of the aircraft's powerplant. This article describes controls used with a basic internal-combustion engine driving a propeller. Some optional or more advanced configurations are described at the end of the article. Jet turbine engines use different operating principles and have their own sets of controls and sensors.

The GM Ecotec engine, also known by its codename L850, is a family of all-aluminium inline-four engines, displacing between 1.4 and 2.5 litres. Confusingly, the Ecotec name was also applied to the final DOHC derivatives of the previous GM Family II engine, the architecture was substantially re-engineered for this new Ecotec application produced since 2000. This engine family replaced the GM Family II engine, the GM 122 engine, the Saab H engine, and the Quad 4 engine. It is manufactured in multiple locations, to include Spring Hill Manufacturing, in Spring Hill, Tennessee while the engine block and cylinder heads are cast at Saginaw Metal Casting Operations in Saginaw, Michigan.

<span class="mw-page-title-main">Redline</span> Revving an internal combustion engine to its maximum speed

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 redline of an engine depends on various factors such as stroke, mass of the components, displacement, composition of components, and balance of components.

<span class="mw-page-title-main">Engine braking</span> Retarding forces within an engine used to slow a vehicle

Engine braking occurs when the retarding forces within an internal combustion engine are used to slow down a motor vehicle, as opposed to using additional external braking mechanisms such as friction brakes or magnetic brakes.

<span class="mw-page-title-main">Nissan QG engine</span> Reciprocating internal combustion engine

The QG engine is a 1.3 L (1,295 cc), 1.5 L (1,497 cc), 1.6 L (1,597 cc) and 1.8 L (1,769 cc) straight-4 piston engine from Nissan. It is a lean-burn aluminum DOHC 4-valve design with variable valve timing and optional NEO Di direct injection.

Lean-burn refers to the burning of fuel with an excess of air in an internal combustion engine. In lean-burn engines the air–fuel ratio may be as lean as 65:1. The air / fuel ratio needed to stoichiometrically combust gasoline, by contrast, is 14.64:1. The excess of air in a lean-burn engine emits far less hydrocarbons. High air–fuel ratios can also be used to reduce losses caused by other engine power management systems such as throttling losses.

Dieseling or engine run-on is a condition that can occur in spark-plug-ignited, gasoline-powered internal combustion engines, whereby the engine keeps running for a short period after being turned off, drawing fuel through the carburetor, into the engine and igniting it without a spark.

Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere.

<span class="mw-page-title-main">Honda F engine</span> Reciprocating internal combustion engine

The Honda F-Series engine was considered Honda's "big block" SOHC inline four, though lower production DOHC versions of the F-series were built. It features a solid iron or aluminum open deck cast iron sleeved block and aluminum/magnesium cylinder head.

<span class="mw-page-title-main">Toyota E engine</span> Reciprocating internal combustion engine

The Toyota E engine family is a straight-four piston engine series, and uses timing belts rather than chains. The E engines were the first multi-valve engines from Toyota designed with economy, practicality and everyday use in mind. Like many other Toyota engines from the era, the E engine series features a cast iron block, along with an aluminium cylinder head. E engines are lighter than earlier Toyota engines, due to the hollow crankshaft, thinned casting of the cylinder block, and several other reductions in auxiliaries as well as in the engine itself. Carbureted versions include a newly designed, variable-venturi carburetor. All of these changes improved economy and emissions. The members of the E engine family, range from 1.0 L to 1.5 L. The E family supplanted the K engines in most applications. A large number of parts in the E engine series are interchangeable between each other.

<span class="mw-page-title-main">Power band</span>

The power band of an internal combustion engine or electric motor is the range of operating speeds under which the engine or motor is able to output the most power, that is, the maximum energy per unit of time. This usually means that maximum acceleration can be achieved inside this band. 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.

Overspeed is a condition in which an engine is allowed or forced to turn beyond its design limit. The consequences of running an engine too fast vary by engine type and model and depend upon several factors, the most important of which are the duration of the overspeed and the speed attained. With some engines, a momentary overspeed can result in greatly reduced engine life or catastrophic failure. The speed of an engine is typically measured in revolutions per minute (rpm).

<span class="mw-page-title-main">Ignition timing</span> Timing of the release of a spark in a combustion engine

In a spark ignition internal combustion engine, ignition timing is the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke.

The following outline is provided as an overview of and topical guide to automobiles:

Diesel engine runaway is an occurrence in diesel engines, in which the engine draws extra fuel from an unintended source and overspeeds at higher and higher RPM, producing up to ten times the engine's rated output until destroyed by mechanical failure or bearing seizure due to a lack of lubrication. Hot-bulb engines and jet engines can also run away via the same process.

Trionic T5.5 is an engine management system in the Saab Trionic range. It controls ignition, fuel injection and turbo boost pressure. The system was introduced in the 1993 Saab 9000 2.3 Turbo with B234L and B234R engine.

References

  1. ↑ "Auto Journalist Sued for $174,000 After Destroying a Rare Porsche's Engine". 21 January 2013.
  2. ↑ "How does a small engine governor work?". Briggs & Stratton. Retrieved 20 August 2022.
  3. 1 2 3 "Rev Limiters - Why Do We Use Them? — FASTuuN". fastuun.com. Retrieved 2015-10-20.
  4. ↑ "High Performance Math". www.hipermath.com. Retrieved 2015-10-21.
  5. ↑ "Power unit and ERS". Formula1.com. Retrieved 2015-10-21.
  6. ↑ "RTA-C Technology Review" (PDF). Wärtsilä. 2004. p. 23. Archived from the original on December 26, 2005.{{cite web}}: CS1 maint: unfit URL (link)
  7. ↑ "Archived copy" (PDF). Archived from the original (PDF) on 2020-10-08. Retrieved 2021-02-02.{{cite web}}: CS1 maint: archived copy as title (link)
  8. ↑ "Diagnose Weak Valve springs". www.aa1car.com. Retrieved 2015-10-20.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.