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From Feed Line
No. 7
NIKOLA TESLA'S DISK TURBINE
Tomorrow's Gas Engine
by Gary Peterson
Since its invention
well over 100 years ago the
reciprocating explosive gas engine has handily served mankind as we have sought to replace
raw muscle power with that of the machine. In this type of motor a linear motion is
imparted
to one or more pistons by the compression and explosion of a combustible mixture of
vaporized fuel and air. The energy released by the explosion is transmitted to a crank
shaft which converts the reciprocating movement into rotation. With the passage of time
the primitive device of the 1860s has evolved into a complex marvel of machinery capable
of propelling an automobile at speeds in excess of 300 mph and yet it still bears the same
basic configuration and the same mode of operation as that of its earliest ancestor.
An alternative to the reciprocating engine is the rotary
engine. The most common form of these machines, the conventional bladed turbine, is used
for everything from the propulsion of aircraft and large ships to stationary power
generation. While working in a somewhat different manner as the machine described above,
the end result of its operation is still the same—the creation of torque.
Among the
advantages to be gained from this design option is a reduction in the number of moving
parts. In the rotary engine the piston, connecting rod, crankshaft, and flywheel are
replaced by a single moving component known as a rotor. In direct contrast to the typical
reciprocating engine, a well balanced rotary engine will operate virtually without
vibration. Other advantages include an increase in power to weight ratio and better fuel
economy. On the other side of the coin, bladed turbines are highly precision machines
built to very close tolerances, and thus are much more expensive.
Nikola Tesla's disk turbine, which is said to approach the
ideal rotary heat engine, can be viewed as an inexpensive alternative to the bladed
turbine. It consists simply of multiple shaft mounted disks suspended upon bearings which
position the rotor system within a cylindrical casing. In operation high velocity gases
enter tangentially at the periphery of the disks, flow between them in free spiral paths
to exit, depleted of energy, through central exhaust ports. The slight viscosity of the
moving gas along with its adhesion to the disks' faces combine to drag them along,
efficiently transferring the fuel's energy to the disks and on to the shaft.
The central component of this unique engine, the rotor, is
built up using eight basic components: ported disks, star washer spacers, ring washer
spacers and rivets, all of which constitute the runner subassembly, and the rotor shaft
with its shaft keys, bearings and lock nuts. Fabrication of the runner is fairly straight
forward. The parts are assembled with the aid of a stub shaft that has three keyways
machined in it to line up with three complimentary keyways machined in the center hole of
each disk. The stub shaft's length should be about three times the intended width of the
runner. One end of the shaft is threaded and a shoulder ring is fastened just over a third
of the way in from that end.
Assembly begins by slipping one of the thicker end-disks on
to the shaft. With the rivets inserted the first set of spacers are installed followed by
the first thin disk. Additional spacers and disks are added in sequence with the second
end disk going on last. (An operational note: In addition to providing spacing and support
to the disks, each ring spacer also adds a small amount of lift that helps to propel the
runner around.) At this point half a dozen or more "C" clamps are used to
compress the subassembly so the rivets can be tightly peened down. The next step is
trueing up of the runner's width with a surface cut across the faces of the two end disks.
While it is not as critical, the runner's outside circumference can also be trued up at
this point. Care should be exercised here to reduce the chance of damage.
Any burrs and
irregularities can next be removed with a narrow cutting tool. Now that the runner
sub-assembly is nearly completed all that remains to be done is to remount it on the
actual motor shaft for dynamic balancing. This is done with the aid of sophisticated
machinery through the removal metal from appropriate locations around the runner's
perimeter by the drilling of shallow holes near or directly into the outer edges of the
end disks.
As a starting point, the thickness of the spacers and thus
the dimension of the interdiscular space can be approximated using the depth of boundary
layer of air adjacent to the disks' surfaces. The boundary layer's true depth will depend
somewhat upon the temperature and density of the propelling gas. Drawing on the science of
aerodynamics we learn that the boundary layer on the skin of an aircraft in flight is
approximately .020 of an inch in depth. So, it can be assumed the layer on each side of
the disks is nearly .020" thick also. If the disk spacing were to exceed .040"
there would be a space through which some of the propelling fluid could flow and fail to
effectively interact with the gas molecules making up the boundary layer.
Reduce the
spacing to .040" and the two layers could be said to come in contact with each other.
This sets the maximum limit of spacing. With a spacing of .030", a standard thickness
of 304 stainless sheet stock, the two layers would overlap by .010". The practical
experience of at least one disk turbine builder lends support to the use of .030" for
the thickness of the spacers and the disks as well. [The turbine was
built with a 11" diameter runner consisting of 35 working disks plus
two end-disks.]
The engine rotor housing or casing as described in Tesla's
turbine patent consists of two basic elements, not counting seals. These are a central
ring casting and two end plate castings to which the flange pillow block bearing
assemblies are bolted. As can be seen from the figure an alternative configuration
involves the use of an upper and lower casting. A third option incorporates four castings,
both left and right, top and bottom. Many independent builders choose the first option,
preferring to bypass the casting process and mill all of their housing components from
commercially available stock. Another important element associated with the casing is the
inlet nozzle through which the propelling fluid is introduced. If reversibility is
desired, a second nozzle can be installed for the introduction of fluid in the opposite
direction. Using compressed air or even steam to operate such a motor as described here is
fairly straight forward. All that is needed is a compressor or a conventional boiler as
the source of pressurized fluid. If, however, this motor is to be run on gasoline or some
other explosive fuel it needs an accessory apparatus or fluid pressure generator into
which the fuel and air are injected, to mix and than be ignited. The products of
combustion that are developed, along with steam, if water is also injected, are than
directed through a nozzle into the rotor housing.
Such pressure generating appliances that are used in
conjunction with upstream compressor stages already exist. In them an ignited fuel air
mixture is continuously burned to provide a nearly uniform flame front.
Another important
creation of Nikola Tesla's, called the valvular conduit, simplifies the design even
further by reducing the need for a compressor while also making possible the introduction
of a modified combustion regime. When incorporated at the combustion chamber inlets the
valvular action of this device makes the turbine more like an internal combustion engine.
While introduction of fuel and air proceeds as usual, immediately upon the point of
ignition all of the inlets are effectively closed. This is due to the action of the
valvular conduit which, without moving parts, has the singular property of permitting free
flow to occur in one direction only. After the hot gases enter into the turbine, natural
venting working in combination with an optional compressor or downstream ventilator clears
the combustion chamber and promotes the introduction of another charge.
In such a manner
successive explosions of the fuel air mixture occur and are projected through the nozzle.
The rapidity of these pulses depends primarily upon the volume of the combustion chamber
and the degree of ventilation. In speaking of their frequency Tesla said, "I have
been able to speed up the rate of such explosions until the sound of exploding gasses
produced a musical note." [Editor's note: While it is
relatively easy to build and operate a steam-driven Tesla turbine, this is
not the case with the high- temperature gas turbine. See the Tesla
FAQ for more details.]
What improvements might be made to the basic disk turbine
design? Between 1906 and 1927 Tesla made real progress optimizing the engine.
Nevertheless, it is reasonable to expect that some further work could have a positive
effect on the machine's performance. A first step might be to evaluate the properties of
the propelling fluid as it exists while inside the engine casing. In this way the
interdiscular spacing might be modified in response to the actual boundary layer depth and
physical conditions at and near the disk surfaces. Another possibility lies in working
with the number, size and distribution of the rivets and more importantly the ring washer
spacers that are positioned between the turbine disks. A third area warranting serious
investigation relates to the materials used in construction of the runner subassembly.
It is well known that any increase in the allowable turbine
operating temperature results in higher engine efficiency. Turbine engineers have long
sought exotic materials out of which to fabricate their turbine blades, the most heat
sensitive component. These efforts have resulted in the development of a variety of
suitable materials. One of the best that is presently being used is a complex superalloy
known as Inconel. Its three principal constituents are: nickel (60%), chromium (16%), and
cobalt (8.5%), with lesser amounts of aluminum, titanium, tungsten, molybdenum, tantalum
and cadmium. Inconel has proven capable of sustaining turbine inlet temperatures of 1,832
F. It is interesting to note that some of Tesla's turbine disks were fabricated out of a
material known as German Silver. This hard alloy, once commonly used for tableware, also
contains nickel along with copper and zinc in varying proportions.
No doubt the super high performance heat engines of the
future will be constructed of even more advanced temperature resistant, high strength
materials. There are a number of promising possibilities in this regard.
One prospect is
injection-molded silicon nitride (Si3N4) strengthened with silicon carbide (SiC) whiskers.
Components formed out of this ceramic composite are processed using a technique known as
Hot Isostatic Pressing (HIP). Another candidate is a metal matrix composite of niobium
(Nb) combined with tungsten mesh, or refractory fibers of Nicalon or FP-Al2O3 for
reinforcement. Components made of niobium matrix composites require an iridium coating for
oxidation protection. A third promising contestant that has been identified is a reaction
milled composite called AlN dispersoid-reinforced NiAl. This nickel-aluminum alloy based
material is produced by milling NiAl powder in liquid nitrogen. While actual performance
data are not yet available for the NiAl/AlN composite, tests show that it compares very
favorably with other superalloys that are presently being used. A related material known
as single crystal NiAl has already been formed into turbine blades and could be adopted
immediately. A near term benefit to be derived from the use of this material, as with the
other NiAl compounds, would be a substantial reduction in weight. In this case weight
savings in a conventional rotor blade and disk system would be about 40%.
Furthermore, it
is expected that techniques will be developed to control high temperature deformation of
these oxidation resistant materials. This will result in heat engines with further reduced
cooling requirements and even higher operating temperatures.
Dr. Tesla's engineering legacy when placed in context with
recent developments in the areas of conventional turbine engine design, tooling, materials
processing and electronics establishes a secure platform for the development of a
radically new type of automobile engine and drive train. By adopting an interdisciplinary
approach that incorporates new light weight carbon fiber composite materials, advanced
power electronics and microprocessors in combination with hydraulics and our best electric
motors we can have a form of personal transportation such as the world has never seen.
The
vehicles of the twenty-first century promise to be more efficient, economical, durable,
better performing and easy on the environment than anything we have on the road today!
Do you want to learn more? A number of fine books about
the Tesla turbine and the turbopump are listed in the bookstore section under the
Turbo Machinery heading. Another excellent resource for
anyone who is considering the construction of a Tesla engine or pump or who would like to
learn more about hybrid electric cars is the Tesla Engine Builders Association. For
membership information send a self addressed stamped envelope to the TEBA, 5464 North Port
Washington Road, Suite 293, Milwaukee, WI 53217. An initial membership in the TEBA is $35
with annual renewals for $30. New members receive a 90 page manual containing engine
drawings, and the current issue of the TEBA NEWS. Back issues of the TEBA newsletter are
available for $8.95 each including postage or $7.95 for multiple issues.
The group can be
reached by e-mail at TEBA@execpc.com. |
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