OT: why do trucks need exhaust brakes? - Page 2 - PeachParts Mercedes-Benz Forum

Stevo · #16 07-03-2005, 12:03 AM

my new too me, 94 Dodge PU with the Cummins has a jake brake which was added by the PO, really saves on the brakes when towing.

ZackaryMac · #17 07-03-2005, 12:16 AM

Quote:

Originally Posted by P.E.Haiges

Bonehead,

"diesels are not throttlerd" and their high compression ratiois is just why Diesels engines have more engine braking than a gasoline engine. If you want a gasoline engine the have more engine braking, turn off the ignition and push the throttle to the floor. Then it will be running unthrottled like a Diesel engine and will have more engine braking.

Don't believe me, try it for yourself.

P E H

What I think he meant was, diesels do not use a throttle plate to restrict intake air, like a gasoline engine. This restriction in a gas engine is what creates vacuum in the intake manifold, and also makes the engine decelerate better with rpm under no throttle, as it tries to draw air into the cylinders but can't because of the throttle plate. Also, good intake valve seals are a must or you will suck oil down past the valve guides.
Diesels don't have this restriction, and compression by itself is not enough, hence the use of various engine brakes.

#18 07-03-2005, 02:15 AM

Quote:

Originally Posted by Stevo

my new too me, 94 Dodge PU with the Cummins has a jake brake which was added by the PO, really saves on the brakes when towing.

Actually, what you have is an exhaust brake. Jakes in the manner being discussed here, have never been available for the 5.9 Cummins engine. The Cummins is actually too small for a jake to work efficiently.

TonyFromWestOz · #19 07-03-2005, 07:23 AM

Quote:

Originally Posted by boneheaddoctor

Diesels are not throttled....thats why they really don't have any engine braking you will have in a gasser.

SI engines, which ARE throttled have significantly less engine braking than diesel engines of the same capacity, which are NOT throttled.
The reasons for the greater engine braking on diesels are that the higher compression of diesels (20 - 25:1) Vs SI engines (8 - 10:1) and that the throttling of the air charge reduces the inlet pressure (manifold vacuum) and there is much less air to compress in SI engines.

It takes more energy to compress the intake air to 1/22nd of it's volume than it takes to compress a throttled air charge in the SI engine to 1/10th of it's volume.
And when you restrict the exhaust flow significantly (exhause brake), there is little expansion of the air, which could otherwise recover some of that energy.

P.E.Haiges · #20 07-03-2005, 08:30 AM

Zackary,

I know what Bonehead meant and he is wrong. Read the JimH and TonyfromOZ explanations of how a Jake Brake works. The more air you get into the engine, the more braking power it has. Even non Jake Brake engines have some braking because of friction in the air some of the heat of compression is lost to the cooling system before the compressed air pushes the pistons back down. And with a Jake brake you get a compression stroke every turn of the crankshaft instead of every other stroke in the normal 4-cycle operation.

If someone here knows how the valves are controlled by the Jake break system, I would be interested how the system works.

P E H

#21 07-03-2005, 08:42 AM

Bolting On Hardware
In the predominant inline six-cylinder diesel, the compression brake housing is bolted atop the cylinder head and its mechanism connected to the engine's valvetrain. Engine oil is the actuating force that engages or disengages the valve action-altering function. Cummins' Signature and ISX engines have two overhead camshafts, one with cam lobes dedicated to the engine brake. This integrated brake claims more retarding power than previous designs, and can itself slow a heavy rig on a steep descent.
Some engines have two-piece heads, with each covering three cylinders, and others three-piece heads, two cylinders per. The compression brake therefore can be made to operate the valves on one, two or three of the heads, and therefore two, four or six cylinders with a three-piece head, or either three or six cylinders with a two-piece head.
The power level can be picked by the driver. He flips one toggle switch to turn the retarder on, then uses another switch to choose between "2-cyl," "4-cyl" or "6-cyl," or between "high" and "low," and gets more or less retarding force. Once on, the retarder will operate every time he takes his foot off the accelerator. It will cut out when rpm's drop toward idle. An exception is a Cat engine; with it the driver has to apply the service brakes to activate a Jake Brake.
Any retarder's output is measured in retarding horsepower. It's possible for a diesel's retarding horsepower to equal its propulsion output, but generally only at higher rpms. As you'd imagine, more power is produced as the engine speeds up and works harder to compress air.
Conversely, retarding force decreases as the engine slows. So drivers will try to choose a transmission gear that keeps revs close to or at the engine's redline for the desired road speed. With the advent of lower-rpm diesels, manufacturers have had to refine their compression brakes to maintain sufficient output.
The Other Types
While the engine compression brake is the principal type used in heavy duty diesels, there are several others:
• Engine-hydraulic — The only one is Caterpillar's BrakeSaver, which bolts onto the rear of the engine ahead of the bell housing. Inside, motor oil is churned between a rotor and stator, creating braking effect. It is less dependent on engine speed than the compression brake, but is not as strong. The BrakeSaver is operated manually with a handle or automatically by releasing the accelerator.
• Exhaust brake — Imposes back pressure in the exhaust system by closing a valve at some point on the exhaust pipe. The valve is actuated by a solenoid or by air on vehicles with an air system. These will vary in power, but those on big-bore engines can be surprisingly effective, especially at high rpm's.
Exhaust brakes can be hung on or near the turbocharger, or downstream a ways. If on the engine, they can work with glow plugs to help speed the engine's warm-up process on cold mornings.
Models for big-bore diesels are offered by Blue Ox and PacBrake, and one Pacbrake model is a combination exhaust and engine brake. Volvo includes a combination exhaust-and-engine brake on its 12-liter Swedish-built diesels sold in Volvo trucks. Several types are available for midrange diesels and are popular on motor homes. Exhaust brakes are standard on some imported midrange diesel trucks.
• Driveline retarder — Installed on the driveshaft between the transmission and rear axle. A hydraulic type absorbs the truck's momentum energy by pumping oil, while the electromagnetic type sets up a magnetic field that grabs at the driveline. Absorbed energy is transferred to the surrounding atmosphere as heat. Switches usually control actuation on modern exhaust brakes and driveline retarders.
No compression brakes are made for midrange diesels, so exhaust brakes and driveline retarders are it for medium-duty trucks. These devices are likewise the only viable possibility for heavy vehicles running at low speeds in stop-and-start service, such as city buses and trash trucks.
Voith, for example, markets its hydraulic retarder to refuse truck operators. Telma has offered an electric retarder, and one of its advantages is that it can also be applied to axles on trailers. But its comparatively high weight has kept it from wide success.
In Europe, driveline retarders have been more popular; some countries require heavy trucks to use some kind of retarder for safety's sake.

http://www.heavydutytrucking.com/2001/12/044a0112.asp

Stevo · #22 07-03-2005, 09:51 AM

Thanks for the great write up though it will take a while to digest. They dont have jake brakes or exaust brakes on marine engines so this is all uncharted waters

TonyFromWestOz · #23 07-03-2005, 10:22 AM

Quote:

Originally Posted by Stevo

Thanks for the great write up though it will take a while to digest. They dont have jake brakes or exaust brakes on marine engines so this is all uncharted waters

Engine braking on marine engines would not be useful, there is no DIRECT transmission to a solid material. The water passing a "stalled" propellor, or one turning significantly slower than the speed required for negligible drag, provides similar retardation to the craft, but not much torque on the engine crankshaft. As there is a fluid medium involved, there is no "engine braking" involved, or possible.
The only means of increasing the drag, is to reverse the rotation of the prop to push water forwards.

I hope that this explanation is comprehensible.

Stevo · #24 07-03-2005, 10:38 AM

"Engine braking on marine engines would not be useful"

Auh damn..there goes my great idea of something to sell to the "yachties" like bow thrusters for little cabin cruisers

Stevo · #25 07-03-2005, 10:59 AM

Brandon

Would you mind if I posted your write up over on the Cummins forum http://www.turbodieselregister.com/forums/forumdisplay.php?s=&forumid=68
I'm sure some folks new to Cummins, like myself, would be interested. I assumed all "jake brakes" were exaust brakes.

boneheaddoctor · #26 07-03-2005, 12:30 PM

Quote:

Originally Posted by P.E.Haiges

Zackary,

I know what Bonehead meant and he is wrong. Read the JimH and TonyfromOZ explanations of how a Jake Brake works. The more air you get into the engine, the more braking power it has. Even non Jake Brake engines have some braking because of friction in the air some of the heat of compression is lost to the cooling system before the compressed air pushes the pistons back down. And with a Jake brake you get a compression stroke every turn of the crankshaft instead of every other stroke in the normal 4-cycle operation.

If someone here knows how the valves are controlled by the Jake break system, I would be interested how the system works.

P E H

Try driveing two GM Full size trucks one with a diesel and one with a gasolene engine....you will see the point I was making..........

Brian Carlton · #27 07-03-2005, 01:17 PM

Quote:

Originally Posted by TonyFromWestOz

SI engines, which ARE throttled have significantly less engine braking than diesel engines of the same capacity, which are NOT throttled.
The reasons for the greater engine braking on diesels are that the higher compression of diesels (20 - 25:1) Vs SI engines (8 - 10:1) and that the throttling of the air charge reduces the inlet pressure (manifold vacuum) and there is much less air to compress in SI engines.

It takes more energy to compress the intake air to 1/22nd of it's volume than it takes to compress a throttled air charge in the SI engine to 1/10th of it's volume.

This is correct with one slight addition:

On a diesel engine, you are compressing air at atmopheric pressure (14.7 psi absolute), so you get the effect of 22:1 compression.

With a gasoline engine at closed throttle, you are compressing air at significantly less than atmospheric pressure (estimated at 4 psi absolute), so the effect of 8:1 compression is effectively reduced by the differential of manifold pressure (4/14.7). The resulting braking force in an equivalent sized gasoline engine is therefore approx. 10% of the braking force in the diesel.

As PEH has mentioned above, the gasoline engine would provide far better braking force if the throttle was open and a full charge of air (14.7 psi absolute) was allowed to fill the cylinders.

This should be quite obvious to all the diesel drivers. We typically don't need to use the brakes much, when the gassers are burning the brake pads at every opportunity.

boneheaddoctor · #28 07-03-2005, 01:46 PM

Quote:

Originally Posted by Brian Carlton

This is correct with one slight addition:

On a diesel engine, you are compressing air at atmopheric pressure (14.7 psi absolute), so you get the effect of 22:1 compression.

With a gasoline engine at closed throttle, you are compressing air at significantly less than atmospheric pressure (estimated at 4 psi absolute), so the effect of 8:1 compression is effectively reduced by the differential of manifold pressure (4/14.7). The resulting braking force in an equivalent sized gasoline engine is therefore approx. 10% of the braking force in the diesel.

As PEH has mentioned above, the gasoline engine would provide far better braking force if the throttle was open and a full charge of air (14.7 psi absolute) was allowed to fill the cylinders.

This should be quite obvious to all the diesel drivers. We typically don't need to use the brakes much, when the gassers are burning the brake pads at every opportunity.

I would have to disagree with that...and my brothers Chevy truck with a small block chevy has far more engine braking than my 6.2 equiped truck....

Also my 1.6 L Honda or my 1.9 saturn has far more engine braking than either benz or my truck....by a huge margin...

I have to use my brakes far more on long downgrades with any of the diesels I own than any gasolene powerd I have ever owned. I only needed brakes on the very steepest of hills to keep my speed down on the gassers I have owned.....on any of the diesels I need the brakes on even moderate long downgrades to keep speeds down. I grew up on the foothills of the Appalacians....so I have spend a large portion of my time either going up or going down one hill or other. True they aren't the Rockies.....but they are high enough to make it an issue.

Brian Carlton · #29 07-03-2005, 02:13 PM

Quote:

Originally Posted by boneheaddoctor

I would have to disagree with that...and my brothers Chevy truck with a small block chevy has far more engine braking than my 6.2 equiped truck....

Also my 1.6 L Honda or my 1.9 saturn has far more engine braking than either benz or my truck....by a huge margin...

The reason that your examples may disagree with the physics are as follows:

The comparison between two different vehicles can only be done fairly if all of the following parameters are identical between the two vehicles:

------weight
------intermediate gear ratios
------final drive axle ratio
------transmission (auto vs. auto or manual vs. manual)

Any other comparison is meaningless and can result in reaching a conclusion that defies the engineering principles upon which the aforementioned post is based.

boneheaddoctor · #30 07-03-2005, 03:10 PM

Quote:

Originally Posted by Brian Carlton

The reason that your examples may disagree with the physics are as follows:

The comparison between two different vehicles can only be done fairly if all of the following parameters are identical between the two vehicles:

------weight
------intermediate gear ratios
------final drive axle ratio
------transmission (auto vs. auto or manual vs. manual)

Any other comparison is meaningless and can result in reaching a conclusion that defies the engineering principles upon which the aforementioned post is based.

BOTH trucks have 4.10 geared axels...BOTH were non-overdrive automatics....both are the same trucks except one has a chevy name slapped on it and the other GMC.

Both Full size 4-wheel drive long beds regular cab..

the 5.7 liter Chevy V-8 gasser has far more engine braking than a larger 6.2 liter diesel in what is effectively identical trucks.

And also if the gassers have less engine braking argument was true why are engine brakes not made for gassers....just for diesels? lots of gasser trucks, lots more than diesels.

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