Most people know Nikola Tesla, the eccentric and brilliant man who arrived in New York City in 1884, as the father of alternating current, the form of electricity that supplies power to almost all homes and businesses. But Tesla was a prodigious inventor who applied his genius to a wide range of practical problems. All told, he held 272 patents in 25 countries, with 112 patents in the United States alone. You might think that, of all this work, Tesla would have held his inventions in electrical engineering -- those that described a complete system of generators, transformers, transmission lines, motor and lighting -- dearest to his heart. But in 1913, Tesla received a patent for what he described as his most important invention. That invention was a turbine, known today as the Tesla turbine, the boundary layer turbine or the flat-disk turbine.
Interestingly, using the word "turbine" to describe Tesla's invention seems a bit misleading. That's because most people think of a turbine as a shaft with blades -- like fan blades -- attached to it. In fact, Webster's dictionary defines a turbine as an engine turned by the force of gas or water on fan blades. But the Tesla turbine doesn't have any blades. It has a series of closely packed parallel disks attached to a shaft and arranged within a sealed chamber. When a fluid is allowed to enter the chamber and pass between the disks, the disks turn, which in turn rotates the shaft. This rotary motion can be used in a variety of ways, from powering pumps, blowers and compressors to running cars and airplanes. In fact, Tesla claimed that the turbine was the most efficient and the most simply designed rotary engine ever designed.
If this is true, why hasn't the Tesla turbine enjoyed more widespread use? Why hasn't it become as ubiquitous as Tesla's other masterpiece, AC power transmission? These are important questions, but they're secondary to more fundamental questions, such as how does the Tesla turbine work and what makes the technology so innovative? We'll answer all of these questions on the next few pages. But first, we need to review some basics about the different types of engines developed over the years. On the next page, we'll get a better idea of the specific problem Tesla was hoping to solve with his new invention.
The Tesla Turbine Engine
The job of any engine is to convert energy from a fuel source into mechanical energy. Whether the natural source is air, moving water, coal or petroleum, the input energy is a fluid. And by fluid we mean something very specific -- it's any substance that flows under an applied stress. Both gases and liquids, therefore, are fluids, which can be exemplified by water. As far as an engineer is concerned, liquid water and gaseous water, or steam, function as a fluid.
At the beginning of the 20th century, two types of engines were common: bladed turbines, driven by either moving water or steam generated from heated water, and piston engines, driven by gases produced during the combustion of gasoline. The former is a type of rotary engine, the latter a type of reciprocating engine. Both types of engines were complicated machines that were difficult and time-consuming to build.
Consider a piston as an example. A piston is a cylindrical piece of metal that moves up and down, usually inside another cylinder. In addition to the pistons and cylinders themselves, other parts of the engine include valves, cams, bearings, gaskets and rings. Each one of these parts represents an opportunity for failure. And, collectively, they add to the weight and inefficiency of the engine as a whole.
Bladed turbines had fewer moving parts, but they presented their own problems. Most were huge pieces of machinery with very narrow tolerances. If not built properly, blades could break or crack. In fact, it was an observation made at a shipyard that inspired Tesla to conceive of something better: "I remembered the bushels of broken blades that were gathered out of the turbine casings of the first turbine-equipped steamship to cross the ocean, and realized the importance of this [new engine]" [source: The New York City Herald Tribune].
Tesla's new engine was a bladeless turbine, which would still use a fluid as the vehicle of energy, but would be much more efficient in converting the fluid energy into motion. Contrary to popular belief, he didn't invent the bladeless turbine, but he took the basic concept, first patented in Europe in 1832, and made several improvements. He refined the idea over the span of almost a decade and actually received three patents related to the machine:
- Patent number 1,061,142, "Fluid Propulsion," filed October 21, 1909, and patented on May 6, 1913
- Patent number 1,061,206, "Turbine," filed January 17, 1911, and patented on May 6, 1913
- Patent number 1,329,559, "Valvular Conduit," filed February 21, 1916, renewed July 18, 1919, and patented on February 3, 1920
In the first patent, Tesla introduced his basic bladeless design configured as a pump or compressor. In the second patent, Tesla modified the basic design so it would work as a turbine. And finally, with the third patent, he made the changes necessary to operate the turbine as an internal combustion engine.
The fundamental design of the machine is the same, regardless of its configuration. In the next section, we'll look more closely at that design.
Parts of the Tesla Turbine
Compared to a piston or steam engine, the Tesla turbine is simplicity itself. In fact, Tesla described it this way in an interview that appeared in the New York Herald Tribune on Oct. 15, 1911: "All one needs is some disks mounted on a shaft, spaced a little distance apart and cased so that the fluid can enter at one point and go out at another." Clearly this is an oversimplification, but not by much. Let's take a look at the two basic parts of the turbine -- the rotor and the stator -- in greater detail.
In a traditional turbine, the rotor is a shaft with blades attached. The Tesla turbine does away with the blades and uses a series of disks instead. The size and number of the disks can vary based on factors related to a particular application. Tesla's patent paperwork doesn't define a specific number, but uses a more general description, saying that the rotor should contain a "plurality" of disks with a "suitable diameter." As we'll see later, Tesla himself experimented quite a bit with the size and number of disks.
Each disk is made with openings surrounding the shaft. These openings act as exhaust ports through which the fluid exits. To make sure the fluid can pass freely between the disks, metal washers are used as dividers. Again, the thickness of a washer is not rigidly set, although the intervening spaces typically don't exceed 2 to 3 millimeters.
A threaded nut holds the disks in position on the shaft, the final piece of the rotor assembly. Because the disks are keyed to the shaft, their rotation is transferred to the shaft.
The rotor assembly is housed within a cylindrical stator, or the stationary part of the turbine. To accommodate the rotor, the diameter of the cylinder's interior chamber must be slightly larger than the rotor disks themselves. Each end of the stator contains a bearing for the shaft. The stator also contains one or two inlets, into which nozzles are inserted. Tesla's original design called for two inlets, which allowed the turbine to run either clockwise or counterclockwise.
This is the basic design. To make the turbine run, a high-pressure fluid enters the nozzles at the stator inlets. The fluid passes between the rotor disks and causes the rotor to spin. Eventually, the fluid exits through the exhaust ports in the center of the turbine.
One of the great things about Tesla turbine is its simplicity. It can be built with readily available materials, and the spacing between disks doesn't have to be precisely controlled. It's so easy to build, in fact, that several mainstream magazines have included complete assembly instructions using household materials. The September 1955 issue of Popular Science featured a step-by-step plan to build a blower using a Tesla turbine design made from cardboard!
But exactly how does a series of disks generate the rotary motion we come to expect from a turbine? That's the question we'll cover in the next section.
Tesla Turbine Operation
You might wonder how the energy of a fluid can cause a metal disk to spin. After all, if a disk is perfectly smooth and has no blades, vanes or buckets to "catch" the fluid, logic suggests that the fluid will simply flow over the disk, leaving the disk motionless. This, of course, is not what happens. Not only does the rotor of a Tesla turbine spin -- it spins rapidly.
The reason why can be found in two fundamental properties of all fluids: adhesion and viscosity. Adhesion is the tendency of dissimilar molecules to cling together due to attractive forces. Viscosity is the resistance of a substance to flow. These two properties work together in the Tesla turbine to transfer energy from the fluid to the rotor or vice versa. Here's how:
- As the fluid moves past each disk, adhesive forces cause the fluid molecules just above the metal surface to slow down and stick.
- The molecules just above those at the surface slow down when they collide with the molecules sticking to the surface.
- These molecules in turn slow down the flow just above them.
- The farther one moves away from the surface, the fewer the collisions affected by the object surface.
- At the same time, viscous forces cause the molecules of the fluid to resist separation.
- This generates a pulling force that is transmitted to the disk, causing the disk to move in the direction of the fluid.
The thin layer of fluid that interacts with the disk surface in this way is called the boundary layer, and the interaction of the fluid with the solid surface is called the boundary layer effect. As a result of this effect, the propelling fluid follows a rapidly accelerated spiral path along the disk faces until it reaches a suitable exit. Because the fluid moves in natural paths of least resistance, free from the constraints and disruptive forces caused by vanes or blades, it experiences gradual changes in velocity and direction. This means more energy is delivered to the turbine. Indeed, Tesla claimed a turbine efficiency of 95 percent, far higher than other turbines of the time.
But as we'll see in the next section, the theoretical efficiency of the Tesla turbine has not been so easily realized in production models.
Barriers to Tesla Turbine Commercialization
Tesla, as well as many contemporary scientists and industrialists, believed his new turbine to be revolutionary based on a number of attributes. It was small and easy to manufacture. It only had one moving part. And it was reversible.
To demonstrate these benefits, Tesla had several machines built. Juilus C. Czito, the son of Tesla's long-time machinist, built several versions. The first, built in 1906, featured eight disks, each six inches (15.2 centimeters) in diameter. The machine weighed less than 10 pounds (4.5 kilograms) and developed 30 horsepower. It also revealed a deficiency that would make ongoing development of the machine difficult. The rotor attained such high speeds -- 35,000 revolutions per minute (rpm) -- that the metal disks stretched considerably, hampering efficiency.
In 1910, Czito and Tesla built a larger model with disks 12 inches (30.5 centimeters) in diameter. It rotated at 10,000 rpm and developed 100 horsepower. Then, in 1911, the pair built a model with disks 9.75 inches (24.8 centimeters) in diameter. This reduced the speed to 9,000 rpm but increased the power output to 110 horsepower.
Bolstered by these successes on a small scale, Tesla built a larger double unit, which he planned to test with steam in the main powerhouse of the New York Edison Company. Each turbine had a rotor bearing disks 18 inches (45.7 centimeters) in diameter. The two turbines were placed in a line on a single base. During the test, Tesla was able to achieve 9,000 rpm and generate 200 horsepower. However, some engineers present at the test, loyal to Edison, claimed that the turbine was a failure based on a misunderstanding of how to measure torque in the new machine. This bad press, combined with the fact that the major electric companies had already invested heavily in bladed turbines, made it difficult for Tesla to attract investors.
In Tesla's final attempt to commercialize his invention, he persuaded the Allis-Chalmers Manufacturing Company in Milwaukee to build three turbines. Two had 20 disks 18 inches in diameter and developed speeds of 12,000 and 10,000 rpm respectively. The third had 15 disks 60 inches (1.5 meters) in diameter and was designed to operate at 3,600 rpm, generating 675 horsepower. During the tests, engineers from Allis-Chalmers grew concerned about both the mechanical efficiency of the turbines, as well as their ability to endure prolonged use. They found that the disks had distorted to a great extent and concluded that the turbine would have eventually failed.
Even as late as the 1970s, researchers had difficulty replicating the results reported by Tesla. Warren Rice, a professor of engineering at Arizona State University, created a version of the Tesla turbine that operated at 41 percent efficiency. Some argued that Rice's model deviated from Tesla's exact specifications. But Rice, an expert in fluid dynamics and the Tesla turbine, conducted a literature review of research as late as the 1990s and found that no modern version of Tesla's invention exceeded 30 to 40 percent efficiency.
This, more than anything, prevented the Tesla turbine from becoming more widely used.
As the Office of Naval Research in Washington, D.C., plainly stated: "The Parsons turbine has been around a long time with entire industries built around it and supporting it. If the Tesla turbine isn't an order of magnitude superior, then it would be pouring money down the rat hole because the industry isn't going to be overturned that easily …" [source: Cheney].
So, where does that leave the Tesla turbine today? As we'll see in the next section, engineers and automotive designers are once again turning their attention to this 100-year-old technology.
The Future of the Tesla Turbine
Tesla always was a visionary. He did not see his bladeless turbine as an end itself, but as a means to an end. His ultimate goal was to replace the piston combustion engine with a much more efficient, more reliable engine based on his technology. The most efficient piston combustion engines did not get above 27 to 28 percent efficiency in their conversion of fuel to work. Even at efficiency rates of 40 percent, Tesla saw his turbine as an improvement. He even designed, on paper, a turbine motorcar, which he claimed would be so efficient that it could drive across the United States on a single tank of gasoline.
Tesla never saw the car produced, but he might be gratified today to see that his revolutionary turbine is finally being incorporated into a new generation of cleaner, more efficient vehicles. One company making serious progress is Phoenix Navigation and Guidance Inc. (PNGinc), located in Munising, Michigan. PNGinc has combined disk turbine technology with a pulse detonation combustor in an engine the company says delivers unprecedented efficiencies. There are 29 active disks, each 10 inches (25.4 centimeters) in diameter, sandwiched between two tapered end disks. The engine generates 18,000 rpm and 130 horsepower. To overcome the extreme centrifugal forces inherent to the turbine, PNGinc uses a variety of advanced materials, such as carbon-fiber, titanium-impregnated plastic and Kevlar-reinforced disks.
Clearly, these stronger, more durable materials are critical if the Tesla turbine is going to enjoy any commercial success. Had materials such as Kevlar been available in Tesla's lifetime, it's quite likely that the turbine would have seen greater use. But as was often the case with the inventor's work, the Tesla turbine was a machine far ahead of its time.
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More Great Links
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