I was reading some interesting threads about turbos. Basically, the turbo increases the volumetric capacity of the engine. As someone mentioned, a 5 cylinder engine will have the capacity of 7 cylinders (almost the same capacity of a little V8!--4.2 or 4.5 liters, I think) when turbo charged. So, to match the increased capacity, the diesel must inject more fuel to maintain proper fuel mixture. This adds horsepower to the engine.

I would say that this added horsepower is most welcomed when accelerating or climbing mountain passes. However, I beginning to think that cruising at a steady speed the extra horsepower is not needed. For example, I heard that future V8 GMC truck engines will shut down half of the cylinders under light load to save fuel. In the past, the idea didn't work very with Cadillacs, but GMC says they finally figured it out.

Turning the turbo off would have the same effect as shutting down cylinders. I figure a vacuum switch could easily be installed to open and close the turbo waste gate. Do you guys think that switching the turbo off at cruising speeds would save fuel?

Based on hooking up a pressure gauge to my intake manifold pressure and driving around some it seems to me that it happens kinda automatic. Try it - hook up a pressure gauge 0-15 psi in line with the tap from the intake manifold banjo fitting and you will see it for yourself. Unless the engine is under load the pressure pretty much falls off.

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Jim

You can't turn the turbo "off" per se, since it runs whenever there's enough exhaust to turn the turbines. Mercedes Benz already engineered a fuel saving device into these motors!

If you look on the rear of the intake manifold, you'll see a hard plastic hose. This line goes to the overboost protection switch. When you are decelerating, you still have lots of unused boost from the turbo, and in order to save fuel, the overboost sensor tells the large valve on top of the intake manifold to bleed off the excess boost. The injector pump then realizes that there is not enough boost at this point, so it doesn't inject unnecessary fuel.

Mercedes already has that Cylinder shutoff on their V12's they'll shut off one side of the engine when your crusing and when you wanna blow past some one the Cylinders kick right in, with out any lag. Pretty nifty setup they have.

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2005 Audi A4 1.8T CVT -Silver/Black "Siegfried"

1982 300D - Silver/Blue "Ralph" -For Sale:
http://www.peachparts.com/shopforum/mercedes-benz-cars-sale/119226-1982-300d-sale.html#post852260

1989 VW Diesel Jetta Blue/Blue "Bodo" RIP

In a diesel engine, power and fuel economy has to do with the amount of fuel injected into the combustion chamber and has nothing DIRECTLY to do with boost. In a gas engine, it has EVERYTHING to do with boost.

The added air pressured in by the turbo allows an adequate amount of air to offset the added fuel that can be injected into the combustion chambers.

Additionally the turbo is driven by exhaust which is wasted energy.

As Jim pointed out, the nature of the turbo basically shuts it down under cruise or coast conditions because there is little exhaust to drive it under these conditions.

Have a great day,

Why is that some people idle their turbo charged engines after high speed driving? The turbo shouldn't be working hard at cruise. Is hot oil the problem?

The idea with idling for a few minutes is to prevent coking, which is oil literally baking onto the bearing surfaces inside the turbo. A turbo typically does work to some degree maintaining highway speeds on most turbo diesel engines.

I see. The turbo is spinning at highway speed, but not adding much boost. Since the turbo is spinning, it still need oil.

On their pre-intercooled turbos, SAAB required a cool down period before shut down. The intercooler system solved the problem of the turbo acting as an oil-filled dutch oven.

The post highway run idle period has nothing to do with oiling a spinning turbo, it has to do with the whitehot turbine dumping all that heat into the main shaft and bearing. The excess heat and no cooling from oil circulation will result in the oil remaining in the bearings baking into scaly carbonaceous crud that will seize and/or grind away at the bearing, ruining the turbo.

Turbochargers run at speeds up to 100,000 rpm, so a special floating bearing is required. Coked oil isn't a good thing.

MB also already has a turbo "shutoff", at least on the 601/602/603 and later turbodiesels called an air bypass valve -- vents the boost back into the intake side. This was partially to make the trap oxidizer work better, and to lower emissions by increasing boost more slowly, but also to give better milage on the highway by reducing boost in light load cruise. Seems to have worked, since I get better milage with it connected than disconnected!

It did, however, also cause serious turbo lag sometimes, and cruise control surge under some condtions, which is why I disconnected it.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

This discussion of turning off the turbo reminds me of someone I knew who had a Porsche 924 turbo with a turbo that went kaput.

These people weren't the brightest bulbs in the pack, for sure, and they proved it when instead of repair or replaement, they had their brain-dead mechanic remove the turbo and not replace it. As if they went directly to a normally aspirated engine by simply removing the turbocharger! They soon learned the car was dangerously slow, as the turbo motor (especially a gas one) is designed with low compression pistons so as not to overpressurize the engine.

I experienced this phenomenon when I first got my 84 300cd last year. I knew these cars were slow, but this thing was DANGEROUSLY slow. I had the turbo tested, and it turned out that the boost control valve was clogged with soot so that it could not function, and my car had no boost whatsoever, explaining the snail-pace performance.

They cleaned out the valve and the boost went up to around 8psi from zero. Quite an improvement.

Oh, and either with or without the turbo activated, my gas mileage remained exactly the same, at 25 mpg.

Since this turbo only compresses air and not an air/fuel mixture like most gas engines do these days, the fuel figure makes sense.

Also, although I am concerned about coking, I still can't figure out why MB did not put a water cooled bearing jacket on this turbo charger. A water cooleled bearing jacket is what preserves a turbocharger's bearings, NOT an intercooler (that's got to do with cooling the air to make it denser and more volatile. No appreciable connection to bearing temperatures)

Anyone have any theories?

Every turbo that I've seen was oil cooled. This includes the Detroit, Cummins, and Caterpillar engines that I've driven during my trucking days. The Cummins in my Dodge is oil cooled. As was the turbo in the late 83 Rabbit. Not to say that a water cooled bearing area was never used, but sounds to me like more of an exception.

Well, it's not really an either or question. You're right, every turbo has an oil line, for lubrication and yes, cooling, but those having water lines do so in addition to an oil system. It's hard to imagine something with that much heat and friction not having an oil system. The water jacketing serves to cool, not lubricate.

-Gil

Gil:

Volvo water cools their gasoline turbo models (and charges something like $150 for the radiator hose with the side tube!), but not the diesel ones.

I have a feeling it has to do with the actual temparature of the turbo under load. Diesel engine exhaust is much cooler than gasoline engine exhaust under load -- the exhaust valves are hotter (and hence sodium cooled) becuase the combustion continues much longer in a diesel, but the gas stream on a gasoline engine at full throttle is much, much hotter. I also think that diesel exhaust is cooler at cruising speeds and loads as well.

Not being an engineer, I won't sepculate on the relative efficiencies, but Volvo still recomends a cooldown for the gas turbo -- and it does make a difference as my brother in law fried his by never doing the cooldown thing!

The lack of power when the turbo goes is amazing -- my Volvo TD lost the turbo on a trip -- nearly got killed when I pulled out in traffic and floored it, and nothing happened. 75 hp in a 3500 lb car is SLOW!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

I have been running my 300sd with the turbo control bipassed is there any problems i will run into? Would not want to cause a problem later on. Bill

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94 dodge cummings,86 ford 6.9,75 mbz 240d[ sold], 82 mbz 300sd,83 mbz 300sd