I know that a Diesel engine has a much higher compression ratio than a gasoline engine.

I also know that Mercedes at least builds Diesels with separate vacuum pumps for the locks and A/C and other stuff, because the engine does not produse the same amount of vacuum as a gas engine.

The question is why? Shouldn't the fact that the Diesel squeezes harder on the upstroke mean that it shound also suck more on the downstroke?

I am not an engineer, obviously, but I would like to know why. Why is there less available vacuum in a Diesel? And if there is a vacuum in a Diesel, why can't it be used?

Does anyone know of a really good Diesel technical website? I have found some, but they are either too simple or too complex.

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Semibodacious Transmogrifications a Specialty

1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)

"Time flies like and arrow, yet fruit flies like a banana"
---Marx (Groucho)

The reason is that a diesel is not "throttled." A gas engine regulates power by varying the amount of air and thus fuel that enters the engine. With the throttle closed or near closed, or with the engine at higher RPM, it cannot get all the air it wants past the throttle, that creates vacuum.

A diesel on the other hand has no throttle restricting the air. The power is regulated by the amount of fuel that is injected into the cylinders. Since it gets a free, easy draw of air, there is very little vacuum created.

Imagine this. Put a vacuum guage near the open end of a vacuum cleaner hose. Since there is no restriction, very little if any vacuum will be measured. Now cover the end of the hose and watch the vacuum reading increase.

Hope this helps,

Richard,

Compression ratio is a pure mathamatical number, no dimensions. It is the size of the combustion volume with the piston at BDC divided by the size of the combustion volume with the piston at TDC. The combustion volume includes volume in the cylinder, volume in the head, volume in the precombustion chamber and volume in any recess in the piston.

So you can see that compression ratio has very little relation to the the amount of air taken in on the intake stroke.

Larry is not quite correct that there is no restriction in the intake path on all Diesel engines. I had Perkins Diesel tractor engine that had a throttle in the intake manifold that created a vacuum that was used to control the injection pump. The hand throttle only controlled the throttle valve in the intake manifold and the vacuum created, controlled the IP. There was no mechanical connection to the IP. Even older MB Diesel engines (190D and maybe others) had a throttle valve in the intake and the vacuum created plus a mechanical connection was used to regulate the IP.

The Detroit Diesel engine in my Chevy PU truck has complete mechanical control to the IP. It will run just as well with the intake manifold removed. All the intake manifold is is an air plenum from the air filter to the cylinder head, it just keeps dirt from entering the engine.

What are the other web sites you found for Diesels. I would be interested to look at them because I have Diesels other than MB.

P E H

Not to belabor the obvious, but I assume y'all are aware that with a turbocharged diesel the manifold is under pressure and not vacuum.

Hmmm, that would mean the vacuum controls would work in reverse if the intake manifold was used instead of a separate vacuum pump? Wonder what THAT would do!

Larry pretty much hit it right on the head. You can see a demo of what he describes here:
http://www.rawbw.com/~xmwang/javappl/dieselCyc.html

When I looked into this, I found this site useful as well:
http://www.howstuffworks.com/diesel1.htm

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick

I see what you mean.

Concerning turbos, isn't the basic idea of a turbo to use the force of escaping exhaust gasses to ram air down the intake of the engine?

It is basically a supercharger turned by exhaust gasses, whereas a true supercharger is driven by a belt or chain directly by the motor.

So a turbo doesn't really take power from the engine, but the supercharger does.

So the supercharger has more effect at over 2000 RPM, while the supercharger/ blower/ kompressor functions at all engine speeds.

A supercharger, a blower and a kompressor are all the same animal, aren't they?

What causes defective turbos to blow so much smoke? A friend's gas-engine Volvo did this for a while and I never found out why.

__________________
Semibodacious Transmogrifications a Specialty

1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)

"Time flies like and arrow, yet fruit flies like a banana"
---Marx (Groucho)

Q1.Yes

Q2.Yes or gears

Q3.No, it takes power to turn the impeller 100,000 rpm... so that is subtracted from the gain to find net power increase.

Q4. I assume you mis typed this question and meant to place 'turbocharger' in the first part...I would say that within reason that is a true statement.... there are always "more and less" efficient rpms, air flow speeds, etc... but in general yes....

Q5. A well designed turbo needs plenty of lubrication to its bearing to survive 100,000 rpms for many years... when that bearing wears or the seals which keep the oil at that bearing give way... then that (hopefully plentiful) supply of oil may be able to get into the exhaust manifold.... which of course makes lots of smoke....

Upon further reflection I suppose that same oil could also escape into the intake air stream... thus actually burning inside the engine... again creating lots of smoke...