Exhaust brake terminology

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Smoky

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I enjoyed reading the messages here about exhaust brakes, jake brakes pac brakes, etc.  But now I need a terminology primer. What is the difference, if any, between these terms?

Also I noticed that on every test drive, the dealer recommended that the brake be turned on and left on all the time.  somehow this does not make sense to me.  If the dealers are correct, then why the on off button?

Smoky
 

Ron

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Smoky,

First to explain the reason for having for an engine brake or exhaust brake on a larger diesel engine vehicle.

Unlike gas engines the same amount of air is drawn into the engine cyclinders on each stroke and the fuel injected is metered to adjust power.? You might say there in no restriction to the air flow through a diesel engine thus with out a exhaust brake or jake brake there is very little back pressure felt from the engine when you lift your foot off the acceletator. It is almost the same as puting the transmission in neutral.? Not much to help you slow down.

In a gas engine, including fuel injected, the air flow iin to the engine is regulated by the butterfly valve in the intake, inaddition to the fuel flow being regulated.? When you take your foot off the accelerator the air inlet is restricted by the butterfly valve being closed and you feel the engine being restricted becuase the iit is pulling so to speak against the restricction in the air inlet.

Some folks will leave the exhaust brake on all the time they are driving.  We normally have it off when traveling on the highway over relatively level roads. We always have it on when going down long hills or any hills that requires braking as well as when driving in traffic like in town.

The pac brake is an Exhaust brake .? They operate by closing a butterfly valve in the exhaust system restricting the flow of exhaust causing a back pressure the engine has to push against,? Thus producing braking action.? A jake brake is a systen which changes the valve timing causing the the engine to act as a compressor of sorts thus producing braking action.? The jake brake is actually a better brake system over the exhaust brake but is more comonly only available on engines over 350HP.? I am amazed that there are some diesel powered motorhomes that are sold without an exhaust brake or a engine brake.? Our 93 Bounder as well as other make coaches with the Cummins 190/230 HP engines did not have any type of engine or exhaust brake.? We added the exaust brake after we got the MH.




 

Tom

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Smoky

Pac brake is an exhaust retard system attached to the turbo. When activated, a flap restricts the flow of exhaust gases, causing an increase in back pressure. Activation of the Pac brake also downshifts the transmission from 6th to 2nd on my coach.

Jake brake opens/closes exhaust valves using solenoids mounted on the head. I'll let someone else describe the operation.
 

blueblood

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Brakes come in two flavors - exhaust and engine. Engine brakes supply retarding HP and require strengthened engine components for the higher levels. Exhust brakes simply block off free flow of exhaust gases. It's counter to names but engine brakes are the type banned by communities because of the loud noise introduced. Occassionally some town gets it wrong and puts up a sign banning exhaust brakes. Saw one not too long ago in WA. As for leaving the brake on all the time, :eek: that is a statement sure to get one into trouble. Brakes should never be on when road conditions can be slippery from ice, water or whatever.

Cummins has a new engine brake called InteBrake (meaning brake built into engine) that uses special lobes on a second cam shaft to provide major retarding HP on its big on hwy engine(ISM). Their other engines use the std Jake E-brake (made by Jacobs)  in smallest engine (ISB) and their own design C-Brake (made by Jacobs) in mid range where the engine has a strenghtened block and roller followers.
 

Ron

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Brakes should never be on when road conditions can be slippery from ice, water or whatever.

Good point Leo.  We try to avoid being anywhere it is cold enough for ice or snow but Sam makes sure its off if road conditions warrant it. ;D
 

Ron

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Karl,

On our coach the transmission selects 4th as the target gear.  It does not shift until the coach is slow enough to allow it to start shifting down.  Some coachs are set up so it will target 2nd but again it only shifts down to the gear approprite for the speed traveling. IOW going 60MPH it wont shift until you are slow enough for the trans to shift to 5th, then on down as speed decreases.  Transmission will protect from over reving the engine.
 

Ned

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The transmission will downshift as the computer allows. ?It won't let you overspeed the engine and it will shift down one gear at a time. ?The preselect only determines how far down it will shift. ?I could never understand why anyone would want to have the preset to 4th gear instead of 2nd, but some have had that done.
 

Ron

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Our coach came with it set to preselect 4th.  A couple coaches we drove had it set to preselect 2nd and we didn't really carone way or the other.  There has only been a few times when we manually preselected 3rd and I don't recall ever preselecting 2nd.
DSFDF ;D ;D
 

Karl

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Thanks all,
That clarifies that point but brings up another question. If the Pac brake is activated, can you still override the gear selection to pick an even lower gear? I realize that you may break the motor or tranny by doing that, but in an extreme emergency, I wouldn't hesitate tossing an anchor out the window!
 

Tom

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Karl

Ned described it the way I should have. Don't know the answer to your second question, and I probably wouldn't want to experiment  :)
 

Ron

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kkolbus said:
Thanks all,
That clarifies that point but brings up another question. If the Pac brake is activated, can you still override the gear selection to pick an even lower gear? I realize that you may break the motor or tranny by doing that, but in an extreme emergency, I wouldn't hesitate tossing an anchor out the window!

The transmission controller will only let the transmission downshift to a lower gear when it is safe to do so without over reving the engine.  One of the transmissions responsibilities is to protect the engine from an overspeed condition. 
 

Ned

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Karl,

In a word, NO.  You can tell the transmission anything you want, but if would cause the engine to overspeed, it won't do it.  If you get in that situation, you need to stand on the service brakes, but you shouldn't have gotten there in the first place :)
 

Ron

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Ned said:
Karl,

In a word, NO.? You can tell the transmission anything you want, but if would cause the engine to overspeed, it won't do it.? If you get in that situation, you need to stand on the service brakes, but you shouldn't have gotten there in the first place :)

Called anticipate driving conditions. ;D
 

BernieD

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kkolbus said:
Thanks all,
That clarifies that point but brings up another question. If the Pac brake is activated, can you still override the gear selection to pick an even lower gear? I realize that you may break the motor or tranny by doing that, but in an extreme emergency, I wouldn't hesitate tossing an anchor out the window!

Karl

Actually you can. When your tranny is "seeking" say 4th gear and you want it to continue braking, you can push the gear selection button down to 3rd or 2nd. The engine won't downshift right away, just wait until the engine speed is low enough for the next gear. The Allison is a very smart transmission, it is very hard to do anything that will hurt it :)

As to the preselection of gear when the brake come on, that seems to work with the size of the engine. My UltraSport had an exhaust brake (PacBrake) and it preselected 2nd. The TS has an engine brake and much more hp and it preselects 4th. I thought I would miss the 2nd gear preselection when we got the TS but it turns out 4th works fine for the engine
 

Gary RV_Wizard

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There is another class of extra braking system not yet mentioned - the transmission retarder.  These are neither exhaust or engine driven; instead they work by impeding (slowing) the revolution of the driveshaft  A few retarders may  use straight-forward friction, essentially a drum brake on the drive shaft, but the most popular type (the Telma Retarder)  is electro-magnetic and slows the shaft via magnetic field action.  Allison also offers a retarder integrated into some of their transmissions. I don't know if it can be retrofitted after the factory or not.

See Telma Retarder
 

Ron

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I think one of the forum members maybe John Wilson had a Telma retarder.  However I have only seen one in Erics display booth at QZ.  Do you have any idea what the cost is for the Telma.  Might be good information for some.  For us the Exhaust brake does fine.
 

Karl

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I did a little research and found that the Telma retarder uses 12V power to create, in effect , a locked rotor motor. The frame is mounted to the chassis and the rotor is mounted on the driveshaft. Heat generated is dissipated by the frame, and it can be moved to a different vehicle. Not sure how it's controlled, but suspect a mechanical hook-up to the braking system or maybe a pressure sensor on the hydraulic or pneumatic brake line. There is another kind of retarder also - a type of torque convertor which is either integral to the transmission (I believe Allison uses this arrangement), or one mounted externally between the output shaft and the final driveshaft with the stator afttached to the frame. Either electrically or hydraulically operated and modulated, it uses (probably) ATF fluid or something similar and requires external cooling like a tranny cooler in front of the radiator and, thru hydraulic bypassing, it acheives a variable stall speed. 
 

blueblood

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We had these on our commuter buses going to/from Denver airport from Breckenridge. Used them coming down from Eisenhower tunnel into Frisco especially when road conditions were marginal. Initiated from a lever on the steering column as I recall. 
 

Gary RV_Wizard

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Industrial Automatic (Telma dealer in my link above) says $5500-$6500, depending on the specific motorhome.  More expensive than a PAC Brake or similar turbo-exhaust-type brake, which is probably why we don't see many of them.  Competitive with Jake or other engine brake, though.
 
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