Diesels at high and low altitudes..

[orezok]

Another thought on your high EGT, maybe the issue is not overfueling and burning excess fuel in the exhaust mainifold, but underfueling at elevations lower than Bonneville (this certainly fits my postulation about excess air at (some lower) elevation). Maybe the EGT runs hot because the F/A ratio has finally reached it's optimum.

I have seen no facts that confirm that time is an issue, just opinion.

 

[Tom_Veatch]

Another thought on your high EGT, maybe the issue is not overfueling and burning excess fuel in the exhaust mainifold, but underfueling at elevations lower than Bonneville (this certainly fits my postulation about excess air at (some lower) elevation). Maybe the EGT runs hot because the F/A ratio has finally reached it's optimum.

I have seen no facts that confirm that time is an issue, just opinion.

I can't speak to the diesel engine EGT case, but your suggestion has merit with respect to horizontally opposed, air cooled, aircraft piston engines. In fact, the EGT is the most accurate way to lean the mixture at altitude.

Assuming the recommended procedure hasn't changed in the several years since I've been in the cockpit, at cruise altitude the mixture is gradually leaned out until the EGT peaks. Then, the mixture is gradually enriched until the engine is running slightly rich of peak EGT - about 50 to 100*C rich as I recall.

Incidentally, this type of engine, at least the direct drive non-geared engines, are usually red-lined in the 2700-2800 rpm range and cruise in the 2200-2500 rpm range so they turn at about the same speed as the small diesel on my tractor and thus have about the same amount of time available for combustion.

Can't say whether or not there is any correlation between diesels and gas engines in this area, but an EGT rise due to excess fuel (rich mixture) burning in the exhaust manifold doesn't fit A/C engine operational characteristics. The amount of flame coming from the stacks of those engines make me think there is significant combustion going on in the exhast gases, outside the cylinder, in high power, rich mixture operation, i.e. full-throttle takeoff and climb, but the EGT is less than peak under those conditions.

 

[KICK]

as diesel engines go, the idea of lean or rich doesn't apply.

they dont run on any particular fuel mixture, as compared to a gasoline engine which will only run when the air/fuel ratio is within a certain range.

fact is the amount of fuel injected on a diesel determines the engine speed and the power output, up to the governed speeds or the limits of the engine and fuel system design.

example--- your running at full throttle and steady load and you start climbing a hill, the governor will allow more fuel to be injected to try to keep that engine at governed speed. if you reach the point where the engine starts losing speed but you still have it at full throttle, you will still be injecting the maximum mount of fuel , keep losing rpms in this ituation and the engine will start to smoke. you could do this until the point where the engine stalls, you will be pouring fuel in it but it just cant make enough power to continue.

 

[Tom_Veatch]

as diesel engines go, the idea of lean or rich doesn't apply.
...

Very well. Everywhere I used the term "rich" substitute the term "mass ratio of fuel to oxygen greater than stoichiometric". Everywhere I've used the term "lean" substitute the term "mass ratio of fuel to oxygen less than stoichiometric".

More verbose, but perhaps less offensive to diesel sensibilities.

 

[SkyPup]

Tom, compression ignition engines combustion chamber are not supplied with a homogenous stoichiometric mixture of fuel and air, instead they are supplied with a dense nonvolatile mixtue of fuel directly into the air already inside the combustion chamber which goes from extremely rich to extremely lean as it vaporizes and mixes with the air before it can ignite, igniting only as it "leans" out and mixes with the air enough to burn at various different areas within the combustion chamber. A diesels EGT is NOT like a gasoline engine, high EGT in diesels occurs under load when raw fuel is escaping past the exhaust valve when it opens and continues buring in the exhaust manifold, as evidenced by the thick black smoke accompanying the relatively "rich" conditions indicative of overfueling......evidence that there was not enough TIME for injection, mixture, ignition, and burning to proceed normally.

 

[SkyPup]

Another thought on your high EGT, maybe the issue is not overfueling and burning excess fuel in the exhaust mainifold, but underfueling at elevations lower than Bonneville (this certainly fits my postulation about excess air at (some lower) elevation). Maybe the EGT runs hot because the F/A ratio has finally reached it's optimum.

I have seen no facts that confirm that time is an issue, just opinion.

Since you don't believe me and cannot understand elementary diesel combustion chemisty, try going to the Banks Power Turbocharged Diesel engine website which you previously referred to and look up high altitude or high EGT so that you can repeat exactly what I've been describing to you quoted off their description.....

Diesel Hop Up | Banks Power: Why Exhaust Gas Temperature is important when doing a turbo-diesel hop up by C.J. Baker

" Anything that restricts intake airflow, or intake air density, limits the air mass that gets to the cylinders. Think of it as the amount of oxygen getting to the cylinders to support the combustion of fuel. This could include: a dirty or restrictive air cleaner, a partially blocked air intake, high outside air temperature, high altitude, restricted airflow to or through the radiator or intercooler, and high water temperature. "

"We mentioned earlier that excessive EGTs are due to a rich air/fuel mixture, which can be caused by too much fuel."

"As we pointed out earlier, high EGTs are the result of too much fuel for the available air."

"Excessively high EGTs mean over-fueling...."

"Why is EGT important? EGT is an indication of how hot the combustion process is in the cylinders, and the amount of “afterburning” that is occurring in the exhaust manifold. EGT is also directly related to the air/fuel ratio. The richer the air/fuel ratio in a diesel, the higher the EGT will be. Two things can create a rich mixture under heavy loads or at full throttle: the first is too much fuel, and the second is not enough air."

 

[Tom_Veatch]

Tom, compression ignition engines combustion chamber are not supplied with a homogenous stoichiometric mixture of fuel and air, instead they are supplied with a dense nonvolatile mixtue of fuel directly into the air already inside the combustion chamber which goes from extremely rich to extremely lean as it vaporizes and mixes with the air before it can ignite, igniting only as it "leans" out and mixes with the air enough to burn at various different areas within the combustion chamber.

I understand that the method of supplying and burning fuel in a compression igniton engine is totally different from a gasoline engine. That is precisely the reason that a diesel engine can run at fuel/air ratios much leaner than is possible with a gasoline engine. I'm not arguing that. In fact, I'm not arguing or trying to advance any point of view whatsoever. I've simply stated things I have experienced with respect to gasoline engines (specifically aircraft engines) and explicitly stated there was no necessary correlation between those characteristics and those of diesel engines.

Note: The ratio I'm speaking of is the macroscopic, overall mass ratio of the fuel injected to the oxygen contained in the cylinder. I'm not referring to the microscopic, localized ratio at the (many) combustion loci.

A diesels EGT is NOT like a gasoline engine.
...

Thank you. I'd hoped someone would respond to the obvious differences between my observations of gasoline engine EGT behavior and what had been stated earlier in the thread.

From the implications of that response, I believe there is no conflict between what you've said and what I said in my first post in this thread. I am certain there is no conflict if you agree there is some critical altitude below which an engine can develop X% of it's maximum power and above which it can not. Pick any percentage less than 100% and the statement holds, although the "critical altitude" will decrease as the selected percentage increases.

If that is true, it obviously also true that an engine which is developing X% power at sea level can continue to develop X% power until it reaches that critical altitude, whatever that altitude might be.

 

[SkyPup]

There is no conflict whatsoever, all internal combustion engines lose about 5% of horsepower and torque for every 3,000 foot increase in elevation above sealevel.

There is no such thing as a "critical altitude" where an internal combustion engine is suddenly "derated" from its output @ sealevel, it is a progressive increment as the altitude increases the power goes down......

So at 3,000 feet, both gasoline and diesel engines have X%-5% the power they once had when they were at sealevel.

The increase in Carbon Monoxide emissions from both gasoline and diesel engines as the elevation increases is directly correlated with that fact.

 

[Dargo]

Wow, I can see that this conversation has been pretty well all over the place since I last visited. So, perhaps I won't be publicly lynched for asking a question. Way back when dirt was new, I used to race motorcycles. To be a good racer, I was taught that you needed to know pretty well how everything worked on your bike. So, I was sent to a technical school for internal combustion engines at a relatively young age. I could tear down a 2 stroke and a 4 stroke engine by the time I was 14. By the time I was 16, I was able to change the clutch in my 1970 1/2 (yes, the 1/2 was included) Camaro on my high school lunch break. At a recent class reunion the MC of our reunion spoke to our class of how he got talked into going to my house at least twice to help change a clutch in my car. Everyone laughed and seemed surprised that I ended up doing nothing related to racing or turning wrenches.

Anyway, we covered the "thermal engine" for about a month. Most people have no idea that the diesel engine was first called a "thermal engine" and Rudolph Diesel's first patent on the engine referred to it as an internal combustion thermal engine. I think the first version ran on peanut oil. Also, from what I recall, the fuel ratio could vary widely. Since it's been a while my memory may be off, but isn't the agreed optimum air to fuel ratio at sea level on a gasoline engine about 14.7:1 and on a diesel engine about 30:1? I ask that because the diesel engine runs quite a bit more air through the engine cycle than a gasoline engine. Therefore, in my mine, a naturally aspirated gas engine and a naturally aspirated diesel engine could be affected differently at altitude since the diesel engine requires more air on which to run. Without the assistance of a turbo to push more air into the cylinders, it seems that a diesel may suffer more power loss at altitude than a gas engine since its output requires more air.

I know that each engine would have equally less oxygen with which to work at high altitudes, but mathematically, the naturally aspirated diesel engine would have a greater percentage of reduction per stroke. Which, if correct, would cause a naturally aspirated diesel engine to suffer more with a loss of air? The difference being that most on the road diesels have turbos which would help alleviate this reduction in air; especially if it is a newer hybrid design turbo. Then, with the aid of a turbo, the diesel engine would "feel" as if it didn't lose as much power. Right?

 

[orezok]

I think that Tom has nailed it dead on. Most diesel engines are derated at sea level (that is underfueled to promote longevity). They most likely do not reach their "rating" until some altitude where the F/A ratio drops to less than optimum. Excess air at a lower altitude will not increase power as it is the fuel combined with the proper amount of air that produces the output. I doubt that there is a diesel engine in a tractor that will not increase output at sea level if you increase the injector pump output.

Inreased injector output would result in a linear decrease in output at higher elevation since the optimum F/A ratio would immediately exceed optimum as soon as you climbed.

Gas engines on the other hand can not be derated in this manner because of the strict F/A ratio required. They will definitely lose output as altitude increases.