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Old 07-10-2001, 06:24 PM
RunningTooHot's Avatar
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Join Date: Aug 2000
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Larry: I just now (days later) saw your response in the thread about diesel longevity – Thank you for sharing your knowledge with us here. I am still on the learning curve about the idiosyncrasies of the MBZ diesels, having previously owned only gasoline engined MBZ’s. I am trying to understand more about the theoretical aspects of diesel combustion as well.

You stated that <<“ Most everything in a diesel is backwards from a gas engine. More fuel versus air causes increased combustion chamber heat in a diesel. In a gas engine increasing air vs. fuel increases combustion chamber heat, and it certainly can increase it to a point where it does much more damage than merely causing a misfire. A lean condition in a gas engine can completely destroy it quickly with holes in pistons and burnt valves.”>>

I’ve ‘been there, done that’ with gas engines at track events. Amazing how quickly you can melt aluminum or toast valves when running a turbo’d engine under severe duty & then get a little transient fuel starvation!

However, its hard for me to comprehend the aspect of diesel operation regarding more fuel = more heat; I mean I can understand this up to a point - but then the lack of enough oxygen from an overly rich mixture should not burn any hotter. That is unless I am not aware of a fundamental difference that can be answered by this question: Do diesels run “lean” even when under *full* load? In other words, does a diesel NOT run out of oxygen before combustion temps get too high? (Thus injected fuel amount is the limiting factor?) Please don’t confuse this question with the obvious – I am aware that diesels inherently run in a lean condition due to no throttling, and that power output at anything less than full load is strictly a function of fuel quantity being injected.

The only other thing that I can think of is the relatively slow burn rate combined with enough oxygen to support it – does an overly rich mixture cause *combusting* (versus *combusted*) gasses to flow over the opening exhaust valve, thus causing the heat induced damage? If this is the case, I can see an opportunity for diesel hot rodders: how about sodium filled Iconel exhaust valves & ceramic exhaust port coatings? .

BTW: I am using a universal grade oil (Rotella T), having learned about it here. I don’t know if the PO did, but most likely not based upon the “quickie lube” type stickers on the car. And you are absolutely right about the staining power of this combo!

Thanks Again!
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Old 07-11-2001, 10:00 AM
Join Date: Aug 2000
Location: Holland, MI
Posts: 1,316
I believe that diesels are 'excess air' machines even at full-rated load. because it's a no-brainer in deciding how much air to flow when too much is OK.

BTW, the ultimate 'excess air' internal combustion engine is the gas turbine. About 10%-15% of the air is used for combustion, the rest is for cooling the combustion by-products and turbine wheel. Their response to a step load is phenominal, as they can produce 4-5 times rated power for a short time! Turbine-powered aircraft have 'throttle' setting that go beyond 100%, for short-time power when it's needed like an emergency go-around. More fuel=more power=more heat. More power & heat for too long=melted parts spewing out the back end.

BCingU, Jim
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Old 07-12-2001, 07:52 AM
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You pose an interesting and logical question. When I taught classes in the Army many years ago, I allowed no "why" questions. Unfortunately this is one of those why questions that I can't answer.

I understand your puzzlement. I am very confident that what I say is true, but I'm not aware of how the heat is increased without adding air. I believe the answer relates to Jim's comment. If there is ENOUGH air for combustion, then adding more fuel to the fire makes it hotter.

A campfire has a virtually unlimited supply of air. As you add more logs to the fire, it gets bigger.

When "turning up" a diesel (as the truck drivers call it) you add more fuel to the cylinder which builds more heat and makes beaucoup smoke. I expect that once you pass the point where the incoming air is inadequate, you are also reaching a point where there is much too much fuel in the chamber. The heat is still excessive, plus you are bringing unburned fuel into the picture. Even though diesel fuel is light oil, it is still a solvent relative to the REAL lubricating oil in the engine. This means the diesel is so excessive that it is washing the oil from the cylinder walls and diluting the crankcase with "solvent".

I expect that it is this combination of excessive heat and oil dillution that cuases the quick demise of a "turned up" engine.

Hope this helps,
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