Editing Redline

{{Short description|Revving an internal combustion engine to its maximum speed}}
{{about|engine speed|}}
{{multiple issues|{{original research|date=April 2015}}
{{Refimprove|date=September 2008}}}}
[[Image:Honda CBR 600 tachometer.jpg|thumb|[[Tachometer]] showing red lines above 14,000&nbsp;rpm.]]
The '''redline''' is the maximum engine [[rotational speed|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.<ref>{{cite web|url=https://www.caranddriver.com/features/a16581035/car-terms-defined/|title=Glossary of Car Terms|website=caranddriver.com|date=15 June 2009 }}</ref> The redline of an engine depends on various factors such as [[Stroke (engines)|stroke]], mass of the components, [[Engine displacement|displacement]], composition of components, and balance of components.

Redlining is riding or driving an automotive vehicle above the redline. The actual term ''redline'' comes from the red bars that are displayed on [[tachometer]]s in cars starting at the rpm that denotes the redline for the specific engine. Straying into this area usually does not mean instant engine failure, but may increase the chances of damaging the engine.

==Variation of redline==
The acceleration, or rate of change in piston velocity, is the limiting factor. The piston acceleration is directly proportional to the magnitude of the G-forces experienced by the piston-connecting rod assembly. As long as the G-forces acting on the piston-connecting rod assembly multiplied by their own mass is less than the compressive and tensile strengths of the materials they are constructed from and as long as it does not exceed the bearing load limits, the engine can safely turn without succumbing to physical or structural failure.

Redlines vary anywhere from a few hundred [[revolutions per minute]] (rpm) (in very large engines such as those in trains and generators) to more than 10,000 rpm (in smaller, usually high-performance engines such as motorcycles, some sports cars, and pistonless rotary engines). [[Diesel engine]]s normally have lower redlines than comparably sized [[gasoline engine]]s, largely because of fuel-atomization limitations; even a small diesel engine, such as a [[Yanmar 2GM20]] found on a sailboat, has a redline of 3400 RPM continuous, with a maximum 1-hour rating of 3600 RPM.<ref name="Yanmar technical reference sheet">Yanmar technical reference sheet</ref> Gasoline automobile engines typically will have a redline at between 6000 and 7000&nbsp;rpm. The [[Gordon Murray Automotive T.50]] has the highest redline of a [[Piston engine|piston-engine]] road car rated at 12,100 rpm. The [[Mazda Wankel engine#13B-MSP Renesis|Renesis]] in the [[Mazda RX-8]] has the highest redline of a production [[Wankel engine|wankel rotary-engine]] road car rated at 9000&nbsp;rpm.

In contrast, some older [[Overhead valve|OHV]] (pushrod) engines had redlines as low as 4800&nbsp;rpm, mostly due to the engines being designed and built for low-end power and economy during the late 1960s all the way to the early 1990s. One main reason OHV engines have lower redlines is valve float. At high speeds, the valve spring simply cannot keep the tappet or roller on the camshaft. After the valve opens, the valve spring does not have enough force to push the mass of the rocker arm, pushrod, and lifter down on the cam before the next combustion cycle. [[Flathead engine]]s can have even lower redlines; for example, the [[Universal Atomic 4]], commonly used as auxiliary power on [[sailboat]]s from the 1950s to the 1980s, has a redline of just 3500 RPM. Overhead cam engines eliminate many of the components and moving mass, used on OHV engines. Lower redlines, however, do not necessarily mean lower performance.

Motorcycle engines can have even higher redlines because of their comparatively lower [[reciprocating mass]]. For example, the 1986–1996 Honda CBR250RR has a redline of about 19,000&nbsp;rpm. Higher yet have been the redlines of some [[Formula One]] cars, with engine speeds reaching over 20,000&nbsp;rpm on the [[Cosworth TJ / CA engine|Cosworth]] and [[Renault RS engine|Renault]] 2.4-liter V8 engines during the [[2006 Formula One season|2006 season]].<ref>{{Cite web|title=Cosworth story|url=http://www.formula1-dictionary.net/engine_cosworth_story.html|access-date=2022-03-09|website=www.formula1-dictionary.net}}</ref><ref>{{Cite web|title=King of Speed: Cosworth's CA 2.4l V8|url=https://www.f1technical.net/features/18858|access-date=2022-03-09|website=www.f1technical.net|date=15 October 2013 |language=en-gb}}</ref><ref>{{Cite web|last=Engineering|first=Racecar|date=2008-09-12|title=Inside an F1 Engine|url=https://www.racecar-engineering.com/articles/f1/inside-an-f1-engine/|access-date=2022-03-09|website=Racecar Engineering|language=en-US}}</ref>

==Rev-limiter and implementation==
Most modern cars have computer systems that prevent the engine from straying too far into the redline by cutting fuel flow through the [[Fuel injection|fuel injector]]s/[[common rail|fuel rail (in a direct-injected engine)]]/[[carburetor]] or by disabling the ignition system until the engine drops to a safer operating speed. This device is known as a [[rev limiter|rev-limiter]] and is usually set to an RPM value at redline or a few hundred RPM above. Most [[Electronic Control Unit]]s (ECUs) of [[automatic transmission]] cars will upshift before the engine hits the redline even with maximum [[acceleration]] (The ECU in a sports car's automatic transmission will allow the engine to go nearer the redline or hit the redline before upshifting). If [[manual override]] is used, the engine may go past redline for a brief amount of time before the ECU will cut power to pull it back or auto-upshift. When the car is in top gear and the engine is in redline (due to high speed), the ECU will cut fuel to the engine, forcing it to decelerate until the engine begins operating ''below'' the redline at which point it will release fuel back to the engine, allowing it to operate once again.

However, even with these electronic protection systems, a car is not prevented from redlining through inadvertent gear engagement. If a driver accidentally selects a lower gear when trying to shift up or selects a lower gear than intended while shifting down (as in a motorbike [[sequential manual transmission]]), the engine will be forced to rapidly rev-up to match the speed of the drivetrain. If this happens while the engine is at high RPMs, it may dramatically exceed the redline. For example, if the operator is driving close to redline in 3rd gear and attempts to shift to 4th gear but unintentionally puts the car in 2nd by mistake, the transmission will be spinning much faster than the engine, and when the clutch is released the engine's rpm will increase rapidly. It will lead to a rough and very noticeable [[engine braking]], and likely cause engine damage. This is often known as a "money shift" because of the likelihood of engine damage and the expense of fixing the engine.

Redlining in a [[diesel engine]] can be caused by the engine receiving fuel from an unintended source, such as flammable vapour in the intake air, or a broken oil seal in a [[turbocharger]]. This is known as [[diesel engine runaway]], and can be stopped by blocking the air intake, or opening the decompression valve.

==See also==
*[[Power band]]

== References ==
{{reflist}}

{{Automotive engine |collapsed}}

[[Category:Engine technology|Redline]]