I have been reading the engine stats on the big boy and the Alleganey and see that the allegany engine is rated at 7500 hp and the big boy is 6300 hp. now the B/B has a bigger piston, bigger stroke and higher steam pressure than the allegany but is rated >1000 hp less…so what gives???..if steam engines are rated at zero rpm…its all about pressure, and piston area/displacement? correct? …given the bb has bigger pistons, higher head pressure…bigger stroke…how is the allegany rated at such a higher HP??
all you mechanical engineers…get back to me please…
You’re thinking of calculated tractive effort-- 135575 pounds for the UP engine and 110200 pounds for the C&O. Nothing to do with power, which at zero RPM is zero horsepower for any engine.
The 2-6+6-6 supposedly peaked at a momentary 7500 dbhp at 46 mph, and the 4-8+8-4 did 6300 dbhp around 40 mph. Neither of them could come close to their maximum power at low speed.
that is well and good but: I found this in a discussion group and it clearly rates HP based on boiler pressure, bore/stroke/rpm…now the b/b engine stats are bigger than the allegenahy…but the boiler of the alleghany is bigger…although a lower pressure 250 psi vs 300 psi.
so my next question is at what rpm does the b/b run out of boiler (ie the boiler cannot maintain pressure at full stoking)…vs when does the allegehany??..it seems that since design speed for the b/b was 80 mph…then the boiler will maintain pressure at 80 mph at what driver RPM?? since the b/b had bigger drivers…then where does that leave the need of the bigger boiler on the allegany ?? if the top speed was less…the b/b has a bigger bore and stroke and higher pressure…therefore the alleghany cannot deliver hp at a given RPM…
THE HORSEPOWER OF AN ENGINE
The unit of power is a “horsepower” and is defined as the amount of power necessary to raise 33,000 lbs. one foot in one minute. The horsepower of an engine is equal to the total pressure on the piston multiplied by the number of feet it travels per minute and divided by 33,000.
Which is a characteristic of every reciprocating engine. Your internal combustion reciprocating engine in your car develops it maximum power several thousand RPM above zero, and likewise develops zero power at zero RPM.
Electrical motors generate their most power and torque at zero RPM with decreasing levels of both power and torque as the motor’s RPM increases.
An electric motor that’s not turning is producing zero power. It’s absorbing a bit of electric power (converting it to heat), and it’s producing torque, but no power at the shaft.
If we could assume 50% pressure in the cylinders of a steam locomotive we could calculate its horsepower same as he did for the Case engine. We can try that assumption for a few locomotives and see how it comes out-- I’ll do some tomorrow.
Steam locomotives produce their greatest tractive effort at low speeds, but their maximum HP at higher speeds. So a steam locomotive is not “rated at zero rpm” if your interest is how much HP the boiler can produce. Stationary, the locomotive produces no HP. As soon as the drivers begin to move lifting some tonnage behind the locomotive, when the ‘cut-off’ is set to the highest setting, the torque about the driver axles is the highest (I think…someone can correct me…)
Most steam locomotives produce their highest rated HP somewhere between 40-60 mph, depending on the driver diameter and how fast it is running. Also, the cut-off setting increases the efficiency of a steamer running freely at speeds in the upper quartile of its speed range, to the maximum speed permitted by physics, metallurgy, lube, and the trailing tonnage. Just because an engine is moving quickly doesn’t mean it is running as much steam out of the boiler as possible without losing pressure. In fact, quite the opposite…a steamer is likely to use the most steam lifting the tonnage from zero because that is where the valve cut-off is set highest (meaning the valve travels in such a way inside its cylinder that it leaves the inlet port open the longest, allowing the most boiler heat to enter the cylinders). It should be noted that the first several laboured revolutions of the drivers at start-up include open cylinder cocks to eject condensate from the cylinders…even more steam loss! As the train gains
No matter-- any horsepower you calculate for a steam locomotive is just for amusement. If you actually want to know its horsepower you have to measure it somehow.
So, just for amusement, try your thresher formula on a 2-6+6-6. If you assume 50% mean eff pressure in the cylinders at 250 RPM (i.e. 49.8 mph) that comes out to 8615 hp in the cylinders. A bit high, but not ridiculous.
But with a 4-8+8-4, 50% MEP at 250 RPM means 10740 hp in the cylinders, and no one would say that was possible.
Any engine with a high calculated TE will look powerful with that formula-- the big IC 2-10-2s come out to 7854 cylinder horsepower (cylinders 30 by 32, drivers 64-1/2, boiler pressure 275 psi says 10/48 Trains.)
thanks for all your help Tim, as you show above, reality can depart from the theoretical pretty quickly and it gets more strange. The big take away for me is that these were really big, powerful locomotives…and that is cool…
The 7,498 DBHP “rating” for the Allegheny is, as timz states, a momentary/peak reading which was achived while the test train was coming out of a sag. This will tend to overstate the reading to a degree. The overall curve established for the H8’s can be found in Gene Huddleston’s book, The Allegheny Lima’s Finest, pg 204. It shows that a more reasonable maximum reading would be about 6,700 to 6,800 DBHP, depending on how you interpret the scatterplot of DBHP and its corresponding scatterplot of DB Pull on the same page. The direct and indirect heating surfaces for the H8 boiler were larger than the Big boy, so even with the lower working pressure, the H8 could evaporate more water per hour, thereby producing more steam for the cylinders to use.
All things put together, the Big Boy could be reasonably expected to produce about 6,200 to 6,300 DBHP if required over-the-road, but the H8 could do more at about 6,700 DBHP or so.
The PRR Q2’s “horsepower” is actually indicated horsepower (IHP) which is measured in the cylinders. It will always be higher than DBHP which is measured at the back of the tender. Of course, PRR had its own wrinkle in the horsepower game. The test plant in Altoona gave locomotive drawbar horsepower, which is measured at the rear of the locomotive and excludes any resistance involved in moving the locomotive or tender. This type of horsepower is somewhere between IHP and DBHP.
interesting, how did they work? as a true dyno can absorb the output of an engine and convert it directly to kilowatts to get the true shaft HP, but in a locomotive, that would be hard as there is no shaft, just the drivers.–so everything would be moving…could they handle the complete output of the engine?
Thanks, BigJim. When I saw rotorhead1871’s question, I thought of the dynamometer car and the (what I call) the standing dynamometer–but I was not in a position to answer, and certainly not able to provide the detailed information which you presented… You produced evidence that the wheel has been invented; it may yet be possible to improve upon it.
Also, my Locomotive Up to Date of 1920 has a discussion of the engine indicator. Dynamometers are not included in the book, perhaps because such would not ordinarily be maintained in locomotive shops?