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DonTom
09-11-2007, 03:13 AM
Being curious about how diesels work, I found this during a web search, showing exactly what happens in a diesel:

http://www.howstuffworks.com/diesel1.htm

It did answer all my questions except for one. It didn't seem to explain fuel divlivery well enough.

I noticed that the diesel fuel is delivered right at the maximum compressed air. Seems an incredible amount of pressure would be required for the diesel to get into the cylinder. How do they manage that? How much fuel pressure is used in the average diesel engine?

-Don-

Dave Brand
09-11-2007, 04:24 AM
I noticed that the diesel fuel is delivered right at the maximum compressed air. Seems an incredible amount of pressure would be required for the diesel to get into the cylinder. How do they manage that? How much fuel pressure is used in the average diesel engine?


Older diesels use around 400 bar pressure. Modern common-rail systems are running at around 1500 bar. As a comparison, petrol engine injection systems need around 4 bar.

DonTom
09-11-2007, 04:47 AM
"Older diesels use around 400 bar pressure. Modern common-rail systems are running at around 1500 bar. As a comparison, petrol engine injection systems need around 4 bar."

I assume that means diesels use several hundred times to fuel pressure of EFI. Is there a way to convert "bar" to PSI?

-Don-

Eric
09-11-2007, 06:32 AM
"I noticed that the diesel fuel is delivered right at the maximum compressed air. Seems an incredible amount of pressure would be required for the diesel to get into the cylinder. How do they manage that? How much fuel pressure is used in the average diesel engine?"


Great post, Don - thanks!

And as Mike noted, fuel delivery in a diesel is via high-pressure injection. For comparison, older (carbureted) gas engines typically run at (an unregulated) 3-6 psi fuel pressure. Anything more overwhelms the needle and seat and causes flooding in the carb!

New car with EFI operate at around 40 psi or so....

DonTom
09-11-2007, 06:41 AM
"New car with EFI operate at around 40 psi or so...."

But what is the typical psi in a diesel?

-Don-

Eric
09-11-2007, 06:46 AM
"New car with EFI operate at around 40 psi or so...."

But what is the typical psi in a diesel?

-Don-


"The extremely precise fuel metering of the E320's Common Rail Direct Injection (CDI) system -- which operates at as much as 23,000 psi -- shoots an ultra-fine, completey atomized burst of just enough fuel into each cylinder. The development of electronic direct injection is chiefly responsible for cleaning up the E320 CDI's exhaust (and muting the traditional diesel rattle), along with diesel-specific oxidizing catalytic converters that chemically scrub the exhalations. "

DonTom
09-11-2007, 06:59 AM
"The extremely precise fuel metering of the E320's Common Rail Direct Injection (CDI) system -- which operates at as much as 23,000 psi -- shoots an ultra-fine, completey atomized burst of just enough fuel into each cylinder. The development of electronic direct injection is chiefly responsible for cleaning up the E320 CDI's exhaust (and muting the traditional diesel rattle), along with diesel-specific oxidizing catalytic converters that chemically scrub the exhalations. "

What do they use for a fuel pump to get 23,000 psi?

-Don-

Eric
09-11-2007, 07:07 AM
"The extremely precise fuel metering of the E320's Common Rail Direct Injection (CDI) system -- which operates at as much as 23,000 psi -- shoots an ultra-fine, completey atomized burst of just enough fuel into each cylinder. The development of electronic direct injection is chiefly responsible for cleaning up the E320 CDI's exhaust (and muting the traditional diesel rattle), along with diesel-specific oxidizing catalytic converters that chemically scrub the exhalations. "

What do they use for a fuel pump to get 23,000 psi?

-Don-


The systems are made (I'm pretty sure) by Bosch and are pretty neat in and of themselves. I will try to get some schematics from the Benz media site and post them - but my Luddite inability to understand high-tech may prove an insurmountable obstacle!

mrblanche
09-11-2007, 07:32 AM
The pump DOESN'T provide the massive pressure required. The injector provides it. The pump just provides a supply of fuel to the injector. The injecter than provides a metered amount of fuel through tiny nozzles. The injector itself is about the size of a small beer bottle. I've tried to find a photo, but I've struck out.

Anway, the piston in the injector is actuated by a lobe on the overhead cam. The injector is controlled electronically, so that how much of the stroke is actually powered is controlled by the computer.

A mechanic who is checking an injector has to be very careful. You put the injector into a holder, then actuate it with a hand pump that pushes the piston down. The fuel leaves the inector with such force that it can easily penetrate skin.

DonTom
09-11-2007, 08:53 AM
"The pump DOESN'T provide the massive pressure required. The injector provides it. "

I know that everything for a diesel costs more, but what can the pricetag be on some of these injectors?

-Don-

chiph
09-11-2007, 09:51 AM
You don't want to have to replace one. $$

The tolerances and dimensions are so tight that diesels typically have two fuel filters to prevent contaminents from clogging them.

Chip H.

mrblanche
09-11-2007, 10:55 AM
You don't want to have to replace one. $$

The tolerances and dimensions are so tight that diesels typically have two fuel filters to prevent contaminents from clogging them.

Chip H.



Yes, the first one is as much a water separator as anything, the second one is a very fine particulate filter. If you clog one, it will be the first one. It's usually not to hard to drain and replace. The second one can be a bear. And if you run out of fuel, it's usually the first one you need to refill with fuel.

An injector is very expensive. But not a whole lot more than an injector on a diesel pickup. I've had a few go bad, but usually the injector itself wasn't the problem, but rather the o-ring around it, which will cause diesel in the oil.

misterdecibel
09-11-2007, 12:00 PM
An injector is very expensive. But not a whole lot more than an injector on a diesel pickup. I've had a few go bad, but usually the injector itself wasn't the problem, but rather the o-ring around it, which will cause diesel in the oil.


Are those made by Morton Thiocol?

:)

mrblanche
09-11-2007, 02:22 PM
Are those made by Morton Thiocol?

:)


You never know.

Dave Brand
09-11-2007, 02:31 PM
I assume that means diesels use several hundred times to fuel pressure of EFI. Is there a way to convert "bar" to PSI?




Oops,I forgot you still used hte old units over there!

1 bar = 14.5 lb/sq in

D_E_Davis
09-11-2007, 06:14 PM
An injector is very expensive. But not a whole lot more than an injector on a diesel pickup. I've had a few go bad, but usually the injector itself wasn't the problem, but rather the o-ring around it, which will cause diesel in the oil.

I spent several hours lapping the parts of a leaky injector only to eventually find that the thick body was cracked through.

And an engine that is "making oil" has one or more leaky injectors.

mrblanche
09-11-2007, 07:03 PM
And an engine that is "making oil" has one or more leaky injectors.




Very true.

We picked up a new truck in Omaha sometime in the mid-90's. We left the terminal and turned right onto I-80. Going down the first ramp, I had occasion to use the Jake brake, and I knew right away it didn't sound right.

I kept an eye on everything, and it first went down about a half-gallon of oil in the first 10,000 miles. Pretty normal. Then it started "making oil." I argued with the shop that there was a problem, but they said no chance. Finally, when it was over a gallon above full, I took it into the shop in North Little Rock. They found that it had a broken rocker arm, and the errant piece had damaged an injector. They changed it and called me to come pick it up. I pulled out to the front gate, and I knew it still had a problem. Very rough-running. They took it back in, and found a bent valve. A few days later, they finally got it running right. New head on it.