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Old January 30th, 2007, 19:12   #1
gcarper
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Old January 31st, 2007, 03:46   #2
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Unhappy Richer Mixture??

OK, Its promising that VW is planning on offering a 2008 diesel in the US. On the other hand, how much is this "hotter" exhaust cycle to burn off the NOx and particulates in the trap going to cost in fuel economy? At one point it was postulated that it would cost half of the fuel saved by going diesel to burn off the trap - that is enough to kill off the economic and greenhouse gas advantage of the diesel. Worse, this cycle is "imperceptible to the driver" so how do you know if it cycles too much, using even MORE fuel. I think I'm glad I got a 2006.
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Old January 31st, 2007, 04:31   #3
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Im glad that its going to have some more power. I chose to wait it out until the 2008 model. One thing that the article fails to mention is fuel economy, when are we going to get some estimates on this thing?
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Old January 31st, 2007, 04:41   #4
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They don't say what engine exactly it is, but by the sounds of it being more HP and less TQ than the B5's 2.0L TDI, I wonder if it is the multivalve version?

I think the PD's actually run a periodic richer mixture for the same reason, as well as MB's current CDIs, but I am not sure. TDImeister could clarify that I'm sure...
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Old January 31st, 2007, 05:11   #5
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Quote:
Originally Posted by TDIDerek
Im glad that its going to have some more power. I chose to wait it out until the 2008 model. One thing that the article fails to mention is fuel economy, when are we going to get some estimates on this thing?
I think when someone on here saw the car in Michigan last year the VW reps were saying 60mpg highway and 45mpg city. Sounds absurd considering what the outgoing model gets, but we don't know what all else was done to this engine to make it 50 state legal.

I don't have any new info on rated mpg though.

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Old January 31st, 2007, 05:40   #6
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0-60 in eight seconds, just wait until it gets a chip or box, low sevens perhaps ??


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Old January 31st, 2007, 06:03   #7
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60 hwy/45 city sounds pretty close to possible, IF the gallons are imperial. Our 2004 Jetta PD would get pretty close to that. Maybe a bit less (40 mpg) city)
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Old January 31st, 2007, 06:19   #8
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http://forums.tdiclub.com/showthread.php?t=166313

THHPPPPBBBTTTTT!!!!! I was here first.
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Old January 31st, 2007, 06:25   #9
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Quote:
Originally Posted by Waldek Walrus
OK, Its promising that VW is planning on offering a 2008 diesel in the US. On the other hand, how much is this "hotter" exhaust cycle to burn off the NOx and particulates in the trap going to cost in fuel economy? At one point it was postulated that it would cost half of the fuel saved by going diesel to burn off the trap - that is enough to kill off the economic and greenhouse gas advantage of the diesel. Worse, this cycle is "imperceptible to the driver" so how do you know if it cycles too much, using even MORE fuel. I think I'm glad I got a 2006.
Richer probably in terms of more oxygen than fuel, I'm betting. You put more fuel than oxygen and you get an incomplete combustion, which results in all that particulate matter. Put the other way around and you get a blast that eats oxygen for breakfast.

Which begs the question: why don't they have this richer combustion on all levels at all times so that they don't have to have a filter? My only guess is that the engine cannot take consistently high temperatures like that (wears down the engine quicker), so it can only do it in bursts.
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Old January 31st, 2007, 06:35   #10
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Going back to college and an engine design class I had (gasser only, sorry) they run engines a little rich as a trade off between NOx emissions and hydrocarbons. The lean mix w/ higher temps led to more NOx. While I don't know the combustion chemistry of the diesel engine (higher pressures do funny things I'm sure), it's possible that they run things a little fuel rich for the same reason.
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Old January 31st, 2007, 06:56   #11
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Anyone know if a cutaway version of that motor is making the rounds of the Autoshows? If so, can anyone post photos?
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Old January 31st, 2007, 07:16   #12
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I'd like to clarify something on this because I keep seeing people make confusing statements. Diesel engines cannot run 'rich' like a gasoline engine. Gasoline engines are throttled and ideally run a stoichiometric ratio of fuel to air (14.7:1). This is controlled by how much air is allowed into the cylinder and how much fuel is injected (or sucked in through a carburator). Therefore gasoline engines can run rich or lean depending on how the computer controls things.

Diesel engines are unthrottled and always have excess air. They never run stoichiometric. At max power (what we call 'wide open throttle'), the fuel/air ration is still probably 20:1. At idle it may be 100:1 and some value between the two at cruise. Power in a diesel is controlled completely by how much fuel is injected. If you run 'richer', you produce more power. More power makes the car go faster, unless you apply the brakes and convert it to heat. More air is added with turbocharging to allow even more fuel to be injected without smoke, but the richest ratio doesn't change significantly.

The only variable is injection timing which effects the efficiency of the combustion and can result in more unburnt hydrocarbons in the exhaust. That is the only way you can inject more fuel into the combustion chamber without producing more power, and the timing effect is small within the normal operating parameters of the engine.
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Old January 31st, 2007, 07:48   #13
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Thanks Ken, Saved me a bunch of typing.
I hope the burn-off doesn't mean a reversion to the 96 B4's supplemental injector for the catalytic converter...
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Old January 31st, 2007, 08:15   #14
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Kenbob speak truth.

However, I would like to point out another technology with gas engines. I will soon be taking delivery of a Passat wagon with the 2.0T FSI engine. The FSI stands for "fuel stratified injection". The beauty of this technology is that a gasser behaves more like a diesel; no longer is a stoichiometric ratio required for combustion. The intake valve ingests pure air like a diesel and the injector injects fuel as required to get the job done, as in a diesel. Compression ratio is less though, and spark ignition is still used.

Of course gasoline needs a stoichiometric ratio to explode when ignited. However the FSI engine works by having an overall air/fuel ratio that is non-stoichiometric (lean), but having stratified areas in the cylinder where there are localized stoichiometric ratios, so it's a "lean-burn" engine. When full power is required, the engine runs with a stoichiometric ratio, but in highway cruise, in max. lean burn mode for excellent efficiency.

Apparently sulphur content in our gasoline is an issue and does not allow full FSI benefits, but when this is addressed (where have we heard this before!) the ECU can be remapped to run in lean-burn mode. Already the 2.0T turbo gets fairly decent highway economy: in the Passat, about 6.7 L/100 km on the highway; I've seen numbers from France that suggest 6.3 in the lighter Jetta (rated a bit higher in N. America).

To put it into context, the closest TDI match in performance in N. America would be the B5.5 Passat (134 hp/247 lb-ft vs 200 hp and 207 lb-ft), rated at 5.7 on the highway. If you think about it, given that diesel contains about 10% more energy than gasoline, the gas engine should have a rating, all other things being equal (weight, etc) of about 6.3 L/100 km. However even FSI is a little less efficient, at 6.7.

So good efficient gas engine design is getting close to diesel in efficiency. Still a few drawbacks though: need premium, and you're still carrying around 70 liters or so of an explosive liquid in your back end, and of course gas is less efficient to refine than diesel.

Still, one wonders if the future isn't in more efficient gas and diesel engine design, NOT hybrids. Take VWs TSI engine, 170 hp an rated at 5.9 L/100 km on the highway, out of just 1.4 liters of displacement. Without the hassles of hybrids (battery packs, underpowered gas engines when the batteries run down, etc).

It's odd how VW has some great gasser designs (2.0T, normally aspirated FSI, TSI), but here in N. America, we only get the 2.0T, the others being the piggish 2.5, the awfully outdated 2.Slow (still sold in Canada in the "City" models), and the "who the heck needs 280 hp in a midsize" 3.6 (although it is FSI).

If you ask me, apart from the Touareg, VWs only need the 100 and 140 hp TDIs, and the 2.0 FSI (normally-aspirated, 150 hp), 1.4 TSI (170 hp) and 2.0T (200 hp) for more than adequate motivation in the Golf/Jetta/Passat/NB/Eos lineup. Actually in Europe the base engine in the Passat is a 1.6L, 102-hp gasser...slow but it can still cruise at European speeds...we in N. America need to rethink our love of high displacement inefficient engines.
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Old January 31st, 2007, 08:46   #15
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Quote:
Originally Posted by Lug_Nut
Thanks Ken, Saved me a bunch of typing.
I hope the burn-off doesn't mean a reversion to the 96 B4's supplemental injector for the catalytic converter...
You won't need it, because the Common Rail system can be made to fire during the exhaust stroke, which does the same thing.

And yes, they will likely be injecting some fuel into the exhaust.

The DPF(Diesel Particulate Filter) systems used to trap soot work by storing all of the little soot particles and burning them off all at once. These systems require ALOT of heat to completely burn the soot. To get this heat, many systems utilize a DOC(Diesel Oxidation Catalyst) to help burn fuel injected into the exhaust stream. In order to help this process along, most systems use some sort of intake or exhaust throttle to provide some extra load on the engine to generat more heat.

So, the process goes generally like this:

1. Measure pressure across the DPF, notice that it is getting clogged with soot.

2. Tell engine to go into "generate more heat" mode.

3. Once exhast temps are high enough, start injecting fuel.

4. Hot exhaust and fuel hit DOC, get even hotter.

5. Resulting hot gasses burn off soot.

6. Measure pressure across the DPF, notice that filter is clean. Stop process and wait for soot to build up again.

Wash, rinse, repeat.

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