Diesels at high and low altitudes..

[RayH]

At altitude the oxygen molecules are spread further apart, so are the nitrogen and 1% of other gasses. What I was saying that per cubic inch of "air" aspirated into the engine there are less oxygen molecules. There are still excess oxygen molecules in respect to fuel molecules until the altitude at which the perfect F/A ratio is reached, hence less loss of power to some given altitude.

Back up your statements with some basis in fact.

Im in agreement with you. I wasnt singling you out at all. Some people were discussing an "excess of oxygen" at lower altitudes. There is no such thing. Oxygen comprises about 21% of the atmosphere, it doesnt matter what altitude you are at. The atmosphere is less dense at high altitude. The perdentage of oxygen verses other gases in the atmosphere remains the same. There is no "excess" or shortage of oxygen at any altitude, only density changes of the air itself. Less "air" is available to an engine at altitude.

 

[SkyPup]

Back up your statements with some basis in fact.

Fact is you've forgotten about how extremely little time the diesel fuel has to mix with the less air molecules in the combustion chamber. It mixes even less with less air molecules present, leading to black smoke from the "rich" conditions. Advancing injection pump timing a couple of degrees for high altitude operation gives a little more time for the mixing to occur and for a little bit longer burn time, but it is still no enough to produce the power or torque which it produces at sea level.

 

[orezok]

I wonder how those formula one engines at 15,000+ RPM manage to find enough "time" while my 2,600 RPM Kubota doesn't? Thats about .002 second for complete combustion (if you assume about a 90 degree usable rotation during the combustion process). Time is not an issue.

 

[cp1969]

You think a diesel engine has any power at 100:1? No, it is idling.

Total different story under load.

I have read it is more like 85:1 at idle; 35:1 at full power. So there is still a lot of excess air (and O2) in a diesel's combustion chamber even after it's gained altitude to support full power combustion.

Somebody tell me what happened to the fuel that went in...it remained constant and there was plenty of air with which it could combine. What became of it's chemical energy in the fuel if the engine lost power?

Mr. Pup, it probably warms the hearts of a few here to see the "King" and "Crown Prince" of Fuel & Oil in a family disagreement like this. IIRC, we hashed this out once before. But that's the beauty of Alzheimer's...seems like a whole new debate to me. You probably won the first one, but I don't remember the clincher.

 

[SkyPup]

I wonder how those formula one engines at 15,000+ RPM manage to find enough "time" while my 2,600 RPM Kubota doesn't? Thats about .002 second for complete combustion (if you assume about a 90 degree usable rotation during the combustion process). Time is not an issue.

LOL, TIME is the HUGE issue in diesel fuel combustion!

And I'll give you a hint -> it does not have anything to do with F1 gasoline engines @ 15,000rpms......

 

[SkyPup]

The clincher is that the fuel isn't burned and both the horsepower and torque are lower at higher elevations.

 

[orezok]

Yeah, there is no doubt that diesel has a slower flame front than the so called gasoline that the F1 boys use, but the principal is the same. Mix the fuel with the available air (oxygen) in a ratio that is combustable and light it off. Diesel will combust at about 2 1/2% less oxygen level than gas will. Given that the diesel engine starts a significantly better F/A ratio than a gasoine engine and has a much wider range of F/A ratio, it has the advantage as altitude is gained. None of my posts have said that a diesel didn't lose power at altitude, it's just a question of what altitude and how much power.

 

[cp1969]

The clincher is that the fuel isn't burned and both the horsepower and torque are lower at higher elevations.

Why isn't the fuel burned? There is still plenty of O2 to go around.

 

[SkyPup]

Why isn't the fuel burned? There is still plenty of O2 to go around.

A gasoline engine uses a highly volatile fuel that is MIXED with air prior to induction into the combustion chamber and is already mixed in the combustion chamber during intake and compression and ignited at the optimum time by a spark, allowing much higher rpms due to the fact the fuel is vaporized and throughly mixed homogenously with the air and ignited at one time and one place from one source.

In TOTAL CONTRAST, a compression diesel engines non-volatile fuel is much higher density and more viscous and is injected into the combustion chamber on the compression stoke where it is ultimately ignited due to the compressed air reaching high enough temp. This gives very little TIME for mixing, vaporization, and ignition, and burning to occur when compared to a gasoline based 4-stroke since the fuel and air is NOT homogenously mixed. The fuel is NOT ignited at one time in one place from one source, it only ignites where the proper air-fuel mixture has occured at various location in the combustion chamber. This is the reason that RPMs on compression ignition engines is limited compared to the F1 15,000rpm engines previously referred to, there is NOT ENOUGH TIME for proper injection, mixing, ignition, and burning to occur. The fact that there is less air at high altitudes means less mixing and less fuel being burned with the resulting black smoke out the exhaust.....an IDI design compression ignition engine is worse in this regard compared to a direct injection diesel engine, most smaller tractors are IDI designs.

Electronically controlled diesel engines have an atomspheric sensor to take into account the higher elevations and electronically reduce the amount of fuel per injection event appox. 5% for every 3,000 foot increase in elevation above sea level. The fuel injection timing is also concurrently advanced to allow for a miniscule increase in time for mixing, ignition and burning to occur. But this still results in less horsepower and less torque in direct correlation to any increase in elevation due to less fuel being burned in the combustion chamber.

 

[SkyPup]

Here is a nice piece from Sir Harry Ricardo who invented the Comet IDI diesel engine combustion chamber that basically all diesel engines produced are based on. It should give you a little insight into what is occuring inside a compression ignition combustion chamber during the fuel injection event, the mixing of the fuel and air, and the ignition and burning of the fuel when it reaches the proper temperature and mixture ratio to allow for ignition and burning to commence.

Gasoline and diesel engines both lose the same amount of power at higher elevations, however diesels are even more sensitive to this loss of this of atomspheric pressure than gasoline for the reasons mentioned above. That is one of the reasons there have not been very many diesel powered aircraft, since aircraft have to perform at a variety of different atomspheric pressures and temperatures, diesels are generally designed as stationary engines to operate at one basic atomspheric pressure during their use....