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Nikola Tesla's Disk Turbine

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From Feed Line No. 7

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

Nikola Tesla's Disk Turbine

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