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  #21  
Old October 14th, 2011, 08:28 AM
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kevin
1994 D90 300tdi #730, SIII 88
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Quote:
Originally Posted by Ren Ching View Post
Define "true engine braking." Again, if I push the clutch in while descending a steep slope, the truck will speed up considerably. So it stands to reason that there is some braking provided by the engine. Are you going to provide some real tech or are we just going to keep referring back to the same wikipedia article and hummer forum posts?

So far this thread is completely devoid of facts. It is nothing more than an internet pissing match.

(apologies to MeatBlanket who actually did provide some useful seat of the pants non-Tdi data)
Sorry, I thought I provided facts and the last thing I care to waste my time with is a "pissing match".
Maybe the fact that compression release brakes aka Jake Brake exist and are commonly installed on diesel engines may be a clue that diesel engines don't do a very good job of providing true braking energy.
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  #22  
Old October 14th, 2011, 08:31 AM
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Skinny Pete
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Thanks Matt. Interesting that you mention vacuum created by the turbo compressor. I hadn't thought of that. I was thinking that the exhaust blades would act as a restriction thus dissipating some of that "air spring" energy into spinning the turbine. As for the air spring effect, I would think it would be a net energy loss as the downward energy of the piston would always be less than the stored energy of the compressed intake charge. Then the combustion is acting against another air spring as the opposing cylinder goes up. 3/4 or so of the way down, the exhaust valve opens, blowing off the remainig "air spring" downward energy though at that point the crank is past centerline and thus not exerting very much force. Force which is needed to push the opposing cylinder up against the umpteen psi of the compressed charge. It's a net loser, which is why you still slow down when you take your foot off the pedal. The diesel won't run at "less than idle" though, and the petrol will. I haven't driven a NA diesel off-road since 1996. I don't recall any steep hills on that day. So I have no real world experience to comment on. But, I know Matt and I know he uses his trucks as nature intended LR's to be (ab)used. I don't know Mike (meatblanket) but I know he lives in CO and has probably got plenty of seat time with his new 2.5 NA 110. So, I put alot of stock in what they are saying. If you will notice, they have not quoted wikipedia or the hummer forum. And on the way to work today, I pushed the clutch in on my Tdi, and I went faster. Alot faster. Then I let off the clutch, and I slowed down. So if that is not "braking," then what IS it?

------ Follow up post added October 14th, 2011 08:39 AM ------

So far you have just quoted wikipedia and the hummer forum. You keep saying "true engine braking" and I keep asking you to define that. Yes Jake brakes exist. I read on wikipedia that the jake brake was invented by by mr. Cummins after his new diesel truck lost its brakes. Like I said, I'm not trying to argue that there is or isn't a need for them. I am just trying to clarify what is reality from internet lore. if you wanted to say "petrol engines provide more engine braking than diesels" then that would seem more in line with what empirical evidence points out. But, you are saying something different, that you are unable, or unwilling, to define.

Quote:
Originally Posted by kevkon View Post
Sorry, I thought I provided facts and the last thing I care to waste my time with is a "pissing match".
Maybe the fact that compression release brakes aka Jake Brake exist and are commonly installed on diesel engines may be a clue that diesel engines don't do a very good job of providing true braking energy.
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  #23  
Old October 15th, 2011, 05:26 PM
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ed angel
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Ok guys im trying to think of this from my simplistic mind.

Engines are large compressors when driven from the crankshafts, IE you could in theory remove the spark plugs or glow plugs and make really large air compressors.

So my thoughts is the more compression a motor makes the harder to turn it over from the crankshaft would be, The reason you wont be hand cranking a diesel very easy.

So isnt it safe to say that the higher the compression the more engine brakeing you would get? I read the thread but am confussed what the vacume has to do with it.

My simple minded thoughts are the higher the compression the more engine braking you will have when not using the cylinders to create energy. Im not even sure it has to do with diesel or gas but more of a 4 stroke thing.

So im trying to understand why when you let off the throttle and you are now using the motor as form of compression brakeing why its not simply a matter of displacement and compression that hold you back.


Just thinking out load as this is allways how i thought of this function.

cheers Ed

------ Follow up post added October 15th, 2011 01:35 PM ------

Quote:
Originally Posted by kevkon View Post
Sorry, I thought I provided facts and the last thing I care to waste my time with is a "pissing match".
Maybe the fact that compression release brakes aka Jake Brake exist and are commonly installed on diesel engines may be a clue that diesel engines don't do a very good job of providing true braking energy.

I allways thought they where installed on diesel engines as a saftey precaution for added brakeing due to the large loads the diesels carry.
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  #24  
Old October 15th, 2011, 11:00 PM
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John B.
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One point that is missed in the internet simplified analysis of the engine is the valve timing.

The theory presented is that the compression stroke is completely cancelled by what would be the power stroke, thus compression ratio differences do not matter. However, depending on the valve timing, the two strokes can have different durations when the valves are closed.

Typically there is longer duration on the compression stroke than the power stroke which means that there is net engine input needed as morevolume is compressed than is expanded. It also appears to be much more prevalent with diesel cams that the intake closes earlier on the compression stroke than on gasoline engines giving the diesel even more advantage.

Really someone that knows what they are doing needs to run the math. It is not such a simple thing to calculate the net effect of valve timing differences, heat transfer effects, pumping loss and intake and exhaust manifold pressure differences. These difference cannot also be generically applied to "diesel" and "gasoline". Every engine is different.

The reality is that diesel engines of the same size on the same vehicle with the same gearing provide more engine braking. No, the difference is not anywhere near the difference in compression ratio. The physics involved are much more complex, but the real world testing does not lie.
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  #25  
Old October 17th, 2011, 06:12 PM
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Chas Strickdr
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I would agree with the folks who think that our diesels actually have a fair amount of compression braking. I know that if I'm dragging my trailer down a hill and back off my throttle it slows down. My calling that "compression braking" may be inaccurate for the dictionary, but, in reallity it is what it is. I don't typically use my brakes unless I've picked the wrong gear or if I'm in a hurry and pushing it.
Just a thought,
Chas
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