Few people today would argue gas turbines are the future of propulsion, after all we are told by the “experts” that electromobility is the future! That eccentric man from Munich is the devil, we must not use his engine! But as always, the experts are wrong. Electromobility is a fraud supported entirely by the unscientific theory of catastrophic manmade climate change. Once the pseudo-scientific monstrosity called global warming is belied, there will be ZERO incentive to develop electromobility, and the billions invested by all the world’s automakers will have been wasted! The future of propulsion is of course burning stuff in heat engines, what better way do we know of? But it turns out there are many different ways of burning stuff in heat engines, one such way is to make wheels spin rather than clunky pistons slide. The gas turbine emerges as the final technological step in mans move towards a perfect techno civilization.
Christophe Pochari Energietechnik is reviving long-forgotten and obscure turbine engine technology. With advancements in single-crystal nickel alloys, turbine designers have been able to increase turbine inlet temperatures to record levels. The downside of this is the need to utilize large amounts of compressor bleed air, reducing engine efficiency. Conventional engineering ceramics including silicon nitride and carbide have been forgotten in part due to the focus on ceramic matrix composites. A relatively elegant and simple design is the use of a composite (carbon-fiber) hoop holding the centrifugal forces acting on the blade. Conventional engineering ceramics have uneven properties, rendering them unable to tolerate concentrated forces found in conventional mating conditions use in current turbine disc designs. Contrary to popular belief, engineering ceramics do indeed have sufficient tensile strength to be loaded in tension, such as a beam for example. Silicon carbide has a tensile strength of up to 390 MPa, more than sufficient to tolerate the tensile loads found in a turbine blade root connection. The primary issue is the lack of uniformity, the material properties are highly heterogeneous, in addition, low flexural strength, and high brittleness further impede its use. The simple solution is to load the blades purely in compression, (where ceramics shine), this idea had been extensively investigated by some of the aerospace OEMs and the U.S military, but rapid improvements in single crystal alloys made it uninteresting. In recent times due to interest in “distribution electric propulsion” and broader “clean propulsion” for aircraft, a company called Exonetik, Sherbrooke, Quebec, has strangely claimed to have developed a “new technology” that they have patented, when the idea dates back to the 1970s in the patent US4017209A, Robert R. Bodman, Raytheon Technologies Corp. https://patents.google.com/patent/US4017209/en
There efforts are dubious because they use carbon fiber as the hoop material, which is obviously impossible to its poor thermal stability. Secondly, they use silicon nitride which has much less tensile strength than silicon carbide less alone inferior oxidization resistance.
“Utilizing the high compressive strengths of ceramics in gas turbines for improving ceramic turbine structural integrity has interested engineers in recent years as evidenced by a number of patents and reports issued on Compression Structured Ceramic Turbines with one as early as 1968”
P. J. Coty, Air Force Aero Propulsion Laboratory
Silicon carbide has one of the highest compressive strengths available, 1600 MPa. In order to take advantage of the excellent high-temperature capacity of these materials, a novel architecture is needed, dispensing the existing orthodoxy of turbine disc design. Using this design, turbine inlet temperatures approaching 1600°C are feasible without air cooling. The blades transfer compression loads to hoop loads, the carbon fiber rim performs the same function as a pressure vessel. The carbon fiber parameter containment hoop is not exposed to high gas temperatures. With this technology, it is possible to design small-scale sub-500 hp turboshafts with the efficiency of diesel engines (40%+), enabling jet-pack propulsion with 1 hour plus range.
The idea to use ceramics in a gas turbine is nothing new. To this date, the idea has been plagued with insurmountable problems and is not taken seriously, mainly the sudden brittle failure of any tension-loaded component. Modern research focuses mainly on ceramic matrix composites, but this technology is far from certain. All high-performance gas turbines today use directionally solidified single crystal blades made of nickel, cobalt, and molybdenum, with trace amounts of rhenium, tungsten, tantalum, and hafnium. The problem with these alloys is that although they are very strong, they rapidly oxidize in the oxygen-rich combustion gases and erode quickly, limiting their useful life to only a few thousand hours in small gas turbines. These single-crystal alloys are also very dense, nearly 9 grams/cm3, which generates severe radial stress. When any moving mass is spun, the object is subject to inertia which produces a force stressing the material, eventually, if the object is spun fast enough there will be a rupturing of the object. This is actually what limits turbine power density, not just temperature, because turbine wheels could spin faster for a given size if radial stresses could be lowered.
During the 1960s at the apogee of technological civilization, on-road trucks were viewed as natural candidates for gas turbines. The tremendous success and reliability of gas turbines in helicopters made it seem almost inevitable that they would find use in trucks. Regrettably, a number of technical obstacles made their failure almost inevitable. Unlike a helicopter or turboprop, where gas turbines are the only option, a truck does not operate at full load 100% of the time, not only does the truck use far less power than ever a small helicopter, it will vary its power output by at least 2-3x fold depending on grade, frequency of acceleration, and load. Firstly, gas turbines were woefully inefficient compared to diesel engines. Secondly, their even more abysmal part load efficiency meant that the truck would either be underpowered or overpowered and operated far below the turbine’s optimal speed band. Thirdly, gas turbines at the time, and to a large extent still, rely on metallic alloys for the mainstay of their blading. Metals possess an intrinsic fatigue limit, the more the metal is thermally cycled, the more the grain structure is disturbed and the higher the chance of failure. This meant that trucks operating frequently with short stops would have shortened engine life. Lastly, gas turbines were and still are far more expensive, largely due to small manufacturing volumes and little use outside of aviation and large-scale power generation from natural gas.
I have been forced to employ the term “hard technology” to refer to technologies that produce actual physical output, such as a train, ship, engine, and not merely “virtual” outputs, such as the flickering color on an LED screen. Of course there is something indeed physical about such technologies, a radio produces an electric current when an oscillating magnetic field passes by the antenna, but the output is merely audible to a human ear, or any other organism which can process the particular frequency of sound produced. A true physical technology is one that changes the basic energetic and material basis of civilization, namely labor augmenting technologies, such as steam or hydraulics. In the realm of physical technologies, I have always remained partial to propulsion and energy technologies, viewing these as the substrate by which the rest of the Technosphere lies. Better engines, drivetrains, and prime movers are a sure way to move civilization forward. Anyone who could invent a lighter and more powerful turbine engine is sure to change the world, much more so than Oculus Rift could ever. Regrettably, like much of modernity, we find ourselves in a strange predicament, with politicians and the world’s best engineering labor force squandering precious time and efforts into developing effectively useless “electromobility” using batteries as prime movers. Analyzed on a strictly technical basis, there is simply nothing favoring electro-chemistry to power anything bigger than a lawn mower. Otherwise “intelligent” people are trying to replace the mighty diesel engine with puny little iPhone batteries, rather than replacing the diesel with a superior powerplant, for example, a gas turbine or an adiabatic engine.
In the 1970 and 80s the U.S Army investigated using “adiabatic engines”, the TACOM/Cummins Adiabatic Engine Program. The U.S Army hired Cummins to perform a study and build a prototype engine, unfortunately nothing came of the program due to a lack of suitable lubrication options. An adiabatic engine is essentially an engine that rejects virtually no heat to its surroundings (the cooling system is eliminated altogether), such as engine would be at least 20% more efficient, but due to its extremely high operating temperature, no lubricant could be found that did not experience excessive oxidation. A conventional piston engine rejects about 30% of the heat input to the coolant, such as exergy flow goes unused. The lubricant would needed to have withstood peak temperatures of 400°C, which is impossible for a mineral or even synthetic lubricant such as polyphenol ether. Solid lubricants and ceramic cylinder lines were considered, but solid or “dry” lubricants like molybdenum disulfide or tungsten disulfide, being powders, do not form durable films and provide inferior protection compared to viscous liquids. If the adiabatic engine is not practical, we are left with only the gas turbine as a candidate to replace the diesel engine, but the competition is fierce and it will only happen if the gas turbine can be made more reliable, cheaper, and equally as efficient.
Today civilization is taking a great step backward, towards a lower power density form of propulsion, an entirely illogical and outright perplexing move. But of course, there is no mystery here, we know exactly why this is happening, it is all happening because of Arrhenius’s demon. Until Arrhenius’s demon is overthrown (the greenhouse effect), we may waste decades in this futile pursuit of electromobility. It is more than certain that consumers of these technical boondoggles, whether they be electric semi-trucks or sedans, will finally realize it was a waste of time. Instead of moving towards and cheaper and cleaner fuel: methane, and finding propulsion systems that better utilize this gaseous fuel, notable turbines, every single automotive, truck, and engine maker is committing billions to building batteries. But let us stop complaining about the banality of the modern post-WW2 era and turn to the fascinating and exciting technical aspects of compression-loaded ceramic gas turbines.
If we want to replace bulky, clunky, and maintenance-intensive reciprocating engines with smooth, compact, and efficient gas turbines, we must find a way to reach BSFC parity with diesel engines at the scales found in heavy-duty truck propulsion. A typical highway semi-truck in Europe, such as a Scania R730 consumes 48.7 liters per 100 km, at a cruising speed of 85 km/hr, the engine burns 41.4 liters of diesel fuel in an hour or 35.18 kg/hr. The Spec fuel consumption of the Scania DC16 at 1/2 load is 189 g/kWh at 1500 RPM and 197 g/kWh at 1800 RPM. This means the engine produces 176 kW or just under 200 hp. To highlight the sheer absurdity of “electromobility”, for just one hour of operation, the battery would weigh one ton! To show just how strong a technology we are up against, this Scania DC16 common rail V8 boasts a break thermal efficiency of 42% at 50% load. We must develop a gas turbine that can operate at a steady state output of just under 200 kW at does not substantially fall under the high 30 percent.
A Scania DC16 is hardly a power-dense powerplant, low-speed diesel engines are notoriously heavy and bulky, by installing a gas turbine, the cabin would be roomier since the entire engine bay can be eliminated. A 200 kW high-temperature ceramic and a recuperated gas turbine is a tiny package, which could fit in the glovebox!
A gas turbine has no cooling system, no fan, radiator, or coolant, no high-pressure fuel injection system, no glow plugs, no exhaust treatment system, and no need for complex failure-prone electronics.
The weight savings would be phenomenal, increasing the revenue for the truck operator. The Scania DC16 engine weighs close to 1400 kg loaded with oil and coolant. The 200 kW gas turbine would weigh only at best 100 kg, allowing the truck to carry 1300 kg of additional payload, itself worth tens of thousands annually of additional revenue.
But the gas turbine is not all rosy, there are a number of tradeoffs that have to be made. The high power density of the gas turbine comes at the cost of poor or virtually no part load capability, with efficiency dropping rapidly as the aerodynamics of the compressor and blades fall out of their optimal band. In order to rectify this limitation, only one practical solution exists, namely, modularity where multiple individual turbines can be installed in discrete drivetrains mounted directly parallel to the axle. When more power is needed, they can be simply switched on. A hybrid drivetrain seems appealing, but there is no way to get around the problem of intermediate energy storage. If the gas turbine is sized merely for the average power usage of 180 kW, there will be a large void that will need to be filled when the truck needs to use say 500 kW for half an hour. While we could install a one-ton battery and simply accept it as our penalty, it eats into the advantages offered by the turbine to begin with. Since the DC16 makes a maximum of 566 kW, we may need to draw this amount of power during short bursts or during prolonged climbs up steep grades. Since the turbogenerator is still only able to produce 180 kW, it will take over an hour to replenish this battery, so we are then left with no reserve to draw over 180 kW. Clearly, the best option is t combine both methods, a fully hybrid drivetrain where the high torque and instant acceleration of the motor can be exploited, while also solving the issue of requiring excessive intermediate storage between peak load and steady-state load. In order to maintain the high efficiency of the turbine, we can simply switch additional compressor sections. The turbine can be designed to have more stages than needed during sub-200 kW operation, and when ramped to 550 kW or more additional high-pressure stages can be put into operation. Either a single oversized combustor can be used or an additional higher capacity combustor can be incorporated. Such a design evidently adds complexity, but it is no heavier than a single 550 kW turbine since the smaller capacity sections do not add additional weight beyond the base unit.
Silicon carbide hoop stress-loaded ceramic turbines
The image above is a CAD drawing of a dual-centrifugal compressor gas turbine with two high pressure turbine stages made of ceramic blades contained within a tension-loaded hoop.
In the category of prime movers, the thermal power cycles used in reciprocating and turbomachinery represent a singular technology with no viable alternative contenders. Fuel cells, batteries, and other electrochemical powertrains are simply worlds apart in power density. Hydrocarbon fuels are in a class of their own, with only liquid hydrogen as a possible alternative, but only practical in large aircraft or trucks, where boiloff losses can be minimized. If we look at the horizon, we see no evidence of early-stage prototypical technology that could replace the hydrocarbon prime movers. Physics and the chemical elements, not human intelligence, is clearly the limit here.
Rotary detonations are perhaps the closest there has come to a new class of powerplant, but they remain laboratory curiosities. Small fission reactors powering Brayton cycles, were it not for radiation concerns, would be feasible for aircraft, helicopters, ships, and many heavy-duty propulsion applications. But since heavy thick radiation barriers would be needed, their power density may not even exceed hydrocarbon-fueled gas turbines.
Small gas turbines can be made as efficient as diesel engines or large gas turbines if higher turbine inlet temperatures are employed along with recuperators. A small <200 kW turbine could easily exceed 45% with over 1600°C TIT and 15:1 pressure ratios combined with recuperation. If such powerplants can be made highly reliable and inexpensive, they would displace many reciprocating powerplants.
Unfortunately, small gas turbines cannot as effectively make use of bleed air blade cooling, which places a large penalty on efficiency. The amount of bleed air needed to cool conventional single-crystal alloy blades, such as CSMX-4, for a small turbine is not practical. Liquid cooling of turbine blades has been considered before, and a number of patents were taken by the major turbine OEMs, including much technical literature, but the inherent leak proneness, sharp thermal gradients, and potential failure points made this option impractical. A better solution is sought whereby the blade itself can sustain a steady-state temperature close to or equal to the gas temperature. Unfortunately, the “low density” alloys of nickel simply melt at these temperatures, so their use is impossible.
Even the best single crystal alloys can hardly tolerate temperatures higher than 1100°C before losing most of their tensile strength. Even with air pumped through channels and veins within the blade core, the exterior surface of the blade is close to the melting point and is rapidly worn off and oxidized with the metal effectively converted to vapor. With silicon carbide, the tensile strength at 1600°C is high enough for a completely un-cooled design.
Metals are perhaps the worst material one could use to subject a blade to thousand-degree temperatures. Metals become much more ductile at high temperatures and are consequentially subject to intense creep when operated at prolonged high-temperature conditions. A hot turbine blade is about as strong as magnesium as room temperature and barely withstand the radial stresses generating by its own mass spinning. Ceramics, on the other hand, are brittle materials, that is they do not deform at all until ultimate failure. A ceramic blade, as long as it hasn’t broken, will merely expand slightly but will remain morphologically stable. But ceramics have the added advantage of being very light, with silicon carbide having a specific gravity of 3.24, 2.7x lighter than CSXM-4, which has a high density of 8.7 g/cm3, generating proportionally more stress when spinning. This means ceramic turbine blades would not experience any creep. Unfortunately, ceramics, being brittle and possessing poor tensile strength, have to be loaded only in compression to prevent tensile failure. A simple and elegant solution to this problem was first proposed by an engineer at Fiat and described in a patent from the 1980s. The solution was very simple, place a hoop around the tips of the blades, so the blades are kept in compression at all times. This hoop could be made of fibrous materials, for example, silicon carbide fibers which can maintain tensile strengths of 2 GPa at 1600°C. The fibrous hoop contains the blades which fit onto a standard metal disk, the disk is not exposed to the hot gases directly so has time to cool and maintains a much lower temperature than the blades. While this setup is indeed more complex than a standard tension load blade mounted in a fir tree groove, it offers remarkable advantages. The ideal ceramic is silicon carbide, because it forms a thin glass protective layer. Since silica is so chemically stable, it is impossible for oxygen to react with the silicon beneath this layer. Silicon carbide also possesses high fracture toughness, around 6 MPa m 1/2, and high tensile strength of 390 MPa. The diameter of a 240 kW turbine disk, for example in the Allison M250, is 150mm. The RPM of the disk is 50,000, and the total tangential stress is 160 MPa, within the limit of silicon carbide.
“Although SiC ceramic demonstrated higher strength at 1200°C (~234.9 MPa) than at RT(~220.0 MPa), the flexural strength decreased at temperatures above 1400°C, and strength degradation from 239.4 MPa at 1200°C to 203.7 MPa at 1600°C. The specimen fractured at 1400°C and 1600°C exhibited semi-brittle fracture behavior with a fairly large amount of plastic deformation. Degradation in flexural strength at 1400°C and 1600°C can be attributed to softening of the glassy phase”.
High-temperature flexural strength of SiC ceramics prepared by additive manufacturing, Teng-Teng Xu
“Ceramic materials offer a great potential for high-temperature application. This, however, means it is necessary to live – even in the future – with a brittle material with small critical crack length and high crack growth velocity. Thus it will not be easy to ensure reliability for highly loaded ceramic components, keeping in mind that for reaction-bonded ceramics the material’s inherent porosity is in the same order of magnitude as the critical crack length. A solution to increase the reliability of ceramic turbines may be a compression-loaded rotor design with fiber-reinforced hooping”
R. Kochendrfer 1980, Institut für Bauweisen und Strukturtechnologie, DLR, AGARD CONFERENCE PROCEEDINGS No. 276 Ceramics for Turbine Engine Applications, pp 22
“A vaned rotor of the type comprising a central metal hub or rotor body carrying a plurality of rotor blades made of a ceramic material, in which the blades are simply located on the rotor body and held in place by a coil of carbon fibers or ceramic fibers which surrounds the blades. To form a support surface for the coil each blade has a transverse part at the radially outer end thereof, which is partly cylindrical and which together with the transverse parts of the other blades, forms a substantially cylindrical support surface for the coil. Although ceramic materials used for such vanes (silicon nitride, silicon carbide, alumina, etc.) have much better physical properties at high temperatures than any metal alloy, especially if undergoing compression loads, they are nevertheless very difficult to couple to metal parts because of their relative fragility, lack of ductility, and their low coefficient of expansion. Because of the lack of ductility of ceramic materials, the driving forces exerted during the operation of the rotor give rise to a concentration of the load in parts of the coupling areas between the ceramic vanes and the metal body of the rotor. This frequently causes breakages in these parts. The various systems presently in use for attaching a ceramic blade by the root to a metal rotor body for a gas turbine are generally inadequate because these systems, including dovetail fixings having both straight and curved sides, do not take sufficient account of the rigidity and relative fragility of the ceramic vanes. This problem is exacerbated by the fact that present manufacturing techniques for ceramic materials are still not able to provide a complete homogeneity of composition and structure of the material so that adjacent areas of ceramic material can vary by up to 200% in tensile strength. For this reason, the known types of coupling between a support disc forming a rotor body and rotor vanes of ceramic material, which rely on a wedging action, are not satisfactory”
R Cerrato Fiat SpA, U.S patent 3857650A, 1973
“A Compression Structured Ceramic Turbine looks feasible. A new engine aerodynamic cycle with effective working fins to offset windage loss, a reduced tip speed to enhance aeromechanics, and the possible utilization of leakage gas to augment thrust should be considered. Also, the prospect for more efficient energy extraction offered by an inverted taper in the span of the turbine blade should be of prime interest to turbine designers in any future engine utilizing a Compression Structured Ceramic Turbine. Material property data and design refinements based on this data will also have to be seriously considered”
“The “Novel” feature of this ceramic turbine rotor design involves maintaining the ceramic rotating components in a state of compression at all operating conditions. Many ceramic materials being considered for gas turbine components today display compressive strengths ranging from three to eight times their tensile strengths. Utilizing the high compressive strengths of ceramics in gas turbines for improving ceramic turbine structural integrity has interested engineers in recent years as evidenced by a number of patents and reports issued on Compression Structured Ceramic Turbines with one as early as 1968. Turbine blades designed to be in compression could greatly enhance the reliability of the ceramic hot section components. A design of this nature was accomplished in this contractual effort by using an air-cooled, high-strength, lightweight rotating composite containment hoop at the outer diameter of the ceramic turbine tip cooling fins which in turn support the ceramic turbine blades in compression against the turbine wheel. A brief description of the detailed structural and thermal analysis and projected comparable performance between the Compression Structured Ceramic Turbine”.
P. J. Coty, Air Force Aero Propulsion Laboratory, 1983
Unfortunately, the situation is not so perfect, there are a number of inevitable technical problems that will arise. The first issue is the risk of sudden brittle rupturing the blades, but since the stresses generated are proportional to the specific gravity of the material, in this regard silicon carbide is well positioned. While the fracture toughness is poor compared to steel, a turbine blade experiences a largely uniform and steady-state loading regime, impact or shock is not an expected event. As long as the blades are retained within the tension hoop, they can easily maintain the structural integrity necessary to withstand the radial stresses generated by their own spinning mass. In summary, a compression loaded silicon carbide turbine paired with recuperators may be seen as the “final” powerplant for mankind’s propulsion needs. The future of heavy duty trucks, heavy equipment, and even cars, ships, and many other mobility propulsion may very well be quite gas turbines. If this technology is ever to see the light of day, it will a bold investor willing to lose a lot of money. One of the reasons technological progress has crawled to a halt is not solely due to the fact that there are simply fewer useful things to invent, this is evidently true, but another powerful factor has been the hesitance to bother develop what I would call “marginal technologies”, which simply offer weak adoption advantage over their replacement. The steam engine compared to the diesel engine was simply so far inferior that no one could possibly justify keeping them in light of the new option. Steam boilers require someone to shovel coal into them, they need to be heated up before the steam engine can operate, they require a constant source of water to keep plenished, and they are incredibly bulky with power densities a tiny fraction of internal combustion. But if we look at the situation a century later, we do have new options, but compared to the current options, they are barely justified. As much as we like gas turbines and think it would be “cool” to replace low speed and heavy diesel engines with them, the fact is it will be very difficult because are doing just fine at the moment.