I disagree, I believe this will yield better mileage. Or at least it is worth trying.
This was the concept Volkswagen used when they created the '91 Jetta EcoDiesel.
A little info on that:
Quote:
About the ECODiesel-
Around 1990, Volkswagen in Germany set out to create an environmentally-friendly diesel engine. In order to save time and money, they decided to modify an existing diesel engine instead of creating a totally new one. So they took the standard VW turbo diesel engine, which had been used in millions of Volkswagens and Audis since 1982, and made some modifications to it in order to clean up its emissions.
With any engine, gas or diesel, one major step in reducing emissions is to add an oxidation catalyst (often called a catalytic convertor). So the major difference between the ECOdiesel and a regular VW turbo diesel engine is that the ECOdiesel has a catalyst. But they didn't stop with just adding a "cat." The VW turbo diesel engine has a thing called a "boost enrichment device" attached to the injection pump. Its purpose is to supply extra fuel to the engine when the turbocharger is making a large amount of boost (i.e. full-throttle acceleration, or going up a steep hill). But often the engine can't burn all of this extra fuel, and some of it ends up going out the exhaust pipe.
If this unburnt fuel in the exhaust was allowed to enter the catalyst, it would damage or even destroy the catalyst -- causing VW's efforts at reducing emissions to (quite literally) go up in smoke. So the second modification VW made was to simply remove the boost enrichment device. Of course, this reduces horsepower a little bit, but the turbocharger is still there so the ECOdiesel is still more powerful than the non-turbocharged VW diesel engine. And the Umwelt diesel's turbocharger has the same maximum boost pressure (9.6 psi above atmospheric pressure) as the regular turbo diesel, so the turbocharger can do the same amount of work; but without the boost enrichment device, there is less fuel reaching the ECOdiesel engine at full throttle. This helps with fuel milage.
These are the two main features that distingiush the ECOdiesel engine. Otherwise it is pretty much mechanically identical to the regular VW turbo diesel engine. In the real world, these modifications do seem to work very well; in most driving conditions, the ECOdiesel emits no visible smoke from the exhaust pipe, at least when the engine is warmed up. In fact, it is clean enough that VW was able to sell the ECOdiesel in California, a state whose tough emissions regulations prevented VW from selling regular ECO-diesels there since the mid 1980s. This also makes the ECOdiesel ideal for GREASE systems, as it reduces emissions from less refined fuels.
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Source:
http://massachusetts.estia.com/cars/60206.html
Quote:
Originally Posted by piper109
It may not make much difference on the highway. With diesels, fuel usage is very closely linked to HP usage. If you are going along at a certain relatively constant speed, the HP usage will be the same. You are moving the same weight at the same speed so energy used would be the same. Fuel mileage might be very close. It could actually be worse as turbo diesels are touted to be more efficient than n/a.
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While power is closely tied to fuel, diesels naturally run lean, and this would just be leaning it out a little more. While there should be a loss in power, VW was able to get more power (at a lower RPM) by adding a turbo and not adding fuel enrichment to an N/A diesel (While also adding a CAT!). Look at the HP numbers:
1.6 D-------------1983–1992 I4 Diesel----- 1588 cc 40 kW (54 PS; 54 hp) @ 4800 rpm 100 N·m (74 ft·lbf) @ 2300–2900 rpm -----(N/A)
1.6 ECOdiesel-----1989–1992 I4 Turbodiesel 1588 cc 44 kW (60 PS; 59 hp) @ 4500 rpm 110 N·m (81 ft·lbf) @ 2400–2600 rpm-----(ECOdiesel)
1.6 TD------------1983–1991 I4 Turbodiesel 1588 cc 51 kW (69 PS; 68 hp) @ 4500 rpm 133 N·m (98 ft·lbf) @ 2500–2900 rpm-----(Turbodiesel /w fuel enrichment)
1.6 TD------------1989–1991 I4 Turbodiesel 1588 cc 59 kW (80 PS; 79 hp) @ 4500 rpm 155 N·m (114 ft·lbf) @ 2500–3000 rpm-----(Turbodiesel /w fuel enrichment)
Source:
http://en.wikipedia.org/wiki/Volkswagen_Jetta
Quote:
Originally Posted by Jason
All that will do is keep you from getting enrichment under boost. You need a certain amount of fuel to keep your car going a given speed. If you are making say 5lbs of boost on the highway, the AFC is giving the engine enough fuel to go with it. If you disconnect that, you will just end up having to give it more throttle to make up for the loss of the automatic fueling the AFC is taking care of. I cant see how you will get any better mileage at a constant speed. Maybe around town you would see a gain, but you would be down on power as you would have no fuel to go with boost, and probably not be able to build much boost, as fuel=heat & engery which makes drive pressure. Thats what spins the turbo.
Jason
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And as for needing to depress the throttle more on the highway because of loss of fuel = loss of power:
I believe this is a good point, one that may be the final cause of my idea not working. Although if Ecodiesels could spool the turbo, I don't believe that our straight 6 varieties will have any problem.
One thing to note is that more fuel in the cylinder can actually lower temperatures. Thus leaning out an engine too much can cause the engine to overheat, or melt things. I'm sure all of you know this, it is not a foreign concept to car guys. I don't believe any damage will be done by simply disconnecting the fuel enrichment hose, but it is something to keep an eye on.
If you are planning to mod your D24/T at all, you should have an EGT (exhaust gas temperature) gauge installed. I have my probe in my exhaust manifold, but my gauge is not installed/wired yet.
I know I have spouted off here a good bit, whilst still not proving anything. I will try this soon to see if my idea has any basis in reality. I can jabber on about this forever, but nothing will speak like actually results.
Thanks guys!