Exhaust Brake?

[XLerate]

Hi guys!

I haven't seen anyone mention exhaust brakes in their builds. I mean either the simple flapper valve in exhaust tube, or the 'Jake Brake' type that upsets valve gear. Anybody using one?

[pietenpol2002]

Do a search for "jake" and you'll find a bit of discussion on the matter. Developing a true "jacobs" design for our purposes is likely beyond the scope of what most of us can pull off. And then let's be honest - for most of us so inclined, it would really come down to auditory porn and bragging rights.

However, since my exhaust is stainless, I've thought of using something on the order of the example below. Sadly, no Jake effect. Just lots of hissing.

http://tinyurl.com/7zefqyf

[XLerate]

Thanks for the link!

Yes, that's the type planned for my truck. I've gotten the parts: a 4" nichrome cylinder sleeve, steel plate flapper valve, mounted to a heavy thru-bolt with lever action. A couple of reducers to weld the sleeve into 3" exhaust, with one T-bar clamp for access for cleaning soot buildup. Bore a hole to allow some gasses to bypass so it doesn't shove exhaust valves into pistons! Most say one should also use heavy duty valve springs to prevent that.

I need one when towing with the truck. Also planning to build a trailer along with my trike, so again handy on steep downgrades for an extra safety factor and sparing the binders to boot.

Love the sound of a Jake but haven't ever heard one of these yet, that I know of. Jake's are illegal in some locales due to their noise.

[andrewaust]

Surprisingly enough you get braking by holding down the decompression lever, you feel the difference, but wouldn't it be cool to have a Jacobs brake system.

I seen an old well looked after Denning Coach come down the street twice, fitted with a GM Diesel and hit the Jakes, music to my ears, I love the 2 stroke GM's.



A

[XLerate]

Yes, the 2 stroke Detroits Diesels used by GM are interesting & unique.

Maybe the decompressor on your engine is performing a similar function to a Jake Brake, as I know they upset the valve system. To be honest I don't quite understand how a Jake Brake works even though I've read several descriptions. Seems exactly backwards to my way of thinking, regardless of how it's explained. If the Jake Brake releases certain valves at certain times, upsetting exhaust valve opening to hold pressure against piston, then I could see it would work.

The simple butterfly valve in exhaust makes sense to me. There the valves are releasing exhaust gas but it's against a road block in the pipe, can't go any farther, so it works against pistons when pressure builds up.

http://en.wikipedia.org/wiki/Jake_brake

[andrewaust]

The Jacobs brake works by opening the exhaust valve just as the piston comes up on the compression stroke close to top dead centre TDC, no or minimal fuel is delivered as there is a sensor or relay that only lets it work while the accelerator pedal is up and also a remote activation switch to turn it on and off. What happens in a diesel engine the compression is so high it actually pushes the piston back down, it you release the compression via the exhaust valve that energy is lost, and also on the downward stroke after the exhaust valve closes again a vacuum is slightly formed draining more energy from the engine, but it's mainly the release of compression that is the most effective.

There are some good spots on the net that show you how it all works, it's engine oil driven with solenoids to allow small pistons to slightly push down the exhaust valve and uses the timing of injector with the older engines, the common rail ones are computer controlled from memory.

Your right, by pushing the decompression lever down its stopping the compression from pushing the piston down, that is why you have very little engine braking with a diesel with no Jacob or exhaust valve, only the smaller engines using a butterfly dashpot pneumatic type diaphragm system on the inlet to injector pump gives a little braking due to developing a vacuum in the inlet manifold.



Cheers


A

[XLerate]

Thanks, Andre, I'll read that over a few times & try to get it to sink in. I know that the mechanics of it are extremely simple but somehow I stumble on understanding the wording of explanations. That's me, not anything wrong with the explanations...

[andrewaust]

I think the best way to describe it is get hold of a push bike pump and hold you finger over the end when you push down on the handle, the handle will want to push back up, if you then start again and push the handle down and release your finger over the end and let the air out of the cylinder no air energy is there to push the handle back, and if you had a solid piston in their and pulled back, resistance would also be felt. Bike pumps are made to suck air when pulling back so you cannot get that effect

It can be demonstrated on a small edger engine etc, but must be careful when playing around with compressed air, you don't want to get it into you bloodstream, the reason why I used the bike pump as a good example.


A

[XLerate]

Okay, that makes more sense!! Thanks!

But for that to work it seems this system must be keyed to valve timing, to open a valve at the 'wrong' time, causing this resistance, then closing it later in stroke than normal? That is, it upsets normal valve timing?

Or is the simple opening of valve good enough throughout the movements of pistons, without changing any valve timing sequence, to accomplish the braking? I'm thinking that's what you mean: opens valve continually to prevent compression spark in cylinder & that's what causes braking.

That would explain another thread & post where the fella said his decompression system acted like an exhaust brake too.

[andrewaust]

Yes just before the piston hits top dead centre on the compression stroke the exhaust valves open releasing the air - the same idea as the bike pump, Jacob brakes do alter the valve timing on the exhaust and I go into it on the previous post, the Jacob brake website had an enimated eplanation which was good, not sure if it's still there, but even do a web search for animated Jacob brake operation, to me it's simple, but yes it's a complex little system !!

A

[XLerate]

Ah HAH! THAT'S the part I've never understood! THANKS!

I was absolutely sure it couldn't work without modified valve timing but never could get a straight answer, mainly because others actually had no idea how the blasted thing worked !???! but wouldn't admit it.

I guess that partially explains why they're so expensive. Thanks for the help!

So, maybe a guy could weld up or have welded a few extra lobes on camshaft & build a system to actuate those lobes to upset normal valve timing for braking. May try that...