Jake Brake on a 300Tdi - Defender Source
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  #1  
Old October 11th, 2011, 03:32 PM
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Jake Brake on a 300Tdi

Never really thought about this until I read this article in an Overland Expo email.

http://www.overlandexpo.com/overland...n-braking.html


Is there such a thing for the 300Tdi, maybe the military uses them.
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  #2  
Old October 11th, 2011, 06:07 PM
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Firstly that article is technical inaccurate. The main factors in engine braking are pumping losses and thermodynamic losses due to cooling of the air charge between the compression and what would be the ignition stroke as well as the vacuum on a petrol. The sum of all this does still make a diesel have more braking energy than a petrol of the same displacement. Obviously a 2.5 liter diesel is smaller than a 3.9 petrol.....and they end up fairly similar.

A Jake brake is a very specific system that is designed into the heads of only some large truck engines. It is not an "add on". You could do an exhaust brake, but I have no idea if a 300TDI could handle it.
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  #3  
Old October 12th, 2011, 08:21 AM
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Quote:
Originally Posted by Red90 View Post
Firstly that article is technical inaccurate. The main factors in engine braking are pumping losses and thermodynamic losses due to cooling of the air charge between the compression and what would be the ignition stroke as well as the vacuum on a petrol. The sum of all this does still make a diesel have more braking energy than a petrol of the same displacement. Obviously a 2.5 liter diesel is smaller than a 3.9 petrol.....and they end up fairly similar.

A Jake brake is a very specific system that is designed into the heads of only some large truck engines. It is not an "add on". You could do an exhaust brake, but I have no idea if a 300TDI could handle it.
If you are saying that a diesel has the same engine braking ability of a gas engine, that's simply not true. No vacuum, no braking. That's why Jake brakes and exhaust brakes were developed for diesels. It's also why most mfg's increase the braking system on diesel equiped trucks.
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Old October 12th, 2011, 09:26 AM
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What has vacuum got to do with engine braking on a diesel?
I will put my 300tdi or 2.5na diesel against any identical rover with a 2.5 gas unit down a hill and I know i will be going slower in the same gear thanks to then engine braking of the diesel.
A "jake brake" releases compression through the exhaust valve just b4 tdc on the compression stroke.
http://en.wikipedia.org/wiki/Compres...e_engine_brake
Compression is why a diesel has better unassisted engine braking then the same size gas engine.
with 17:1+ compression ratios it takes that much more energy to compress that air. the engine has more ability to slow a vehicle faster then a gas engine of the same displacement at what 7:1 to 10:1 compression these days?
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Old October 12th, 2011, 10:03 AM
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Originally Posted by exmod110 View Post
What has vacuum got to do with engine braking on a diesel?
I will put my 300tdi or 2.5na diesel against any identical rover with a 2.5 gas unit down a hill and I know i will be going slower in the same gear thanks to then engine braking of the diesel.
A "jake brake" releases compression through the exhaust valve just b4 tdc on the compression stroke.
http://en.wikipedia.org/wiki/Compres...e_engine_brake
Compression is why a diesel has better unassisted engine braking then the same size gas engine.
with 17:1+ compression ratios it takes that much more energy to compress that air. the engine has more ability to slow a vehicle faster then a gas engine of the same displacement at what 7:1 to 10:1 compression these days?
http://www.hummerknowledgebase.com/engine/engbrake.html
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Old October 12th, 2011, 10:31 AM
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Quote:
Originally Posted by kevkon View Post
If you are saying that a diesel has the same engine braking ability of a gas engine, that's simply not true. No vacuum, no braking. That's why Jake brakes and exhaust brakes were developed for diesels. It's also why most mfg's increase the braking system on diesel equiped trucks.
Perhaps you should read what I wrote. Some braking is from vacuum, some from pumping of teh intake and exhaust, but most is from the loss in gas volume due to cooling between the compression and expansion strokes. On a diesel, the higher compression increases this type of braking a lot. It is easy to compare a petrol and diesel of the same displacement and you will see higher braking with the diesel.

On a petrol, the vacuum actually prevents gas flow and thus the pumping action is neglidgable and the heat of compression is very little as the pistons are acting under vacuum meaning there is little gas to compress. On a diesel, the full air flow pumping occurs since there is no vacuum and the heat of compression is high.

To test, take a petrol down a hill, with the ignition off, so there is no combustion. Test the engine braking with no throttle (max vacuum) and full throttle (no vacuum). See what happens....
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Old October 12th, 2011, 01:56 PM
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Quote:
Originally Posted by Red90 View Post
Perhaps you should read what I wrote. Some braking is from vacuum, some from pumping of teh intake and exhaust, but most is from the loss in gas volume due to cooling between the compression and expansion strokes. On a diesel, the higher compression increases this type of braking a lot. It is easy to compare a petrol and diesel of the same displacement and you will see higher braking with the diesel.

On a petrol, the vacuum actually prevents gas flow and thus the pumping action is neglidgable and the heat of compression is very little as the pistons are acting under vacuum meaning there is little gas to compress. On a diesel, the full air flow pumping occurs since there is no vacuum and the heat of compression is high.

To test, take a petrol down a hill, with the ignition off, so there is no combustion. Test the engine braking with no throttle (max vacuum) and full throttle (no vacuum). See what happens....

I did.
I have.
No vacuum, no engine braking.
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Old October 12th, 2011, 02:08 PM
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so, if there is no engine braking with a tdi engine, how come we Tdi'ers don't just go careening down hills out of control? What mysterious phenomena is keeping that from happening? I know this has been covered somewhere but please remind me.
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Old October 12th, 2011, 04:22 PM
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The person who wrote the above article forgets that most gasoline engines have numerous vacuum ports downstream of the venturi butterfly of a carburetor or EFI throttle body. These would negate the mere closure of the throttle plates.

4 cylinder engines will always have at least two bores resisting external rotation forces. which has mare resistance a diesel or gas? Duh.
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Old October 12th, 2011, 04:51 PM
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Quote:
Originally Posted by Ren Ching View Post
so, if there is no engine braking with a tdi engine, how come we Tdi'ers don't just go careening down hills out of control? What mysterious phenomena is keeping that from happening? I know this has been covered somewhere but please remind me.
You are making a common mistake of confusing resistance with true engine braking which is a function of engine vacuum. No my Tdi and Powerstroke don't careen down a hill, but they also don't offer as much ( or any) engine braking either.
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Old October 12th, 2011, 08:09 PM
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Ok, so what causes the resistance? Let's get into some real tech here. I want good answers not a link to some guy on a hummer forum. Because I am pretty sure if I push the clutch in going down Daniel @ Uwharrie that things are going to get a whole lot more exciting. So, which is it, not as much engine braking, or not any engine braking?

Quote:
Originally Posted by kevkon View Post
You are making a common mistake of confusing resistance with true engine braking which is a function of engine vacuum. No my Tdi and Powerstroke don't careen down a hill, but they also don't offer as much ( or any) engine braking either.
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  #12  
Old October 12th, 2011, 08:20 PM
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I am with Bobek on this one.... (I need more technical info)

I get the whole compress on one stroke and release on the next so I am ready to beleive that the "compression" is not the source of the breaking.

The vacuum bit sounds plausible but like the other dude said - there have to be so many other sources of air.

I drive a V8 petrol and I have lots of engine breaking going down hill in first gear.

I also drive a fire engine that has a selectable Jake brake. (Detroit diesel 6V92 engine) I can turn the Jake "off" and down shift the truck and get some (what I presume to be) engine braking. Naturally, the truck brakes more with the Jake turned to 'high' but it does not simply 'free-wheel' with the Jake turned off.
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  #13  
Old October 12th, 2011, 08:43 PM
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I have driven the 300 tdi for over six years now. Same hill as an NAS 90 my 300 goes slower down the hill. Both trucks have the same gears, same trans, same transfer case but different motors. Now explain why my diesel holds back more?
I also have a Cummings 6.7 with the exhaust brake. How come my Cummings holds back on the mountain better than the gas trucks? That is with the Exhaust brake off. I too drive a few large Fire trucks with diesels. The diesels hold back better than any gas motor, same size or even larger.
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  #14  
Old October 12th, 2011, 11:59 PM
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Quote:
Originally Posted by kevkon View Post
I did.
I have.
No vacuum, no engine braking.
Bullshit. I'm stuck in China. When I get back, I'll make you a video.

------ Follow up post added October 12th, 2011 10:17 PM ------

Quote:
Originally Posted by MC22958 View Post
I have driven the 300 tdi for over six years now. Same hill as an NAS 90 my 300 goes slower down the hill. Both trucks have the same gears, same trans, same transfer case but different motors. Now explain why my diesel holds back more?
I also have a Cummings 6.7 with the exhaust brake. How come my Cummings holds back on the mountain better than the gas trucks? That is with the Exhaust brake off. I too drive a few large Fire trucks with diesels. The diesels hold back better than any gas motor, same size or even larger.
Look, stop letting reality get in the way of a stupid internet myth.
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Old October 13th, 2011, 07:44 AM
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Quote:
Originally Posted by Red90 View Post
Bullshit. I'm stuck in China. When I get back, I'll make you a video.

------ Follow up post added October 12th, 2011 10:17 PM ------



Look, stop letting reality get in the way of a stupid internet myth.
The term engine braking usually refers to the braking effect caused by throttle position-induced vacuum in gasoline engines. While some of the braking force is due to friction in the drive train, this is negligible compared to the effect from vacuum. When the throttle is closed, the air flow to the intake manifold is greatly restricted. It is the work the engine has to do against this restricted air flow that provides the braking effect.
Diesel engines do not have engine braking in the above sense. Unlike petrol engines, diesel engines vary fuel flow to control power rather than throttling air intake and maintaining a constant fuel ratio as petrol engines do. As they do not maintain a throttle vacuum, they are not subject to the same engine braking effects. But here is what you are forgetting and why the Diesels can hold back better than the gas motors.....A mechanism related to the exhaust brake is back-pressure from a turbocharger. In turbodiesels with variable-vane Turbos, the vanes will close when the accelerator is released, which creates a back-pressure braking effect similar to an exhaust brake. Even fixed turbos, (300 Tdi) especially larger ones, will cause some back-pressure when they are below the turbo threshold (albeit not to the same extent as a variable turbo) and contribute to the braking effect.
As I said earlier my diesels, Hold Back, better than the gas motors.
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  #16  
Old October 13th, 2011, 08:20 AM
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Surely the reason that we have all retrofitted diesels is for increased downhill control under engine braking? The low end torque is also nice.
That's my reason anyway!
All comes down to higher cyliner compression - simple.
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Old October 13th, 2011, 08:48 AM
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Hey, if it makes some of you happy to believe you have true engine braking with your diesels then so be it. It's your right to believe whatever you like.
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Old October 13th, 2011, 09:06 AM
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Just as a point of comparison, the 2.25 petrol in my 88 offered much more resistance going downhill than the 2.5NA in my 86. It's a pretty dramatic difference. The 2.5NA in my exMOD 110 doesn't engine brake worth squat either. So I have to assume that the turbo on a tdi works as an exhaust brake.

Just another reason to upgrade to a tdi.
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Old October 13th, 2011, 10:23 AM
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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)

Quote:
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Hey, if it makes some of you happy to believe you have true engine braking with your diesels then so be it. It's your right to believe whatever you like.
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  #20  
Old October 14th, 2011, 07:48 AM
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Mr Meat is right on. NA diesels have very little engine "braking" . A turbodiesel will have an engine braking effect due to the turbo creating vacuum. It has nothing to do with compression ratios etc.
The difference in the engine braking between a 2.25 petrol and a 2.25 na diesel is in fact quite alarming (when the engines are running). I often drive like a fawkin twat for the first few minutes of getting in a petrol powered series rig being used to driving the diesel. The effective engine "braking" of the 300tdi is quite respectable but I really don't think it as good as the Rover V8. I do own and drive at least one example of all these combinations so this is "real" seat of pants not the made up internet kind.
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