and that he could develop engines that would weigh between one and four pounds per horsepower.

1 to 4 pounds per horsepower.....that sounds like a pretty big spread.

While the 1 to 4 HP figure sounds impressive, that doesn't take into account the source to drive it.

Many of the DIY experimental versions of this motor use compressed air to drive small turbines, but to generate enough power to drive a car requires some very large source of compressed gas or a source of flowing fluid.

This is why you do not see these motors driving vehicles. While I think there are one or two examples of these motors actually employed as a working device, the rest are cosigned to novelties of science. However, they are certainly fun to build.

I have doubts about the 60% gasoline and 95% steam efficiencies, both of which I suspect exceed the Carnot efficiency, and thus would not be possible.

This type of turbine has been public knowledge for a long time, has been thoroughly investigated, but is still not widely used.

So, what's the reason? A vast evil conspiracy or they don't actually work very well?

The link for Laminar-flow disc pumps:

http://www.discflo.com/

These are popular in paper mills.

I think the Italian Tesla Pump is the same idea.

Can someone please explain to me what "125 pounds of steam pressure" is? Is it actual pressure (PSI), mass flow rate (lb/sec), or another measure.

It is a shorthand (and imprecise) way to to describe PSI, which are correct units (as are SI equivalents).

Thankyou for the clarification; as a soon to be Engineer in the rail industry, knowing the terminology associated with steam will be an entertaining history lesson. However something seems to be missing from the design calculations expressed, how are the horsepower ratings of the steam (not the turbine) being calculated to give the efficiency? It would be an interesting excercise to put my hand at to test my knowledge as a near graduate engineer. I understand that steam tables can be used for this calculation, but the whole concept of horsepower to pressure seems an unreliable measure of a power to weight ratio (what is typically the desired metric for an engine); is this perhaps because the infrastructure required to produce a higher pressure steam represents greater weight?

A "boiler horsepower" is defined as 34,025 Btu/h. I think this is because of history (boilers maybe some 20% efficient at the time this definition was adopted). A more modern analysis would compare thermal energy input (so many Btu_in of fuel value) with dynamometer output converted to its Btu_out equivalent. Then efficiency = Btu_out/Btu_in. I haven't run boilers much or recently, so these comments might not be entirely right.

Tesla claimed 5 to 10 horsepower per pound, not 1 to 4 pounds per horsepower! That is a big difference.

"I can build a steam engine that will develop one horsepower for every one tenth of a pound of weight," was the instant and amazing response. "I am now building a double turbine, one with two wheels which must revolve in opposite directions. It is for a special purpose, and I cannot talk much about that, but each wheel develops 200 horsepower, that is 400 horsepower, and it weighs 88 pounds." - Nikola Tesla, from "Motor World", September 18, 1911

Tesla's claim when he started his Tesla Turbine company, was that he could develop 5 horsepower per pound of engine weight.

With recent high-temp carbon nanotube and ceramic materials, it may soon be possible to make over ten times that amount, i.e. 50 horsepower per pound of engine weight, with a Tesla turbine.

In contrast, a modern airplane turbine makes about 2.5 horsepower per pound of engine weight.