Poll : How much you know about diesels??? - PeachParts Mercedes-Benz Forum

#1 08-24-2002, 11:32 AM

OK here is the big question
is the injection timing on all diesels ATDC or BTDC???
Not talking about the specs you see in the book
use your heads...

P.E.Haiges · #2 08-24-2002, 12:10 PM

You will have to define at what engine speed you are asking about because the injection timing varies with engine speed.

P E H

leathermang · #3 08-24-2002, 01:07 PM

Well, the " all" in your question makes me wary of guessing....

But I will guess ATDC as compared to before because I think spraying before may cool the air too much to have complete combustion....

But what about those two cycle diesels ? Wouldn't they have to be BTDC if they are using ports to introduce the mixture ?

#4 08-24-2002, 02:04 PM

Lets stick to 4 cycle diesels
at idle rpm

P.E.Haiges · #5 08-24-2002, 03:40 PM

Leathermang,

2 cycle Diesels do not introduce a mixture thru the ports. They only admit air thru the ports and the fuel is injected same as a 4 cycle Diesel.

The Detroit 2 cycle Diesel has overhead intake valves. The air is pushed thru the intake valves into the cylinders by the supercharger at the same time the exhaust is going out the ports at the bottom of the cylinder.

P E H

P.E.Haiges · #6 08-24-2002, 03:44 PM

I would think the timing at idle or stop would be ATDC because otherwise the engine would want to run backward because it would fire before piston reached TDC.

My old 180D would run backwards if it was roll started with the car moving backward.

P E H

The Warden · #7 08-24-2002, 03:50 PM

Quote:

Originally posted by P.E.Haiges
Leathermang,

2 cycle Diesels do not introduce a mixture thru the ports. They only admit air thru the ports and the fuel is injected same as a 4 cycle Diesel.

The Detroit 2 cycle Diesel has overhead intake valves. The air is pushed thru the intake valves into the cylinders by the supercharger at the same time the exhaust is going out the ports at the bottom of the cylinder.

P E H

Actually, it's the other way around. The overhead valves on 2 stroke Detroits are exhaust valves (there are 2 valve versions and 4 valve versions). The blower (yes, they're supercharged, but it's necessary for their operation, so most people consider non-turbo'ed Detroits to be naturally aspirated) blows air into the crankcase, and when the piston gets below the ports in the cylinder walls, the blower pushes the air into there, forcing the exhaust out of the exhaust valves and into the exhaust manifold.

Only problem is the positive crankcase pressure and the ports combined with oil control rings make for an engine that has a bit of an appetite for oil. As many people have put it, the only two-stroke Detroit that doesn't leak or use oil is the two-stroke Detroit that has been freshly rebuilt and has yet to be fired off.

As to the first question, at idle, theoretically, it should be after TDC. Given the higher pressures with a diesel and the fact that the fuel's introduced independently of the air, it's actually possible to start and run a diesel backwards. I read somewhere that a guy accidentally did this when he push-started his VW diesel with the gearshift in reverse instead of 1st. Engine ran, until he realized that he had 5 reverse gears and one forward gear. With the fuel firing before TDC at idle, I can imagine it firing the cylinder and pushing it back down the wrong way...and a healthy diesel may wind up being able to actually run based on that.

Just a thought.

P.E.Haiges · #8 08-24-2002, 04:14 PM

The Warden.

I stand corrected on the intake valves in Detroit Diesel engines.

I would have designed a 2 cycle with intake valves (less chance of burning them) and the exhaust thru the ports out the side of the engine into an exhaust manifold. I guess that's why I thought it that was the way it was done on the Detroit Diesels.

I don't get the idea of blowing the air into the crankcase. Why not directly from the supercharger thru the ports into the cylinders? Seems like that would eliminate the leaking and oil consumption problems. Also with the air going thru the crankcase, you would tend to get more dirt in the crankcase and lubrication system.

P E H

190D22 · #9 08-24-2002, 11:37 PM

I would have never thought of a diesel running backwards. But it makes sense and is quite a funny concept. Does this hurt the engine?

I obviously know nothing about diesels because all that you're talking about has me lost!

autozen · #10 08-24-2002, 11:56 PM

Mclarenf1,

I'm too dumb to understand all the hyperbole in this thread. All I know is that if you remove the # 1 injector line and install a transducer in series and hook up a timing light, you read about 13 degrees BTDC. I hope this answers your question.

Peter

mreid · #11 08-25-2002, 12:57 AM

The crankcase on a detroit diesel is not pressurized by the blower. There is a 'air box' that surrounds the cylinders and the air is forced into the ports from this which is not part of the crankcase.

BlueBabyBenz · #12 08-25-2002, 04:01 AM

Ok, here's this newbie's thinking....

I ask myself, what makes the explosion occur?

Isn't it the fact that the fuel/air mixture has been heated/compressed? And if that is true, wouldn't the fuel/air mixture have to be in the cylinder before the compression reaches maximum? And therefore, doesn't that imply that the injection timing has to be BTDC, which is the point of maximum compression?

After all, after TDC, isn't the compression lessening?

Or am I oversimplifying?

The Warden · #13 08-25-2002, 04:36 AM

Actually, on a diesel, the air is compressed by itself; the air hasn't been mixed with anything unless you have a propane injection setup installed, or have a bad turbo seal and have oil getting into the intake

. The fuel is introduced into the system, through the injector that actually sticks into the combustion chamber (or, in our case, the pre-combustion chamber that also houses the part of the glow plug that actually heats up), at the point in the cycle where you want the cylinder to "fire". Theoretically, at the point that the fuel's injected into the combustion chamber, the air will be hot enough to ignite the fuel right then and there. Actually, Rudolf Diesel's original concept was a "slow-burn" where the fuel was slowly injected into the cylinder beginning at TDC and ending right before the exhaust valve opened. This didn't work very well, and they switched to the brief fine-mist injection of fuel that they use today, shortly after he died.

P.E.Haiges · #14 08-25-2002, 09:09 AM

Mreid,

That makes sense that the intake air from the supercharger does not go into the crankcase in a Detroit diesel engine. Possiblly it is better to have the cooler air surrounding the cylinders than the hot exhaust thus the exhaust goes out the exhaust valves in the head.

I think that most of the larger Diesel engines (locomotives, marine, ETC) are 2 cycle engines. You can get more power out of a 2 cycle engine than a 4 cycle engine of the same displacement and number of cylinders because a 2 cycle engine fires twice as often as a 4 cycle engine.
P E H

#15 08-25-2002, 09:58 AM

I'm not a pro tech, but I've been messing with engines since was in grade school. I have yet to see an engine, diesel OR gas, that is timed ATDC. It seems to me that if you are firing ATDC, you are wasting energy, there is less compression in the cylinder upon ignition.

My $0.02,