Diesels at high and low altitudes..

[Tom_Veatch]

Oxygen doesn't produce power. Fuel makes power.
...

It is true that oxygen (alone) doesn't produce power. But, it is not true that fuel (alone) makes power, either. As a point in fact, perhaps pedantic, the combustion of fuel in oxygen doesn't make power, it releases energy. The function of the engine is to transform that energy into work and power.

The maximum energy that can be released in the combustion process is when all available oxygen combines with all available fuel. If there is either unburned fuel or free oxygen remaining in the exhaust stream, above some minimum amount determined by the combustion efficiency of the particular engine, additional usable energy could be obtained by increasing the fuel/oxygen ratio in the cylinder.

An engine can be "de-rated" or limited/governed by restricting the amount of fuel that can be supplied such that it develops significantly less power at low altitude than it is capable of developing. If that is the case, then as altitude increases (manifold pressure decreases) it is possible that a constant power output could be developed by increasing the fuel/oxygen ratio (constant fuel flow rate/decreasing oxygen) until the point is reached at which all available fuel burns in all available oxygen. Note that, due to combustion inefficiencies, "available" doesn't necessarily mean "all that is present".

The point I'm trying to make is that an engine running an excessively lean mixture at sea level can maintain it's power output up to some maximum altitude where the fuel/oxygen ratio is optimum. Above that altitude the power output will decrease with increasing altitude.

But, it that is the case for a particular engine, it means the engine at low altitude was developing less power than it was capable of producing. In general, if an engine is running at it's maximum power at sea level, it's power output, whether it be Diesel or Otto Cycle, will decrease as the mass of oxygen in the cylinder decreases with altitude.

 

[cp1969]

The "obvious" is that a gas engine has a fuel/air ratio range that, outside of which, it will not run. On a naturally-aspirated gas engine, once the rich limit is reached, the only option available to the operator is to reduce the amount of fuel going in and consequently produce less power because of...??? Buehler? Buhler? Anyone? Ans: Less fuel going in.

This "combustible mixture range", if we want to call it that, is much, much, much, much wider for diesels. (Go to a tractor pull and watch the smoke being generated by the diesels.) At full power, a naturally-aspirated diesel has a fuel air ratio of something like 35:1, a range far beyond which a gas engine would cease to run.

--No naturally aspirated diesel engine that I'm aware of has any means of reducing fuel injection as a function of reduced atmospheric pressure, so the same amount of fuel is injected at altitude as was at sea level.

--Although the amount of oxygen is reduced, there was a surplus to begin with. One would have to climb to an altitude of 20,000 feet just to get the diesel down in the lean range of the gas engine. So...same amount of fuel went in, enough oxygen is there to meet with it, and contrary to Skypup's assertion that there isn't enough time, there is--diesels typically have very long rods in proportion to the stroke of the engine which produces a longer time at TDC than a short-rod gas engine.

Don't introduce supercharged engines--not what we're talking about here. Don't talk about less than full power, either, because if you're using less than full power, you'd never know you'd lost any.

 

[cp1969]

The point I'm trying to make is that an engine running an excessively lean mixture at sea level can maintain it's power output up to some maximum altitude where the fuel/oxygen ratio is optimum. Above that altitude the power output will decrease with increasing altitude.

That's exactly what I've been trying to say. Thanks Tom.

What I meant by "fuel makes power" is that it is the source of energy. As I said in an earlier post, the engine is just a chemical energy converter.

 

[bx23barry]

Oxygen doesn't produce power. Fuel makes power. A gas engine makes less power at altitude because it can send no more fuel than what the max rich fuel air ratio dictates. Now tell me how a diesel engine running 35:1 doesn't have enough oxygen to combine with the available fuel, which did not decrease.

If you don't believe O2 produces power I have a little day hike up Mt. Whitney you can try. Ok at sea level enough fuel is injected (or even more if you like) to ignite all the avalable O2 in the engine (or the other way around if you like) it produce maxium power. At 6000' the same amount of fuel injected would have less O2 available so less of the fuel/O2 will ignite. = less power available. The only way the diesel wouldn't lose power at altitude is if it wasn't fueling enough to produce maximum power at sea level. The same applies to supercharged and turbo engines too.

 

[KICK]

it sounds like the Masses are trying to convert Moses.

 

[ozzie tractor]

ok here is a possibilty which no one has mentioned. maybe the lower atmospheric pressure at the higher altitudes is not pushing the air into the engine as efficiently. now before you start saying that the engine sucks the air in, it doesnt. it creates a low pressure zone in the chamber which th air pressure tries to equal out by pushing air into it. i can see this causing a power drop.

now just because a engine runs at 35:1 it doesnt mean there is a excess of air. if the 35 is dropped to 32 due to altitude you suddenly have too much fuel.

note: I have a poduction dirt bike which i dirt track with, 400cc and 13000 RPM.
the guy next door has a 250 honda road bike which redlines at 19500 RPM. why does my pickup only rev to 5000 RPM, because diesel burns slower. just about all light diesel engines are governed to no more than 5000 rpm for a reason, if the engineers who built these things could physically rev them harder they would.

interesting issue with my pickup, mechanical injected 2.5 ltr turbo, intercooled diesel. now when i get some altitude on it, the performance drops noticeably. after some temp tests i found that the turbo is working near the end of it efficiency range. once it has to spin harder due to atmospheric change it starts to really increase the inlet temp, to small an intercooler cant scrub it away. so it looses power. in this case my diesel and high altitude dont mix.

 

[cp1969]

If you don't believe O2 produces power I have a little day hike up Mt. Whitney you can try.

The Power Bar (made from apple cores and waste newspaper) I ate at the bottom would be what propels me to the top of Mt. Whitney, not oxygen.

Ok at sea level enough fuel is injected (or even more if you like) to ignite all the avalable O2 in the engine (or the other way around if you like) it produce maxium power.

Where is it stated that that is the case with a diesel engine? The amount of fuel injected is chosen by many factors, engine durability being probably the foremost one. MUCH more power can be extracted from a diesel engine by simply adding more fuel. To illustrate this point, look at the immense columns of black smoke coming from a pulling tractor by simply injecting more fuel, you can see that for a normal diesel engine, the maximum amount of fuel is not being injected. In some cases, they even inject additional fuel (propane) into the intake air. How is it that this propane burns if all the available oxygen is already consumed? More fuel could not be added if there wasn't sufficient oxygen for it to burn.

At 6000' the same amount of fuel injected would have less O2 available so less of the fuel/O2 will ignite. = less power available. The only way the diesel wouldn't lose power at altitude is if it wasn't fueling enough to produce maximum power at sea level. The same applies to supercharged and turbo engines too.

As I have illustrated above, normal diesels do not fuel enough to produce maximum power at sea level because we want them to last a while.

 

[BTDT]

I guess along the same lines, that's why if your air filter is dirty you lessen the air to fuel ratio and it smokes more with dimished power. Lot's of interesting input here folks. Good job.

 

[Doc_Bob]

A turbo pumps more oxygen into the combustion chamber. More oxygen plus fuel equals more power then less oxygen plus less fuel.

At altitude, when the air has less pressure and therefore less oxygen (percent stable, but total mass of oxygen decreased). Turbo does not does change oxygen percent (20.9%) but increases the mass of oxygen in the combustion chamber.
Bob

 

[SkyPup]

No problem, your welcome to your opinion that diesel engines are LESS affected by altitude change than gas engines.

I could only get my Volkswagen TDI up to 108 mph during the 4 miles on the top end run at Bonneville due to the elevation when I raced it there, but it easily made it up to 135 mph at sea level........

At Bonneville, I had to change out my fuel injector nozzles from 0.205 microns down to 0.150 microns due to excessive smoke, high EGT and lower power.

At sealevel the 0.205 micron nozzles put out lots more power and torque, plus the EGT is no problem.

Can't run that much fuel at Bonneville though or it would burn a hole through the piston from raw fuel being sprayed on it due to not enough air to mix with the fuel......