Talk:Engine braking

vacuum

Can someone write up more information on how and why this "vacuum" inside an engine magically slows a vehicle more than friction with the drive train? As someone who's generally competent--although inexpert--with physics, this sounds like a load of bullshit, so I'd appreciate more actual detail with useful information here. :)

Vacuum or compression has nothing to do with engine braking. Diesel or Otto engine never mind. Energy spent to create vacuum or compression is conservative and returned in the very next expansion. Just like any spring. Engine braking comes mainly from internal friction. Small part of it comes from pumping losses (air flow). Completely different thing is engine brake system on a big truck engine. It basically converts engine into a big compressor while braking. This whole page has to be rewritten. As it is it doesn't belong to any encyclopedia.206.116.101.49 (talk) 17:04, 21 February 2012 (UTC)

I'm not certain the article as-is reflects what's happening. There is an easy test for this, which I have done in the past on a friend's classic Mini which I had to turn over by hand to get the engine turning.

With the carburettor in place, and the spark plugs removed, I was able to turn the engine by hand. This was because the engine cannot compress any air when the plugs are removed. With the carburettor removed, and the spark plugs in place, it was impossible to turn the engine by hand. This experiment would appear to suggest that out of the "Suck Squeeze Bang Blow" of the four stroke cycle, it is the "Squeeze" rather than the "Suck" that causes the majority of the engine braking effect in a gasoline engine.

If you have a carburetted engine the same test can be carried out while on the move. Shut off the ignition while travelling down a hill. If you open the throttle at this point, the article suggests that any engine braking will be lost. This is not the case.

The rotational energy of the drivetrain is being converted into heat energy by compressing the air, and that energy is not fully recovered by the subsequent downstroke since the engine is designed to remove as much heat from the pistons as possible at all times, causing the temperature to drop, and the piston to be at a slight vacuum by the bottom of the downstroke. That's on a gasoline engine. The same is true on a diesel engine.

Any confusion here is with the addition of the "Jake Brake", which according to their website cleverly actuates the exhaust valve much earlier in the cycle, where "bang" normally would be. This releases the compressed air when it holds the most energy, and then closes the exhaust valve again, causing that piston to have to work against much more of a partial vacuum in the cylinder on the downstroke. That massively increases the braking effect.

Davepoth (talk) 22:12, 17 January 2011 (UTC)

Where does waste heat go when creating a vacuum? While friction and drag from accessories contribute, much of the force in engine braking is due to pumping losses, with the cylinders trying to draw a vaccum against the closed throttle (as well as pushing air past the valves). Instead of the pistons pushing on the connecting rods, the connecting rods pull the pistons down. Vacuum braking force for each cylinder would be Patm-Pcyl * piston area. Drawing a vaccum actually cools the air though. The kinetic energy of the car must ultimately be converted to heat and be disappated into the air (presumably through the radiator and/or exhaust), but where in the process of creating a vaccum is the heat made and how is it rejected.--129.238.237.96 (talk) 18:22, 15 September 2011 (UTC) — Preceding unsigned comment added by 129.238.237.96 (talk)

Engine braking in a gasoline engine has nothing to do with intake vacuum; it is due to compression. This can be easily verified in your car by shutting off the ignition while keeping the throttle wide open. The car slows down just as much with the throttle open as with the throttle closed. — Preceding unsigned comment added by 70.71.27.207 (talk) 09:02, 29 December 2011 (UTC)

Your experiment will actually support the opposite of your argument. If you shut down a gasoline engine and open the throttle, the engine will run on longer than with a closed throttle - this is very noticeable if the vehicle is moving when you do it (don't try this at home, folks!). It IS the restriction in the inlet that creates engine braking, not compression effects. Yes, it is hard to overcome compression effects if starting from stationary, but once an engine is running and the flywheel has stored energy to smooth out the lumps, the 'effort' you put into compressing a charge of air will almost all be got back as a 'spring' effect. You will lose a little energy through friction and heat losses, but negligible compared to the throttling effect in the intake. Weasley one (talk) 15:33, 17 December 2012 (UTC)

A major loss is from pumping air (moving mass is doing work). If the carb was sealed shut it would drag less because no air would be pumped. So 'vacuum' from a closed throttle certainly does not slow it down 'more'. Think of putting your hand on the end of a vaccum cleaner hose, the motor speeds up because it's doing less mechanical work. It would be helpful if someone would demonstrate this on youtube, drive an unfueled engine via prime mover whose wattage can be measured, try with throttle open, throttle shut, and valves stuck open. 146.115.72.218 (talk) 00:52, 1 April 2016 (UTC)

reworking

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)

I agree - the article as it currently stands is completely wrong. Engine braking is the term used to describe the retardation effect when the driver lifts off the accelerator. As has been stated above and below, this is NOT from compression of the combustion charge, but from throttling effects in the air flow pathway. It appears that I am commenting nearly 3 years after the above and yet the article currently continues the phallacy of engine braking and compression. There are compression braking mechanisms, as described elsewhere, but the simple "lift-off" braking effect commonly known as engine braking is not correctly described. —Preceding unsigned comment added by Weasley one (talk • contribs) 15:24, 25 February 2008 (UTC)

Just checked back here and now fully happy with the article as written today!  :) Weasley one (talk) 13:14, 4 June 2010 (UTC)

Brakes, brakes, brakes

OK, we've got Engine brake (AKA Compression brake), Exhaust brake, Air brake (rail), and Jake brake (plus multiple suffixes for each e.g. -e , -es, -ing). Is there a better way to organize it? Just adding a category might not elucidate one as to the differences, if there are any, between the various types. Ewlyahoocom 10:51, 30 July 2005 (UTC)

- Jake brake seems like a popular name for an exhaust brake made by Jacobs, and could be a subset of that article. However, the jake brake article seems to describe the mechanism in clearer detail (and perhaps more correctly) by saying how the energy is dissipated by the release of the air from the cylinders.

The exhaust brake article is more extensive in mentioning the competitor of Jacobs, Pacbrake but seems confused as to the mechanism, and makes it seem the air never leaves the engine.

An air brake (rail) is very different from everything else and should remain in its own article. It mentions trucks, but refers to applying pneumatic pressure to activate regular friction braking. Also, compression brake probably is fine as a redirect. Exhaust brake/jake brake is a mechanism that does not exist in regular cars, but engine braking is possible in any car, so they are also distinct.

To remedy the number problem, I guess I'll, or someone else will come back to merge parts of exhaust brake and jake brake, hopefully maintaining all the information in the jake brake article. But Jake brake is actually an indivudual product, and merits its own article.Raazer 23:01, 23 August 2005 (UTC) -

Could we merge a lot of these pages together, including the section from Air brake (rail), into a new page about truck braking technology? Or maybe "large vehicle" braking systems? In some respects trucks and trains have more in common than trucks and cars. Ewlyahoocom 10:35, 4 February 2006 (UTC)

Agree that the section Other applications from Air brake (rail) be removed from that article; material not about railroad/railway air brakes does not belong in an article titled Air brake (rail). While I am certain that the section is out of place where it sits today, I'm not certain where the material does best belong. Since truck braking is not an area of particular interest or expertise for me, I won't offer an opinion on this material's eventual home — those with more knowledge about that subject matter should decide.

I thought I'd point out that the disambiguation page for "Air Brake" lists a page (which is not created) for Air brake (commercial vehicle) which would be a good place to put all the truck-related air brake information. There is quite a bit over at Semi-trailer (now semi-trailer truck ENeville), which would be better put there, in addition to the stuff on Air brake (rail) and elsewhere. I don't think it should be moved to the Engine brake page though, since it's not really engine-brake related (it's about air brakes, which aren't engine brakes). I think there is enough stuff to have separate pages for the air braking systems of trains and trucks (and planes, and a separate page for anything else that uses them). We just need to condense all the truck stuff onto one page. There's definitely enough content, I think. -Kadin2048 02:26, 1 May 2006 (UTC)

I concur, Air brake (rail) should be just that, and not involve trucks at all.UbiquitousGeep 22:20, 15 February 2006 (UTC)

I will add, however, that the term Engine braking may deserve to be better qualified. There are a lot of different types of engine in the world. — JonRoma 18:59, 4 February 2006 (UTC)

I'll add to the mix by saying I added an article on retarders a little while ago(Retarder (mechanical engineering)), which also covers an overlapping area of this topic. I just thought I would bring it to people's attention in case a major rewrite/clean up of this topic area takes place.WLD 08:49, 15 February 2006 (UTC)

My two pence:

• Rename air brake (aircraft) to speed brake, which as far as I'm aware is the more commonly used name.
• Rename air brake (rail) to air brake, put a disambig link to speed brake at the top, and leave the section on trucks there. Air brakes on trucks are basically the same thing as air brakes on trains, and a completely different subject from engine braking.

FiggyBee 23:47, 9 June 2006 (UTC)

I disagree. In my experience, "air brake" is used far more often than "speed brake" to refer to brakes on aircraft. Balfa 18:16, 11 December 2006 (UTC)

The braking effect is actually as a result of the compression stroke of the engine, both Petrol and Diesel. In fact the Diesel engine has more braking ‘power’ than the petrol because, as the wikipedia.org author states, the throttle of the petrol intake limits the air intake thus limiting the amount of air that can be compressed, and a vacuum doesn’t compress so well! Furthermore the diesel engine has much higher compression since it needs high compression for diesel ignition. (AfricaLionHeart) — Preceding unsigned comment added by AfricanLionHeart (talk • contribs) 06:01, 21 June 2011 (UTC)

question

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)

You guys need to come over to Nairobi Kenya sometime, if you think the engine braking in your place is loud. we got commuter service mini-buses with modified exhausts that would rapture your ear drums. It sounds similar to passing all the exhaust gases from the engine through a whistle. there is no transport law governing exhausts so they try out doing each other as far as loudness is concerned. But at times there are police crackdowns on the said passenger vehicles to have the exhaust modifications removed But again, its what we like about them. Drop a line if you have any questions- +254 724394525. —Preceding unsigned comment added by 62.24.112.246 (talk) 13:01, 12 January 2008 (UTC)

Don't Merge

I don't understand why some of the Wikigeeks love to combine pages. This page is fine on it's own for several reasons:

• Jake Brakes are a distinct item in trucking and vehicular braking
• Jake Brake is a genericized trademark
• The issue of compression brakes being banned on certain roads due to their noise has become a hot-button issue in many communities

I can't believe I'm actually taking the time to write my opinions on a Wikipedia article on friggin' JAKE BRAKES, but come on guys. It seems like every other article on here has some knob pushing for merging with another, semi-related article. Why not combine the articles on lard and apples, because together they make a pie? —Preceding unsigned comment added by 72.57.241.84 (talk • contribs)

"Compression Braking"

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)

Risks?

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)

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)

Clarification

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)

Confusion

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)

fuel?

There should be information in the article regarding whether engine braking affects fuel economy. --Joy [shallot] (talk) 23:57, 15 December 2007 (UTC)

I was under the impression that injected vehicles will in fact not inject fuel when the throttle is closed and the engine is still turning. --GSchjetne (talk) 12:21, 20 May 2008 (UTC)

That's ridiculous. Then the engine would die every time you're at a stoplight and the foot is only on the brake. Novasource (talk) 13:17, 20 May 2008 (UTC)

It does indeed die if you do that - unless you have automatic transmission, where the torque converter performs the same effect as disengaging the clutch in a conventional transmission car. But this behaviour is completely unrelated to fuel injection. --GSchjetne (talk) 05:35, 30 May 2008 (UTC)

Fuel injection resumes once the engine speed is near the idle speed, of course. This behavior was documented in the shop manuals for my Honda Civic del Sol and my MINI Cooper S. It's certainly not "ridiculous". -- Coneslayer (talk) 13:55, 20 May 2008 (UTC)

I don't see how that would work without a direct connection between the rear wheels and the engine, and that can only happen with an automatic that is at coasting RPMs (torque converter lockup engaged) or a manual transmission. With an automatic and a disengaged torque converter clutch, the engine would immediately fall to 0 RPMs without any fuel.

Also, are you sure that "shut off" is literal? Could it not be euphemistic for "turned down to normal idle fuel consumption"?

Novasource (talk) 14:16, 20 May 2008 (UTC)

I'm not sure what the rear wheels have to do with anything in the cars I listed, and I'm sure that it's a full cutoff. The del Sol manual specifically indicated the engine speed at which fuel flow resumes (something like 1,000 rpm, from memory). I don't know a lot about slushboxes, and I don't really follow your argument. You seem to be suggesting that the engine is completely decoupled from the wheels during engine braking. As far as I can tell, the wheels are still turning the engine. Otherwise, what do you think is keeping the engine speed synchronized with the wheel speed? -- Coneslayer (talk) 16:58, 20 May 2008 (UTC)

PS: See, for example, [1] (look for "Over-run fuel cut-off"), or [2] (search for multiple occurrences of "Overrun"). -- Coneslayer (talk) 17:10, 20 May 2008 (UTC)

Ah, you're correct. My bad. I was forgetting you would have to shift out of gear to cause the drop to 0 RPM, and that would apply regardless of transmission type. Novasource (talk) 18:17, 20 May 2008 (UTC)

It's good to see this information added, thanks. But, there is no explanation of what "overrun fuel shutoffs" are (or a link to an appropriate article). Does the common car made these days typically have this feature? --Joy [shallot] (talk) 01:27, 6 September 2008 (UTC)

What about addressing the simple problem/question; when going downhill should people driving an automatic transmission downshift to 1st or 2nd gear or will they blow their transmission? —Preceding unsigned comment added by 76.242.91.183 (talk) 02:48, 13 October 2008 (UTC)

That would very much depend on the hill and the type of autobox; Generally it's best to let the car figure it out. If it's a really steep hill and the car is quite stupid, manually overriding it for a lower gear would be beneficial for the same reasons it is on a manual. It's worth noting that Toyota Hybrids will not engine-brake unless they are in B-mode (Normally the automatic regen-brake is enough, but as it stops working when the battery is full, B-mode can be needed on long hill descents). It's not recommended to use downshifts for engine-braking on CVTs as it can fuck up the belts, but you're usually okay with semi-autos (MMT, DSG etc.) and 'real' autoboxes (i.e. torque converters). Note there are very few circumstances you'd want to go to 1st gear unless the hill is dangerously steep; I don't think I've ever gone lower than 3rd personally except once where I got lost in Wales and had to go down a 45 degree slope (!!). That was f'ing scary lemme tell you...! 92.234.58.169 (talk) 21:07, 30 May 2014 (UTC)

Physics and Mathematics

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)

Legal implications section

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)

Using engine braking and brakes together

Does it improve or reduce braking efficiency if using both regular brakes and engine braking at the same time?