Engine Design RPM

[russkey]

I've been reading quite a bit on steam engine design, including a fine text from 1910 Russia titled Heat Engines (Тепловые Двигатели) (a very thorough treatise on piston steam power). And yet, nowhere have I seen the way that the engine's RPM is calculated. It is used in power calculations, and yet (at least in that book) the author just throws out a seemingly random RPM number and moves on. Is there a guideline for engine RPM calculation? How does one go about designing an engine for 120 RPM vs. 980 RPM? Or is this something that is figured out empirically once the engine, or a scaled prototype perhaps, is built? What am I missing?

[gondolier88]

Engine RPM in a steam engine is a very hazy field- the length of the con-rod and the resulting angularity means that 4 steam engines with the same components other than differing con-rod lengths will all have different max/cruising RPM's.

However, identical engines can have differing RPM's too; an all cast iron engine with solid piston/piston rod/con-rod and solid shaft journals will have a different rotating characteristics to an engine with hollow piston/hollow piston rod/hollow con rod and hollow shaft journals.

Again, it also depends on it's application- identical engines driving a 30" x 60" 4 blade prop' and a 20" x 60" prop for example would have different max/cruising RPM's.

For a guide to RPM's you can do no worse than going through as many manufacturer's catalogues as possible and comparing engine sizes and RPM's of different designs.

That said, at all sizes anything above cruising RPM and the engine becomes noisy; so if possible size an engine bigger than required and run it slower for the same amount of power a small engine would be giving you at max. RPM, giving you better steam consumption, a quieter run and power in reserve if you need it.

Greg

[fredrosse]

Engine RPM is determined based on the mechanical limitations of the engine, (mostly stresses associated with engine pressure forces and inertia loads), fluid flow limitations in getting the steam into and exhaust out of the cylinder, and by the requirements of the driven machinery.

In general, with acceptable valve porting, there is no reason why a steam engine would not run as fast as modern gasoline Internal Combustion (IC) engines. When IC engines were first built, they followed typical steam engine practice, and ran at a few hundred RPM. Today, most IC engines are rated at several thousand RPM, and smaller engines into the 10,000 - 20,000 or more RPM.

Higher RPM means more power strokes per unit of time, and, if porting is OK, a nearly linear increase in power with increasing RPM.

A few examples:

Racing Steam Hydroplanes, these little engines are similar to model airplane IC engines, and run at similar speed, around 10,000 RPM.

The Whitecliffs Solar Steam Power Project, a 3 cylinder Uniflow engine with bash valves. This engine ran at 1500 RPM, the speed selected because it directly drove an AC 4 pole generator. The machinery was designed for this RPM level (it was a Diessel engine block crank, rods, etc), and the high steam inlet pressure (600 PSI, 40 atmospheres) allowed small high speed inlet valves. The uniflow exhaust ports were also suited to allow enough exhaust port area during the very short duration of engine exhaust. The Whitecliffs engine is one of the the most efficient reciprocating engine designs I have ever seen.

The Domestic Heat-Power Module that powered my house had a single cylinder uniflow steam engine, with automotive poppet inlet valve, and made best power at 1040 RPM. This engine was based on an industrial IC engine block, crank, rods, etc.

On the other side of the spectrum, Liberty ship engines (2500 Hp) were designed to run at about 100 RPM, because this speed best suited the propeller RPM for direct drive of the propeller. Sidewheel steamers usually have very low RPM, again to suit the proper speed of the driven equipment. Large paddle boats usually have design well below 60 RPM.

For our typical steam launches, proper propeller RPM is in the vicinity of a few hundred RPM, so that is where most of us drive directly with traditional reciprocating steam engines. Currently there are reasonably efficient and inexpensive ratio changing devices, such as gearboxes, belt and chain drives, etc. so one could use a high speed engine to drive a low speed propeller, but some of the steam community consider this to be improper.

In summary, the designer can pick virtually any RPM for a steam engine, so long as proper port area is considered, and the mechanical design allows enough strength for the forces encountered. For high speed engines the inertia forces can become very large, and would shake apart many slow speed engines that were not designed for high RPM.

[barts]

There are a variety of factors affecting what rpm a designer selects for a steam plant. For our typically smaller boats, selecting the design RPM to be pleasant is perhaps the most important. I'll go out on a limb to suggest that somewhere between 200 and 400 rpm is a reasonable compromise between size of engine, smoothness of turning effort, "buzziness" and human-scale speeds.

- Bart

[russkey]

...so long as proper port area is considered...

Could you suggest a reference or text which provides more information on this? How is the proper port area calculated?

[fredrosse]

A good reference, although rather old, is the book "Internal Combustion Engines" by Lester Lichty, 1939, McGraw Hill Publishing. It has good discussion on valves and porting, and fluid flow follows the same fundamentals for air, exhaust gasses, or steam. I have been collecting technical books on engines for about 40 years (mostly steam, but IC engines as well), and this one is very good. I am sure there are other more modern texts, however this one would serve your purposes well. I think it is available new in paperback format.

[artemis]

I've got a LOT of books about steam and steamboats, but for engine design the best is Steam Engine Design last published by the Lindsay Publications http://www.lindsaybks.com in 1983 (originally by International Correspondence Schools) but now out of print. Copies may still be found online: try eBay or others like Amazon, etc. As of Sept 30 try:
http://www.ebay.com/itm/Steam-Engine-De ... 51935982f3

Most of the questions you've asked about steam passages, valve opening, rpms, etc. are all covered in detail in this text. Good illustrations with/and part definitions.

By the by, maximum engine speed is determined in large part by the piston speed in feet per minute

[gondolier88]

As well as Audel's Guides from the turn of the 20th century which have many details regarding relationships between steam engine components.

Greg

[russkey]

Thanks for the suggestions! Can't wait for my copy of Steam Engine Design to arrive!
Just to be clear, my current concern is the theoretical calculation of an engine's RPM, not the required RPM of a boat power system. The question is: given "these" materials, mechanical configurations, and steam conditions, what would be the estimated RPM of the engine under some arbitrary load. I gather that this is a difficult question indeed.

[steamboatjack]

A Ron says the usual governing factor is piston speed (average) Rule of thumb would be 500 feet per minute for launch engines, e.g. a stroke of 6ins is 1 foot per rev or 500 RPM. Of course much faster speeds are suitable for enclosed engines and special cases.

Regards
Jack