The Wankel engine is a type of internal combustion engine using an eccentric rotary design to convert pressure into rotating motion. Over the commonly used reciprocating piston designs, the Wankel engine delivers advantages of: simplicity, smoothness, compactness, high revolutions per minute, and a high power to weight ratio. The engine is commonly referred to as a rotary engine, although this name applies also to other completely different designs. All parts rotate moving in one direction as opposed to the common piston engine which has pistons violently changing direction. The four stroke cycle occurs in a moving combustion chamber between the inside of an oval-like epitrochoid shaped housing, and a rotor that is similar in shape to a Reuleaux triangle with sides that are somewhat flatter.
The concept of the engine was conceived by German engineer Felix Wankel. Wankel received his first patent for the engine in 1929, began development in the early 1950s at NSU, and completed a working prototype in 1957. NSU subsequently licensed the design to companies around the world, which have continually added improvements.
The Wankel engine has the advantages of compact design and low weight over the most commonly used internal combustion engine employing reciprocating pistons. These advantages have given rotary engine applications in a variety of vehicles and devices, including: automobiles, motorcycles, racing cars, aircraft, go-karts, jet skis, snowmobiles, chain saws, and auxiliary power units. The point of power to weight has been reached of under one pound weight per horsepower output.
Mazda and NSU signed a study contract to develop the Wankel engine in 1961 and competed to bring the first Wankel-powered automobile to market. Although Mazda produced an experimental Wankel that year, NSU was first with a Wankel automobile for sale, the sporty NSU Spider in 1964; Mazda countered with a display of two- and four-rotor Wankel engines at that year's Tokyo Motor Show. In 1967, NSU began production of a Wankel-engined luxury car, the Ro 80. However, NSU had not produced reliable apex seals on the rotor, unlike Mazda and Curtiss-Wright. NSU had problems with apex seals' wear, poor shaft lubrication, and poor fuel economy, leading to frequent engine failures, not solved until 1972, which led to large warranty costs curtailing further NSU Wankel engine development. This premature release of the new Wankel engine gave a poor reputation for all makes and even when these issues were solved in the last engines produced by NSU in the second half of the '70s, sales did not recover. Audi, after the takeover of NSU, built in 1979 a new KKM 871 engine with side intake ports and 750 cm3 per chamber, 170 HP @ 6500 rpm, and 220 NM @ 3500 rpm. The engine was installed in an Audi 100 hull they named "Audi 200", but the engine was not mass-produced.
Mazda, however, claimed to have solved the apex seal problem, and operated test engines at high speed for 300 hours without failure.[1] After years of development, Mazda's first Wankel engine car was the 1967 Cosmo 110S. The company followed with a number of Wankel ("rotary" in the company's terminology) vehicles, including a bus and a pickup truck. Customers often cited the cars' smoothness of operation. However, Mazda chose a method to comply with hydrocarbon emission standards that, while less expensive to produce, increased fuel consumption. Unfortunately for Mazda, this was introduced immediately prior to a sharp rise in fuel prices. Curtiss-Wright produced the RC2-60 engine which was comparable to a V8 engine in performance and fuel consumption. Unlike NSU, by 1966 Curtiss-Wright had solved the rotor sealing issue with seals lasting 100000 miles.
Mazda later abandoned the Wankel in most of their automotive designs, continuing to use the engine in their sports car range only, producing the RX-7 until August 2002. The company normally used two-rotor designs. A more advanced twin-turbo three-rotor engine was fitted in the 1991 Eunos Cosmo sports car. In 2003, Mazda introduced the Renesis engine fitted in the RX-8. The Renesis engine relocated the ports for exhaust from the periphery of the rotary housing to the sides, allowing for larger overall ports, better airflow, and further power gains. Some early Wankel engines had also side exhaust ports, the concept being abandoned because of carbon buildup in ports and the sides of the rotor. The Renesis engine solved the problem by using a keystone scraper side seal, and approached the thermal distortion difficulties by adding some parts made of ceramics. The Renesis is capable of 238 HP with improved fuel economy, reliability, and lower emissions than previous Mazda rotary engines, all from a nominal 1.3 L displacement. However, this was not enough to meet more stringent emissions standards. Mazda ended production of their Wankel engine in 2012 after the engine failed to meet the improved Euro 5 emission standards, leaving no automotive company selling a Wankel-powered vehicle. The company is continuing development of the next generation of Wankel engines, the SkyActiv-R with a new rear wheel drive sports car model announced in October 2015 although with no launch date given. Mazda states that the SkyActiv-R solves the three key issues with previous rotary engines: fuel economy, emissions and reliability.Mazda announced the introduction of the series-hybrid Mazda2 EV car using a Wankel engine as a range extender.
In the Wankel engine, the four strokes of a Otto cycle piston engine occur in the space between a three-sided symmetric rotor and the inside of a housing. In each rotor of the Wankel engine, the oval-like epitrochoid-shaped housing surrounds a rotor which is triangular with bow shaped flanks (often confused with a Reuleaux triangle, a three-pointed curve of constant width, but with the bulge in the middle of each side a bit more flattened). The theoretical shape of the rotor between the fixed corners is the result of a minimization of the volume of the geometric combustion chamber and a maximization of the compression ratio, respectively. The symmetric curve connecting two arbitrary apexes of the rotor is maximized in the direction of the inner housing shape with the constraint that it not touch the housing at any angle of rotation (an arc is not a solution of this optimization problem).
The central drive shaft, called the "eccentric shaft" or "E-shaft", passes through the center of the rotor and is supported by fixed bearings. The rotors ride on eccentrics (analogous to crankpins) integral to the eccentric shaft (analogous to a crankshaft). The rotors both rotate around the eccentrics and make orbital revolutions around the eccentric shaft. Seals at the corners of the rotor seal against the periphery of the housing, dividing it into three moving combustion chambers. The rotation of each rotor on its own axis is caused and controlled by a pair of synchronizing gears A fixed gear mounted on one side of the rotor housing engages a ring gear attached to the rotor and ensures the rotor moves exactly 1/3 turn for each turn of the eccentric shaft. The power output of the engine is not transmitted through the synchronizing gears. The force of gas pressure on the rotor (to a first approximation) goes directly to the center of the eccentric, part of the output shaft...
The easiest way to visualize the action of the engine in the animation at left is to look not at the rotor itself, but the cavity created between it and the housing. The Wankel engine is actually a variable-volume progressing-cavity system. Thus, there are three cavities per housing, all repeating the same cycle. Points A and B on the rotor and E-shaft turn at different speeds—point B circles three times as often as point A does, so that one full orbit of the rotor equates to three turns of the E-shaft.
As the rotor rotates orbitally revolving, each side of the rotor is brought closer to and then away from the wall of the housing, compressing and expanding the combustion chamber like the strokes of a piston in a reciprocating piston engine. The power vector of the combustion stage goes through the center of the offset lobe.
While a four-stroke piston engine completes one combustion stroke per cylinder for every two rotations of the crankshaft (that is, one-half power stroke per crankshaft rotation per cylinder), each combustion chamber in the Wankel generates one combustion stroke per driveshaft rotation, i.e. one power stroke per rotor orbital revolution and three power strokes per rotor rotation. Thus, the power output of a Wankel engine is generally higher than that of a four stroke piston engine of similar engine displacement in a similar state of tune; and higher than that of a four-stroke piston engine of similar physical dimensions and weight.
Wankel engines generally can sustain much higher engine revolutions than reciprocating engines of similar power output. This is due to the smoothness inherent in circular motion, and the absence of highly stressed parts such as crankshafts, camshafts or connecting rods. Eccentric shafts do not have the stress related contours of crankshafts. The maximum revolutions of a rotary engine is limited by tooth load on the synchronizing gears. Hardened steel gears are used for extended operation above 7000 or 8000 rpm. Mazda Wankel engines in auto racing are operated above 10000 rpm. In aircraft they are used conservatively, up to 6500 or 7500 rpm. However, as gas pressure participates in seal efficiency, racing a Wankel engine at high rpm under no load conditions can destroy the engine.
National agencies that tax automobiles according to displacement and regulatory bodies in automobile racing variously consider the Wankel engine to be equivalent to a four-stroke piston engine of 1.5 to 2 times the displacement. Some racing series have banned the Wankel altogether.
Engineering:
Felix Wankel managed to overcome most of the problems that made previous rotary engines fail by developing a configuration with vane seals that had a tip radius equal to the amount of "oversize" of the rotor housing form, as compared to the theoretical epitrochoid, to minimize radial apex seal motion plus introducing a cylindrical gas-loaded apex pin which abutted all sealing elements to seal around the three planes at each rotor apex.
In the early days, an specially devoted production machine had to be built for an individual housing dimensional arrangement, however, patents as US3824746, G J Watt, 1974, for a: 'Wankel Engine Cylinder Generating Machine', or US3916738: 'Apparatus for machining and/or treatment of trochoidal surfaces', US 3964367: 'Device for machining trochoidal inner walls', solved the issue.
Rotary engines have a thermodynamic problem not found in reciprocating four-stroke engines in that their "cylinder block" operates at steady state, with intake, compression, combustion, and exhaust occurring at fixed housing locations for all "cylinders". In contrast, reciprocating engines perform these four strokes in one chamber, so that extremes of "freezing" intake and "flaming" exhaust are averaged and shielded by a boundary layer from overheating working parts.
The boundary layer shields and the oil film act as thermal insulation, leading to a low temperature of the lubricating film (maximum ~200 °C/400 °F) on a water-cooled Wankel engine. This gives a more constant surface temperature. The temperature around the spark plug is about the same as the temperature in the combustion chamber of a reciprocating engine. With circumferential or axial flow cooling, the temperature difference remains tolerable.
During research in the 1950s and 1960s problems arose. For a while, engineers were faced with what they called "chattered marks" and "devil's scratch" in the inner epitrochoid surface. They discovered that the origin was in the apex seals reaching a resonating vibration, and solved the problem by reducing the thickness and weight of apex seals. Scratches disappeared after the introduction of more compatible materials for seals and housing coatings. Another early problem of the build-up of cracks in the stator surface was eliminated by installing the spark plugs in a separate metal insert in the housing instead of it being screwed directly into the block housing. Toyota proved that substituting the leading site spark plug with a glow-plug improved low rpm, part load SFC by 7%, emissions and idle (SAE paper 790435). A later alternative solution to spark plug boss cooling was provided with a variable coolant velocity scheme for water-cooled rotaries which has had widespread use being patented by Curtiss-Wright,[39] with the last-listed for better air-cooled engine spark plug boss cooling. These approaches did not require a high conductivity copper insert but did not preclude its use. Ford tested an RCE with the plugs placed in the side plates, instead of in the housing working surface that was the usual way (Patent CA1036073, 1978).
Four stroke reciprocating engines are less suitable for hydrogen. The hydrogen can misfire on hot parts like the exhaust valve and spark plugs. Another problem concerns the hydrogenate attack on the lubricating film in reciprocating engines. In a Wankel engine, this problem is circumvented by using a ceramic apex seal against a ceramic surface: there is no oil film to suffer hydrogenate attack. The piston shell must be lubricated and cooled with oil. This substantially increases the lubricating oil consumption in a four-stroke hydrogen engine.
Increasing the displacement and power of a Wankel RCE by adding more rotors to a basic design is simple, but a limit may exist in the number of rotors, as power output is channeled through the last rotor shaft, with all the stresses of the whole engine present at this point. For engines with more than two rotors, the approach of coupling two bi-rotor sets by a serrate coupling between the two rotor sets has been tested successfully.
SPARCS in the UK found that idle stability and economy was obtained by supplying an ignitable mix to only one rotor in a multi rotor engine in a forced-air cooled rotor, similar to the later Norton designs.
Materials:
Unlike a piston engine, where the cylinder is heated by the combustion process and then cooled by the incoming charge, Wankel rotor housings are constantly heated on one side and cooled on the other, leading to high local temperatures and unequal thermal expansion. While this places high demands on the materials used, the simplicity of the Wankel makes it easier to use alternative materials, such as exotic alloys and ceramics. With water cooling in a radial or axial flow direction, with the hot water from the hot bow heating the cold bow, the thermal expansion remains tolerable; top engine temperature has been reduced to 129 °C, with a maximum temperature difference between engine parts of 18 °C by the use of Heat Pipes around housing and in side plates as a cooling means (SAE paper 2014-01-2160).
Among the alloys cited for Wankel housing use are A-132, Inconel 625, and 356 treated to T6 hardness. Several materials have been used for plating the housing working surface, 'Nikasil' being one. Citroen, Mercedes-Benz, Ford, A P Grazen and others applied for patents in this field. For the apex seals, the choice of materials has evolved along with the experience gained, from carbon alloys, to steel, ferrotic, and other materials. The combination between housing plating and apex and side seals materials was determined experimentally, to obtain the best duration of both seals and housing cover. For the shaft, steel alloys with little deformation on load are preferred, the use of Maraging steel has been proposed for this.
Lead is a solid lubricant with leaded gasoline linked to a reduced wear of seals and housings. Leaded gasoline was the predominant type available in the first years of the Wankel engine's development. The first engines had the oil supply calculated with consideration of gasoline's lubricating qualities. Leaded gasoline was phased out, with Wankel engines needing an increased mix of oil in the gasoline to provide lubrication to critical engine parts. Experienced users advise, even in engines with electronic fuel injection, adding at least 1% of oil directly to gasoline as a safety measure in case the pump supplying oil to combustion chamber related parts fails or sucks in air. The SAE paper by D W Garside describes extensively Norton's choices of materials and cooling fins.
Several approaches involving solid lubricants were tested, and even the addition of MoS2, one cm3 per liter of fuel is advised (LiquiMoly). Many engineers agree that the addition of oil to gasoline as in old two-stroke engines is a safer approach for engine reliability than an oil pump injecting into the intake system or directly to the parts requiring lubrication. A combined oil-in-fuel plus oil metering pump is always possible.
Sealing:
Early engine designs had a high incidence of sealing loss, both between the rotor and the housing and also between the various pieces making up the housing. Also, in earlier model Wankel engines, carbon particles could become trapped between the seal and the casing, jamming the engine and requiring a partial rebuild. It was common for very early Mazda engines to require rebuilding after 50,000 miles (80,000 km). Further sealing problems arise from the uneven thermal distribution within the housings causing distortion and loss of sealing and compression. This thermal distortion also causes uneven wear between the apex seal and the rotor housing, evident on higher mileage engines. The problem is exacerbated when the engine is stressed before reaching operating temperature. However, Mazda Wankel engines have solved these problems. Current engines have nearly 100 seal related parts.
The problem of clearance for hot rotor apexes passing between the axially closer side housings in the cooler intake lobe areas was dealt with by using an axial rotor pilot, radially inboard of the oils seals plus improved inertia oil cooling of the rotor interior ( C-W patents 3,261,542, C. Jones, 5/8/63, 3,176,915, M. Bentele, C.Jones. A.H. Raye. 7/2/62), and slightly "crowned" apex seals (different height in the center and in the extremes of seal).
Modern Wankel engines have fully sealed mainshaft cases. Many engines do not require oil changes as the oil is not contaminated by the combustion process.
Fuel economy and emissions:
The shape of the Wankel combustion chamber is more resistant to preignition operating on lower-octane rating gasoline than a comparable piston engine. The combustion chamber shape also leads to relatively incomplete combustion of the air-fuel charge, with a larger amount of unburned hydrocarbons released into the exhaust. The exhaust is, however, relatively low in NOx emissions, as combustion temperatures are lower than in other engines, and also because of some inherent exhaust gas recirculation (EGR) in early engines. Sir Harry Ricardo showed in the 1920s that for every 1% increase in the proportion of exhaust gas in the admission mix, there is a 45 °F reduction in flame temperature. This allowed Mazda to meet the United States Clean Air Act of 1970 in 1973, with a simple and inexpensive 'thermal reactor' which is an enlarged chamber in the exhaust manifold. By decreasing the air-fuel ratio until unburned hydrocarbons (HC) in the exhaust would support combustion in the thermal reactor. Piston-engine cars required expensive catalytic converters to deal with both unburned hydrocarbons and NOx emissions. This inexpensive solution improved fuel consumption, which was already a weak point for the Wankel engine, at the same time that the oil crisis of 1973 raised the price of gasoline.
Mazda improved the fuel efficiency of the thermal reactor system by 40% by the time of introduction of the RX-7 in 1978. However, Mazda eventually shifted to the catalytic converter system. According to the Curtiss-Wright research, the factor that controls the amount of unburned HC in the exhaust is the rotor surface temperature, with higher temperatures producing less HC. Curtiss-Wright showed also that the rotor can be widened, keeping the rest of engine's architecture unchanged, thus reducing friction losses and increasing displacement and power output. The limiting factor for this widening being mechanical considerations, especially shaft deflection at high rotative speeds (SAE paper 710582). Quenching is the dominant source of HC at high speeds, and leakage at low speeds.
Automobile Wankel rotary engines are capable of high speed operation. However, it was shown that an early opening of the intake port, longer intake ducts, and a greater rotor eccentricity can increase the amount of torque at low RPM. The shape and positioning of the rotor recess -combustion chamber- influences emissions and fuel economy, the MDR being chosen as a compromise, but which shape of the combustion recess gives better results in terms of fuel economy and exhaust emissions varies depending on the number and placement of spark plugs per chamber of the individual engine.
In Mazda's RX-8 with the Renesis engine, fuel economy met California State requirements, including California's low emissions vehicle (LEV) standards. This was achieved by a number of innovations. The exhaust ports, which in earlier Mazda rotaries were located in the rotor housings, were moved to the sides of the combustion chamber. This solved the problem of the earlier ash buildup in the engine, and thermal distortion problems of side intake and exhaust ports. A scraper seal was added in the rotor sides, and by use of some ceramic-made parts in the engine. This approach allowed Mazda to eliminate overlap between intake and exhaust port openings, while simultaneously increasing the exhaust port area. The side port trapped the unburned fuel in the chamber, decreased the oil consumption, and improved the combustion stability in the low-speed and light load range. The HC emissions from the side exhaust port Wankel engine are 35–50% less than those from the peripheral exhaust port Wankel engine, because of near zero intake and exhaust port opening overlap. Although peripheral ported RCEs have a better mean effective pressure, especially at high rpm and with a rectangular shaped intake port (SAE paper 288A). However, the RX-8 was not improved to meet EuroV emission regulations and was discontinued in 2012.
Mazda is still continuing development of the next generation of Wankel engines, the 16X. The company is researching engine laser ignition, eliminating spark plugs, and direct fuel injection to which the Wankel engine is suited. This leads towards a greater rotor eccentricity, equaling a longer stroke in a reciprocating engine, for better elasticity and low rpm torque. These innovations promise to improve fuel consumption and emissions. To improve fuel efficiency further, Mazda is looking at using the Wankel as a range extender in series-hybrid cars and announced a prototype, the Mazda2 EV, for press evaluation in November 2013. This configuration improves fuel efficiency and emissions. As a further advantage, running a Wankel engine at a constant speed gives greater engine life. Keeping to a near constant, or narrow band, of revolutions eliminates, or vastly reduces, many of the disadvantages of the Wankel engine.
Advanteges:
-A far higher power to weight ratio than a piston engine (it is approximately one third of the weight of a piston engine of equivalent power output)
-It is approximately one third of the size of a piston engine of equivalent power output
-No reciprocating parts
-Able to reach higher revolutions per minute than a piston engine
-Operates with almost no vibration
-Not prone to engine-knock
-Cheaper to mass-produce as the engine contains fewer parts
-Superior breathing, filling the combustion charge in 270 degrees of mainshaft rotation rather than 180 degrees in a piston engine
-Supplies torques for about two thirds of the combustion cycle rather than one quarter for a piston engine
-Wider speed range gives greater adaptability
-It can use fuels of wider octane ratings
-Does not suffer from "scale effect" to limit its size
-On some Wankel engines the sump oil remains uncontaminated by the combustion process requiring no oil changes. The oil in the mainshaft is totally sealed from the combustion process. The oil for -----Apex seals and crankcase lubrication is separate. In piston engines the crankcase oil is contaminated by combustion blow-by through the piston rings.
Wankel engines are considerably lighter and simpler, containing far fewer moving parts than piston engines of equivalent power output. Valves or complex valve trains are eliminated by using simple ports cut into the walls of the rotor housing. Since the rotor rides directly on a large bearing on the output shaft, there are no connecting rods and no crankshaft. The elimination of reciprocating mass and the elimination of the most highly stressed and failure prone parts of piston engines gives the Wankel engine high reliability, a smoother flow of power, and a high power-to-weight ratio.
The surface-to-volume-ratio is so complex that a direct comparison cannot be made between a reciprocating piston engine and a Wankel engine. The flow velocity and the heat losses behave quite differently. Surface temperatures behave absolutely differently; the film of oil in the Wankel engine acts as insulation. Engines with a higher compression ratio have a worse surface-to-volume ratio. The surface-to-volume ratio of a diesel engine is much poorer than a gasoline engine, but diesel engines are well known for a higher efficiency factor. Thus, engines with equal power should be compared: a naturally aspirated 1.3 L Wankel engine with a naturally aspirated 1.3 L, four stroke reciprocating piston engine with equal power. But such a four-stroke engine is not possible and needs twice the displacement for the same power as a Wankel engine. When comparing the power-to-weight ratio or physical size to a similar output piston engine, the Wankel is superior.
The extra or "empty" stroke(s) should not be ignored, as a four stroke cylinder produces a power stroke only every other rotation of the crankshaft. This doubles the real surface-to-volume ratio for the four-stroke reciprocating piston engine and the demand of displacement. The Wankel, therefore, has higher volumetric efficiency and a lower pumping loss through the absence of choking valves. Because of the quasi-overlap of the power strokes that cause the smoothness of the engine and the avoidance of the four-stroke cycle in a reciprocating engine, the Wankel engine is very quick to react to throttle changes and is able to quickly deliver a surge of power when the demand arises, especially at higher rpm. This difference is more pronounced when compared to 4 cylinder reciprocating engines and less pronounced when compared to higher cylinder counts.
In addition to the removal of internal reciprocating stresses by virtue of the complete removal of reciprocating internal parts typically found in a piston engine, the Wankel engine is constructed with an iron rotor within a housing made of aluminium, which has a greater coefficient of thermal expansion. This ensures that even a severely overheated Wankel engine cannot seize, as would be likely to occur in an overheated piston engine. This is a substantial safety benefit of use in aircraft. In addition, valves and valve trains that do not exist cannot burn out, jam, break, or malfunction in any way, again increasing safety. GM tested an Iron Rotor and Iron Housing in their Wankel RCEs, that worked at higher temperatures with lower SFC.
A further advantage of the Wankel engine for use in aircraft is the fact that a Wankel engine generally has a smaller frontal area than a piston engine of equivalent power, allowing a more aerodynamic nose to be designed around it. The simplicity of design and smaller size of the Wankel engine also allows for savings in construction costs, compared to piston engines of comparable power output.
Wankel engines that operate within their original design parameters are almost immune to catastrophic failure. A Wankel engine that loses compression, cooling or oil pressure will lose a large amount of power and fail over a short period of time. It will, however, usually continue to produce some power during that time, allowing for a safer landing when used in aircraft. Piston engines under the same circumstances are prone to seizing or breaking parts that almost certainly results in catastrophic failure of the engine and instant full loss of power. For this reason, Wankel engines are very well suited to snowmobiles, which often take users into remote places where a failure could result in frostbite or death, and aircraft, where abrupt failure is likely to lead to a crash or forced landing in a remote place.
From the combustion chamber shape and features, the fuel ON requirements of Wankel RCEs are lower than in reciprocating ICEs, maximum road octane number requirements were 82 for a peripheral intake port RCE, and less than 70 for a side inlet port engine (SAE paper 720357), from the point of view of oil refiners this may be an industrial advantage in fuel production costs. ('Lubricant and Fuel Requirements and General Performance Data of Wankel Rotary Piston Engines', R D Behling and E Weise, BP, SAE paper 730048; 'A Refiner's Viewpoint on Motor Fuel Quality', W M Holaday and J Nappel, Socony-Vacuum Oil Co, SAE paper 430113).
Due to a 50% longer stroke duration than a reciprocating four-cycle engine, there is more time to complete the combustion. This leads to greater suitability for direct fuel injection and stratified charge operation. A Wankel rotary engine has stronger flows of air-fuel mixture and a longer operating cycle than a reciprocating engine, realizing concomitantly thorough mixing of hydrogen and air. The result is a homogeneous mixture, and no hot spots in the engine, which is crucial for hydrogen combustion.
Disadvantages:
Many of the disadvantages are in ongoing research with some advances greatly reducing negative aspects of the engine. However, the current disadvantages of the Wankel engine in production are:
Rotor sealing. This is still a problem as the engine housing has vastly different temperatures in each separate chamber section. The different expansion coefficients of the materials gives a far from perfect sealing. Additionally, both sides of the seals are being exposed to fuel, and the design does not allow for a dedicated lubrication system, as in two-stroke engines. In comparison, a piston engine has all functions of a cycle in the same chamber giving a more stable temperature for piston rings to act against; additionally, only one side of the piston in a (four stroke) piston engine is being exposed to fuel, allowing for oil to lubricate the cylinders from the other side. To overcome the differences in temperatures between different regions of housing and side and intermediary plates, and the associated thermal dilatation inequities, the use of a heat pipe, transporting heat from the hot to the cold parts of engine, has been shown to reduce, in a small displacement, charge cooled rotor, air-cooled housing RCE, the maximal engine temperature from 231 °C to 129 °C, and the maximum difference from a hotter to a colder region of engine, from 159 °C to 18 °C.
Apex seal lifting. Centrifugal force pushes the apex seal onto the housing surface forming a firm seal. Gaps can develop between the apex seal and troichoid housing in light-load operation when imbalances in centrifugal force and gas pressure occur. In low engine-rpm ranges, or under low-load conditions, gas pressure in the combustion chamber can cause the seal to lift off the surface, resulting in combustion gas leaking into the next chamber. Mazda has identified this problem and have developed a solution. By changing the shape of the troichoid housing, the seals remain flush to the housing. This points to using the engine at sustained higher revolutions eliminating apex seal lift off, in applications such as an electricity generator. In vehicles this leads to series-hybrid applications of the engine.
Slow combustion. The combustion is slow as the combustion chamber is long, thin, and moving. The trailing side of the combustion chamber naturally produces a "squeeze stream" that prevents the flame from reaching the chamber trailing edge. Fuel injection in which fuel is injected towards the leading edge of the combustion chamber can minimize the amount of unburnt fuel in the exhaust. Kawasaki proposed a triangular tail extension of the plug hole, pointing to the combustion chamber trailing side to solve this.
Bad fuel economy. This is due to seals leakages, and the "difficult" shape of the combustion chamber, with poor combustion behavior and mean effective pressure at part load, low rpm. Meeting the emissions regulations requirements sometimes mandates a fuel-air ratio that is not the best for fuel economy. Acceleration and deceleration as in direct drive average driving conditions also affect fuel economy. Running the engine at a constant speed and load eliminates excess fuel consumption.
Poor emissions. As unburnt fuel is in the exhaust stream, emissions requirements are difficult to meet. This problem may be overcome by implementing direct fuel injection into the combustion chamber. The Freedom Motors Rotapower Wankel engine, which is not yet in production, met the ultra low California emissions standards. The Mazda Renesis engine, with both intake and exhaust side ports, suppressed the loss of unburned mix to exhaust formerly induced by port overlap.
Although in two dimensions the seal system of a Wankel looks to be even simpler than that of a corresponding multi-cylinder piston engine, in three dimensions the opposite is true. As well as the rotor apex seals evident in the conceptual diagram, the rotor must also seal against the chamber ends.
Piston rings are not perfect seals: each has a gap to allow for expansion. The sealing at the Wankel apexes is less critical, as leakage is between adjacent chambers on adjacent strokes of the cycle, rather than to the crankcase. Although sealing has improved over the years, the less than effective sealing of the Wankel, which is mostly due to lack of lubrication, is still a factor reducing its efficiency. Comparison tests have shown that the Mazda rotary powered RX-8 sports car may use more fuel than a heavier vehicle powered by larger displacement V8 engines for similar performance results.
The fuel-air mixture cannot be pre-stored as there are consecutive intake cycles. The Wankel engine has a 50% longer stroke duration than a piston engine. The four Otto cycles last 1080° for a Wankel engine (three revolutions of the output shaft) versus 720° for a four stroke reciprocating piston engine, but the four strokes are still the same proportion of the total.
There are various methods of calculating the engine displacement of a Wankel. The Japanese regulations for calculating displacements for engine ratings use the volume displacement of one rotor face only, and the auto industry commonly accepts this method as the standard for calculating the displacement of a rotary. When compared by specific output, however, the convention results in large imbalances in favor of the Wankel motor, an early approach was rating displacement of each rotor as two times the chamber.
Wankel rotary engine and piston engine displacement and corresponding power output can more accurately be compared by displacement per revolution of the eccentric shaft. A calculation of this form dictates that a two rotor Wankel displacing 654 cm3 per face will have a displacement of 1.3 L per every rotation of the eccentric shaft (only two total faces, one face per rotor going through a full power stroke) and 2.6 L after two revolutions (four total faces, two faces per rotor going through a full power stroke). The results are directly comparable to a 2.6 L piston engine with an even number of cylinders in a conventional firing order, which will likewise displace 1.3 L through its power stroke after one revolution of the crankshaft, and 2.6 L through its power strokes after two revolutions of the crankshaft. A Wankel rotary engine is still a four stroke engine and pumping losses from non power strokes still apply, but the absence of throttling valves and a 50% longer stroke duration result in a significantly lower pumping loss compared to a four stroke reciprocating piston engine. Measuring a Wankel rotary engine in this way more accurately explains its specific output, as the volume of its air fuel mixture put through a complete power stroke per revolution is directly responsible for torque and thus power produced.
The trailing side of the rotary engine's combustion chamber develops a squeeze stream which pushes back the flamefront. With the conventional one or two spark plug system and homogenous mixture, this squeeze stream prevents the flame from propagating to the combustion chamber's trailing side in the mid and high engine speed ranges, Mazda engineers described the full process. Kawasaki addressed this problem in their US patent nº 3848574, and Toyota obtained a 7% economy improvement by placing a glow plug in the leading site and using Reed-Valves in intake ducts.[64] This poor combustion in the trailing side of chamber is one of the reasons why there is more carbon monoxide and unburnt hydrocarbons in a Wankel's exhaust stream. A side port exhaust, as is used in the Mazda Renesis, avoids one of the causes of this because the unburned mixture cannot escape. The Mazda 26B avoided this issue through a three spark-plug ignition system. (At the 24 Hours of Le Mans endurance race in 1991 the 26B had significantly lower fuel consumption than the competing reciprocating piston engines. All competitors had the same amount of fuel available due to the Le Mans limited fuel quantity rule).
A peripheral intake port gives the highest mean effective pressure, however, side intake porting produces a more steady idle, as it helps to prevent blow-back of burned gases into the intake ducts which cause "misfirings": alternating cycles where the mixture ignites and fails to ignite; peripheral porting (PP) gives the best mean effective pressure throughout the rpm range, but PP was linked also to worse idle stability and part load performance. Early work from Toyota led to the addition of a fresh air supply to the exhaust port and proved also that a Reed-valve in the intake port or ducts improved the low rpm and partial load performance of Wankel RCEs, by preventing blow-back of exhaust gas into the intake port and ducts, and reducing the misfiring-inducing high EGR, at the cost of a small loss of power at top rpm; this is according to David W. Garside, the developer of the Norton rotary engine, who proposed that an earlier opening of the intake port before top dead center (TDC) and longer intake ducts improved low rpm torque and elasticity of RCEs, also described in Kenichi Yamamoto's books. Elasticity is also improved with a greater rotor eccentricity, analogous to a longer stroke in a reciprocating engine. Wankel engines operate better with a low pressure exhaust system, higher exhaust backpressure reducing mean effective pressure, more severely in peripheral intake port engines. The Mazda RX-8 Renesis engine improved performance by doubling the exhaust port area respect to earlier designs, and there is specific work about the effect of intake and exhaust piping configuration on RCEs performance.
All Mazda made Wankel rotaries, including the Renesis found in the RX-8, burn a small quantity of oil by design, metered into the combustion chamber to preserve the apex seals. Owners must periodically add small amounts of oil, thereby increasing running costs. Some sources (rotaryeng.net) claim that better results come with the use of an oil in fuel mixture rather than an oil metering pump. Liquid cooled engines require a mineral multigrade oil for cold starts, and RCEs need a warm-up time before full load operation as reciprocating engines do. All engines exhibit oil loss, however the rotary engine is engineered with a sealed motor, unlike a piston engine that has a film of oil that splashes on the walls of the cylinder to lubricate them, hence an oil "control" ring. No oil loss engines have been developed, eliminating much of the oil lubrication problems.As the rotor's apex seals pass over the spark plug hole, compressed charge can be lost from the charge chamber to the exhaust chamber, entailing fuel in the exhaust, reducing efficiency, and giving high emissions. This may be overcome by using laser ignition, eliminating traditional spark plugs, which may give a narrow slit in the motor housing the rotor apex seals can fully cover with no loss of compression from one chamber to another. The laser plug can fire its spark through the narrow slit. T Kohno et al. proved that installing a glow-plug in the leading site improved in 7% part load and low rpm fuel economy. Direct fuel injection of which the Wankel engine is suited, combined with laser ignition in single or multiple laser plugs, will enhance the motor even further reducing the disadvantages.
