Tdi engine braking - Page 2 - Defender Source
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  #21  
Old August 17th, 2015, 05:17 PM
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I will say this. When I had my 2.5na in my truck I thought I would try a KN style performance air filter. I noticed less engine braking than when I had the stock one on. It led me to think that there is a direct relationship between ease of airflow through the engine and engine braking. For example: less air in the intake will increase engine braking. Less air through the exhaust, more engine braking and vise versa.
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  #22  
Old August 18th, 2015, 06:34 PM
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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.
So I went and did a test in the Lightweight. I don't have a super steep hill to test on, but you could tell that the engine braking increased with the throttle open. So the whole theory about the intake throttle on a gasser being a large contributor to engine braking is crapola.
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  #23  
Old August 18th, 2015, 07:48 PM
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Very interesting


So much for theory!
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  #24  
Old August 19th, 2015, 11:02 AM
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I decided to try it out for myself and take on the Red90 challenge, so on the way to work this morning, I turned the engine off in my CRV and rolled down two hills - Barton and also on the way down the parking garage ramp.

I couldn't really tell if there was a difference between engine braking on the hills with the throttle close or at WOT - at least not significant enough to tell. One would expect a big difference - the car would accelerate very slowly at closed throttle and start accelerating rapidly at WOT, but that wasn't the case. However, the engine braking didn't increase at WOT either.

One very noticeable difference between engine braking at WOT vs fully closed was intake noise. At a fully closed throttle position, there was very little noise coming from the engine. At WOT, I could clearly hear air resonating - the same way the intake resonated when I'm accelerating with the engine on. So clearly there was more air moving through the engine (as expected) but the effect of this increase in air movement was very small with respect to the other forces involved.

Another observation is that engine braking is very, very poor with the engine off as opposed to with the engine on. It was clear that on the parking lot ramp that the engine would "run away" while off and descending down the ramp (in 1st gear).
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  #25  
Old August 19th, 2015, 11:10 AM
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Quote:
Originally Posted by sonoronos View Post
Another observation is that engine braking is very, very poor with the engine off as opposed to with the engine on. It was clear that on the parking lot ramp that the engine would "run away" while off and descending down the ramp (in 1st gear).
Do you mean in neutral? Otherwise, it makes no sense. This is a manual, with a cable throttle?
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  #26  
Old August 19th, 2015, 11:15 AM
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Do you mean in neutral? Otherwise, it makes no sense. This is a manual, with a cable throttle?
Result was surprising. Honda CRV with the old DOHC B20B, 5MT, cable throttle. Result was definitely in 1st gear. Mind you - this was a steep parking lot ramp.
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  #27  
Old August 19th, 2015, 11:19 AM
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You are saying that with the engine running, doing the same thing results in good engine braking? I saw zero difference with the ignition on or off in my tests.
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  #28  
Old August 19th, 2015, 11:33 AM
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You are saying that with the engine running, doing the same thing results in good engine braking? I saw zero difference with the ignition on or off in my tests.
Yes, I am saying that engine braking with the engine on was noticeably better with the engine on than off.

I can retest later to verify.
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  #29  
Old August 19th, 2015, 12:06 PM
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So to further clarify on engine on engine braking, engine on engine braking is much better with the engine on? That seems to make sense. I can't see how having the engine off would help with engine on engine braking. Conversely, if you were engine off engine braking, having the engine on would cause the engine off engine braking to be off. If you're engine on engine braking and your engine is off, your engine on engine braking is off.

I do appreciate this most informative thread on engine off and engine on engine braking I've ever read.

Let us know how the retest goes. Thanks!

Quote:
Originally Posted by sonoronos View Post
Yes, I am saying that engine braking with the engine on was noticeably better with the engine on than off.

I can retest later to verify.
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Present:
1960 SII 109"- "Red Square"
1984 90 Tdi- "Yamelo"
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1987 RRC- "Chewy 2"
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Past:
1959 SII 88"- "The Little Green Beastie" last seen in NY
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1988 90 "Eric the Half a Bee" half a truck, sold for parts
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  #30  
Old August 19th, 2015, 01:04 PM
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lol!
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  #31  
Old August 19th, 2015, 01:08 PM
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Quote:
Originally Posted by Ren Ching View Post
So to further clarify on engine on engine braking, engine on engine braking is much better with the engine on? That seems to make sense. I can't see how having the engine off would help with engine on engine braking. Conversely, if you were engine off engine braking, having the engine on would cause the engine off engine braking to be off. If you're engine on engine braking and your engine is off, your engine on engine braking is off. I do appreciate this most informative thread on engine off and engine on engine braking I've ever read. Let us know how the retest goes. Thanks!
does anyone know what he's talking about? .
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  #32  
Old August 19th, 2015, 01:21 PM
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  #33  
Old August 19th, 2015, 01:24 PM
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Quote:
Originally Posted by 1of40 View Post
does anyone know what he's talking about? .
Too many experts in the conversation. I'm keeping an eye on this debate to see who argues best.
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  #34  
Old August 19th, 2015, 01:41 PM
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Too many experts in the conversation. I'm keeping an eye on this debate to see who argues best.
Nick Naylor: That's the beauty of argument, if you argue correctly, you're never wrong.
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  #35  
Old August 19th, 2015, 02:04 PM
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I sure don't!


Quote:
Originally Posted by 1of40 View Post
does anyone know what he's talking about? .
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Present:
1960 SII 109"- "Red Square"
1984 90 Tdi- "Yamelo"
1988 RRC- "Chewbacca"
1987 RRC- "Chewy 2"
2008 RRS SC- "The Supersofa"

Past:
1959 SII 88"- "The Little Green Beastie" last seen in NY
1972 SIII 88"- "GreenHELL" now in NC
1988 90 "Eric the Half a Bee" half a truck, sold for parts
1991 RRC- never got a name- long since recycled
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  #36  
Old August 20th, 2015, 01:16 AM
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Ok, I'm no expert but I slept in a Holiday Inn last night......

As I understand it, there are 3 ways to use the engine for braking.

1) starting with petrol, letting off the throttle, engine on or off the piston travels to TDC and as it does so it compresses what ever is in there, then the exhaust valve opens and it goes out. on the next stroke because there is no fuel/air mix coming in it creates a vacuum which slows the rise of the piston thereby slowing the cranking of the engine.

2) diesel, on big rigs they use a jake brake which as the throttle is released and the jake turned on the piston rises to TDC and the jake opens the exhaust valve spilling out the compressed gas and forcing the next stroke to compress air again with no return of energy to the downward stroke of the piston.

2a) in a small diesel such as a 200tdi the throttle is let off and the piston travels to TDC compressing the fuel/air mix which due to the nature of diesels detonates and adds energy to the downward stroke of the piston. As far as I know the throttle doesn't push more fuel into the engine like a carb, but instead allows more fuel as the throttle opens. IF that is true then the diesel engine braking effect is due to insufficient fuel allowed in the mix to push the piston downward as in accelerating, it is more like idling down the hill.

Then there is the exhaust brake, which is external of the engine, being a butterfly valve in the exhaust after the turbo and when closed creates back pressure slowing the turbo and the engine.

Now back to our regularly scheduled programming.

Jeez did I just type all that? Where did I learn that?
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  #37  
Old August 20th, 2015, 03:27 AM
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FINALLY !!!
Someone comes up with the answer I'm waiting for.
Hallelujah ! Amen !
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  #38  
Old August 20th, 2015, 09:20 AM
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I have a friend with a jake brake on a dump truck he owns. He once told me he could lock up the rear tires with the jake brake and that it was more effective than his air brakes when towing a heavy trailer with a back hoe or small dozer
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  #39  
Old August 20th, 2015, 09:46 AM
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This thread is useless. If you've driven a Tdi you know it has enough engine braking for the vehicle to get sideways going down a slippery incline. So you end up having to give it some skinny pedal to straighten it out.

Interesting tech above for those who haven't got it sorted yet.
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Present:
1960 SII 109"- "Red Square"
1984 90 Tdi- "Yamelo"
1988 RRC- "Chewbacca"
1987 RRC- "Chewy 2"
2008 RRS SC- "The Supersofa"

Past:
1959 SII 88"- "The Little Green Beastie" last seen in NY
1972 SIII 88"- "GreenHELL" now in NC
1988 90 "Eric the Half a Bee" half a truck, sold for parts
1991 RRC- never got a name- long since recycled
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  #40  
Old August 20th, 2015, 11:40 AM
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Uncle Doug -

I have driven a variety of fire engines (brands, types, sizes) for many years.

The Jacobs brakes are great but I have never "locked up" the tires using it unless you are talking about weird cases of snow or wet roads and then only for an instant.

Still, the Jake Brakes do work really well. If you drive with a light foot and anticipate your stops - you can drive around using the engine braking with minimal use of the actual air brakes. For heavy footed response driving - the jake brakes are almost a must-have to augment the traditional air brakes.
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