Editing Engine braking (section)

=== Gasoline engines===
The term "engine braking" refers to the braking effect that occurs in gasoline engines when the [[accelerator pedal]] is released. This causes fuel injection to cease and the [[throttle valve]] to close almost completely, greatly restricting forced airflow from, for example, a turbocharger. The restriction causes a strong [[manifold vacuum]] which the cylinders have to work against, sapping much of the potential energy out of the system over time and producing the majority of the engine-braking effect.<ref>{{cite web |last1=Megli |first1=Thomas |title=Increased engine braking with adjustable intake valve timing |url=https://patents.google.com/patent/US6951198B1/en |website=Google Patents |access-date=December 15, 2019}}</ref> This vacuum manifold effect can often be amplified by a down-shift, which induces a faster spinning drivetrain to engage with the engine.

Engine braking is a viable method of controlling the speed at which a vehicle travels downhill. By shifting to a lower gear in a [[manual transmission]], or applying "low" mode on an [[automatic transmission]], engine braking reduces the need to repeatedly apply the [[Car controls#Braking|foot brake]], lowering the risk of the brakes overheating.<ref>{{cite web |title=Controlling your vehicle |url=https://www.safedrivingforlife.info/drivers-and-riders/drivers/driver-knowledge-centre/controlling-your-vehicle |website=Safe Driving for Life |publisher=[[Driver & Vehicle Standards Agency]] |access-date=12 August 2019}}</ref>

While some of the braking force is produced due to friction in the [[drive train]], this is negligible compared to the effect from the manifold vacuum caused by the air-flow restriction. {{citation-needed|date=April 2019}}

On an automatic transmission, engine braking often spontaneously increases the engine RPM, causing a sudden revving to occur even without applying the accelerator pedal. This is due to the fact that an automatic transmission, as its name implies, automates the inter-mechanical actions in a manual transmission vehicle that would normally be produced from a human driver controlling the [[Car controls#manual|clutch pedal and gear shifter]] with their foot and hand, respectively. As such, the spontaneous revving is the automated reproduction of a manual transmission driving technique called [[Manual transmission#Other driving techniques|rev-matching]]. 

Rev-matching is used when down-shifting (shift to lower gear) to match the incoming gear's rotational frequency or speed (RPMs) to that of the driveshaft already in motion, requiring a quick "blip" of the throttle while in neutral during the gear shift. This in turn gets the larger (lower) gear up to rotational speed first before engaging the gear. Down-shifting without doing this results in a sudden jerking of the vehicle as the mismatched rotational speeds being engaged produce conflicting forces and directions of momentum.