Short cutoff doesn't generate more power. The more live steam, the greater the power. Short cutoff is more efficient, as it uses more of the energy of the steam, thus using less steam, less water, and less fuel. But power is actually lower. To continue the car analogy, it takes power to get the car moving to speed, but once at a speed, it takes less power to maintain that.
It's also the reason for many compound locos having starting vales that ran both sets of cylinders on live steam, instead of using live steam in the high pressure cylinders and then extracting the still abundant energy of that exhaust steam in the low pressure cylinders. It's much more efficient to use as much enrgy out of a given volume of steam as possible, but you get a lot more power by using more live steam as opposed to extracting more energy by allowing more expansion.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
I don't have a *great* understanding of this (still learning -- see below) so forgive me if I'm telling you something you know. The issue, as I understand it, is not just that early cutoff makes more efficient use of the expansion of steam, but that at higher piston speeds admitting steam for too much of the travel becomes counterproductive.
I understand it to be a bit like the transmission in your car. From a snippet I read (one of the Stauffer NYC books), high throttle at advanced cutoff can be really rough on the equiment. Like trying to start your car from rest in 4th gear. (I believe NYC had reverser positions indicated on the speedometer.)
If no one here can answer your questions, can I suggest this book:
https://www.amazon.com/Steam-Locomotives-Really-Popular-Science/dp/0198607822/ref=sr_1_1
I'm only part-way through it, and it's an excellent book with a lot of information to absorb for into my little pea-brain... which is why I can't answer your question. :)
Aaron
At what throttle setting? If severely pinched, just cracked, your pressure swings would be severe with each admittance event. If substantially wider, there'd be less amplitude swings because more refill flow would be pushing in behind the steam being admitted.
as far as i know, a steam engine throttle is nothing more than a valve controlling the size of an opening. it doesn't regulate the pressure (i.e. limit the pressure). it controls the pressure only to the extent that it limits the flow and depends a lot on the consumption of steam by the cylinders
reducing cutoff more efficiently uses steam by allowing the lesser amount of steam to expand. in other words, when cutoff is reduced by half, there may only be a ~~20% reduction in power from the steam.
(the other advantage of decreasing cutoff is reducing the consumption of steam which means conserving water, fuel and manpower).
my questions are:
does a reduction in cutoff without any change to the throttle result in an increase in steam chest pressure (since less steam is being drawn into the cylinders)?
can reducing cutoff w/o a throttle change result in an increase in power (i.e. tractive effort) due to higher steam chest pressure?
and does this mean that if cutoff were reduced should there be a corresponding reduction in the throttle to limit to the increase in steam chest pressure to maintain a desired speed?
greg - Philadelphia & Reading / Reading