Why do trucks need exhaust brakes? My SDL won't run away down a 6% grade. Is it a 722 feature? Is there a built-in exhaust brake I don't know about? Is there some feature of the Bosch IP that Ford and Cummins IPs don't have (besides being able to go 300K miles without attention)?

Thanks,
Sixto
95 S420
87 300SDL

You car dosen't weight 80,000 to 100,000 lbs with drum braks to slow it down.

The Jake brake (or engine brake) lets the engine share part of that load in the mountains.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

http://or.essortment.com/jakebraketruck_raio.htm

"...With the fuel flow terminated, the upward moving first stroke still compresses the air to very high pressure. As we said above, this transfers mechanical energy into heat as the air becomes highly compressed. If nothing else were done, most of this energy would be recovered, except for frictional losses, as the cylinder moved back down and the compressed air expanded.

The Jake brake, however, opens the exhaust valve just as the air reaches maximum compression, dumping all of that energy in an almost instantaneous explosive release. The result is a very effective slowing of the vehicle as mechanical energy is converted to heat and then dumped. The Jake brake effectively transforms the internal combustion engine into an air compressor.

It has only one drawback: it is very noisy...

...Because it extends the life of wheel brakes and saves money, trucking companies generally lobby against the bans and some towns are compromising by allowing the Jake brake to be used in daylight hours. ..."

Those of us that drive in the mountains frequently you will see enough trucks over time with brakes on fire , smoking or gotten so bad they become runaways you would see a good reason to allow their use....get the brakes hot enough the drums swell and no longer hold the truck back.....then you have a 100,000 pound rollorcoaster with no hope of stopping. When the driver is lucky there is nobody in front of him and he gets to an upgrade to get stopped....when he isn't he loses control in a turn or plows through vehicles in front of him...

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Eisenhower Memorial Tunnel is located approximately sixty miles west of Denver, Colorado on Interstate 70 with a 7% grade going west.

That's why there is a 'ski ramp' the other side of the Eisenhower tunnel in Colorado.

You'll see three 'ski ramps' on the north side, going north from the Mont Blanc tunnel in France.

( Nearly had to use one of those.... )



.

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[http://languageandgrammar.com/2008/01/14/youve-got-problems-not-issues/ ]

"A liberal is someone who feels they owe a great debt to their fellow man, which debt he proposes to pay off with your money."

yeah if nothing else they just sound cool.

My old job had 2 "mountain" busses that had electric brake retarders and jake brakes. With those and gearing you could come down the mountains without touching the brakes. So cool.
-Nate

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'82 240D, 4-speed, 617 turbo engine

I've driven trucks with them, and without. IMO, the jakes should be mandantory on all big trucks. You're SDL has four nice disk brakes on four tires to slow a ~4,000 pound car. A big truck has 10 drum brakes on 18 wheels to slow down 80,000 pounds of truck, trailer, and load. Going down a 7% grade in a truck without jakes is a white knuckle experience while you tiptoe very slowly down the hill, hoping that you've picked the right gear to help hold the truck back, so that your brakes don't end up smoky charcoal at the bottom of the hill. Give me the same truck, down the same hill, at the same speed, in the same gear, with jakes, and I can ease on and off the service brakes only as needed to scrub off a few mph.

As for those signs, I never obeyed those stupid "no jakes" signs. IMO, keeping my truck, myself, and even more importantly, the other drivers that I was sharing the road with from becoming bent up metal at the bottom of the hill, was FAR more important than waking up some whining idiot stupid enough to buy a condo 15 feet away from an interstate right of way. Also, keeping a proper muffler in the exhaust system, instead of running straight pipes does alot to mitigate the jake brake bark.

When I had my car loaded up to probably around 5000lbs traveling through pennsylvania I had to keep the tranny in 3rd gear (S) 95% of the time both for the engine braking as well as the climbs....which were difficult even in third. Some of the grades were so steep that the engine/car would quickly accelerate down them even in 3rd I had to keep shaving off mph's with the brakes.....it was quite an experience, I did that for about 65 miles nonstop.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

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

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

I have owned several Mercedes L1117 trucks and they all had inline 6 cyl turbo diesel engines. They also all had a small 'exhaust brake' (basically a small air-actuated flap that blocks the exhaust I think) to help preserve the wheel brakes. They work very nicely and are not very noisy. Of course, these Mercdes trucks had a gvw of 25k lbs as opposed to 80k+ for the big rigs.

I am sure that the Mercedes 'tractors' have some type of 'jake' brake set up.

If I were doing much driving in Maryland, Penn, or other mountainous states, I would prefer the same 'engine flap' set up on my 300sdl. It isn't loud, doesn't get the brakes hot, and just works great on those long steep hills.

I've always been curious as to how jake brakes interrupt the normal valve timing. Something is pushing that valve in at a point in the cycle that is out of sync with the camshaft. I've seen pictures of the jake brake mechanism and it's not very big or complicated.

Anybody know?

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Te futueo et caballum tuum

1986 300SDL, 362K
1984 300D, 138K

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.

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Tony from West Oz.
Fatmobile 3 84 300D 295kkm Silver grey/Blue int. 2 tank WVO - Recipient of TurboDesel engine.
Josephine '82 300D 390kkm White/Palamino int.
Elizabeth '81 280E, sporting a '79 300D engine.
Lucille '87 W124 300D non-turbo 6 cylinder OM603, Pearl Grey with light grey interior


Various parts cars including 280E, 230C & 300D in various states of disassembly.

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

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.

As for those signs, I never obeyed those stupid "no jakes" signs. IMO, keeping my truck, myself, and even more importantly, the other drivers that I was sharing the road with from becoming bent up metal at the bottom of the hill, was FAR more important than waking up some whining idiot stupid enough to buy a condo 15 feet away from an interstate right of way.

Well put! Whats better: Waking up some yuppie and his ski bunny or having a pileup at the bottom. Screw 'em, hit the jake! RT

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When all else fails, vote from the rooftops!
84' Mercedes Benz 300D Anthracite/black, 171K
03' Volkswagen Jetta TDI blue/black, 93K
93' Chevrolet C2500HD ExCab 6.5TD, Two-tone blue, 252K