Editing Engine braking (section)

===Diesel engines===
Diesel engines in personal cars provide little engine braking as they are not equipped with a throttle body and thus cannot draw a vacuum in the intake manifold.

In heavy vehicles the engine is often made to provide extra braking power to take some strain off the vehicle's regular brake system and to help avoid overheating the brakes. In its simplest form this consists of a butterfly valve that restricts the exhaust flow. This is referred to as an exhaust brake and mostly found on older trucks. It has a limited effect, and more advanced systems as described below are near universal on newer heavy vehicles.

==== Compression release brake ====

[[File:No Jake brakes within city limits.jpg|thumb|No Jake Brake Sign in [[Unadilla, Georgia]]]]

A [[Compression release engine brake|compression release brake]] (also known as a Jacobs brake or "jake brake"), is the type of brake most commonly confused with real engine braking; it is used mainly in large diesel trucks and works by opening the exhaust valves at the top of the compression stroke, so the large amount of energy stored in that compressed air is not returned to the crankshaft but is released into the atmosphere. It is a very effective method of braking, creating large amounts of braking force which significantly extends friction brake life – a {{convert|565|hp|abbr=on}} diesel engine can produce up to {{convert|600|hp|abbr=on}} of braking power at 2,100 RPM.<ref>{{Cite web |title=Understanding Tractor-trailer Performance |url=http://electracold.com/CAT.pdf |url-status=deviated |archive-url=https://web.archive.org/web/20220314154105/http://electracold.com/CAT.pdf |archive-date=2022-03-14 |access-date=2023-10-13 |website= |publisher=[[Caterpillar Inc.]] |page=20}}</ref>

Normally, during the compression stroke, energy is used as the upward-traveling piston compresses air in the cylinder; the compressed air then acts as a compressed spring and pushes the piston back down. However, with the jake brake in operation, the compressed air is suddenly released just before the piston begins its downward travel (this sudden release of compressed air creates audible sound waves similar to the expanding gases escaping from the muzzle of a [[firearm]]). Having lost the energy stored within the compressed air, there is no "spring back" so the engine must expend yet more energy pulling the piston back down again.

This type of brake produces extreme amounts of [[noise pollution]] if there is no [[muffler]] on the exhaust manifold of the engine, loud enough to disturb the surrounding area. Anecdotally, it sounds similar to a [[jackhammer]], however, the [[loudness]] is between 10 and 20 times the sound pressure level of a jackhammer (10 to 13 [[decibel|dB]] greater). Numerous cities, municipalities, states, and provinces have banned the use of unmuffled compression brakes, which are typically only legal on roads away from populations. In Australia, [[traffic enforcement camera]]s are currently being tested that automatically photograph heavy vehicles that use compression braking.<ref>{{cite web|publisher=[[Roads & Traffic Authority]]|year=2007|title= Annual Report 2007|url=http://www.rta.nsw.gov.au/publicationsstatisticsforms/downloads/2007_rta_annualreport_mainbody.pdf|access-date=2008-06-30}}</ref>

==== Exhaust brake ====
An [[exhaust brake]] works by causing a restriction in the exhaust, much like the intake throttle causes in a gasoline engine. In simple terms, it works by increasing the back-pressure of the exhaust. Nearly all of these brakes are [[butterfly valves]] similar to a throttle valve, mounted downstream of the [[turbocharger]] if there is one.

==== Exhaust obstructions ====
Modern diesels are subject to many strict controls on emissions and often have many obstructions in the exhaust, which cause them to feel like they have some engine braking like a gasoline engine. The main ones are:
* [[Turbocharger]] creates some back-pressure when it is stalled
* [[Exhaust gas recirculation|Exhaust gas recirculator]] (EGR) valve redirects exhaust gas back into the engine intake, often through a restricted/narrow pipe.
* [[Diesel particulate filter]] (DPF) is designed to capture soot particles that would otherwise be released into the atmosphere; it greatly obstructs exhaust flow and can sap almost as much power as a small air conditioning compressor.