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  #1  
Old August 17th, 2015, 09:00 AM
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Tdi engine braking

Talk about an irate dude confronting a truck driver 'jake braking' through town. NSFW.

Given the Tdi does not have this system, how effective is engine braking? Reads if diesels without it would be worse than the V-8.
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  #2  
Old August 17th, 2015, 09:17 AM
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I believe it more depends on if you are running a manual or automatic transmission.
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Old August 17th, 2015, 09:28 AM
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  #4  
Old August 17th, 2015, 09:52 AM
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The engine braking is great on the tdi. I know from first hand experience needing it haha.

But yeah you can pretty much go 45 degree downhill in first and it maintain the exact same speed. Nothing like I was doing in my d2 with the v8 auto
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Old August 17th, 2015, 10:11 AM
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With the same gearing the V8 and TDI are about the same, IME.
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Old August 17th, 2015, 10:58 AM
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How so John as Tdi compression is about twice that of a gasser? The swept volume still has to be compressed does it not?
One reason for appreciating diesels offroad (Nissan and Land Rover) over many years has been the control I've experienced under engine braking. Or so I thought?
Eric.
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Old August 17th, 2015, 11:14 AM
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Quote:
Originally Posted by Whiesel View Post
How so John as Tdi compression is about twice that of a gasser? The swept volume still has to be compressed does it not?
One reason for appreciating diesels offroad (Nissan and Land Rover) over many years has been the control I've experienced under engine braking. Or so I thought?
Eric.
probably because the v8 has the advantage of a throttle plate.
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Old August 17th, 2015, 11:24 AM
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So engine braking has nothing to do with compression. Think about it, every action has an equal and opposite reaction, so while the compression of air will inhibit the motor from spinning on the compression stroke, the downstroke right after would negate that as it pushed the piston back down.


Engine braking has everything to do with vacuum.


On a gas engine, you are trying to suck ALOT of air into the engine through a pretty much closed off throttle body. Like trying to shoot a pump water gun the effect is resistance (engine braking). If that throttle plate were open (even with the car off), you would have much less engine braking.


On a Diesel, there is no throttle body, so really you get less engine braking (although there is still some resistance in sucking air through the intake). A "jake brake" is essentially a throttle body on the back side of the engine, slowing the exhaust and thus slowing the engine down.


A lot of the "engine braking" you feel is just drag on the driveline
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Old August 17th, 2015, 11:26 AM
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Quote:
Originally Posted by Whiesel View Post
How so John as Tdi compression is about twice that of a gasser? The swept volume still has to be compressed does it not?
One reason for appreciating diesels offroad (Nissan and Land Rover) over many years has been the control I've experienced under engine braking. Or so I thought?
Eric.
Engine braking is a very misunderstood thing. Engine compression itself does not directly affect braking. The factors that contribute to engine braking are quite complex. If everything was perfect, the compression and ignition strokes would cancel each other out.

The braking is influenced by pumping losses, thermodynamic losses, drivetrain resistance and intake and exhaust pressure differentials....

Regardless of all of that, IME, the same displacement diesel will give more engine braking than the same displacement gasser. In this comparison, the gas engine is larger displacement and the effect is about equal. Anyone that has driven diesel and gassers know this is true.
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Old August 17th, 2015, 11:31 AM
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Quote:
Originally Posted by bossman429 View Post
On a gas engine, you are trying to suck ALOT of air into the engine through a pretty much closed off throttle body. Like trying to shoot a pump water gun the effect is resistance (engine braking). If that throttle plate were open (even with the car off), you would have much less engine braking.

On a Diesel, there is no throttle body, so really you get less engine braking (although there is still some resistance in sucking air through the intake).
What is missed in this evaluation of the braking effect is that with a closed throttle plate, there is very little mass flow of air moving through engine, so although you have the intake/exhaust pressure differential, there is very little pumping loss. The diesel does not have the pressure differential, but has the maximum pumping loss. It is a complex situation. If you have a gasser that you can safely shut off the fuel flow while going down hill, try opening the throttle. You will find little difference in engine braking throttle open or closed.
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Old August 17th, 2015, 11:36 AM
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I would think you are going to feel the biggest difference based on gearing. The 110 3.5 is much slower going down a steep hill in low range than is the 4.6 DII on the same hill. The hill is my back yard if you want to try it!
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Old August 17th, 2015, 11:40 AM
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Quote:
Originally Posted by down_shift View Post
Talk about an irate dude confronting a truck driver 'jake braking' through town. NSFW.

Given the Tdi does not have this system, how effective is engine braking? Reads if diesels without it would be worse than the V-8.
My Tdi engine braking is so good the truck goes back up the hill. The guy in the video is such a douchebag I'm surprised he doesn't put the sunglasses on the back of his head.
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  #13  
Old August 17th, 2015, 11:51 AM
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Wiki's overview. I figured the Tdi falls into paragraph 1) with little to no engine braking. The 'jake brake" is the system of opening the exhaust valves in paragraphs 2) and 4) which the Tdi does not have.


1) When the driver releases the accelerator on a moving vehicle powered by a diesel engine, the vehicle's forward momentum continues to turn the engine's crankshaft. With the example of Mercedes diesels being a notable exception, diesel engines, by design, have no throttle butterfly in the intake so regardless of throttle setting a full charge of air is always drawn into the cylinder. As such, each time air is compressed in a cylinder virtually 100% of that energy is returned to the crankshaft providing very little in the way of engine braking to the vehicle.

2) The typical compression release engine brake, as originally developed, uses an add-on Hydraulic system utilizing engine oil. When activated, the motion of the fuel injector rocker arm is transferred to the engine exhaust valve(s). This occurs very near TDC and releases the compressed air in the cylinder so that that energy is not returned to the crankshaft. Energy is now absorbed and engine becomes an excellent "brake." If used properly, this energy can be used by the truck driver to maintain speed or even slow the vehicle with little or no use of the service brakes.

3) Contrast a petrol engine, where the mechanics are different and a special valve is not necessary for engine braking to happen when the driver releases the accelerator. In a gasoline engine, when the accelerator is released, the throttle prevents the free flow of air into the cylinders, so there is little pressure to release at the top of the compression stroke. The closed throttle provides engine braking by forcing the engine to generate a vacuum between the throttle and the cylinders.

4) Diesel compression release brake controls consist of an on/off switch and, often, a multi-position switch that controls the number of cylinders on which the brake is active. Throttle and clutch switches are integral with the system. Activation occurs when both the clutch is released with transmission in gear and the throttle is released. It is the driver's job to ascertain the correct transmission gear to use depending on, for example, the steepness of the grade and whether the truck is loaded or empty. The name is derived from the manufacturer, Jacobs (of drill chuck fame), and was patented 1962–1965 by Clessie Cummins.[1]
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Old August 17th, 2015, 11:58 AM
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Quote:
Originally Posted by down_shift View Post
Wiki's overview. I figured the Tdi falls into paragraph 1) with little to no engine braking.
Unfortunately, the Wiki article is incorrect as I've explained. Seriously, if you have a gasser that you can safely turn off the ignition, try engine braking throttle on and off.

The whole thing is very misunderstood and perpetuated by the internet.

What always amazes me about this whole "diesel engines have no engine braking" theory is that all you need to do is go drive one to see it is false.
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Old August 17th, 2015, 12:00 PM
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Originally Posted by Red90 View Post
Unfortunately, the Wiki article is incorrect as I've explained. Seriously, if you have a gasser that you can safely turn off the ignition, try engine braking throttle on and off.

The whole thing is very misunderstood and perpetuated by the internet.

What always amazes me about this whole "diesel engines have no engine braking" theory is that all you need to do is go drive one to see it is false.
Hence my question!
Talk about loud: https://youtu.be/z3bLqjPBlx8?t=22s
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Old August 17th, 2015, 12:07 PM
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Originally Posted by RBBailey View Post
I would think you are going to feel the biggest difference based on gearing. The 110 3.5 is much slower going down a steep hill in low range than is the 4.6 DII on the same hill. The hill is my back yard if you want to try it!
Does the 110 have a steering lock? If not, you are the perfect candidate for my ignition off, throttle closed and open engine braking test.
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Old August 17th, 2015, 12:10 PM
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Back to Russ's question, on my 200tdi, I'll give a concrete example:

Remember going to Huff's old place there was that big hill that you would go down right before the guy with the homebrew fortress? First gear in the 200tdi, 1.222 transfer case, 3.54 gears, the truck would roll down naturally at around 1800-2000 rpm.

In second gear in the 200tdi, same hill, the truck would roll down and accelerate without stopping. I could easily exceed 4000 rpm on that same section of downhill in second gear. Whatever braking the engine was providing was not sufficient to slow down the truck in second gear.

While off road, low range on the transfer case seems to provide plenty of engine braking in 1st gear.
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Old August 17th, 2015, 01:07 PM
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Yeah, I agree that the whole "diesels have no engine braking" isn't very true.

The Mitsubishi Fuso engine I swapped into my 80 series Cruiser has an exhaust brake that is just a valve in the exhaust system, which increases back pressure. They call this a "Pac Brake". The braking is unbelievable. I took it off since it was way too much on a 5000 lbs SUV. They offer the same thing for the 6BTs, but you need to upgrade the valve springs. Might be an option for the wannabe trucker 300TDI driver - but it won't give you the noise that a Jake will!

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Old August 17th, 2015, 01:09 PM
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With like a 25:1 compression ration on a diesel I would expect them to have much better engine braking than a gasser
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Old August 17th, 2015, 02:19 PM
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I think earlier posts have made the point that compression ratio has overall little contribution to engine braking (beyond the basic thermodynamic loss)
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