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Topic: A twister for thinking . ISX 650 Comes with a Jacobs Brake (Read 1092 times) previous topic - next topic

Re: A twister for thinking . ISX 650 Comes with a Jacobs Brake

Reply #25 –
Quote from: Rusty Socket –
I just looked up the specs on the ISX 650
There are 600 HP available for braking.

The way we operate our trucks is when descending a hill the jake switch is in full (position 3), the transmission is in lockup (there is a switch to keep it locked up manually).
The driver will start down the hill in the gear selected and will apply the retarder via the foot pedal.  While he is descending he keeps an eye on the trans temp and when the temp starts to get up into the crispy zone, he will release the retarder and the jake will automatically cut in.  Once the trans temp drops back down he will engage the retarder again with the foot pedal and the jake cuts out.

Now these are big trucks designed for long steep slopes.  The trans coolers used to be cooled with engine coolant, much the same as your MH's, but most have been converted to different system to avoid the dreaded glycol in trans problem.

The retarder put the heat into your transmission and the jake puts it into your engine, either way you are going down hill so your not putting heat into your engine from fuel.

By swapping back and forth between retarder and jake the driver is able to keep his trans temp in the operating zone.  The retarder in these trucks is a little different.  They are designed to hold back way more than the ones on a motor home but the idea is still the same.

I don't really see any reason why a motor home that has both jake and retarder could not be hitched up the same way.

I would think that with 600 braking horse in a motor home on jake, that you would not even need the retarder.
Just my opinion ;)

I like the operational design.  Well thought out.  Not the first rodeo
"Riding and rejoicing"
Bob
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4

Re: A twister for thinking . ISX 650 Comes with a Jacobs Brake

Reply #26 –
Quote from: Rusty Socket –

The retarder put the heat into your transmission and the jake puts it into your engine, either way you are going down hill so your not putting heat into your engine from fuel.



Sorry, but incorrect.  A Jake brake= engine compression brake does NOT put heat in the engine or engine cooling system.  There is ZERO fuel injected with the throttle closed/engine compression brake on. No fuel, not extra heat.

An engine compression brake works as follows:

The pistons does all the "work" of compressing 17 or so volumes of air into one (a lot of work).  Multiply by number of revolutions divided by two (4 stroke engine) times number of cylinders.  OK-- a LOT of HP needed.

Now if the engine compression brake is not on, but throttle closed, as soon as the piston passes TDC, all that compression pushes the piston back down-- so most of that "stored energy" is returned.  Therefore little braking.

BUT, when the engine compression brake is turned on, as the piston comes to TDC (with all that stored energy), the exhaust valve is opened, and that stored energy is  sent out the exhaust system.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: A twister for thinking . ISX 650 Comes with a Jacobs Brake

Reply #27 –
Yes Brett
But when you compress air, what happens to it.
On a diesel engine, you don't have spark plugs but rely on the heat of the air in the cylinder that has been compressed by a ratio of, oh I don't know for sure but lets just pick a number of say 17:1.
The air gets hot enough to ignite the diesel fuel as it is injected in to the cylinder

The dynamic energy of forward motion is reduced by the jacobs brake by turning the engine into a large compressor, so the energy is absorbed by the engine and removed via the coolant.

I agree that there is no where near the heat that is produced by the engine when fuel is added, but you are still creating heat
Foretravel owner wannabe

Re: A twister for thinking . ISX 650 Comes with a Jacobs Brake

Reply #28 –
No question, the engine/compression has to follow the Ideal Gas Law (PV=nRT).  But the heat produced, particularly since the compressed and therefore hot combustion chamber contents are sent out the exhaust is minuscule compared with that generated by burning diesel fuel.

So, I agree, you are right theoretically, but question that anyone operating an engine compression brake actually saw a coolant temperature rise using one.

Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: A twister for thinking . ISX 650 Comes with a Jacobs Brake

Reply #29 –
I'll make a quick apology here to Brett

For some reason I had exhaust brake stuck in my head (I'm blaming several sleepless nights)

With and exhaust brake, more heat is generated then a proper jacobs brake.

My bad
Foretravel owner wannabe

Re: A twister for thinking . ISX 650 Comes with a Jacobs Brake

Reply #30 –
I reported in an earlier thread that at the Ladies Driving Class, a technical told me that the FT comes already with the retarder and the jake brake install, so to speak, available.  He did not say it only the ISX.  I also said that I may have misunderstood and one of the Forum members, per out PMs, I think was going to check on it.

Anyway, the FOT fellow told me that a FT owner in Canada had both activated, that is usable.  For mountains.  I did not hear that the owner could or would activate both at the same time, and perhaps the intent was/is to select between the two depending the conditions.

If the other Forum member did not check this out and put in better information than I got in this casual conversation, I would be glad to go back and see if I misunderstood.  Some member said they doubted both would be fully installed already due to extra expense.

The whole thing seemed simply a curious answer when I asked the FOT fellow about his opinion of jake vs retarder.  But now I read  this is in the ISX.  Maybe really in some others?  Not really sure it good to have though, retarder seems great.
Mike
2001 U320 4010 Build 5878 (Gus)
Wrangle Unlimited Toad
Nacogdoches
 

Re: A twister for thinking . ISX 650 Comes with a Jacobs Brake

Reply #31 –
Rusty and Brett,

So, with the Jake, the air is compressed by the piston on the compression stroke until it reaches somewhere around 600 degrees and just before top dead center, the Jacobs Brake opens either one, two or four exhaust valves in each cylinder. Since most of the air then goes out the exhaust valve(s), no energy is transferred to the crankshaft on the downstroke so a braking effect occurs.

Since the Jake's effectiveness increases with the RPM, it stands to reason the most advantageous way to use it is at higher RPMs. The water pump is also pumping more coolant at higher engine RPM so circulates the coolant more rapidly than at lower RPM. In other words, while the Jake may generate a little heat, the cooling system will be very effective in removing it.

Yes, I am the culprit in questioning whether Foretravel would install both a Jake and retarder on the same coach but not activate the Jake installation. In the case of the Detroit two cycle engines found on the U300, the Jake requires a taller valve cover for clearance. The retarder only valve cover can be identified by the raised letters, Detroit Diesel, a very handsome addition in my opinion. I wish our Jake equipped Detroit had these embossed covers in a taller version. Since the Jake is an expensive installation (not nearly as expensive as a retarder), I can't imagine a, "for profit company", installing it but not utilizing it. The wiring to the operator's station would have to be also installed at the time of manufacturer as a later installation would be much harder and more expensive. If any member with a retarder can find evidence of a Jake installation but not functional in their coach, please post it.

If a Cummins or CAT has a Jake installed, the wiring can be seen leading under the valve cover.

Pierce
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)