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 > Diesel Engine Auxiliary Brake Discussion.... Pac and Jake Type

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JohnnyT

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Posted: 08/02/04 02:02pm Link  |  Print  |  Notify Moderator

Diesel Engine Auxiliary Brake Discussion.

To many what will follow is old news. This post is intended for folks new to DP ownership... There are a lot of individuals on this forum with far greater expertise than I, so do not hesitate to raise issues or questions...

Additional Braking beyond the service brakes is needed for diesel powered motorhomes since unlike a gasoline engine the air intake is not restricted when the throttle is released so the air intake being un restricted on a diesel engine gets compressed as the intake valves are closed and the piston travels up to the top of the cylinder This compressed air when it is not released creates a spring effect forcing the piston back down... On a gasoline engine when the throttle is released the air intake gets severely restricted which created a vacuum like effect resulting in braking action to coin a phrase by tyoungs in his postings on this issue "the gasoline engine is tying to suck air through a straw" when the throttle is closed.


Motorhomes with diesel engines will generally come equipped with an exhaust brake such as a Pac Brake which is basically a baffle to retard the escape of exhaust gases or an Engine compression brake which otherwise know as a Jake brake, either of which are installed on diesel coaches to compensate for the natural engine braking action missing from a diesel in comparison to a gas engine. Either Brake system will also work in conjunction with the transmission which will automatically gear down as RPMs permit... to reach a preselect gear... On our coach that gear is 2nd... But it can be reprogrammed for another gear..

The Jake (engine compression) type brake will deliver a consistent braking force down to a preset MPH which on our coach is set for 15 MPH.

The Pac ( exhaust) type brake will deliver the most braking force when the rpm is high since the backforce is greater the higher the rpm...

For either brake system, most recommend to let the ECM control the gear selection as it will not over rev the engine. Manual gear selection is of course possible but caution is advised since one could over rev the engine or damage the torque converter in the transmission...



The pack type brake or exhaust brake creates the "straw" effect by closing off the exhaust to the air trying to be released is therefore severely limited in escaping and thus serves to create a braking action...The components of an exhaust brake is basically an air operated actuator that controls a baffle that has been inserted in the exhaust pipe.

The operation of the Jake brake basically turns the engine into an air compressor.. By opening the exhaust valves as the piston reaches the peak of the compression cycle..and preventing the spring affect.. For explanation purposes the components of a Jake brake are a solenoid with an attached control valve and two pistons, a master and a slave with a check valve between them.. All of this is generally inside the valve cover including the solenoid. When the solenoid is energized the solenoid control valve permits regular engine lube oil to flow through the control valve to both the master piston and the slave piston. The Oil pressure causes the master piston to move down, coming to rest on the injector arm clevis as the injector rocker arm clevis begins upward travel (as in normal injection cycle) forcing the master piston upward which causes the oil to become highly pressurized which then directs the high pressure oil to the slave piston. A ball check valve imprisons high pressure oil in the master-slave piston system. This high pressure oil causes the slave piston to move down, momentarily opening the exhaust valves, while the engine piston is near top dead center position, releasing compressed cylinder air to the exhaust manifold.Compressed air escapes to atmosphere completing a compression braking cycle..

Jake brakes can be single stage using all cylinders or 2 stage where the first stage will only affect half of the cylinders and the second stage will utilize all cylinders. There are also 3 stage Jake's so on a 6 cylinder each stage will add two cylinders to the braking function...

Some coaches will also have transmission retarders which work in conjunction with the Engine brake...The transmission retarders is not just the transmission gearing down as rpm permits but an additional or modified converter utilized to enhance braking...

The bottom line of all this is that for coaches thus equipped in most situations going down a grade there will be no need to use the service brakes or in some instances significantly less application of the service brakes will be required.. For safety sakes any time the engine brake is activated the brake lights on most coaches will be illuminated... In fact in normal driving many drivers will use the engine brake in almost all cases to slow the coach just using the service brake to accomplish the final stopping action... Most motor home engine brakes are not very noisy by comparison to the sound that big trucks make since the engine brake on most motor homes is muffled... Barely audible from inside the coach... The Pac (exhaust) type brake will have sort of a hissing sound and the Jake (engine compression) type brake will have a low rumble sound...

One other point, even though when the auxiliary brake is activated it operates the brake light, the dingy supplemental brakes if installed and of the type that do not activate solely on the brake lights being activated are unaffected. The type dingy supplemental brake we use is only activated when the coaches service brake is applied... So in addition to saving wear and tear on the coaches service brakes, the dingy brakes also benefit...

Having driven coaches with both types of auxiliary brakes The best way to compare them is that the Jake brake with two stages set on stage one is very similar to the braking action provided by an exhaust brake.. The braking action on stage two is much more aggressive.. On coach thus equipped the switch will have a high and low setting..Since our coach is on the heavy side I would be reluctant to operate it without an auxiliary brake..


Since the Jake (Engine compression) type brake components are internal to the engine there are no routine maintenance items to be performed. However for the Pac (exhaust) type brake there is routine maintenance... The best advice I have seen on this was posted by wolfe10 "Pacbrake has a special high-temp lube. Apply it to the pivot for the activating cylinder, pivot for butterfly valve and along the shaft for the butterfly valve so it runs down into the brake housing." Other maintenance information can be found at www.pacbrake.com

JohnnyT

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Posted: 08/02/04 03:38pm Link  |  Print  |  Notify Moderator

Excellent work, Johnny. Very good explanation, should clear things up for those who wondered...

Steve


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Posted: 08/02/04 06:28pm Link  |  Print  |  Notify Moderator

Good job Johnny.

I was just on Pac and Jake's websites to learn more. Looks like each of them make both Engine Brakes and Exhaust Brakes. Am I reading it right?

Jerry


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JohnnyT

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Posted: 08/02/04 06:57pm Link  |  Print  |  Notify Moderator

I believe your are correct.. PacBrake, and Jake Brake both offer an engine compression brake and an exhaust type... I am not sure what the advantages are of one company over the other except that Jake is best known for their engine compression brake and PacBrake for their exhaust brake... We have a Cummins engine with the Jake Engine compression brake..

It would be interesting to know about any experience anyone has with the PacBrake engine brake application in a motorhome.. I do know that there are some motorhomes that do come with the Jake exhaust brake...I am just not sure which ones...

Johnny

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Posted: 08/02/04 07:14pm Link  |  Print  |  Notify Moderator

JohnnyT:

Great info. You may have to elect some input from some of the smarter people on the BB here and maybe make a sticky about this sort of thing.

Im under the impression that the Jake Brake is FAR superior to the Pac Brake in actual braking...but that a lot of engines cant handle the stresses involved and cant use a Jake Brake device.

JanLiz

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Posted: 08/02/04 07:42pm Link  |  Print  |  Notify Moderator

Thank you for the great explanation. If we could touch on cruise control use with the exhaust brake, that would be appreciated.


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Posted: 08/02/04 07:45pm Link  |  Print  |  Notify Moderator

Pac brakes work better on some engines than others. Example, a Cat 3126 can withstand back pressure of 55 PSI, but my 3208TA will only withstand 25PSI. I can only assume that my system does not provide as much braking as the 3126. I feel very little help from the system, and I would think that the low amount of back pressure a 3208 can withstand is the reason. Do you agree? Thanks. PS. I have a Pac-brake chart on engines should anyone wish to learn more on their engine.

dleslie125

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Posted: 08/03/04 09:18am Link  |  Print  |  Notify Moderator

Does anyone know what impact an exhaust brake has (or can have) on fuel consumption? Comments on MPG by owners of similar units vary all over the map. I've been wondering if some of the difference might be due to braking habits. I'm new to a DP and have been using my exhaust brake a lot (have it on almost all the time and often I don't have to even touch the service brakes until I'm very close to to a stop light). Am I paying a big or small fuel consumption penalty as a result of the higher rev's to slow it down?

Don

ps. Great thread by the way. Thanks Johnny.


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Steve Val

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Posted: 08/03/04 07:41am Link  |  Print  |  Notify Moderator

Quote:

Thank you for the great explanation. If we could touch on cruise control use with the exhaust brake, that would be appreciated.


Your cruise control should override the Pac brake on your rig, and not engage until you hit the brakes to disengage it. Then it will work normally. If you have any doubts, just try it out - you can't hurt anything, or check out your manual -the info will be in there as well.
Congrats on the new rig...

Steve

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Posted: 08/03/04 08:19am Link  |  Print  |  Notify Moderator

Johnny, this is one of those that should be locked to the front, ahead of the microwave recall.

SUPER !


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