Talk:Engine braking

message: This page needs reworking, in my opinion, and I'll try to do so soon if there are no objections.  In an engine, the compression of air in the cylinder is approximately a reversible process.  Yes, it takes energy to compress it on the compression stroke, and the gas heats up, but on the downstroke that energy is returned to the crankshaft and the gas cools back down.  In a gasoline engine, engine braking during closed-throttle operation comes mainly from maintaining a partial vacuum in the intake manifold (between the closed throttle and the cylinders).  Diesel engines are unthrottled, so they don't incur these "throttling losses" and don't provide much engine braking (it's also one reason they tend to be fuel-efficient); a jake brake on a (presumably diesel) truck provides engine braking by opening the exhaust valves at the top of the compression stroke, so that the energy used in compressing the air is lost, instead of being returned to the crankshaft.  - Coneslayer 05:01, 2005 Mar 5 (UTC)
Sounds like you know about the subject, no objections here. Raazer 16:44, 8 Mar 2005 (UTC)
replies: text: Just checked back here and now fully happy with the article as written today!  :) Weasley one (talk) 13:14, 4 June 2010 (UTC)
thread_title: reworking

message: Why are compression release engine brakes so loud?  I'm also confused about whether there's combustion going on when the exhaust valve is opened at compression stroke (If anyone would care to answer). —The preceding unsigned comment was added by 71.10.233.117 (talk • contribs) 05:47, 9 September 2005.
:I realise this answer is a bit late.  In a normal engine cycle, the exhaust valve opens at the bottom of the stroke, when the gasses in the cylinder are under relatively low pressure.  The rising piston then pushes the exhaust gasses out.  However, with the compression release brake, it pops the exhaust valve at the top of the compression stroke, when the air inside is at many times normal atmospheric pressure.  This sudden release of pressurised gas is what makes all the noise - it's like  firing a gun.  The compression release brake also turns the fuel injection off (and there's no pressurised air in the cylinder any more to ignite it anyway), so no, there is no combustion. FiggyBee 23:15, 10 June 2006 (UTC)
thread_title:  question 

message: Is "compression braking" just a bad synonym for "compression release braking"?  Compression release makes sense, but as the Jake Brake article tells, the compression itself is entirely countered by the spring effect in the inevitable expansion that follows, thus no net braking occurs. Balfa 18:23, 11 December 2006 (UTC)
thread_title:  "Compression Braking" 

message: Are their any possible risks to the engine when breaking?  For example if you use engine breaking on an steep hill, is there a point where you should use your regular breaks to prevent too much strain on the engine?
amRadioHed 23:41, 10 January 2007 (UTC)
replies: text: Actually that's a good point; May be worth adding to the article.  The risk is basically that you can over-rev the engine. If you were, for instance, in 2nd gear going down a really steep hill, there is a good chance the engine braking won't be enough to stop gravity accelerating the car down the hill - The engine RPM will continue to increase until something breaks. In you're in that situation, it's important that you go up a gear and/or engage the friction brakes to slow the car down and stop the RPMs shooting up. (It'd have to be a bloody steep hill tho') 92.234.58.169 (talk) 20:56, 30 May 2014 (UTC)
thread_title:  Risks? 

message: Yes.  Please do clean this up. I was actually confused about the explanation.  It seemed at first that all you were doing was explaining how a diesel engine works compared to a gasoline engine.  I scrolled down looking for the description of the actual topic and realized you were trying to do that already.  A bit confusing.—Preceding unsigned comment added by Theillien (talk • contribs) 11:40, 30 March 2007
*I have tried to rework the design section to get to the procedure and the results of the procedure.Raazer 15:44, 7 April 2007 (UTC)
thread_title: Clarification

message: The description could be misleading to whoever reads it.  You should definitely go thru it and clarify the explanation.  —Preceding unsigned comment added by Hbettis (talk • contribs) 14:34, 2 October 2007 (UTC)
thread_title: Confusion

message: What I am missing in the article is something about how the amount of engine break torque or power can be calculated. Maybe there could be a reference to some related article if this one is supposed to only describe the meachanics. There should be information available about how to calculate this, or at least some graphs from real life testing. Without the physics/mathematics behind the topic, I find it incomplete.  —Preceding unsigned comment added by 85.227.143.102 (talk) 14:56, 27 February 2010 (UTC)
thread_title:  Physics and Mathematics 

message: This section only makes reference to legal implications in the last paragraph. The first two are irrelevant to legalities.  — Preceding unsigned comment added by Abrusletten (talk • contribs) 17:44, 14 September 2013 (UTC)
thread_title:  Legal implications section 

message: There are various problems associated with the idea that it is the closed throttle that creates engine braking:
  
Conservation of energy: On a descent where  if we used solely the brakes to remove the vehicles gravitational potential energy we would expect and do see very hot brakes(red hot discs) and heat emitted from the wheels etc.  If it is the restriction through the throttle plate that removes/transfers the energy, we should be seeing red hot areas around the throttle intake regions.......but alas we don't, a law of physics would have to be violated for the closed throttle theory to be correct. 
"Power/force" of a vacuum is often mentioned:   If the throttle was completely closed then the piston only has to overcome 15psi of outside pressure to move against the "power" of the vacuum, this is small compared to the pressure that occurs on the compression cycle where we can easily get 120-200psi(depending on revs and cam profile), this is where the serious engine braking occurs. 
Also depending on the revs and profile, the more air that is sucked into the motor, the higher the dynamic compression ratio, this is why when testing the compression of an engine it must be done at WOT
Many have not tried this, but if have an old school cable throttle where one can switch fuel and/or spark off, then there will be slightly more engine braking applied if you use WOT. 
The energy used in compression is returned in the power stroke, just like a spring?  Well a spring with incredibly high damping factor would be more accurate! the compression is non adiabatic and the process of returning the stored energy in an internal combustion energy is well less than 20%.  One only has to try turn a small engine(like on a motor mower) over by hand with open throttle and no fuel and see how quickly it stops spinning, and then compare when a spark plug is removed, the friction of the moving parts counts for little.  One can also see this if you turn off a petrol engine doing say 2000rpm, it stops within a second, even if you give it WOT whilst turning it off.
So where does the energy go that would have turned the brakes red hot, it goes into heating the air within the engine when compression occurs, some getting transferred to the cylinder walls and some out the exhaust.  
 
The proof is in the pudding, go from closed throttle to WOT whilst engine braking with no fuel/spark and there will be basically small noticeable increase in braking going to WOT, rather than zero engine braking when full throttle is applied.
The article should be pulled down, I am not the first person to make these points and so far the original author has not made any attempt to address these issues.
In regard to the references cited, they are wrong, being published simply does not make them correct, and wiki is simply just perpetuating the myth of those publications. I have a citation that is more powerful, the conservation of energy law, which is violated by the closed throttle idea, here is a reference:  https://en.wikipedia.org/wiki/Conservation_of_energy  Sacspeed (talk) 03:19, 21 January 2017 (UTC)
replies: text: The thing to understand is that Wikipedia articles should contain no original research (please click & read). Improving this article should consist of finding reliable sources, and citing them. The article should reflect whatever the sources say, nothing more. When you start reading the published experts (here's a couple examples , they all say closing the throttle increases engine braking. If you wanted the article to say anything that contradicts this, you would need to find and cite reliable sources (read Identifying reliable sources) which state this opposite position.<P>Your arguments might serve as a motivation for editors to carry out further research into the sources to see if there is a controversy or disagreement among published experts on engine braking, but that's all unsourced opinions can do. Editor opinions are not "proof" of anything. You can believe it, but it's not relevant to article editing until there's sources to go with it.<P>By all means, if you know of a published source that says it, then do cite it in the article, or identify the source here so other editors can make use of it. --Dennis Bratland (talk) 01:33, 20 January 2017 (UTC)
thread_title: January 2017