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Old 15-04-2006, 08:39   #31
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Gord,

Thanks for the clear explanation. Do you think engaging the old-fashioned belt-driven mechanical refrigeration system (as well as the high output alternator) would put sufficient load on the diesel to avoid the problems you list?

Jay
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Old 15-04-2006, 08:58   #32
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Old 15-04-2006, 16:29   #33
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Gord,

That was a pretty decent explanation for those of us who don't know the ins and outs of diesels. Thanks!

I am still not sure exactly how the load determines the amount of fuel in the cylinder at a given RPM... Perhaps you might elaborate on that a bit.

For example... what is the difference in fuel consumption motoring at say 2500rpm in calm conditions or in 25knots on the nose with 4 foot seas? Obviously you move slower.... but is the engine working harder under the latter situation and burning more fuel... same RPM?
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Old 16-04-2006, 02:47   #34
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There are a number of diesel fuel delivery (injector/governor) systems*, operating on slightly differing mechanisms. I’m just an uneducated shade tree mechanic, and I’m pretty much worn out ton his topic.
* ie: Yanmar uses a Direct injection, Centrifugal weight governor system

There’s a decent discussion at “Fundamentals of Gasoline and Diesel Fuel Systems”
Part II -- Diesel Fuel Systems: http://www.free-ed.net/sweethaven/Me...Mech01_TOC.asp
specifically chapter 5, “Governors”:
http://www.free-ed.net/sweethaven/Me....asp?iNum=0405

HTH,
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Old 16-04-2006, 17:44   #35
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Forgive me for intruding into this "heated"(pun intended) conversation

I don't know much about diesel engines, but what I do know is that they work differently from gasoline engines in that they basically use the compression of the fuel/air mixture in the cylinder to provide enough heat to ignite it(when you compress something, it creates heat, touch a dive tank right after it's been filled for an example.)

I'm thinking since engines are designed to have loads on them, not having a load on it screws with the compression.

When the fuel gets injected into the chamber, the piston compresses it, creating heat, the glowplug provides the last little bit of heat to push it over the edge and ignite it.

When there isn't a load on the engine, the piston gets pushed down easier, lowering the pressure(and as a result the heat) before all the fuel can be burnt.

when there is a load on the engine, the fuel all has to burn before the pressure lowers since there is more work to be done to push the cylinder down


I may have gotten it completely wrong, like I said I've never done any real research on diesel engines, that's just how they work in my head from the brief descriptions of their workings I've recieved over the years.
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Old 16-04-2006, 18:30   #36
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I am still foggy about the whole cylinder temperature / volume of fuel / and load relationship. Sorry for being dense on this... and thanks in advance for listening.

I now understand that inder load conditions more fuel is delivered to the cylinder and this is more fuel to compress and combust and so the fuel in the cylinder "explodes" with more force and perhaps hotter... more gas compressed inside the same cylinder volume. This is the optimal ratio to insure complete burning and temperature.

I am not clear how the increased load changes the amount of fuel in the cylinder but I can understand why more fuel combusting would yeild more power... and if there was too little fuel the explosion would be weak and perhaps could cause the engine even to stall???

But why don't the engineers design the fuel delivery to be optimal for both idle and load? What happens when you provide more fuel and there is no load? Wouldn't the cylinders burn hotter? Clearly the mechanism to add fuel via the throttle results in higher RPMs and not more fuel burned per RPM... and this will not sufficiently burn hot enough.

No?
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Old 16-04-2006, 20:11   #37
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and, to the rescue comes the modern diesel engine and things like "common rail" fuel injection with electronically controlled injectors and horrendous fuel pump pressure, the requirement of multi-stage fuel filtration systems and very expensive parts.

edit: did I mention "no water"...
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Old 17-04-2006, 01:44   #38
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DefJeff, Gord and a few others here have given you very good answers. Go take a good long look at some of the links posted. I also suggest you take a look at the site "how stuff works" and look at the deisel engine.
To quickly explain about the load a fuel dose relationship however. With a Petrol engine, the throttle governs the amount of Air entering the engine. Carburators work by that air volume sucking fuel in, thus controlling engine speed and power. Modern day engines inject the fuel in and the air and fuel ratio is carefully controlled by the CPU. This is becuase a Petrol engine has a very fine balance between lean and rich fuel mixtures and an extreme either way is harmful to the engine.
A Deisel engine is compleatly different. It has a constant Air supply. The volume of Air is governed purely and simply by the speed of the piston taking in a gulp of fresh air. The speed and power is derived by controlling the fuel delivered. So the throttle in this instance is controlling the fuel pump and allowing a set amount of fuel to be delivered to each cylinder. OK, so now we have to controll the speed and power. This is simply done by a part in the fuel pump called a Governor. It is actually the Gonernr that is being changed by the throttle. But once the throttle has set the govenor to a postion, the engine RPM then continues to change the Governor, so as the two settle to a constant RPM. If a load is applied to the engine, the govenor alters and opens the fuel rack and allows more fuel to be delivered to the engine and it's speed picks up to maintain RPM.. Load comes off and the reverse happens.

The answers to the "engine under load" argument have been given to you in great detail. They are NOT MYTHS! To not head them is at the peril of your engine. Most all engines you see started from cold in your marina, that have the large amounts of blue smoke issuing from them, will most likely be as a result of people that allow their engines to warm up under idle or no load. The replies given so far have been extremely accurate. There is one other simpler answer. Back to the Petrol engine, Gasoline is very "dry". It has absolutely NO lubricating properties at all. So the the cyclinders of the petrol engine do not easily lubricate at cold, untill the engine oil is splashed up around the piston. The other detrimental thing about petrol engines is the heat during combustion is very much hotter than Deisel. So valve seats and other direct contact areas with teh extreme heat must be heated to operating temp much slower. Otherwise they crack.
Desiel does not have these issue to the same extent. Deisel fuel is much cooler and slower burning. Infact it is very poor at burning and requires a lot of compression heat to cause it to burn efficiently through out it's power stoke cycle. This is partly where the load issue comes in, but only partly. The main issue is not producing the heat, but simply getting the engine warm as fast as we can. the hotter the engine, the more efficiently the fuel burns and the less carbon remains in the burn after the combustion cycle. Desiels are high compression, heavy lumps of very high tolerance peices of metal moveing at high speed. No1 impotance on the list is no leakage past rings. This is partly maintained by wear. Unlike a petrol engine at cold start up, a Deisel is actually required to have a little wear at start up. It keeps the rings bedded into the Hard metal bores. If they don't, the high lubricating properies of the Deisel fuel cause the ring to skate on the surface of the bore and the bore will become contaminated and will glaze. This causes the sump oil to mix into the combustion and create the blue smoke you see from engines.
I am not going to go over others posts. Just add whatr they have said into this and I think you should get a clear picture.
I recomend however, that you follow the Start up procedure of your engine manufacturer, should it be given in the manual. This is very important, as some designs have very specific things taking place to allow the engine to start. For example, one real quirky engine, is the old Volvo 2000 series. You need to pull and release the engine stop before you fire it up. It releases a special "lock" inside the pump that allows an extra dose of fuel to help the engine to cold start. As soon as the engine starts, this "lock" goes back to normal postion. Anyway,
as soon as the engine is started, give it a minute to allow oil pressure to come up and then place it into gear. this places more load on the engine than a compressor will. Of course, it also depends on size of engine. A compressor can take about 5hp. To a 10hp engine, that is 50% of it's power, but to a 50hp engine, it is of little consequence.
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Old 17-04-2006, 06:19   #39
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Alan,

Thanks... that was a terrific explanation. Thank you for sharing that.

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Old 17-04-2006, 08:41   #40
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defjef --

The answer is: with load and proper combustion pressure the rings will flex (like a belleville spring) and the ring face at the cylinder wall will be slightly flexed allowing more of a 'point to point' contact of the ring and the cylinder wall. With no load, the piston ring wont flex as much and the face of the ring to the cylinder wall will be parallel to the cylinder surface.
As an engine ages and wears the end gap of a piston ring becomes larger and larger due to increased cylinder diameter ... and this increasing leakage of combustion gases and lubricants between the rings is what glazes the walls - the glaze is an accumulation of insoluables and deposited hydrocarbons, etc. that gets 'pushed' into the intersticies of micro-grain structure of the exposed base metal of the cylinder wall.

As Gord stated earlier, if you want the sources go take a 4-5 year mechanical engineering course.
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Old 17-04-2006, 09:05   #41
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Excellent summary

I think you salts are doing an excellent job at demystifying the workings of diesels. I must admit I have been horribly deficient in learning about them, basically doing only regular maintenance on mine for 20 yrs.

But the load thing was new to me, if it is in my Volvo manual I missed it. Having just had the top end of the diesel rebuilt I wandered onto these forum to get a little learnin... and the above posts have really begun to connect the dots... dots that I never bothered to even look at.

I suspect that my lower end is not in the greatest shape... 20 yrs of use (and some abuse). I do see a bit of uncombusted oil in the exhaust water and I do see some bit of whitish/bluish smoke in the exhaust which I attribite to problems descibed above... but probably not severe enough to do the bottom end now.

Other than that the engine works reliably. Let's see for how much longer.

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Old 20-04-2006, 12:18   #42
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Defjef-
"Does this apply to all diesels? I see many buses and trucks idling their diesels. One would presume that owners of fleets of diesels would be very careful about these matters."
From the dean of our engineering school, who had multiple PhD's and a large income from "combustion" patents, I was told that the extended idling you see is bad for the engines--but good for the trucks and busses. The problem is that any time you start or stop an engine (diesel or otherwise) you put wear on the bearings (no oil pressure when it starts), wear on the starter and starter relay contacts, spikes in the alternator and starter and electrical system, and the heating and cooling thermal cycles everything, also corroding the exhaust system as moisture builds up from condensate.
So while extensive idling is bad for the engine, the alternative (very frequent starting and stopping) would be worse for the overall maintenance picture. Those engines get torn down and rebuilt routinely anyway, and according to some modern manufacturers (for new pickup trucks) the newer diesels are built differently in order to somehow tolerate it.
But on the typical sailboat auxiliary diesel? Idling under no load is still considered a bad idea. Better to shut it down, or put a load on it. (As long as you're not shutting it down 100x per day.<G>)
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Old 20-04-2006, 13:47   #43
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Trucks and Buses are a little different again. I'll come back to that.
Once an engine is hot, the idling is not....hmmm...so much...an issue.In fact, idling before shut down is good, as this allows the engine to reach a cooler uniform temperature before cooling flow is stopped. Especially for engines that a turbo charged.
OK, back to the Trucks etc. Actually, the idle times of these vehicles are not much when viewed in the bigger scheme of there running life. But a Buss that is used extensively in a city where stop start is the norm, infact doesn't have as long a service life as any Buss or truck that is in long haul service. But on the other side of that card, the start and stopping is counteracted by hard acceleration. A boat engine doesn't change gears, It is set at a constant throttle and will stay there unaltered for a long period. A Buss will be accellerating and the engine put under hard work constantly.
Now here is one of the biggest problems with idling a Deisel engine when hot. Injector shock! This is a shock wave that is sent through the engine drive train, every time the injector inserts a dose of fuel and it ignites. Some engines are designed to cope better than others. But at idle, this shockwave will transfer itself down to the gearbox. For most boats, we don't have complicated gear boxes. So there is not as much "gear slack" as in a complicated gear box. Hydraulic boxes are totaly disconnected from this problem. But a complex multi gear box will have this "wiplash" effect all the time and it causes sever wear on gears. So idleing a truck or buss can be an issue in other area's you don't have on a boat. Although, a simple gear type box on a boat will be seeing some of this issue at idle.
Hope that wasn't boring or over anyones head.
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Old 07-05-2006, 14:46   #44
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Love all the pointers to more information and ideas on this board for engines!

Anyone want a job working on an old Perkins in Darwin in a couple months?
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