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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #11  
Old 11-11-2007
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Since doffe indicated in his second post that the engine compartment had air intakes and they were not blocked, I really doubt that is the problem. It would take a very significant vaccum in the engine compartment to have any measureable effect on the air density and thus the air flow. A dirty air filter would have a much more significant effect. I really suspect something there is something else going on with the engine.

SD, I have to disagree with your statement that air flow through an engine is directly related to compression ratio. Compression ratio has almost no effect on the air drawn in on the intake stroke. The air intake during the intake stoke is determined almost entirely by the bore and stroke (i.e. displacement) and the intake air density. Compression ratio (volume at BDC divided by the volume at TDC) really doesn't affect the intake stroke.
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  #12  
Old 11-16-2007
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Further developments

Thanks for all the thoughts. Let me complicate this further. The reason for my question and thoughts is that I from time to time can see that the exhaust includes a trace of oil or diesel. My initial thought was that it was running rich. However, I have noticed the following symptoms that I would like to get you input on. There is oil dripping from the airfilter for the aftmost cylinder, which is also the airfilter to which the vent from the oil filler cap is connected. So a new thought is that the cylinder is pulling through some oil that is not fully combusted. The question is where it is coming from and why? It is quit a bit of oil and seems to be to much to be coming through the cylinder (also don't think that I have a compression problem. One possibility is that there is oil being sucked in through the vent. Why would that be? One idea is that there should be an o-ring on the oilstick to seal and avoid that any air is pulled through. Is that feasible. Any other thoughts.

As for the lack of air, I have ruled that out as the problem, though I think that could be have been a culprit.

All your thoughts are very much appreciated!

Mattias
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  #13  
Old 11-16-2007
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I have the MD-17C in my boat too. That's the crankcase breather hose that recirculates some of the combustion chamber blowby back to the intake manifold. As the engine wears, there's going to be more blowby. I've seen others put a piece of foam rubber between the breather tube and the intake to capture some of the oil held in suspension in the blowby gasses on other engines. You might be able to rig something like that on yours and then occasionally wring it out. Other than that, it's an old engine that probably has some hours on it and you have to expect a little more smoke.
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  #14  
Old 11-16-2007
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Sorry SD

Yep I have to agree with JimsCAL. The amount of air an engine consumes is directly associated with the swept volume of the cylinders and the RPM. The compression ratio is the ratio of swept volume to compressed volume. An 18:1 compression ratio simply means that the swept volume of the cylinder is compressed into a space 1/18th the size (more or less).

And given the above, a diesel engine does not consume any more air than a gasoline engine of the same size except to say that diesel engines generally turn at a lower RPM.

The oil in the air cleaner is blow-by past the piston rings that is pressurisng the crank case and blowing oil-laden air up through the breather on the filler cap. And the richness is probably being caused by a lower-than-spec compression ratio that is also the result of the blow-by. In other words, this engine is tired and in need of major surgery.

When the oil from the air cleaner is drawn into the engine and burned, it will smoke white/grey. If the injection system is faulty and causing richness, it will smoke black.

Hope this helps
Andre
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Old 11-17-2007
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Omatako and JimsCAL are right on the air volume. A 2.0 liter engine is only 2.0 liters gas or diesel. The diesel just compresses the same volume a lot further than the gas.
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  #16  
Old 11-17-2007
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As a diesel mechanic for 30+ yrs I will make a few suggestions.


1, Add some Diesel fuel treatment. Silo, P.S. or Howes are the best IMO but any will do the same basic job. Put 1.25 to 1.5 the recommended treatment if it hasn't been done in a long time. Run the engine at a load for at least 1/2 hr. after this.

1a, At your own risk on this one Dextron II automatic transmission fluid at the rate of 1 pint per 50 gallons of fuel is an alternative to clean really dirty injectors BUT this will boost the Cetane (diesel equiv. of Octane) rating which is good if you have a bad load of fuel and BAD if your engine is really old and close to an overhaul. Run the engine at a load for at least 1/2 hr. after this method also. At your own risk on this one


2, Make sure the air cleaner (if any) is clean AND make sure any rubber hoses are not collapsing at higher RPM's.

3, idle/no load diesels as little as possible and keep them under a load when running or they will load up w/ fuel. Always let them come to full operating temp. before shutting them down if you need to run it only for a little while for charging etc..

HTH
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Last edited by MacGyverRI; 11-17-2007 at 02:14 AM.
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  #17  
Old 11-17-2007
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Sailingdog...

We have had our disagreements inthe past but you are definitely wrong on the air consumption bit about your diesel engine. Compression ratio has very little to do with the appetite that a motor has for air.

Swept volume, number of inlet valves, and volumetric efficiency are where it's at, my friend, and the diesel is not inherently better at any of them.

At the same revs, and both without a turbo, a 3 litre motor will have about twice the appetite for air that a 1500 cc motor will have. At the same swept volume, the diesel just compreses the air more, that's all, really. There are subtleties betwen the two, but compression ratio really does not give the effect you describe.

Rockter.
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  #18  
Old 11-17-2007
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I'm one of the others who stated that a diesel consumes more air than a gas engine. It's something I heard years ago in a class on diesel engines. At the time, the difference was explained by the fact that a diesel motor has unrestricted air flow on each cycle, while a gasoline engine has a restricted air flow. After reading some of posts, I'm not sure now whether a diesel consumes more air or not.

I do know that I've adjusted a few carburetors in my days and a gas engine at an idle seems to sip air, evidenced by the sound of the air passing the throttle plate, the size of that itty bitty hole that the engine is sucking the air through and the high vacuum in the intake manifold. I also know that some of the older Detroit Diesels used a device that blocked the intake air as an emergency shut-off in case the engine "ran away" which was a problem with Detroit Diesels if one of the injectors in the rack seized up in the open position. My guess is that if you blocked the air intake on a diesel motor to the same extent as an idling gas motor, the diesel would stall or run like crap.

Having said all that, my intuition is that a diesel consumes more air for the same displacement at lower rpms, but that it may be a wash once the throttle plate on a gas engine is wide open but I frankly don't know that to be a fact.
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  #19  
Old 11-17-2007
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erps..

Yes, partial throttle settings will alter the breathing, and a diesel does not have such a throttle principle, but that's nothing to do with compression ratio.

Twice the compression ratio will not mean twice the air consumption. That would mean twice the oxygen in there too, with no turbo needed to do it. Oh I wish. I wish!

It won't happen, and doesn't happen. No.

Rockter.
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  #20  
Old 11-18-2007
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Quote:
Originally Posted by erps View Post

1, I do know that I've adjusted a few carburetors in my days and a gas engine at an idle seems to sip air, evidenced by the sound of the air passing the throttle plate, the size of that itty bitty hole that the engine is sucking the air through and the high vacuum in the intake manifold.


2, My guess is that if you blocked the air intake on a diesel motor to the same extent as an idling gas motor, the diesel would stall or run like crap.

3,Having said all that, my intuition is that a diesel consumes more air for the same displacement at lower rpms, but that it may be a wash once the throttle plate on a gas engine is wide open but I frankly don't know that to be a fact.
1, Throttle plates give that illusion, a diesel controls RPM via the injector or Injection pump, it can't go any faster or take in more air than the injector will allow the engine to spin.

2, that's how an emergency shutdown handle works on a Detroit Diesel (most are 2 cycle). A plugged air filter or soft intake tube (in my 1st post) will cause that also.

3, any exact same sized gas engine will use the same air volume as a Diesel as long as both are w/o a turbo or blower (naturally aspirated).
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