Slowing down your truck & trailer while towing. - Diesel Truck Forum - TheDieselGarage.com
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post #1 of 25 (permalink) Old 12-23-2008, 05:34 PM Thread Starter
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Slowing down your truck & trailer while towing.

Like the big rigs (Class8) all diesel trucks have an inherent lack of stopping power add 10,000lbs or more on a down grade and the truck will want to start to runaway, causing you to ride your service brakes. Riding your service brakes causes heat, brake fade and premature wear.

With the big rigs they use what is called an engine brake, these brakes work by opening the exhaust valve on the compression stroke, forcing the gases out of the exhaust thus causing the piston to return to the bottom of its stroke under its own lag. It is these gases being pushed out the exhaust that causes the burping noise you all hear as the truck is going down hill.

With the smaller displacement diesel engines (as in the Ford Powerstroke, Chevy/GMC Duramax, and Dodge Cummins Ram) an engine retarded or engine brake is not available due to the amount of compression and space developed.

This is where an exhaust brake comes in, used widely in the medium duty truck market (5 tons) Class A Motorhome’s tow trucks and such exhaust brakes are also available for the personally use vehicles.

Unlike an engine brake an exhaust brake works by closing the exhaust gases off that are trying to escape the engine. This is done by placing a butterfly valve in the flow of the exhaust; once the brake is activated the butterfly closes to restrict the flow of exhaust. By restricting the flow of the exhaust no noise produced like and engine brake in the larger trucks. When you step on the throttle the exhaust brake shuts off and the engine returns to normal.

Each engine manufacture has its own set of backpressure specifications that exhaust brake manufactures must adhere too to ensure no engine damage can or will occur. The maximum back pressure limit is set by ether placing a hole in the butterfly or only partially closing the butterfly off at the engines maximum RPM.

Now with this being said all exhaust brakes are created equal at the maximum engine RPM’s. Unfortunately, with these types of designs, as the engines RPM’s decreases below1900rpms the Backpressure (holding power) fades away as the hole in the butterfly or partially closed butterfly do not change to compensate for the lose of retarding performance.

What sets some exhaust brakes apart from others is the ability to regulate the engines back pressure throughout the entire RPM Range. By regulating the back pressure the result is a longer duration of braking thus saving your service and trailer brakes from heat that can cause brake fade and premature wear. Add this to a vehicle with or without the tow haul mode and towing anything above 10,000lbs becomes a dream. The exhaust brake can be used even when not towing thus saving your brakes during every day driving.
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post #2 of 25 (permalink) Old 12-25-2008, 05:43 AM
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great and very useful info!!! Learned somethin I didn't already know about the exhaust brakes.

Is really is amazing how well this type of thing works!!! Innovative programmed it into my turbo on the towing tune I got and it's awesome! I would reccomend exhaust braking to anyone that tows much.

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post #3 of 25 (permalink) Old 12-25-2008, 05:51 AM
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I also have the turbo brake tune on my truck and it is amazing how good it works.

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post #4 of 25 (permalink) Old 12-25-2008, 07:39 AM
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Is anyone even making an exhaust brake for 6.4'S?

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post #5 of 25 (permalink) Old 12-25-2008, 11:40 PM
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Anyone make an effective exhaust brake for older mechanical DT466's?
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post #6 of 25 (permalink) Old 01-02-2009, 08:45 PM
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Diesel Engine "Compression Braking"...

EDIT: My statement at the end of this post about a Jake brake is totally wrong, and some of what I say concerning compression braking is also mostly incorrect as well. I plan on doing a corrected post on this subject!!!

Quote:
Originally Posted by pacbrake-harold View Post
...all diesel trucks have an inherent lack of stopping power... With the big rigs they use what is called an engine brake, these brakes work by opening the exhaust valve on the compression stroke...
I think there's some misconceptions concerning these two statements. Now don't get me wrong here I'm a firm believer in exhaust brakes for diesels and I towed a 21K GCW for 10 years using a US Gear exhaust brake on my F350 and now I've got a Pac-brake which was OEM on my Freightliner and I'm towing a 32K GCW.

However there seems to be a basic misunderstanding about diesels not providing "compression braking". Diesels actually provide better "compression braking" than gassers do because diesels ingest more intake air while coasting in gear than gassers do since diesels don't have a closed throttle butterfly blocking their intake manifold. Then diesel's compress this extra amount of air using a higher compression ratio than for gassers, and this takes more work which at a given RPM produces a larger braking HP on the crankshaft.

The "Physics" of this above description of diesel compression braking HP is illustrated in the graph below which shows the compression stroke of my CAT C7 at 2,200 RPM under conditions of coasting in gear with no load on the engine so the BP=0 psi and the initial cylinder fill conditions at BDC of the intake stroke are determined by the nominal atmospheric pressure of 14.7 psi.

As indicated by the "blue" HP curve the higher PS=Piston Speed at 2,200 RPM causes an instantaneous peak piston braking HP of 130.8 HP per piston at 20* BTDC, and the average piston braking HP over the 180* compression stroke is 36.8 HP per piston, and this results in a total continuous average flywheel braking HP of 55.2 HP which is the HP required to overcome the pumping loss of the compression strokes.

Now 55.2 HP doesn't sound like a lot of braking HP, but that's 18.4% of my maximum 300 HP under full load, and I can easily judge the relative amount of inherent diesel engine compression braking by coasting in neutral and getting none, coasting in gear with my exhaust brake off and feeling the 55.2 HP of inherent compression braking, and applying my exhaust brake which adds additional pumping loss to the inherent compression breaking.



I'm 99% sure that the Jake brakes used on big rigs don't "work by opening the exhaust valve on the compression stroke" as stated. I think a Jake brake converts a diesel engine into an even better energy absorbing air compressor by modifying the valve timing to keep the exhaust valve closed much longer than is normal during the "exhaust stroke", and then high pressure oil is used to activate a slave piston to pop open the exhaust valves near TDC of the exhaust strokes to release the compressed air into the exhaust manifold. This in effect allows for two compression strokes, and by releasing the trapped air at the end of this second "pseudo" compression stroke you avoid the "spring back" effect and this gives even more compression braking!

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Last edited by ernesteugene; 01-06-2009 at 10:16 PM. Reason: Brain fart!
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post #7 of 25 (permalink) Old 01-03-2009, 01:25 AM
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Jake brakes(and Pac brakes) aren't nearly that complicated. There is no adjustment of valve timing. They are a simple oil actuated piston that pushes down on the exhaust valves. This will also open the exhaust valves at /near TDC of the compression stroke.
The reason for doing so is to release the compressed air from the cylinder before it can start pushing down on the piston. That way the engine uses energy to compress the air, but does not gain energy from the compressed air pushing on the piston.
Mercedes motors use an even simpler system for their engine brakes. They have an extra valve in the head, and it is held open by air or oil pressure whenever engine braking is desired.


Quote:
Originally Posted by ernesteugene View Post

I'm 99% sure that the Jake brakes used on big rigs don't "work by opening the exhaust valve on the compression stroke" as stated. I think a Jake brake converts a diesel engine into an even better energy absorbing air compressor by modifying the valve timing to keep the exhaust valve closed much longer than is normal during the "exhaust stroke", and then high pressure oil is used to activate a slave piston to pop open the exhaust valves near TDC of the exhaust strokes to release the compressed air into the exhaust manifold. This in effect allows for two compression strokes, and by releasing the trapped air at the end of this second "pseudo" compression stroke you avoid the "spring back" effect and this gives even more compression braking!
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post #8 of 25 (permalink) Old 01-03-2009, 03:20 AM
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Quote:
Originally Posted by tc2 View Post
... will also open the exhaust valves at /near TDC of the compression stroke..
It seems to me that if the compressed air gets expelled at the top of the normal compression stroke that there'd be little air left to compress during the exhaust "pseudo" compression stroke, and that you'd get more compression braking by reusing the air from the compression stroke before letting it escape on the following exhaust "pseudo" compression stroke. Yes there's a "spring back" effect if you don't release the air but a lot of the energy used to generate the "heat of compression" is absorbed and only some is left over to push the piston back down.

It also seems that if too much air is released at the top of the normal compression stroke you might even get a reverse flow of dirt into the cylinder during the exhaust stroke? I know several truckers have commented that the oil needs to be changed more often with extensive use of their Jake brake.

The 3 graphs below show why the Mack 18 wheeler definitely needs a Jake and why the F350 and Freightliner can get by with an exhaust brake. Note that at 60 MPH the 6% grade braking HP required to maintain a steady speed for the F350 is 200 HP but the aero drag is 100 HP so you only need a 100 net braking HP! Also note that the 50 to 60 MPH speed is the worst case and at over 90 MPH you don't need any brakes at all! If you want to climb a 6% grade at 60 MPH you need 300 RWHP!

The Freightliner requires 310 braking HP but it only gets back 125 HP from aero drag so it needs a net of 185 HP, and the Mack requires 760 braking HP but it only gets back 200 HP from aero drag so it needs a net of 560 HP.






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post #9 of 25 (permalink) Old 01-03-2009, 02:42 PM
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Quote:
Originally Posted by ernesteugene View Post
It seems to me that if the compressed air gets expelled at the top of the normal compression stroke that there'd be little air left to compress during the exhaust "pseudo" compression stroke, and that you'd get more compression braking by reusing the air from the compression stroke before letting it escape on the following exhaust "pseudo" compression stroke. Yes there's a "spring back" effect if you don't release the air but a lot of the energy used to generate the "heat of compression" is absorbed and only some is left over to push the piston back down.
The compression brake turns the engine into a big air compressor. It pulls in air like it normally would to run. Once the air is compressed, the exhaust valve is opened right before the fuel would be injected. The air being compressed is what is slowing the engine down.



Quote:
Originally Posted by ernesteugene View Post
It also seems that if too much air is released at the top of the normal compression stroke you might even get a reverse flow of dirt into the cylinder during the exhaust stroke?
Nope. There is valve overlap as well. The intake is starting to open when the exhaust valve is starting to close to help scavenge the cylinder.

Quote:
Originally Posted by ernesteugene View Post
I know several truckers have commented that the oil needs to be changed more often with extensive use of their Jake brake.
On an old mechanical motor, maybe. But from fuel, not dirt.

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Last edited by bmoeller; 01-03-2009 at 02:45 PM.
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post #10 of 25 (permalink) Old 01-03-2009, 04:30 PM
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Pacbrake Harold,

How much braking HP can you get me for an older DT466? From what I've read, it's not enough to be worthwhile, otherwise I'd buy one in a heartbeat.

Last edited by Duke; 01-03-2009 at 06:27 PM.
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