[Genma Saotome]

I stumbled upon a chart showing showing much oxygen is available at different altitudes. It data values ranged from 20.9% at sea level to 12.7% at 4000m -- a 39% drop. Obviously 4000m is rather high but the percentage change across that rise certainly suggests some measurable impact on locomotive performance, even at lower altitude (e.g., it's a 12% drop going from sea level to 1000m).

Would it make sense to vary locomotive performance based on altitude?

[atsf37l]

Let's see. 4000 meters is 13,123 feet. The highest mountain railroad passes are in the neighborhood of 10,000 feet, or 3048 meters. What is the measurement for that? From walking around Cumbres and Lizard Head I know the air is rarified but not nearly as much as at 13k.

[PerryPlatypus]

The following is info that we were given in a university class I took this past semester.

"For naturally aspirated engines operation at altitudes above sea level will cause a significant decrease in available engine power. This power decrease is caused by the decrease in air density associated with increased altitude."

Altitude effect
• Naturally aspirated engines:
– Two-cycle diesel engine, reduce rated rimpull
by 1.5% per 1,000 ft between sea level and
6,000 ft. Above 6,000 ft reduce rimpull by 3%
per 1,000 ft.
– Four-cycle gasoline and diesel engines
reduce rated rimpull by 3% for every 1,000 ft
above 1,000 ft.

• Turbocharged engines:
– Two- and four-cycle diesel engines -- usually
very little or no loss in rated power up to
10,000 ft.
– Turbocharger: a mechanical component
mounted on the engine which forces air to the
piston.

[longiron]

Here are one discussions on the topic:
I've dealt with this issue with regard to stationary steam power - both recip and turbine, located at high altitudes.Rich is correct about the pressure inside the boiler being what matters most, and that is not affected by atmospheric pressure. However, the boiler pressure is a steam engine is dumping its exhaust steam through the cylinders into the atmosphere, which being slightly lower pressure at, say, 15,000 feet, would make the locomotive slightly more efficient - at least on paper. Theoretically at altitude the ratio of boiler to atmospheric pressure would be slightly better, and depending on the design of the pressure control valves for the boiler (some types use a spring to maintain, say, 200 pounds above atmospheric pressure, some are absolute, maintaining 200 pounds, period, etc., and don't vary a tiny bit as atmospheric pressure does). In either case there would be a tiny, tiny advantage, theoretically, at altitude (probalby not enough to make up for what is lost in the combustion, I suspect), and in the latter valve case, there might actually be something approaching a half percent higher net power gain. The effect would be tiny - what I would classify as a tertiary, not even, secondary,effect as altitude changed. I suspect overall if run well it just would not matter up to a reasonable altitude, probably 11 to 12 thousand feet anyway.

And another discussion
The answer to your question is that the efficiency of the "engine" is only slightly affected by the altitude, and is improved at higher altitude. The exhaust pressure is lower, which improves efficiency, but the boiler pressure also has to be lower (not the same as at sea level as some others have posted) and that partially offsets the improvement. However, because the air density at 15,000 feet is about half that at sea level, the amount of oxygen that can be delivered to the fire to burn fuel ( pounds of oxygen per hour) is reduced to about half that at sea level, so the locomotive power output would be significantly less, overwhelming any efficiency improvement.

A steam engine (locomotive is what I assume you mean) consists of several important major component parts, each of which has an influence on overall efficiency and operation. There is the engine itself, which is the cylinders, pistons, valve gear, rods, crank, etc. When folks talk about articulated locomotives, they sometimes refer to the “front” engine and “back” engine, so that might give you an idea of what is actually the “engine”. Then there is the boiler, which must supply the steam to the engine. And since we are talking about a combustion engine (external combustion in this case) there is a third important component that is required to move air into the combustion chamber (firebox), and that is the apparatus in the smokebox (basically a venturi ejector) that uses low pressure exhaust steam to create a draft and draw air into the firebox. Depending on operating conditions, either the engine or the boiler can limit the locomotive's performance, and the boiler can be fuel or air limited (usually it’s air).

The theoretical maximum efficiency of a heat engine is the temperature of the working fluid (steam) at the inlet of the engine, minus the temperature at the outlet, divided by the temperature at the inlet (in absolute temperature units). The inlet is the steam going into the cylinders, and outlet is the steam exhausting from the cylinders. Efficiency of the engine is the measure of the amount of heat energy in the steam that is converted into useful work (moving the train). Efficiency of the locomotive is the measure of the amount of energy in the fuel that is converted into useful work, and includes not only the efficiency of the engine itself, but the efficiency of the boiler in absorbing the heat released by combustion and transferring it to water and steam, as well as the impact of the exhaust jet, which must do work to move air into the firebox. This last part is important, because a portion of the work the steam is capable of doing in the cylinders must be used to drive the “engine” that moves air into the firebox – the venturi jet in the smokebox. All steam locomotives operate with some backpressure on the cylinders, which reduces potential efficiency, in order to have enough pressure at the exhaust nozzle that drives the venturi to generate sufficient draft, which in turn pulls air into the firebox.

At sea level, air pressure is about 14.7 psi absolute (0 psi gauge). Water boils at 212 degrees F. At 15000 feet altitude, air pressure is about half of the sea level pressure, so about 7.3 psi absolute. Water boils at about 180 degrees. Unless your pressure gauge is recalibrated to read 0 at 15000 feet, it will read negative 7.3 psi gauge at 15000 feet when the boiler is not operating and open to the atmosphere.

Some of the posts above, I believe, are a little misleading and/or confusing in regard to the distinction between absolute and gauge pressure. First of all, the boiler design must include the effect of the external pressure, so it’s not correct to say that a boiler designed to operate at 200 psi gauge at sea level can be safely operated at 200 psi gauge at 15000 feet (assuming you have not recalibrated your pressure gauge), because the difference in pressure that the boiler shell sees is 207.3 psi at 15,000 feet, not 200 psi, and the shell steel has to be able to contain the added stress. It’s a small effect, but real. If you want to operate a boiler with a 207.3 psi differential, it has to have a thicker shell. Secondly, most safety valves operate based on the difference in pressure between the inside and outside of the boiler. The difference is determined by the tension in the spring that holds the valve closed, but the outside air pressure also helps to keep it closed. So at 15000 feet, a safety valve set to pop at 200 psi gauge at sea level would pop at about 193 psi gauge, again assuming you have not recalibrated your pressure gauge when you moved the engine from sea level to the higher altitude.

For some illustrative efficiency calculations, let’s assume your engine is running on saturated steam at 200 psig at sea level, with 13 psig backpressure. The inlet steam is at 214.7 psia and the exhaust is 27.7 psia. The corresponding steam temperatures at these conditions (from the steam tables you learned about in Thermo) are about 387.5 degrees F and 245.7 degrees F. To do the efficiency calculation, add 460 degrees to all the temperatures (to convert to absolute temperature degrees), and the math works out to 16.73 %. This is the maximum theoretical efficiency that can be achieved by the engine with these steam conditions. If you now move the engine to 15,000 feet (and do not recalibrate your gauges) the inlet and outlet steam pressures are both reduced by the same amount, so your 200 psig sea level boiler would now operate at about 207 psia and the exhaust steam would be at about 21 psia. The corresponding temperatures are 384.5 and 229.2. The efficiency works out to about 18.4%. So there is a slight improvement in maximum theoretical efficiency of just the engine component of the locomotive.

The boiler component is more difficult to give numbers for, but suffice it to say that the physical size of the boiler and boiler tubes, and the dimensions of the exhaust nozzle and venturi, determine how much air per hour can be drawn through the firebox, and that in turn determines how much fuel can be burned and how much steam can be generated per hour. You could design a boiler specifically to operate at higher elevation, but it would be significantly larger and I would guess you would more than offset any gains in efficiency from operating at the lower pressure. Most importantly, you would need to redesign the smokebox and exhaust system to draw twice as much air into the firebox on a cubic foot per hour basis in order to get the same number of pounds of oxygen per hour to the fire as at sea level, and therefore generate the same number of pounds per hour of steam for the engine. You would need more exhaust steam or exhaust steam with significantly more energy to operate the venture ejector in the smokebox, and that means either operating at higher back pressure, which hurts efficiency, or putting some live steam (200 psi) into the ejector, which also hurts efficiency.

chris

[ATW]

I agree with this view as locomotives don't perform well at higher altitudes vs lower an engineer told me not long ago. Take for example being on top of a mountain on flatland where it gets very cold, the conditions change. In ORTS condition % change the more foggy it gets regardless of dry or wet.

So answer would be for ORTS to be able to change the adhesion conditions or prime mover power based on altitude by option settings, eng parameters or both optional?

I yet have been experimenting an prepping custom ORTS include file physics an have a variable pack of models started with modern GE-EMD 89% complete needing additional parameter includes. But latest I have been giving was there represented ORTSCurtius_Kniffler ( A B C D ) values an stumbled upon some interesting finds. D is pretty interesting an it relates to wheel turns vs cab speeds (needle/digital) but they say it should always be 0.7 but try it in the hundreds an you will see the speedometer an wheels go higher then actual speed but no wheel slip warnings.


Who is in charge an experienced at the adhesion code?

[Mike B]

Related to "density altitude" - i.c. engines are derated at higher altitudes unless supercharged in such a way that there's excess inlet pressure at sea level - and even then, there's some altitude where the supercharger (usually a turbo) runs out of excess boost and engine output starts to drop with additional altitude. The drop is minor but noticeable on land, but very significant for aircraft. There are a number of density altitude calculators online, most oriented at the aircraft crowd but some at hotrodders and others who want to know what their engine is actually producing. Some examples: http://wahiduddin.net/calc/calc_da.htm

Using the calculator from that page for engine tuning and relative humidity:
At sea level (0), 24C, 30.07inHG altimmeter, and 100% RH, you get a correction factor of 1 (100% of rated engine power). Interestingly, that's a "density altitude" of 1257', so the formula can be odd. Raise the density altitude to that of Denver (about 5000' MSL, 26.9inHG altimeter), and your engine produces 87.4% of what it did at sea level (all else being equal). Raise it again to Tennessee Pass (10,424' MSL, 22.82inHG altimeter) and your engine drops to 72% of sea level power.

Again, that's for an engine without a supercharger (usually a turbo) that produces excess boost (which is wasted through some kind of a blowoff device or controlled with a wastegate) at sea level. You can probably figure the excess boost needed to produce rated power at some arbitrary altitude using other parts of those calculators. Which of course is why diesels with turbos were what D&RGW and SP preferred to use over Tennessee Pass. For your car, in the old days, there were alternative carburetor jet settings for high altitude use that partially compensated for the situation; these days, the computer reads the local barometric pressure and makes adjustments in fuel flow - not always with complete success as my Mazda shows when driven into the mountains.

[Genma Saotome]

All very interesting. I know little about diesels and had been thinking about steam locomotives when i asked my question. Obviously water boils at a lower temperature so the amount of heat put into the boiler is less... but as air pressure is also lower does that mean the heat loss is made up (to any degree) elsewhere... smokestack and cylinder exhaust should be a bit easier.

No doubt its all a complex problem... which takes me back to the original question: Is it worth adding tot he code... or would it just be a lot of work for very little noticeable difference?

[B & O GUY]

At 13,000 ft. you'd need an Alpine Sherpa for a fireman. :rotfl: Any thing over 10,000 ft. in an aircraft requires an oxygen supplement mixed with the air to keep passenger's and crew from succumbing to oxygen deprivation. The human element could play a big role in steam locomotive performance at high altitudes. That 10,000 ft. limit on aircraft could have a big safety factor built in, I'm sure, as people have survived at much higher altitudes without an oxygen supplement and survived. But you have to figure in those with health problem's who would ride the plane or train. Heart disease and respiratory problem's would come to mind as limiting factor's. That fireman would have to be very healthy.

Allen :D

[Genma Saotome]

10,000 ft altitude is pretty extreme... maybe Peru into Bolivia; But even 1000 foot altitude has less O2 and anyone who has wandered around the mountains above 5000 feet has noticed the difference. Many, many railroads operate at high altitudes... and all operate above sea level so there is potentially a factor for all of our routes, especially using equipment older than, say, 70 years from the present.

[longiron]

Dave,

I've consulted B&O official steam locomotive trailing tons reference guide and there is no difference between sea level areas (say between Brunswick and Cumberland) and at the Summit of West End grades at 2,258 feet for the same locomotive. So from an operational POV, that much change in altitude doesn't make a difference.

chris