Engine Breaking Performance

[SlyBones]

I was on a 4hr bus ride today coming back from a 6 day company workshop, done off-site over at Lake Chelan. On the ride back several people got into the debate about engine breaking performance of diesel engines versus gas engines. I sat in the middle getting quite a kick out of this. And of course I added fuel to the fire to keep it going. ( hehehe, it was a long ride and I had a week without the TDR )



One side was talking about the fact when you let off the throttle the diesel does not slow down the vehicle as fast, and provides less engine braking power. Thats why we all buy exhaust brakes. And that if we stuff two equal trucks were both on a large grade with equal loads and all that the diesel would provide less peformance and resistance to gravity, etc. Ultimately accelerating faster than the gasser would. No technical babble just offering his RV experience.



The other side was sitting there claiming that in college they calculated all sorts of crap, and that since a diesel has 18:1 compression and a gasser 8:1 ( for example ) the diesel therefore had to have more engine braking performance simply because it has more compression. No experience, all babble about back when he was in college.



So... How about the experts here fill me in, so I can straighten these 2 guys out and maybe learn a little something myself.

 

[CCBrady]

SlyBones,

I just read an article about the PacBrake, it was writen for Camping World. It explaned that when you let off the accelerator on a gaser you close the butterfly cutting off the air and causing a vacuum on the cylinders, causing the engine to work like a brake. Where as when we let off the accelerator we cut off the fuel and leave the air free to flow, so that only the parts of the engine moving against each other is all the reistance we get. That is why we need the exhaust brakes to help us slow down. Hope this helps.

Chris

 

[HC]

I believe that what the "theoretical" fellow was missing is this:

Yes, the diesel does work more on the compression stroke, due to the higher compression ratio. (call that stroke #1 for this discussion) He got that part right. But with or without fuel, that compressed air still wants to expand again on the next stroke, so a lot of that energy is recovered, even if you don't squirt in any fuel and get a big bang (stroke #2). So you get very little net braking power. The exhaust brake adds resistance to stroke #3, the one that should be "easily" pushing out the last of the combustion gasses. That's where we're getting the increased braking power from.



Take that, plus what Chris said about vacuum in the gassers, and I believe you get the following in increasing order of braking resistance:

1) Diesel (least resistance)

2) Gasser (some resistance)

3) Diesel + exhaust brake (good resistance)

4) Diesel + true Jake Brake (valve timing changed, most resistance)



Either something like that, or I'm all messed up too.

Either way, RV man was right; wish I could have listened in too!

 

[SlyBones]

So how is it the true Jake Brakes do this? They are made into the cylinder head right?

 

[Doc Tinker]

Why not put a butterfly on the intake of the diesel, instead of in the exhaust?



Doc

 

[M Barnett]

The Jake Brakes that I know of aren't built in on big trucks. They are an add on accessory that goes on the head and when engaged, hold the valves closed.

Michael

 

[baby.driver]

Jake Brake

A Jake brake works from injector timing. When the piston comes up on the compression stroke, the exhaust valve opens at the same instant the injector sprays, then snaps closed again. The effect is that the work of compression is wasted and the piston draws a partial vacuum on what would have been the power stroke. They are very effective.



Jay

 

[Deezul1]

Gassers use the closed throttle butterfly to create vacum which makes the engine a vacum pump. Diesels have very little or no engine vacum do to open air flow so closeing the exhaust with a flapper turns the engine into a air compressor which slows the engine.

 

[illflem]

A Jake Brake is a device that mounts on the engine overhead. It changes the action of the exhaust valves, turning your engine into a giant air compressor. A Jake isn't an add on feature, it must be built into the engine



Read all about Jake brakes and exhaust brakes here and you'll be an expert http://www.jakebrake.com/askus/faq/faq.htm

 

[Sled Puller]

I thought you were going to talk about how much hp they would make before they BROKE!

 

[Utah Willie]

Engine braking performance

HC is dead on with his explanation. The added advantage of an exhaust brake is that it pressurizes the engine, I read somewhere... . 35 PSI. That maintains the oil distribution. The gasser, which when on deceleration, creates a vacuum which tends to draw oil up past the rings and valve stems... ... result: blue smoke from the exhaust of a worn engine. An advantage of the Jake (engine) Brake is that it can be set to 2, 4 or 6 cylinders on a six cylinder engine which gives a variety of braking force for the driver. The blast of fresh, heated air into the exhaust system is said to "scrub" the system as well. I also understand that an exhaust brake will work on a gasser... ...

 

[Steve M]

I just wish Jacobs would make an exhaust brake with comparable retarting HP as a gasser has with it's butterfly throttle body. That way I could have comparable engine braking with a gasser and not have to throw gobs of money at a transmission that works like a champ. I don't hear the gasser crowd complaining about burning up their trannies by using engine braking on the downhills nor do I hear them complaining about not having enough engine braking to keep thier rigs from running away from them. Seems like whatever meager retarding HP the gassers have is what we should be able to get out of ours so we don't rip our trannies apart. I don't really think I need 190hp to assist my service brakes on the truck and trailer to keep things under control. I wonder what the retarding hp in on a 8. 1 gasser or 8. 8L Dodge V10 gasser at 2500-3000rpm? I use second gear on mine up to 50mph downhill without an exhaust brake and it doesn't do much but it does help even if very little. Without the trailer, I use second gear to coast downhill and don't use the service brakes at all so it doesn't completely freewheel. It does just a little and I'd like for it to do just a little more but not enough to heat things up too much.

I got it! I could re route the exhaust to exit just outside the passenger window. When we start to descend, the ol' lady can cram a sock in the exhaust tip til we hit the bottom... . Wait 'til I tell 'er!

 

[Utah Willie]

Engine braking performance

Steve, take a 5. 9 liter gasser (Dodge 360 CID) down the same slope with the same load as a CTD with exhaust brake installed and you will see that the exhaust braked diesel VS the throttle plate retarded gasser will be a "no contest". The E brake is much more efficient ( 80%... + or-... of the HP back as a brake) than the throttle block of the gasser. An 8. 0 liter V10 has a lot more CID... thus a lot more braking effort than a mere 5. 9 liter. Just the friction alone should account for some increase in retardation... ... What we experience in power going up the hills with a turbo-diesel Cummins, we can experience on the other side with an exhaust brake. I have had both hauling and towing the same rigs... ... . believe me... . a 5. 9 CTD with an E-brake is IT!! I have 60K on my brakes..... go figure! BTW... Utah has a vehicle inspection program..... I just passed.

 

[Steve M]

I agree with what you're saying. If you have a 14,000 lb 5th wheel you wouldn't use a 5. 9 gasser to pull it or hold it back on a 6* grade. If as in my case, you have a 7,000 lb GVWR 5th wheel, well within the tow rating of a 5. 9 gasser or my CTD, my CTD will pull it up the hill better but the 5. 9 gasser will hold it back going down hill if they're both stock. I'd like to duplicate the holding power of the 5. 9 or V10 gasser so I'm not pushing the limits of my transmission but still have enough engine braking power to handle our light trailer. We do fine as it is on the downhills but just a little more assistance holding back would be enough for our application. I could be totally wrong in my thinking but it seems as though the gassers have no trouble holding back moderate sized trailers on the grades with thier lesser engine braking capabilities.

I've towed with a "94 Ford gasser with 5spd/3. 55 which held back great if I could get up the hill. Then a '95 CTD auto/3. 54 that went downhill like it was on ice. Then a '98 12v CTD that did great uphill and ok downhill without an Ebrake. My '01. 5 has minimal hold back abilities. It wouldn't take a whole lot more to satisfy me. If we ever decided to go with a big 5th wheel, I'd go for another 5 or 6spd and install an Ebrake with it's full potential.

 

[JJW_ND]

Manual serves EBrake best

We have pulled the same 13K trailers (21K Total) through Wyomings bigger hills with both V10 Dodges, 454 Chevys, and Dodge Cummins. Every truck was a manual (5sps and 6sps). Gassers were 4:10 and diesels are all 3:54.



Even gassers had a handfull on the steeper grades keeping the brakes from fading out. This is with very good 8K trailer brake setup. Diesels without a Ebrake had to us as much or more caution then the gassers. Diesels could desend the same hills and NEVER touch the service brakes. Increadable difference.



Much easer on the trailer and truck brakes. Allows them to be available if you really need them!



jjw

ND

 

[Dave Zuber]

Originally posted by Doc Tinker

Why not put a butterfly on the intake of the diesel, instead of in the exhaust?



Doc

Believe it or not, the most vacumn an engine can pull is -15psi (one atmosphere). But, the exhaust can generate pressure upto the max compression, about 200 to 300 psi.



The systems create force by doing work. If the exhaust butterfly COMPLETELY closed the exhaust off, it would not retard the engine for very long (maybe only 10-20 revolutions), the pressure build up would not allow any exhaust and or intake of gasses. It would be like having a spring on top of the pistion getting compressed and released, any work done would be by engine friction alone.



All these exhaust brakes have a way of bleeding off some of the built up pressure. My Cummins/Jackobs/Dodge brake has a 3/4" dia hole in the butterfly for the engine to force all the exhaust through. You could vary the size of this hole and reduce/increase engine braking. One of the exhaust brake kits has a variable bypass (Gear Vendor?) that you can preset to a maximum of 35 psi. There is no reason you couldn't set it at a lower value for an automatic trans.



QUIZ: Why do the first gen 12v engines need to install larger exhaust springs to run the exhaust brakes?



The pressure built up in the exhaust will blow open the exhaust valve and blow the exhaust back into the cylinder during the intake stroke. This will happen on all of them if the pressure gets high enough.



I've found most 'book learned' engineers can remember formulas very well, but can't visualize the relationships or apply them to real world problems. I can only remember about 10% of the formulas from school but I learned the relationships.



Hope I explained this well.

 

[Chipstien]

The Signature Series Cummins has a separate camshaft devoted entirely to engine braking.

 

[carson6]

i love my pac brake

instead of out of control in first gear with the brake's burning! know i can go 25 in second in control with the pac brake. . o-ya

 

[jpolston]

What are you saying Dave Zuber; engineers are stupid????:{

 

[E-9]

Jake Brake

The Jake Brake on large engines are add on units. When I say large, I mean over the road trucks. The rocker cover is removed, the Jake unit installed and the rocker cover placed on top of the Jake unit. It does manipilate the valves, turning the engine into an air compressor. This is done on each head of the engine. The units can be used in sequence, depending on the amount of braking you need, 1 , 2 , or all three. There is a switch on the clutch and accelerator pedal that dissengages the unit if either are pressed.