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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old February 8th, 2012, 10:56   #61
tdipoet
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i watched all of these videos while driving, and my mileage didn't go up at all.
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Old March 14th, 2012, 15:11   #62
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http://www.youtube.com/watch?v=qVEFs...ature=youtu.be

Long time since the last video. This video shows how stupid it is to drive with small safety gaps and waste fuel on the motorway. I try to post more videos in the next couple of weeks.
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Old March 16th, 2012, 06:45   #63
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Excellent! You and I have very similar driving styles. Thanks for sharing.

My MPG has gone up significantly since following your YouTube videos.
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Old April 8th, 2012, 11:08   #64
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Someone on the first page stated that while under engine braking the injectors do not fire any fuel at all...as in zero fuel use. Is that really true? I would think with the compression on any diesel you would just grind to a halt if that was the case. I have seen Dodge Cummins trucks with the manual tranny try to "push start" a manual truck and the only result is that the truck skidded to a halt like you had mashed the brakes. I just wanted some clarification for those in the know. I know for a fact that the common rail cummins (cp3 injection pump), is still firing fuel when you are coasting in gear with zero throttle.
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Old April 8th, 2012, 12:16   #65
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You're absolutely right, except for one thing... inertia. Had you read the entire thread and watched the videos, you would have learned the following...

While in gear, the inertia of the moving vehicle keeps the engine rotating while no fuel is injected - engine braking. If you press the clutch, the injectors fire to maintain idle speed - no engine braking.

Engine braking and coasting are very useful tools for obtaining high MPG. Engine brake while coming to a stop rather than using your brakes. Coast in neutral down slight grades (highway, rolling hills, etc) rather than setting the cruise control or applying fuel to maintain speed... you'll be surprised by how far you can coast without having to use fuel (the difference is ~200 mpg vs ~80 mpg, coasting vs light load).

Remember that true hypermiling requires a lot of attention, planning and patience. It's a pretty fun game - see how high you can get the MFA or Scanguage to display.

Welcome to TDIClub! If you start using some of these techniques, let us know what your results are!
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Old April 8th, 2012, 21:19   #66
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Well a diesel engine doesnt actually "engine brake" as would a petrol engine. A petrol engine brakes itslef via restricting the air intake thus creating a vacuum on the intake stroke. Petrol engines are air throttle engines, air and fuel are always in the the same ratio. Though I imagine some of the newer DI petrol engines operate over a moderate range of air fuel ratios, nonetheless they are still air throttle engines.

Diesels are fuel throttle engines. They take a full gulp of air on every intake stroke, there is no butterfly valve to restrict air intake. Thus a diesel will run drastically different air fuel ratios at idle vs full throttle. And you get no intake stroke engine braking.

The only ways I kow of to "engine brake" a diesel are:

1. a "jake brake" which releases the pressure of the compression sroke so the cylinder has the drag of compressing air but not the power provided by the power stroke because the cylinder doesnt fire. This is the most effective form of "non friction" braking, but its loud and somewhat costly.

2. "Exhaust brake" basically a butterfly valve placed after the turbo, usually right after the turbo downpipe. This device simply creates a back pressure on the exhaust stroke, kind of analagous to the petrol engine creating a vacuum on the intake stroke via an air intake restriction.

3. Turbos, by nature are both gas pushers (they stuff air into cylinders) as well as gas restrictors (they inhibit exhaust gasses from escaping to the atmosphere.

And in my opinion this is what we are experiencing in our VW's...a nice little variable geometry turbo that creates an exhaust restriction when under light or no throttle, which creates a braking effect. I still find it hard to believe that there is NO fuel injection event at all during a "no throttle, still in gear" situation. My feeling is that there is about the same fuel injected as would be injected at idle. That why its more efficient to coast in neutral vs coasting down in gear, which has more frictional losses and that lil variable geo turbo in the way.
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Old April 8th, 2012, 21:42   #67
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i have done this on an alh jetta with vag-com. when slowing down with the accelerator pedal at 0%, injected quantity is 0 mg/stroke and fuel usage is 0l/hr. It really does cut off all fuel, that is, until the revs drop to near idle.
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Old April 9th, 2012, 00:57   #68
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This has been discussed over and over here, the debate of staying in gear in overrun or using nuetral for coasting. In my experience using N for coasting allows for more distance to be covered saving fuel than staying in gear in overrun unless you are going down a very steep grade.

The best way to use less fuel on city loops is by watching road conditions to try to avoid overacceleration which requires brake use which wastes fuel. Gravity can also be used to save fuel on city loops if you have rolling terrain. A pulse of energy from burned fuel to get moving then allow gravity to get you up to your desired speed. If you drive the same route over and over with a down grade you can judge just how much energy needs to be derived from the engine to allow available gravity on the grade to propel you up to your desired speed.

On the question of bus and truck speed limits, in the western US all traffic can legally and is allowed to go 65-70 mph on two lane rural highways and 75-85 mph on rural freeways. Everyone travels these speeds regardless of what we are in!!

In diesels and gassers with both automatics and manuals I have clocked as much as a 10 and more mpg difference when using N for coasting over just staying in gear when a grade is available. I have pushed my Passat TDI as high as the mid 60s mpgUS using these techniques. While if you just put the car in gear and spend as little time as possible out of gear coasting low to mid 40s are the best.

In my current 05 Accord just getting into gear and staying there around town high 10s to maybe 22 mpgUS at best. With as much N use as conditions allow over the same routes at the same speeds it 27+ to 34 mpgUS, the last tanks were 31.3 and 27.4 with ~30 % alcohol (splash bleeded to E-30). I have close to 3 years 40k miles of data on this now in this car.
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Old April 9th, 2012, 03:59   #69
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Quote:
Originally Posted by rmalone68 View Post
...
I still find it hard to believe that there is NO fuel injection event at all during a "no throttle, still in gear" situation. My feeling is that there is about the same fuel injected as would be injected at idle.
...
It even says it in the car's owner manual - no fuel injected when in overrun. Take a look at the VW leaflet in my signature, page 12, which describes when each technique should be used.

(Note also that they mention engine braking... the engine braking effect is obvious if you flip between neutral & overrun). The reason that neutral is better than overrun, despite using fuel to spin the engine at 850rpm, is that in overrun the engine is having to overcome the resistance of spinning at 2200rpm or whatever, which takes a lot more energy than 850rpm. It is this frictional resistance which causes engine braking in diesels.


PS read the entire leaflet. There are a lot of good tips in there from VW.
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Last edited by MikeMars; April 9th, 2012 at 04:06.
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Old April 9th, 2012, 07:04   #70
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Quote:
Originally Posted by rmalone68 View Post
Well a diesel engine doesnt actually "engine brake" as would a petrol engine. A petrol engine brakes itslef via restricting the air intake thus creating a vacuum on the intake stroke. Petrol engines are air throttle engines, air and fuel are always in the the same ratio. Though I imagine some of the newer DI petrol engines operate over a moderate range of air fuel ratios, nonetheless they are still air throttle engines.

Diesels are fuel throttle engines. They take a full gulp of air on every intake stroke, there is no butterfly valve to restrict air intake. Thus a diesel will run drastically different air fuel ratios at idle vs full throttle. And you get no intake stroke engine braking.

The only ways I kow of to "engine brake" a diesel are:

1. a "jake brake" which releases the pressure of the compression sroke so the cylinder has the drag of compressing air but not the power provided by the power stroke because the cylinder doesnt fire. This is the most effective form of "non friction" braking, but its loud and somewhat costly.

2. "Exhaust brake" basically a butterfly valve placed after the turbo, usually right after the turbo downpipe. This device simply creates a back pressure on the exhaust stroke, kind of analagous to the petrol engine creating a vacuum on the intake stroke via an air intake restriction.

3. Turbos, by nature are both gas pushers (they stuff air into cylinders) as well as gas restrictors (they inhibit exhaust gasses from escaping to the atmosphere.

And in my opinion this is what we are experiencing in our VW's...a nice little variable geometry turbo that creates an exhaust restriction when under light or no throttle, which creates a braking effect. I still find it hard to believe that there is NO fuel injection event at all during a "no throttle, still in gear" situation. My feeling is that there is about the same fuel injected as would be injected at idle. That why its more efficient to coast in neutral vs coasting down in gear, which has more frictional losses and that lil variable geo turbo in the way.
You are completely forgetting (or ignoring) friction and pumping losses.

Try a little experiment. First, get up to 60 mph and take your foot off the accelerator. See how long it takes to drop to 50 mph. Then do it again in neutral.
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Last edited by VeeDubTDI; April 9th, 2012 at 10:35.
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Old April 9th, 2012, 08:28   #71
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Fair enough, I stand corrected. I knew there were pumping losses, and obviously friction losses. I was just under the impression that the primary braking was provided by the vgt turbo. Well live and learn, thanks for the clarification guys.

On another note I have definately seen better milage by accelerating fast to speed, featering the throttle over hills and coasting in neutral down grades and while coming to a stop. Traffic often makes it impossible to drive that way but when possible I sure am on board with the principles espoused by this thread.

At 5k mi on my 2012 I am seeing about 42-44mpg, and we still have a bit of #1 diesel in our blend.
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Old April 9th, 2012, 10:36   #72
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Not bad mileage for still being on winter fuel.
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Old April 9th, 2012, 12:59   #73
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From this chart you can see your fuel consumption at idle and choose the point where its wiser to start engine braking. Its only wiser if you have to stop anyway or slow down faster...

https://www.facebook.com/photo.php?f...type=3&theater


MPH 12,4 9,3 6,2 3,1
MPG 94,1 70,6 47,0 23,5
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Last edited by Vekke; April 9th, 2012 at 13:02.
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Old April 9th, 2012, 13:57   #74
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Awesome I really appreciate your effors Vekke.
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Old August 28th, 2012, 21:46   #75
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If I coast down hills in neutral, then put it back in drive with my DSG at highway speeds, will I cause any damage to the DSG tranny.
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