[railguy]

I live at nearly 8,000 ft. elevation and regularly go to 10,000 feet plus. Humans, unlike engines, can somewhat acclimate to high elevation. The main difference is that those of us who live at high elevation continually have considerably higher red blood cell counts--more cells to carry more oxygen at high elevation. My Dr. tells me that, for example, my red blood cell count would be considered abnormal for a person living at low elevation. By the way, people who spend their whole lives at high elevations typically have fewer heart-related problems, not more.

Now, as to internal combustion engines. The horsepower losses for naturally aspirated engines are pretty much as quoted above. Fuel efficiency in gasoline engines is another matter. In the old days of carburetors, unless the carburetor was equipped with high altitude jets, the engine would run rich and inefficiently at high altitudes. Today's electronically controlled fuel injected gas engines compensate for high altitude by automatically leaning the fuel mixture to fit the available oxygen supply. As a result a modern electronically fuel injected gas engine will have less power at high altitude, but will consume less fuel, too.

Naturally aspirated diesel engines with mechanical fuel injection could also overfuel at high altitudes, especially under load. Supercharged diesel engines--the normally aspirated 2-cycle EMD engines with Roots Blower as the prime example, would pretty much behave like a normally aspirated diesel because the blower was sized to provide boost at near sea level altitude. Superchargers are powered by the engine itself, and are thus considered "parasitic" on the engine--they take engine power to operate, while turbochargers operate off of exhaust gases and are considered "non-parasitic."

Turbocharging uses exhaust gases to spin the turbocharger, which then pressurizes the air charge entering the engine. Most modern turbochargers are oversized--designed to provide an adequate air charge up to whatever altitude the engine might be expected to operate, probably 12,000 to 15,000 feet elevation. A wastegate is employed to "exhuast" whatever excess air charge there is to atmosphere. The older wastegated turbos were mechanically controlled, but most current generation turbos feature electronically controlled wastegates and variable-vane blades to deliver precisely the right air charge at any altitude, engine RPM or load condition. Simply stated, most turbocharged diesel engines couldn't care less about the altitude at which they are running--they are, insofar as air charge goes, operating at way below sea level altitude. From what I've read, the new Tier 4 GE's have multiple electronically controlled variable vane turbos to help them meet the rigorous Tier 4 emission standard--which, of course, adds a lot of electrical and mechanical complexity, at considerable extra cost, to the locomotive.

So, as to the question posed in this thread, personally, I wouldn't worry much about an altitude-based logarithm for OR physics realism insofar as diesel locomotives are concerned. I would much rather see programming time spent in emulating other diesel locomotive features currently un-modeled in OR--things like variable idle RPM based ambient and prime mover temperature factors (plus others), and Automatic Engine Start Stop (AESS), based on some of those same parameters. Those things affect diesel prime mover operation, regardless of the altitude.

[Mike B]

I think the steam locomotive point originally contemplated was answered, too. Essentially, the engine component would be slightly more efficient at high altitude in terms of power output per steam use. The boiler otoh isn't as clear. And as with i.c. engines I would expect the fire to be affected by the lower oxygen content of the air, effectively running "rich" and smoky, but not as hot, if sea-level firing rates are used. Cleaning up the stack with a reduced firing rate to match the oxygen available, the boiler hp would likely be lower. Given higher engine efficiency and lower boiler output, would total system output increase up to some moderate altitude, then decrease? Sounds like an interesting thermo problem.

Agree, though, that there are probably more important things to work on in OR unless this is a really quick, easy tweak. For diesels, it might be (whatever the adjustment is, it would apply only to normally aspirated engines; assume turbos have enough excess boost to provide sea-level performance up to any practical altitude). Steam could be a little more difficult.

The highest railroads in the U.S. reached a bit over 10,000' altitude (e.g. Tennessee Pass). There are higher ones, but I don't think anybody's working on them for OR; e.g. the China-Tibet line (somewhere above 15K'? they have pressurized cars and oxygen for the passengers), and some lines in South America that also get up to near 15K'.

Humans in good health can usually acclimate up to about 15K'. The altitude sickness zone, however, starts as low as 8K' for people with some lung problems. 10K' limit in the aviation rules is a compromise - problems at that altitude are rare, though they do happen. That's why the 787 is recommended for people who want a comfortable flight; it maintains a maximum of 8K' internally when cruising at 40K' or even a bit more, while most other planes have their ceiling limited by as much by the internal altitude (10K') as by their real flight capabilities. Essentially everybody, even those acclimated to high altitude, needs supplemental oxygen above 15-20K' (including those climbing Everest, which tops out just under 30K').

Edit: obligatory Wikipedia - https://en.wikipedia...ighest_railways