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  #31  
Old 08-27-2002, 02:27 PM
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Injection needs to be timed such that ignition is after top dead center. Injection timing would have to be set BTDC to have any chance for ignition to occur at or slightly after TDC. So what is critical is the timing of the ignition which is going to lag the injection timing by several degrees. Injection timing and ignition timing relationships are a function of engine design and speed, but I would think this lag is always big enough to say that injection must occur before top dead center.

My $.02 worth.

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  #32  
Old 08-27-2002, 07:24 PM
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On the subject of "reversing the cam" -- timing is everything, valve timing included, and while an MB diesel will run backwards (at least the old ones, anyway), it won't run WELL, just stagger along. Probably a good thing if you ever got the clutch in too late on an abrupt start and the car went backwards when you let it out again......! To get decent power and running, the cams have to be changed round so that the valve timing is correct in the "wrong" direction, usually done by rotating the cam on the gear. Rather complicated, I'd guess, but not all that hard. This is why many DEs later had a clutch and reversing gear. Not much better operationally, perhaps, but easier and faster.

Ship diesels don't have electric starters -- they are started with compressed air driven into a cylinder on the power stroke, or a separate engine vis a vis train engines. An electric starter for a 50,000 hp engine boggles the mind, let alone the battery required.

TDC on crank indicators is ALWAYS TDC, absolutely no point in putting a pointer on if it isn't. Accurate within a degree or two Unless bent) even on the cheapest crud current motor out there. Not true of past american engines, where the crankpins were often 0.030" out or more and the lifter bores and cylinder bores could be 0.100" off, as well as up to 5 degrees off prependicular. Check with a dial indicator on the #1 piston if you don't believe me. Did you ever wonder why Chevy 350s used to run rough all the time, even when rebuilt?

The MARKER can be anywhere (it's on the flywheel on the Volvo, about 30 degrees off the top of the engine). This has notrelationship whatsoever with the actual position of the piston -- somebody just picked a convienent (or not, on the Volvo) place to put the pointer.

Pressure in a prechamber will be almost identical to that of the combustion chamber at idle -- the holes in the chamber are large compared to the volume. I don't know that anybody would care if they were different, the injector is in the prechamber and pressure there will certainly be higher than the combustion chamber when the fuel is injected. The point is high velocity rotation of the air mass, not pressure, anyway.

You can get a diesel to run with overly fast injection -- listen to a Ford Powerstroke and you will hear why it isn't a good idea -- very loud knock and excess smoke -- any fixed timing diesel will be loud and smoky except at the design rpm.

Peter
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  #33  
Old 08-27-2002, 09:25 PM
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To understand the timing question you have to understand that there are 4 distinct stages of diesel combustion. They are:

1. Ignition Lag: Fuel is being injected into the cylinder but it has not yet began to burn. This can be shortened by higher injection pressures in new engines (with pilot injection) and good injector spray patterns in our old engines.

2. Uncontrolled burn: This is where all of the unburned fuel in the chamber catches on fire at once. This is where the characteristic diesel clatter comes from. This is why pilot injection has quieted diesels significantly, and on the other hand why a bad spraying injector makes so much noise.

3. Controlled burn: Fuel is burning as it is injected into the cylinder.

4. Afterburn: This is the period after injection has stopped and some remaining fuel continues to burn in the cylinder.

Diesel are theorized as a constant pressure engine (whereas gas engines are theorized as a constant volume engine) this means that in an idealized diesel engine the pressure would remain the same throughout the combustion process. This does not happen in our engines exactly it is just a theory/model. In gasoline engines the combustion happens very quickly (constant volume). This is because all the fuel mixture is there and compressed and when the spark happens all the fuel burns up quickly.

So diesel combustion is a much slower process than gasoline combustion. So the start of injection does not mean the start of combustion, there are many other factors.

The 89-93 Dodge Cummins engine uses a Bosch VE rotary pump and this pump has an automatic rpm-dependent timing advance. The later Bosch P1700 inline injection pump equipped models (94-98) do not have a timing advance. If they did it would be mounted external to the pump much like it is on our Mercedes Diesels. Cummins did not do this because they weren't concerned with the excess noise at idle. The later Cummins Dodges use an electronic rotary pump which does have an injection advance. So only 4 years of the Cummins Dodge trucks didn't have an injection advance.

It is a long jump to say that because an engine does not have an injection advance and is direct injected that it is "constant rpm." Direct injection coupled with extremely high injection pressures make for a fairly wide powerband and efficient running. Someone made the comment that the Powerstroke Ford is smoky and associated it with the fact that they are direct injected. They must not be very familiar with the older IDI Ford/International Diesels. These (which I own) are the smokers.

Direct Injection makes diesels much more efficient. The pre-combustion chamber was something that was invented to use lower pressure injection systems in our light duty diesels to make them quieter. The pre-combustion chamber came about for necessity and the weakness of the technology. Now with new technology it is no longer needed. For example the new CDI Mercedes and the TDI Volkswagen, which are wonderful direct injection engines.

I hope this enhances understanding.
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  #34  
Old 08-27-2002, 09:47 PM
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Kerry,

They probably used the same starter but reverse it electrically as you can do with any DC motor.

I wonder about how the oil pump running backward can pump oil to the engine. That's one reason you don't want to run your MB Diesel backward: no oil pressure.

P E H
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  #35  
Old 08-27-2002, 10:29 PM
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You are assuming a gear pump... how about a piston pump ... or several... and a damping cylinder.... that would not care which way the crank was turning...
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  #36  
Old 08-28-2002, 07:15 AM
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The engines that I spoke of, on which my Dad "reversed the cams" were indeed started by air. I remember this because he told a story of an officer on the bridge that was bringing the ship into port and had no clue about what he was doing. He kept calling from the bridge for reverse, forward, reverse, forward at a very rapid rate. My Dad kept trying to warn him that he would run out of air at this fast cycling rate, and finally did. The ship ran into something as a result, I don't remember what it was.

I don't know if the compressed air pressurized a cylinder or ran an air operated starter motor, but I think it pressurized a cylinder.

This thread has become very convoluted. It seems to me it can be simplified by the fact that the ignition is "timed" BTDC, so that it has time to truly begin the ignition process. The confusion is because there is more than one answer and none of them are simple yes or no.

Have a great day,
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  #37  
Old 08-29-2002, 12:21 AM
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psFred: You must be Yen, as I learned TDC isn't TDC on a particular 6 cylinder gas job farm truck. It was 10 degrees away. But remember the holes or orifices on the pre-chamber hold up changes in pressure like they hold up temperature when glowing. The burn is slow in a diesel compared to a gas job, so the swirl is needed more to keep the unburned fuel near the flame than to keep the flame away from the cylinder after it all gets sucked out of the pre-chamber because of the difference in pressure.

You cam turners need to put the cam shaft back into the block. LarryBible probably remembers his dad calling an engine block "the frame". Put straight gears on the crank and cam shafts, and a pinion gear in between. One end of a shaft on the pinion gear, the other out the frame. A lever with a yoke on one end and a sailor on the other pulls the pinion shaft and a spring puts it back. One end of the cam shaft also through the frame. The non-regulation way was to pull the pinion before the engine stopped, as a cam shaft and push rods were kind of heavy on those big engines. The hard part was setting the propellor shaft brake on the few WWII DEs that didn't have a clutch. A ship coasting through the water will turn the propellor the wrong way with respect to reversing the engine, so the brake had to be hard enough to stop the propellor but light enough to slip when the engine started. They had a small boiler for things like the laundry, galley, and ship's whistle, so the stuff like engine lube oil, fuel oil, and coolant pumps were either run from either baby steam turbines or engine operated piston pumps. I don't know about the gun hydraulics or winches. The ships I served on (DDG and DE/FF) were steamers. Auxilary pumps for the plant were powered by baby turbines but the gun hydraulics and winches were run by 3ph 440V electric motors with sailor power for standby.

A virtual cold one for the person who can tell us how the marine diesel in the attached pic works. You'd need big springs and a wide hood to fit it into your benz. I'll wait a couple days before confirming the answers.
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  #38  
Old 08-29-2002, 01:33 AM
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Larry, if memory serves me I bought a new 77 Dodge 3/4 ton Power Wagon with a 360 and the factory setting was TDC ,I set it 10 degrees BTDC and it ran much better and got better Mileage witch is not saying much.........
William Rogers......
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  #39  
Old 08-29-2002, 09:00 AM
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DaBenz, Ok I will go for the virtual cold one with this guess:

1. the pistons go in at the same time and out at the same time sharing the ignited fuel power stroke.

2. They rotate in opposite directions.

3. This allows them , since they are on opposite sides of the output shaft, to be geared to the output shaft in a manner which would allow, by choosing the size of the gears on their shafts relative to the main output shaft, a transmission effect... ie, they could fire many times in the course of the turning of the final output shaft to provide very high torque without a " transmission" to gear them down...

I am guessing from looking at the picture.....
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  #40  
Old 08-29-2002, 09:19 AM
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The picture looks similar to a triangular train diesel I saw in the train museum in York, England. There were three crankshafts at the corner points of the triangle and at each journal there were two pistons going into the two legs of the triangle for a total of six pistons for each band of cylinders. I can't remember how many sets of cylinders there were but I'm guessing six. This would make a total of 36 pistons. The compression was created by the pistons compressing against each other. I believe the crankshafts were geared to a single output shaft in the center. It was one of the most aesthetically pleasing engine designs I have ever seen.
Can anyone comment on he power production of these opposing cylinder engines as opposed to single pistons compressing against a fixed surface?
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  #41  
Old 08-29-2002, 02:43 PM
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There were three crankshafts at the corner points of the triangle

That was the General Electric "Deltic", developed from the Jumo 205 concept.

In the 1930´s the Junkers "Jumo 205", a 6-cylinder, vertical, opposed-piston supercharged 2-stroke Diesel aero-engine, was used in scheduled transatlantic air service between Europe and South America

Sixty-five years later several lightwieght diesel Aero-engines are currently under developement, one with no wrist-pins in the pistons - now work that one out!!
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  #42  
Old 08-29-2002, 03:16 PM
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I found a good site on the 'Deltic' diesel with a good drawing of the configuration:
http://www.lexcie.zetnet.co.uk/delticengine.htm
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  #43  
Old 08-30-2002, 12:59 AM
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DaBenz:

That is a vertial opposed diesel engine, sometimes used in ships. Works just like any other two-stroke. There is a shaft not shown that runs between the two crankshafts to synchronize them and usually had another gear on it as the output shaft. There were several valveless and sleeve valve variations on this, usually with eccentric rings rather than wrist pins in attempts to eliminate some of the recipricating parts of internal compustion engines. Usually failures, for obvious reasons, but at least some horizontal examples were actually built.

The really fun one to see is a doubleacting diesel -- relatively thin piston running in a LONG cylinder, rings on the piston shaft (no wristpin, either, as the "wrist" action is below the bottom of the whole mess), valves top and bottom, injectors top and bottom, fires on both sides. I don't know if these were two stroke or four stroke, but at least MAN and one Italian manufacturer made them during and after WWII. There is at least one twostroke version that was VERY common is smaller vessels -- characteristic thonk thonk thonk noise.

And by the way, I think the Bosch VE type pump (actually a Yanmar design) is available with and without internal advance. Not sure -- the key would be external feed pump or not. The "usual" VE has an internal feed pump, but I don't know about the Yanmars.

Prechambers are still very much in evidence -- the TDI engines just have them in the piston crown rather than in the head, and use pilot injection, too. Reduction in noise, as usual in mechanical things, equals increase in efficiency. The reason that prechambers are quieter is that fuel ignition is smoother and faster as a result of the high swirl velocity, and better burn effciency is also obtained from the burning mixture being blown into the main combustion chamber. They provide a number of benefits not directly related to low noise. Incidentally, both the newer Volvo and Cummins diesels use prechambers and timed injection. Watch them in the highway sometime, compared to an older diesel.

Peter
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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!

Last edited by psfred; 08-30-2002 at 01:05 AM.
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  #44  
Old 08-31-2002, 11:35 AM
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You folks did well. Remember, I’m not frugal - I’m cheap. So you’ll have to share that virtual cold one.

With the engine in the pic, the lower piston leads the upper by a few degrees, so it’s called a lower crank lead, opposed-piston engine. 2-cycle, “naturally supercharged,” with the cranks rotating in opposite directions. You can see the shafting that connects the cranks, but not the single output shaft at the bottom. Upper part of cylinder is cooled with intake air. Water jacket around the center portion of the cylinder. Not shown is a water-jacketed exhaust manifold mounted to lower part of cylinder. Used in rear engine cargo type vessels, but they never figured out how to get lube oil away from the top “wrist pin.” Sideways orientation was tried in a few civilian ships, but the sailors couldn’t sleep with the sideways vibration. But they had the highest power/weight of any diesel engine.

The modern european diesel aircraft engine probably won't get US certification, at least any time soon. Regulators are justifiably concerned about inconsistent American fuel quality. I have a cousin itching at the bit to put one in his home-built pusher.
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  #45  
Old 08-31-2002, 12:09 PM
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Ok, guys...... since we are talking about precombustion chambers ... can anyone tell me why one of my old Briggs and Stratton gas engines has one ? It is usual except for that... I think it is a K model Briggs.... pre WW2.....how does that help it's working ? It has a normal carb.... but the sparkplug is recessed into the end of a precombustion chamber .... Actually I have two of them... one from a David Bradley walking tractor and the other on an olive green Army generator also WW2 vintage...

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