Why do some diesels have glowplugs and some not? - Page 2 - PeachParts Mercedes-Benz Forum

#16 02-03-2007, 03:18 AM

Quote:

Originally Posted by cessna5354

I am wondering as to why ???

With the turbo forcing air into the cylinder under pressure, the actual compression ratio goes up. A lower static compression ratio allows more air to be stuffed into the cylinder without pushing compression pressures beyond what's best/safe/needed.

Shorebilly · #17 02-03-2007, 06:27 AM

Quote:

Originally Posted by Brian Carlton

All of the 2 cycle Detroits that I'm familiar with never had any glow system. But, they wouldn't start below 32F. without ether unless they were brand spanking new.

I believe the need for glow plugs is mandatory on an IDI diesel due to the very cold environment of the prechamber. A DI diesel has an even chance of starting without glow plugs, but, it's no certainty on any of them when it gets really cold. Ether is the mode of choice to get them going and it doesn't appear to have any negative consequences if used judiciously.

The OM617 type of injection uses a pre-combustion chamber (Indirect Injection).....

The injector does not spray a pattern as most do...instead it "spritzes" a straight stream....

The stream of fuel oil is designed to hit a little ball within the pre-combustion chamber....

This little ball is also designed to retain heat when the engine is running....

The straight shot of fuel oil is vaporized upon contact with the HOT little ball......

The pencil type of glow plug heats this little ball...unsure as to how the older loop style work, for certain....

So when the engine is cold, the glow plug heats the little ball (initially), the fuel is squirted onto the ball, and vaporized.....and combustion occurs when the compression rises to it's ignition point.....after the engine is running the heat of combustion maintains the little ball temperature.....

SB

Note: I think (note think!) that the reason we get the nailing issue is because a piece of carbon (or other trash) causes a portion of the injected fuel to miss the little ball....and to ignite further into the combustion chamber.....and at a slightly later time.....

cessna5354 · #18 02-03-2007, 06:58 AM

Forced Induction, Thanks
So if some one was to install a blower on a std DD then he runs the risk of damage,
How do they "lower" the compression, shorter connecting rod? Diff piston head? You got me to wondering.. But not like Stevie yet,,,

Pequod · #19 02-03-2007, 08:31 AM

Quote:

Originally Posted by cessna5354

Forced Induction, Thanks
So if some one was to install a blower on a std DD then he runs the risk of damage,
How do they "lower" the compression, shorter connecting rod? Diff piston head? You got me to wondering.. But not like Stevie yet,,,

There is already a supercharger or blower on all naturally or turbocharged Detroit Diesel 2 cycle engines. By utilizing the blower and the scavenger ports that are in the piston liners as well as cast into the engine block, the need for exhaust valves are eliminated. Everyone should google Detroit 2 Cycle and there are more indepth explanation of the processes involved.

Brian Carlton · #20 02-03-2007, 10:17 AM

Quote:

Originally Posted by Shorebilly

The OM617 type of injection uses a pre-combustion chamber (Indirect Injection).....

The injector does not spray a pattern as most do...instead it "spritzes" a straight stream....

The stream of fuel oil is designed to hit a little ball within the pre-combustion chamber....

This little ball is also designed to retain heat when the engine is running....

The straight shot of fuel oil is vaporized upon contact with the HOT little ball......

The pencil type of glow plug heats this little ball...unsure as to how the older loop style work, for certain....

So when the engine is cold, the glow plug heats the little ball (initially), the fuel is squirted onto the ball, and vaporized.....and combustion occurs when the compression rises to it's ignition point.....after the engine is running the heat of combustion maintains the little ball temperature.....

SB

Note: I think (note think!) that the reason we get the nailing issue is because a piece of carbon (or other trash) causes a portion of the injected fuel to miss the little ball....and to ignite further into the combustion chamber.....and at a slightly later time.....

In looking at the diagram for the prechamber, glow plug, and ball, you'll notice that the ball is probably 6-9 mm away from the tip of the glow plug. The tip resides in the air space between the bottom of the injector and the ball.

So, with the plugs on for 15 seconds, the capability of getting the ball to anything close to "hot" is non-existant. It's the airspace above the ball that gets most of the benefit from the glow plug. The fuel must pass through this space on its way to the ball. While the ball certainly helps with dispersion, it can't be the main source of heat to light the charge. The steel could never soak up enough heat within the short glow plug cycle.

rwthomas1 · #21 02-03-2007, 10:30 AM

Knightrider,
Any way you could provide links or any other information on these high compression diesels? I would like to see what they are all about, things like how they are started, fuel used, etc. A search on the internet as well as speaking to some buddies that are mechanics, and a USCG diesel tech have all drawn blanks looking for info on these high compression diesels.....

Thanks, RT

Shorebilly · #22 02-03-2007, 10:54 AM

Quote:

Originally Posted by Brian Carlton

In looking at the diagram for the prechamber, glow plug, and ball, you'll notice that the ball is probably 6-9 mm away from the tip of the glow plug. The tip resides in the air space between the bottom of the injector and the ball.

So, with the plugs on for 15 seconds, the capability of getting the ball to anything close to "hot" is non-existant. It's the airspace above the ball that gets most of the benefit from the glow plug. The fuel must pass through this space on its way to the ball. While the ball certainly helps with dispersion, it can't be the main source of heat to light the charge. The steel could never soak up enough heat within the short glow plug cycle.

I would think that the surface of the ball would pick up a bit of radiant heat in a small space.......and as you pointed out the air within the pre-combustion chamber would be warm/hot.......

Perhaps I am missing something, but that's how I thought this type of pre-combustion chamber works....perhaps I am mistaken, would you care to enlighten me???

SB

rwthomas1 · #23 02-03-2007, 11:17 AM

Quote:

Originally Posted by Shorebilly

I would think that the surface of the ball would pick up a bit of radiant heat in a small space.......and as you pointed out the air within the pre-combustion chamber would be warm/hot.......

Perhaps I am missing something, but that's how I thought this type of pre-combustion chamber works....perhaps I am mistaken, would you care to enlighten me???

SB

Don't know what the little ball does. Other IDI diesels I have looked at, GM and VW, IIRC don't have the little ball. The precombustion chambers don't protrude into the combustion chambers in the GM either. The MB has 5 little holes that all that hot gas has to pass through. The GM has an oval port.... I'd like to know what the difference is. RT

Brian Carlton · #24 02-03-2007, 11:48 AM

Quote:

Originally Posted by Shorebilly

I would think that the surface of the ball would pick up a bit of radiant heat in a small space.......and as you pointed out the air within the pre-combustion chamber would be warm/hot.......

Perhaps I am missing something, but that's how I thought this type of pre-combustion chamber works....perhaps I am mistaken, would you care to enlighten me???

SB

I believe the ball definitely picks up radiant heat from the tip of the plug. But, being that it's steel, I can't heat up to anything close to ignition temperatures in the short period that the plugs are on.

You can easily duplicate this with a glow plug and a battery. Put a piece of steel about 1/4" away from the red hot glow plug. In 15 seconds, I'll bet that you can still touch that piece of steel without burning yourself. If so, then the ball cannot be the ignition source.

The air surrounding the tip of the plug will probably climb up to 200 degrees or so within 15 seconds. Couple this with the added heat of compression and you've got the temperature you need to light off.

The temperature of the ball won't climb 5 degrees during the compression stroke........the temperature of the air will climb hundreds of degrees.

kerry · #25 02-03-2007, 12:00 PM

This is getting to be an interesting thread. I still don't understand why the need for glowplugs in some and not other diesels. It isn't the compression ratio apparently.
My CAT 3208 is in a Bluebird Wanderlodge. It came from the factory with a block heater but no ether injection system, so it seems there was a fair amount of optimism about its ability to start in cold weather unaided, although it does have a Perkins glow-plugged diesel generator which can be used to operate the block heater. I believe the 4 cycle Detroit 8.2 diesel also did not have glow plugs. I've seen an ether injection system on one of those. How about the Cummins 555?

rwthomas1 · #26 02-03-2007, 12:16 PM

My buddy has two Detroit 8-71TI's in a Hatteras. Old engines but way cool with two turbos intercooled and feeding the blower. 425hp and ridiculous torque. No glowplugs and no cold starting system. Dunno if they even have block heaters. They are very cold-blooded and smoke enough to kill every mosquito in the state for a few minutes until warm. They take a good bit of cranking to get them to light off. I know its old tech but the noise they make is great. RT

dannym · #27 02-03-2007, 12:19 PM

Yes, very interesting. Apparently the explanation isn't as simple as one would think.

Brian, your idea of a test wouldn't work unless it was made within the same small space as the precumbustion chamber. Doing it in open air wouldn't be conclusive.
I bet the results of this test would be interesting.

I would imagine the answer lies somewhere between Shorebilly and Brians explanation.

Brian Carlton · #28 02-03-2007, 12:24 PM

Quote:

Originally Posted by dannym

Brian, your idea of a test wouldn't work unless it was made within the same small space as the precumbustion chamber. Doing it in open air wouldn't be conclusive.
I bet the results of this test would be interesting.

The results would be conclusive if you agree that the ball gets heated via radiant heat from the glow plug.

If you believe that the ball gets heated via conduction through the heated air space, then the open air test would not be valid.

The chances of the ball getting much heat via conduction within 15 seconds are just about nil, IMHO.

kerry · #29 02-03-2007, 12:24 PM

Could it have anything to do with the length of the stroke? Shorter stroke, faster compression more heat? Longer stroke slower compression less heat?

fj bertrand · #30 02-03-2007, 12:25 PM

Older cat's (1950-1960's) usually had pony engines that always started, if you could get the pony started...

In the army, lots of mechanics paid for multifuel engines when they used smelling salt (ether) on manifold heater engines...As an investigating officer, I cited quite a few idiiots in Korea in the winter...

so it goes...