Editing Redline (section)

==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.