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Internal Combustion Engines
For over a century internal combustion engines have been and still are one of the most successful compact portable autonomous power sources being used in the transport of people and goods over relatively long distances relatively quickly. It is also in many products such as chainsaws and Lawnmowers.
Alternate power sources such as electric motors powered by batteries or hydrogen fuel cells or wind farms, suffer from energy storage problems, high capital costs, possible environmental problems, irratic winds affecting wind-turbines and a lack of energy-delivering infrastructure. Substantial improvements in such technologies are arguably at least a decade away. A "Well-to-Wheel" analysis shows that efficiency comparisons between electricity, fuel-cells, hydrogen and gasoline are in reality more complex. And there are concerns about the green credentials and safety of electric cars, hybrid-technology, and hydrogen-fueled cars.
Moreover hydrocarbon fuels pack at least 10 to 20 times (i.e. 1000% to 2000%) more energy than batteries and are also lighter to carry around. For instance octane has a specific energy of 12.3 kWh/kg which is roughly 18 times that of Li-SOCl2 batteries (660 Wh/kg) and 33 times that of Li-S batteries (370 Wh/kg).
So for the foreseeable future it is very likely that internal combustion engines will continue to be the dominant source of portable autonomous power. As the worldwide engines market is huge, it was worth $235 billion in 2001, there is a great incentive to improve the performance and efficiency of the internal combustion engine or see it eclipsed by alternate power sources. Also, as crude oil reserves are dwindling, more efficient use of this resource is required. To sustain the internal combustion engine for use in transportation well into the distant future, renewable fuels offer a possible solution.
Why the Wankel Engine? Rotary vs Reciprocating
Reciprocating engines have been the choice of the automotive and allied industries for many many years. However converting reciprocating motion into rotary motion (e.g. to turn wheels) is not as mechanically efficient as pure rotary motion. Despite this disadvantage the reciprocating internal combustion has achieved legendary status for well over a century with no obvious competitor in sight.
The reciprocating engine powers our automobiles reliably and takes us over long distances mostly without trouble. Thousands are made each day with precision and at reasonable cost. Its only possible competitor has been the rotary engine developed by Dr Felix Wankel in the 1950s. A variety of novel engines that have been developed over the years have yet to topple the mighty reciprocating engine from its lofty perch.
Rotary engines have fewer moving parts than reciprocating engines. A high torque engine, rotary engines also run more smoothly. The rotary engine and ceramic would be an ideal blending of technology and material, and we believe would do justice to the exceptional properties of ceramics. We have chosen the Wankel rotary engine to be made out of ceramic because it is a known, tried and tested design. In other words, CRE is NOT developing a new or novel engine; our intention is to ceramicize the Wankel engine.
The metal Wankel engine is an elegant design with much to offer but development efforts to improve it have been limited. For instance Mazda, with its cadre of highly qualified highly skilled automotive and mechanical engineers, has been trying to improve its Wankel engine since the 1970s. Debatable though the scale and scope of their improvements are, one thing is certain - Mazda cars with their Wankel-powered engines have not penetrated the worldwide automobile market significantly. Many other automobile companies have essentially abandoned efforts to improve the metal Wankel engine. Without the devoted and passionate support of diehard Wankel engine enthusiasts all over the world the Wankel engine automobile market might have dwindled even more. Perhaps the last effort to significantly improve the Wankel design is to make it out of ceramic.
Suggested viewing. Two YouTube Mazda Videos: #1 and#2
The choice of metal as the material from which heat engines are made is an unfortunate one. This is because metals are relatively low temperature materials for heat engines and they are also good thermal conductors, two properties that are detrimental to efficient combustion.
Historically, when the internal combustion engine was first being developed, it was an unavoidable choice because metals were the only suitable material available at that time. Recognising this problem, development of new materials for IC engines is now being encouraged by the US Government.
The maximum service temperature of many metals is less than 600 C, and thus metal engines are required to operate at temperatures too low for fuel to be burnt completely. Also, as metals are good thermal conductors, the heat generated within the metallic combustion chamber is easily conducted through the metallic casing. Liquid cooling is thus required to prevent the metallic engine from overheating and this hastens heat loss (about 30% of the heat generated is lost to the coolant or radiator water). Furthermore, resulting incomplete combustion products are discarded through the exhaust adding to airborne pollution.
These temperature trade-offs required of metallic internal combustion engines result in low combustion and low thermal efficiencies. Thus, metallic internal combustion engines suffer primarily from three problems: 1. Low combustion efficiency (due to the lower operating temperatures of metals), 2. Substantial heat loss (due to the high thermal conductivity of metals), and 3. Some wear (resulting in some limited metal component life).
As ceramics are high temperature materials, a ceramic engine should be able to operate at higher temperatures enabling combustion of fuel to be more complete resulting in increased combustion efficiency. This should increase performance, decrease fuel consumption and reduce pollution. This should also enable various fuels to be used (i.e. multi-fuel capability).
What Ceramic to Choose
Silicon Nitride: Among the various engineering ceramics that have been developed over the decades, silicon nitride has received the most attention for use in internal combustion engines and turbines. It has good thermal shock resistance (ΔT ~ 600 C) and good creep resistance. Though very desirable as an engine material, their poor mechanical strength (low fracture toughness) has precluded their use in load-bearing applications. As the brittleness of silicon-based ceramics is considered an intrinsic characteristic of such materials (ref 15) by virtue of their strong bonding, covalent and ionic, only limited increases in the fracture toughness of silicon nitride is believed to be attainable. The development of ceramic matrix composites (CMC) is considered to be a more attractive alternative (ref 15), but success in this approach has been limited.
Although some progress has been made over the years, the processing of silicon nitride remains a problem (ref 20) and larger higher-strength silicon nitride components have yet to be fabricated. Silicon nitride cannot be heated over 1850 C to densify because it dissociates into silicon and nitrogen. Also its covalent bonding does not allow it to easily sinter and fully densify.
Furthermore, silicon nitride ceramics in a hot, corrosive and humid oxidizing atmosphere (such as during fuel-air combustion in internal combustion and turbine engines) are prone to degradation. When they are subject to oxidation, water vapour and high temperatures they form a thermally-grown silicon oxide layer which continually volatilises as hydroxide species affecting the integrity of the silicon-based ceramic surface. For more about silicon nitride and silicon carbide degradation see references 7-13 and 17-18 below.
Despite the persistent and seemingly intractable problems of degradation and poor mechanical strength as well as the difficulties in fabricating and processing larger higher-strength load-bearing components, silicon nitride ceramic remains surprisingly the preferred high temperature material for turbines.
With only a limited budget, Ceramic Rotary Engines Inc (CRE) simply cannot afford the very high costs that are required in developing a silicon nitride engine given the length of time silicon nitride has been under development (i.e. since the 1960s) and given the great uncertainities in the fabrication of durable and reliable silicon nitride components for our ceramic engine.
Silicon Carbide: A material with a very high hardness, silicon carbide has, in the last few years, been receiving some attention from the Micro Electro-Mechanical Systems (MEMS) community in their quest to develop a miniature engine. However, the same problems that plague silicon nitride would also apply to silicon carbide, and the fracture toughness of silicon carbide is even lower than that of silicon nitride. Silicon carbide still has many other uses that do not require mechanical integrity and strength.
Aluminas: A much used ceramic, mainly as electrical insulators, they have seldom been considered as suitable materials for engines possibly because of their low fracture toughness and high thermal conductivity. However, there has been some recent interest in fabricating alumina components for micro-engines (ref 19). Zirconias: These engineering ceramics were once dubbed "ceramic steels" (ref 21) because of their very high fracture toughness among ceramics. Also, zirconia ceramics have one of the highest maximum service temperatures (~2000 C) among all of the ceramics (ref 25) and they retain some of their mechanical strength close to their melting point (2750 C). However, their low creep resistance and their low thermal shock resistance (ΔT ~ 350 C) could pose a problem.
Zirconiaceramics have been used in heat engines because of two very notable properties they possess: a high temperature capability and a low thermal conductivity. None of the other ceramics possess a thermal conductivity as low as the zirconias. This means that engines made out of zirconia would retain much of the heat generated in the combustion chamber instead of loosing it to the surroundings (approaching near adiabatic conditions). Thus the need for a cooling system could also be eliminated.
No wonder the Ford Motor Company used zirconia engineering ceramics for their ceramic engine development program in the 1980s (see next section for more on this R&D program). Aircraft jet engines currently use a zirconia-based coating to increase their turbine inlet temperatures to improve performance.
CRE's Choice: The choice of a ceramic is complicated by particular attributes that are present in some ceramics and not in others. Furthermore, our choice of ceramic is made more difficult by the dearth of uses, short history and only a small dataset of ceramic applications. In contrast, metals and alloys have an extensive and long history of activity and a considerable amount of data to rely upon. On careful consideration we have decided on a ceria-based zirconia engineering ceramic for the following reasons: (1) its relatively good mechanical strength (i.e. combination of high fracture toughness and high bend strength); (2) the twin properties of the zirconias that are clearly useful to heat engines; (3) tests which show that CeTZPzirconia ceramic does not suffer from hydrothermal degradation which plagues several other ceramics; (4) being an oxide, zirconia is unlikely to be further oxidized. Considering that many fuels contain chemically-active and corrosive substances, resistance against hydrothermal degradation and resistance against oxidation are highly desirable attributes of a ceramic for an internal combustion engine.
Low Heat Rejection Engines
Approaching Adiabatic Conditions: About a third of the heat generated by an internal combustion engine is lost to its surroundings. Insulating the combustion chamber with zirconia-based ceramics to reduce heat loss has been the focus of some research for over thirty years but progress has been slow. An overview of such low heat rejection (LHR) engine programs has been published by S.Jaichandar and P.Tamilporai (reference 16 given below).
Some Useful LHR Engine Studies
[A] During the 1980s, in a development program at Ford Motor Company USA led by Dr Arthur McLean, zirconia-based ceramic components have been successfully tested in reciprocating internal combustion engines. Dr McLean stated that their testing was conducted in a single cylinder, 80 mm bore by 80 mm stroke, high speed direct injection diesel engine at full load conditions over the complete speed range and at speed/load conditions representative of the EPA urban driving cycle. His results showed that the fuel consumption of the uncooled ceramic insert engine was 5% to 9% less than the baseline water-cooled engine generally confirming computer modelling predictions. Dr McLean also reported that after several design/material interations a zirconia ceramic cylinder head plate was successfully tested for 120 hours and a zirconia ceramic short cylinder liner survived over 500 hours. [reference 4 given below].
[B] A 1984 NASA SBIR Phase One study of an "Adiabatic Wankel-type Rotary Engine" by Dr Roy Kamo of Adiabatics Inc indicated that progressive performance improvements in Wankel rotary engines could be achieved when the combustion chamber components were insulated. It was claimed that the advanced concepts of turbocompounding, higher compression ratio, reduced leakage and faster combustion could decrease specific fuel consumption by 25% and increase power output by 34%. Also eliminating the cooling system could produce another 5% reduction in fuel consumption. A follow-up NASA SBIR Phase Two study reported in 1988 that a thermal barrier coating (TBC) on the Wankel housing is unlikely to be successful as the thermal stresses were excessive for a direct injection stratified charged Wankel engine. It was also concluded that the correct choice of a TBC material was of crucial concern. From a reading of this report it appears that there is cause for some optimism for achieving a low heat rejection engine using TBCs.
[C] In diesels, it has been reported that a 1 mm thick coating of ceramic on the cylinder head and a 2 mm thick coating of ceramic on the piston caps reduces the heat losses into the water coolant by 9%, while increasing the heat losses into the oil by 3% for an overall reduction of 6%.
[D] Dr Alan Bentz and Professor Andre Boehman of the Energy Institute at Pennsylvania State University found not only significant morphological and composition changes in particulate emissions but also total particulate mass reductions when thin thermal barrier ceramic coatings were applied to the combustion chamber surfaces. They said that the thermal barrier coating enhances the oxidation of condensable hydrocarbons that agglomerate with the diesel soot.
[E] A NASA study by Dr Harold Sliney (1990) showed that a solid lubricant, designated PS212, can be used successfully in a metal Wankel engine at high temperatures. He reported that a plasma-sprayed composite coating of metal-bonded chromium carbide with additions of silver and fluorides and used in combination with a zirconia ceramic thermal barrier coating (TBC) on the inner surface of a metal Wankel engine provided sufficient lubrication at temperatures up to 900 C. The zirconia ceramic TBC provided some insulation.
[F] Another NASA study by Drs DellaCorte & Wood (1994) reported that soft metals such as gold or silver, could act as solid lubricants at high temperatures. They reported that thin gold-chromium films exhibited outstanding wear in friction tests lasting more than 200,000 sliding passes, and these duplex metal films continued to lubricate even at temperatures of 1000 C.
Though not definitive, these studies show that insulating the combustion chamber of internal combustion engines with a ceramic thermal barrier coating could reduce heat loss and increase combustion efficiency. Furthermore, one of the NASA studies showed that solid lubricants can be used to reduce wear between apex seals and the inner housing surface of Wankel engines at high temperatures where liquid lubricants could not operate.
Making An All-Ceramic Wankel Engine
Over the last century only three combustion engines have been commercialized - the turbine, the reciprocating and the Wankel rotary. Making a ceramic engine will be a paradign shift in IC engines technology, an innovation in marrying engine technology with state-of-the-art materials and design.
Note that a simple one-to-one metal-ceramicsubstitution is not considered a prudent or workable approach because of concerns with contact stresses, thermal shock resistance mismatch, stress rupture life and ceramic oxidation resistance (see e.g. Report of NASA/TM 2006-214220, Pg 6).
Metals are softer and more ductile than ceramics and thus easier to machine into precision components. As ceramics are harder and more difficult to machine, the design and fabrication of the ceramic components for a ceramic engine would be a major challenge, requiring new and innovative approaches. For instance fabrication problems were encountered in the MEMS Berkeley project for their micro-Wankel engine made from silicon carbide ceramic.
Adequate sealing and lubrication at high temperatures are clearly major issues. One choice is to dispense with the apex seals. Instead, choke flow could be invoked by introducing turbulence and thus eddy currents at the rotor tips, but studies of the choke flow effect are not encouraging. Applying this effect could prove impracticable as the gap required between tip and housing is likely to be very small (probably < 2 microns) or the sealing could be inadequate. Another choice is to use apex seals but liquid lubrication is not an option at high temperatures (> 600 C).
Solid lubricants could be the solution. Following on the work of Drs DellaCorte & Wood (1994) in their NASA study soft metal (e.g. gold or silver) coating on the inner surface of the ceramic housing with nickel-based apex seals, is a possibility.
Environmentally Friendly Engine?
A Ceramic Engine is expected to be capable of using various fuels because of its high operating temperatures. Renewable fuels such as cellulosic bio-ethanol or straight vegetable oils (SVO) could be used in this engine. SVOs are unprocessed or unmodified vegetable oils.A Ceramic Engine as a portable power source using SVOs as fuel for rural communities and villages in the third world could enhance the quality of life of the citizens in these communities.
Ceramic engines could power water pumps for drinking and irrigation and could also power electricity generators for lighting and cooking in rural communities (which should reduce the need for firewood, saving trees).BioDiesel, from vegetable oils, has received the attention of various government agencies such as the Department of Energy in the USA and the UK Department of Transport. B5 and B20 BioDiesel is already being sold in some states in the USA. Experimental programs using biodiesels in cars are drawing a lot of public attention. However the adoption of alternative fuels presents a challenge for the present.
An excellent comparative study of the environmental impact of various fuels (including fossil fuels and biofuels) was given by Professor Mark Holtzapple in a video presentation* at Texas A&M University on Tuesday 25 April 2006 as part of their Distinguished Lecture Series on Sustainable Energy and Transportation Engineering The 21st Century. He also discusses the production of biofuels from the MixAlco Process and introduces the Star Rotor Engine (*click here for a slide presentation).
Engines As A Substitute for Batteries?
According to Professor Alfred Pisano of the MEMS project at the University of California at Berkeley liquid hydrocarbon fuels like butane, kerosene and propane pack at least 10 times more energy, pound for pound, than batteries do even after taking into account how inefficiently a metal IC engine burns fuel. That means an engine could be 10 times smaller than a battery and still deliver the same amount of energy. Or the engine could be the same size as a battery and last 10 times longer.
Decentralized Electrical & Heating Systems And Batteries
Electricity distribution over very large distances (e.g. the national electricity grid in the USA) is not as efficient as distributed power generation produced near the point of use (e.g. micro-Combined Heat and Power, or micro-CHP, using a combination of technologies).Engines currently being used in micro-CHP, including reciprocating engines, are inadequate for today's needs. They are noisy, requires too much maintenance, do not meet emissions standards, needs vibration dampening, and are not sufficiently fuel efficient. A better engine is needed (1) in the range of 0.5 kW to 30 kW for the micro-CHP market, and (2) in the range of 0.5 kW and less to replace battery power as, pound for pound, liquid hydrocarbon fuels like butane when used in metal IC engines pack at least 10 times more energy than batteries.
References & Useful Reading:
 R.Kamo and W.Bryzik 1979 SAE Technical Publication Number 780068 "Adiabatic Turbocompound Engine Performance Prediction".
 W.Bryzik and R.Kamo 1983 SAE Technical Publication Number 830314 "TACOM/Cummings Adiabatic Engine Program".
 R.Kamo, R.M.Kakawani and W.Hady 1986 SAE Technical Publication Number 960616 "Adiabatic Wankel-Type Rotary Engine".
 W.Bunk and H.Hausner 1986 Proceedings of the Second International Symposium; 14 to 17 April, Lubeck-Travemunde, Germany "Ceramic Materials and Components for Engines".
 W.Dworak and D.Fingerle 1987 Journal of Materials Science; Volume 86, pages 170-178 "Ceramic Materials for Engines".
 R.Kamo 1991 Ceramic Acta, Volume 3, pages 49-65 "Ceramic Engine and their Cost Effectiveness".
 B.J.Opila and R.B.Hann 1997 Journal of American Ceramic Society; Volume 80(1), pages 197-205.
 B.J.Opila et alia 1999 Journal of American Ceramic Society; Volume 82(7), pages 1826-1834.
 J.L.Smialek et alia 1999 Advances in Composite Materials; Volume 8(1), pages 33-45.
 R.C.Robinson and J.L.Smialek 1999 Journal of American Ceramic Society; Volume 82(7), pages 1817-1825.
 "The degradation of silicon carbide in hot humid environments" ORNL Review Volume 33 (#1) 2000.
 H-S Rho,N.L.Hecht and G.A.Graves 2000 Journal of Materials Science; Volume 35, pages 3631-3639 "Oxidation Behaviour of Hot-Isostatically Pressed Silicon Nitride Containing Yttria".
 H.F.Eaton et alia 2001 "EBC Protection of SiC/SiC Composites in the Gas Turbine Combustion Environment - Continuing Evaluation and Refurbishment Considerations" Proceedings of ASME TURBOEXPO 2001, 4-7 June 2001, New Orleans, LA, USA.
 by A.K.Shukla "Cars beyond Otto's internal combustion engines", November 2001, .
 S.Dutta (NASA Glenn, Cleveland, OH) - Bulletin of Materials Science 2001 Volume 24(2), pages 117-120 "Fracture Toughness and Reliability in High Temperature Structural Ceramics and Composites: Prospects and Challenges for the 21st Century".
Disclaimer *Ceramic Rotary Engines Inc (CRE Inc) is an independent corporation incorporated in the State of Texas and is neither part of nor associated with Encompass Holdings Inc. Also, CRE Inc is neither part of nor directly associated with any other company*.