Which Scenario Wears Diesel Engine Less?

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It still strikes me that this makes more sense as a fuelling option. Like a comment above says, the exhaust temp is hotter. Note; hotter exhaust doesn't necessarily mean there is more heat (as in thermal energy) passing through the exhaust manifold, because there are less combustion gases flowing.

My guess for what you end up with is; a) a more enriched combustion in those cylinders that are fuelled (guaranteeing better cold weather combustion), and b) a lower throughput of combustion gases, thus hotter exhaust (because you still have to generate the same power to idle the engine at the same speed, you're just doing it with less exhaust gas volume), thus maximising transfer of exhaust thermal energy to the exhaust side of the engine - thus faster warm-up.

So there is extra fuel going into each cylinder, but not necessarily the engine as a whole. In fact, I think the total fuel going in would probably be less (if we were talking about equivalent idle speeds) because the volumetric efficiency of the fuelled cylinders would be equivalently higher with more fuel [higher combustion temperature] being fed into each.

I do feel the 'extra-load-on-the-engine' argument for why it is used to be illogical, because you either have enough power generated that matches the idling loads, or you don't in which case the idle speed changes. The idling loads are much the same, same pumping losses, same compressive loads, same friction loads for the idling speed. What extra loads can there be when you don't fuel some of those cylinders?

 

[Jasongreat]

I do feel the 'extra-load-on-the-engine' argument for why it is used to be illogical, because you either have enough power generated that matches the idling loads, or you don't in which case the idle speed changes. The idling loads are much the same, same pumping losses, same compressive loads, same friction loads for the idling speed. What extra loads can there be when you don't fuel some of those cylinders?

No need to 'feel' anything. Let's try this, fill a five gallon bucket with water, use both hands to lift over your head, now try it again using only one hand, which warms you up quicker? Why, the total load is the same?

Here is a clip from a dodge technical service bulletin discussing the new high idle option: Symptom/Condition:

Extended idle operation, especially in cold weather, can lead to stuck valves and bent push-tubes due to insufficient cylinder heat. This allows varnishes/oils to condense on the exhaust valve stems, leading to stuck valves, and damaged valve train components. A new software feature, enabled or disabled through the DRB III (See instructions under Repair Procedure), reduces the chance of valve sticking and improves cab heat warm-up time.

 

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No need to 'feel' anything. Let's try this, fill a five gallon bucket with water, use both hands to lift over your head, now try it again using only one hand, which warms you up quicker? Why, the total load is the same?

I'm with you on that analogy. It's a 'different way' to convert the same amount of energy to deliver the same 'useful' work [albeit idling], but because all the [same] work is done with a half of the resources [hands, or cylinders], those particular resources end up with all the waste heat - viz twice as much. I'm getting pretty confident here that the reason this works out for an engine is because you have a half of the flowing gases, so less heat goes out the tailpipe meaning more must stay in the engine.

 

[Jasongreat]

I'm with you on that analogy. It's a 'different way' to convert the same amount of energy to deliver the same 'useful' work [albeit idling], but because all the [same] work is done with a half of the resources [hands, or cylinders], those particular resources end up with all the waste heat - viz twice as much. I'm getting pretty confident here that the reason this works out for an engine is because you have a half of the flowing gases, so less heat goes out the tailpipe meaning more must stay in the engine.

Would we have half the flowing gasses though? Doesnt it take a certain amount of fuel and air to produce a certain amount of power? Let's say it takes 10hp to rotate the engine, it is always going to take 10 hp to turn the engine, be it through 6 cyl or 3. So the gasses would stay the same, i would think, they would just all be coming from 3 cyl instead of six and on top of that, wouldn't the 3 dead cyl's also be flowing air, adding even more airflow?