How a 'Jake Brake' works

Originally published at: https://boingboing.net/2020/02/12/how-a-jake-brake-works.html

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By converting forward motion into a hell of a racket.

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Isn’t this just engine braking?

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Yes and no. Just laying off the gas still produces a compression power cycle. This is a compression release, which doesn’t produce a power cycle at all. This was popular on two stroke dirt bikes back in the 70’s for a few years.

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True, but as not stimulating your date’s imagination with technical explanations amounts to giving up, fair.

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The video specifically states that modern exhaust systems “virtually eliminate the noise”.

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I understand most of what’s going on in the video, but where does the wasted fuel wind up? Gas tank? On the ground?

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The fuel/air mix is still vaporized, and partial combustion still occurs, though necessarily less than if the full four stroke cycle completes. Most of the un-combusted fuel will end up in the diesel particulate filter. On a four-stroke engine, the compression-release brake AKA “jake brake” is simply a mechanism that opens the release valve on the second instead of fourth stroke, pushing the mostly combusted fuel through the release valve. Chemical potential energy is still converted into the kinetic energy of thermal expansion, but deprived of the opportunity to push the piston back out, being pushed instead through the release valve in all its unsapped glory.

Diesel burns slower than petrol, but in an engine cycling at 4800 RPM this means that explosive release is happening 80 times a second. This is why the driver still has to engage the friction brakes, otherwise the downhill kinetic energy of the vehicle will accelerate the cycle and push the engine past its max RPM, causing it to overheat.

Even used properly, the sound is very much like a machine gun, which is why the use of jake brakes is banned in many densely populated jurisdictions. The muffler absorbs some of the acoustic shockwave prior to the exhaust, but the sound is still pretty unpleasant, especially inside the vehicle. The video is claiming new technology all but eliminates the sound. Not sure what they’re referring to, maybe sound baffles around the release valve, but I’ll believe that claim when I don’t hear it.

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I see the signs banning the use of motor deceleration in the US all the time. I would like it if it were enforced here in TJ.

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I love vintage motorcycles, but I once had a neighbor who rode an old two-stroke diesel bike with a compression-release brake (different implementation, but the same basic principle as on a four-stroke). He was not popular, though I took some satisfaction in knowing he was damaging his bike because two-stroke engines rely on fuel compression to lubricate the engine.

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How long before the filter get useless? (because saturated)

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I feel like they had the assets to do so, but didn’t quite manage to illustrate the difference well enough to make the video worthwhile.

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It shouldn’t be much longer than the filter being saturated by the full cycle. Most of the combustion occurs as the piston is compressing the fuel/air mix. Some combustion is continuing as the piston is pushed outward, and the compression-release brake opening the release valve early lowers the pressure pushing it which means not as much of the mix is combusted on the outward stroke, but it shouldn’t be that much more. If the jake brake operates correctly then nearly as much fuel is combusted and the emissions are nearly the same; the kinetic energy released just isn’t used as efficiently by the engine since it’s allowed to escape through the release valve rather than being contained.

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I had the same original question as @knoxblox, but with the thought that there is a whole cycle’s worth of fuel that doesn’t get burned, and those raw hydrocarbons end up in the atmosphere. I think that might be true in a regular gasoline engine, but this is a diesel burner, and I’m far less familiar with the process in those machines. I seem to recall something about the burn happening because of the crazy-high compression, something about glow plugs, and perhaps that’s what @GulliverFoyle is referring to when the combustion happens in the compression stroke.

Does the fuel injection shut off during engine braking?

I know gasoline fuel injection stops if the engine is turning fast enough and there’s no throttle. Which is why you shouldn’t coast in neutral or with the clutch in, that forces fuel injection on.

Maybe they’ve changed something but they have a long uphill battle.

jake-brakes

People say a lot of things. Log trucks come through my area. They’re all late-model, though they look old because lumbering beats the hell out of them. From a mile away, jakes are LOUD. From a quarter mile away, they rattle the windows in our house. I don’t know if these trucks have less exhaust muffling than they should, but I don’t think so, as they don’t sound that loud otherwise. But maybe that’s the dif. But, my experience is, cheesy promotional video assertions aside, jakes are loud as all get-out.

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This reminds me of the signs that say “no horn except in emergency.” As The Bear put it, why not just put up signs reminding people not to murder anyone?

I once stayed with some friends who lived near the Gowanus Expressway in Brooklyn – not right by it, but close enough where I could hear trucks braking all night. Between that and the heating pipe rattling, I felt like I barely slept at all. My hosts had gotten used to it and had trouble sleeping when they came back to Austin.

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