Paul,
I’ve worked with gas bearings. They are excellent for applications with small radial loads and high rpm, which this valve system doesn’t appear to resemble. High pressure admission steam pressing down on your conical valve will collapse any reasonable gas film. Moreover, for operational reasons, we definitely do not want any gas film involved. High pressure steam engines suffer from severe blowby – steam leaking past the piston. You state that your expansion ratio is to be 200 to 1 (we’ll go into that later) which means your acceptable leakage is essentially zero. So, while blowby is a problem in any high pressure steam engine, it’s fatal in yours.
Lots and lots of people have tried rotary valves and, so far, none of have been commercially successful; and none of them were striking for the sort of performance that you claim. Maybe you could make it work if you aren’t running high pressures and temperatures but there’s not a lot of reason to build a commercial steam engine working at low steam conditions since internal combustion already possesses an inherent efficiency advantage over the more efficient high pressure and temperature and pressure engines. Anyhow, Dutcher industries built the most efficient piston steam engine ever encountered and they had significant blowby in the high-pressure cylinder. After doubling the piston rings, they found that they had improved things … a little bit. (And after spending a lot of money on their project, they never did match a typical gasoline engine, let alone a diesel). Their engine still had unacceptable amounts of blowby. And that’s with rings rubbing the cylinder wall, imagine how much steam will leak by if you are intentionally leaving even a small gap.
I’ll accept the 4-link motion but wonder why one wouldn’t use a typical master rod, which is much simpler and has fewer wear points, and is cheaper due to lower parts cost. All those tens of thousands of Pratt & Whitney, Wright, BMW, Nakajima, Bristol, Armstrong-Siddeley, Lycoming, Continental and other engines argue that the current method is highly effective. I’ve been involved with a number of engineering development programs over the years and have found that it pays to minimize the number of new features. That’s what killed Cyclone, they put a lot of ‘bright ideas’ into one basket and never could get the whole thing working.
I’m sorry, 12 cylinders are more costly to manufacture. With an inline engine, we can deck all the cylinders in one pass whereas you require 12 passes by indexing the part each time. The same applies for other operations which can normally be performed in gangs, such as boring, honing and drilling. As you increase the number of operations, the cycle time rises, as does the cost. Manufacturers strive to minimize these times. In any case, you need to manufacture extra pistons, linkages, bearings, screws, pins, and so on. And if you think tolerances aren’t all that important, then you haven’t seen all the CMM machines in an engine plant or its suppliers. Efficiency and durability are far more important today and you need to be holding tolerances measured in a handful of microns.
A Z06 Corvette can generate 670 horsepower on 8 cylinders, and some production 4-cylinder engines can make 300, or better. So, we can assume that 75 hp/cylinder is roughly an upper end for current engines, with roughly 35 on the lower end. These are all 4 stroke engines, whereas a single-acting steam engine is effectively a 2 stroke. Therefore, a 12-cylinder steamer should make as much power as a comparable 24-cylinder ICE having the same mean effective pressure. Twenty-four cylinders is an extremely rare configuration only used when large amounts of power are needed. I noticed that the industry trend is to reduce cylinder count for the number of horsepower, for good reasons. In any case, above about 500 HP, steam turbines become competitive with, and then surpass, piston steam engines. Everything else being even, turbines are simply a better deal for a number of reasons if you are trying to compete with engines producing the same power as a 24-cylinder motor.
If you are looking at a small engine putting out something like 100 horses, then the 12 cylinder is problematic due to the square-cube law. For a given displacement, the surface area goes up rapidly as you increase the number of cylinders and make the cylinders smaller. A 12-cylinder engine has 44 percent more cylinder wall surface than a 4-cylinder engine having the same displacement and the same ratio of bore to stroke. That 44 percent represents added friction and heat loss through the cylinder wall. Heat loss is a bad thing in a steam engine. Something similar happens to the connecting rod bearings. Oh yeah, I forgot, rotary cylinders have more external surface area than inline cylinders, which means you have more surface through which heat can leave the engine (a bad thing).
As for the 200 to 1 expansion ratio, that’s utterly impossible. You are using rotary valves having ports into the cylinder head, which is functionally similar to a slide or piston valve. These ports have height, width, and depth – especially depth; in other words, they contribute to clearance volume. Your rotary valve and cylinder head surfaces must be perfectly conical otherwise they will either leak or bind up. In order to resist bending due to the high pressure and temperature steam sitting inside the cylinder, these ports are going to be fairly deep, so that you have sufficient strength to prevent distortion. For a 200 to 1 expansion, your total volume at steam cutoff has to be ½ of 1 percent of the volume at bottom dead center. The volume in your ports will need to be larger. Just obtaining a 0.5 percent clearance is problematic enough if we ignore the ports, assuming thermal differential expansion as the engine operates and flexing in the lower end components at high output.
There’s another reason that a 200 to 1 expansion volume won’t work; you totally forgot about FMEP (Friction Mean Effective Pressure); this is the mean effective pressure in the cylinder needed to overcome all engine friction and auxiliary loads. At 200 to 1, your mean effective pressure will be much less than 1/200th of the admission pressure. This is because the steam pressure will drop by much more than a factor of 200 during expansion. The average steam pressure during the stroke won’t be enough to overcome the friction of your cylinders (half of which are pushing steam out the cylinder head, which is another force you need to overcome) not to mention the friction on all those bearings. The, of course, you need to drive the feed water pump, burner blower, condensate pump, oil pump, alternator, valve friction, and so on. You are far from the first person to realize that extreme expansion offers high efficiency – but this is theoretical efficiency and not what you will see in the real world.
Even if we ignore all the above, the 200 to 1 STILL won’t work due to ‘port blocking phenomenon’. Steam doesn’t accelerate instantly, nothing accelerates instantaneously. Since it takes a small amount of time to get up to speed, the steam experiences an extreme pressure drop. This is no big deal in an engine with longer cutoff since the following steam makes up most of the deficit. Unfortunately, you are making sure that there is no following steam. There are two consequences to this, the first being that your mean effective pressure is much lower than boiler pressure would indicate, and your power is going to drop dramatically (and you were already having problems with FMEP).
Furthermore, if we yet once again ignore all the above, there’s the matter of temperature drop during expansion. You can only expand steam just so far before it begins to condense. Such condensing is one reason the steam pressure drops in the cylinder more rapidly than the expansion ratio, water is far denser than steam and every volume of steam turning into just a drop of water produces just that much less pressure against the cylinder wall. This is the sort of thing that gets out of hand because steam is a relatively poor thermal conductor while water is pretty good; as water hits the cylinder wall it draws off heat which is then pushed out the exhaust. The incoming steam has to give up energy to reheat the cylinder, which is heat that no longer drives the engine. Turbines having very extreme expansion use reheaters between stages to prevent condensation; Abner Doble also found this necessary and his engine had nothing vaguely close to the expansion you state.
Regards,
Tom
