BACKGROUND

Field of the invention

[0001] The present invention concerns an internal combustion engine with three rotors.

Prior and related art

[0002] Small engines used in, for example, cars, boats and propeller aircrafts should be efficient, environment-friendly and inexpensive. Numerous designs have been developed over the years to achieve these aims. Engines with reciprocating pistons still dominate this market.

[0003] Although two stroke engines are still in use, we will consider only four stroke engines in the following. In otto and diesel engines, a rotating crankshaft moves a piston 'up' during an intake stroke to allow air into a cylinder. During a subsequent compression stroke, the piston moves 'down' to compress the air. This defines a compression ratio. Next, a mixture of compressed air and fuel is ignited to expand during a power stroke, which performs work on the piston and defines an expansion ratio. Finally, an exhaust stroke expels the combustion gases. Thus, the four strokes of a cycle complete in two revolutions of the crankshaft.

[0004] In an otto engine, the fuel is ignited by a spark plug. The fuel is composed to avoid autoignition when it enters a hot combustion chamber. According to current standards, petrol with an average nominal composition C ₆ H8 is such a fuel. This fuel has a stoichiometric ratio 14.7: 1 air to fuel for complete combustion. 'Lean' engines designed for higher ratios, e.g. 22: 1, compensate for losses due to throttling. The air- fuel mixture will be termed 'charge'.

[0005] Combustion is never complete, and the combustion gases contain several pollutants. As the exhaust from small engines typically is released at ground level, emission of toxic or hazardous pollutants are particularly important. Specifically, nitrous oxides (NO _x ), volatile hydrocarbons (including methane - CH ₄ ), and dangerous airborne particles (PMio) are estimated to cause several tens of thousands premature deaths in Europe and the US every year. Carbon dioxide (C0 ₂ ) contributes little to ground level air pollution, and only fossil fuels add carbon to the atmosphere. Thus, biofuels or synthetic fuels that recycle carbon from the troposphere are likely to remain important energy sources in the future. Methods for producing synthetic fuel, e.g. Fischer-Tropsch conversion, methanol to petrol conversion and syngas fermentation are not part of the invention, and need no detailed description herein. [0006] Regardless of fuel, the combustion gases contain small amounts of hydrocarbons (HC) and soot or ash due to incomplete combustion. HC, especially methane, react in the atmosphere to form smog. Soot contributes to the ground level concentration of dangerous particulate matter, PM^. Air contains about 78% nitrogen, and combustion in air produces NO _x in amounts that increase with increasing temperature. To limit emission of these and other toxic or hazardous pollutants, the combustion gases pass through a catalytic converter in the exhaust system. Catalytic converters have been mandatory in most countries for several decades, and need no detailed description herein. However, we note that current three way catalytic converters do not work well with lean engines, so modern otto and diesel designs typically operate close to the stoichiometric ratio.

[0007] In a diesel engine, the compression causes the fuel to autoignite. Thus, the diesel fuel has different properties than a fuel for a spark ignition engine, and the diesel engine has many of the properties of a lean engine. Specifically, the combustion occurs close to droplets of fuel at higher peak temperatures than in an otto engine. As higher temperature implies more NO _x , a diesel engine generates more NO _x than an otto engine of similar size.

[0008] Makers of diesel passenger cars have reported low emission of NO _x due to advances in combustion control, e.g. direct injection and non-uniform concentration of fuel in the combustion chamber. However, a study from 2013 funded by the International Council on Clean Transportation (ICCT) showed that two Volkswagen models emitted 5 - 35 times the ΝΟχ emission reported by the manufacturer. In a broader study from 2014, the ICCT found an average NO _x emission of 560 mg/km from 15 diesel cars from different manufacturers during real- world driving. All cars where approved under Euro 6 or US Tier 2 Bin 5/ULEVII, and thus had a 'type approved' emission of 80 mg/km or less. Further details are available at "theicct.org" or in Franco et al.: "Real-world exhaust emissions from modern diesel cars", ICCT 2014. In 2015, Volkswagen had to admit it had installed software to defeat emission tests in several diesel car models. Thus, it seems difficult to adapt diesel engines to strict emission standards despite advances in combustion technology.

[0009] A modern Atkinson-cycle engine is a modified otto engine in which the intake valves are open longer than normal. Hence, some air escapes during the compression stroke and reduces the compression ratio while the power stroke remains unchanged. This reduces the pressure at the end of the power stroke. For example, the pressure at the end of a combustion stroke in an otto engine is about five bar, whereas the corresponding pressure in an Atkinson- cycle engine approaches one bar. Thus, less energy is lost as pressure through the exhaust, and correspondingly more fuel energy is converted to useful kinetic energy or work. On the other hand, the reduced compression ratio means less fuel and air per cycle. Thus, an

Atkinson-cycle engine achieves thermal efficiency at the expense of power density.

[0010] In 1997, Toyota introduced its Prius family of hybrid cars with Atkinson-cycle engines. In these cars, an electric motor is connected to the power train when extra power is needed. Since 1997, other car manufacturers have introduced similar hybrid cars, and some car manufacturers offer engines that run in an Atkinson cycle when the need for power is low and in an Otto cycle when power is more important than thermal efficiency.

[0011] US 2,817,322 discloses a "method of operating a supercharged intercooled engine", later known as the Miller cycle. The original engine from 1957 has reciprocating pistons and a compression control valve in the cylinder head that provides a variable compression ratio. In modern versions, the intake valves are kept open for the initial 20-30% of the compression stroke, reducing the compression rate to 70-80% of the maximum stroke volume. Similar to the Atkinson cycle, this improves thermal efficiency at the expense of power density. In a Miller-cycle engine, a supercharger supplies compressed air to compensate for the reduced compression ratio. An intercooler removes the heat caused by the compression. This yields a lower charge temperature than if the charge was compressed directly by a piston, and reduces the NO _x emission from large diesel engines used aboard ships and in power plants. The Miller cycle is also well suited for spark- ignition and gaseous fuel, e.g. biogas.

[0012] The supercharger in a Miller-cycle engine is typically a Roots or screw design driven by a belt, a chain or gears, as these designs are effective at low angular frequencies. A typical Roots design achieve peak torque around 1800-2000 rpm, and uses 15-20% of the input fuel energy to compress the air. In general, the Miller cycle achieves less thermal efficiency and better power density than the Atkinson cycle. A typical intercooler comprises a water cooled heat exchanger between the supercharger and the engine. Usually, a fan and/or speed wind passing the engine dissipates the heat removed from the heat exchanger.

[0013] A fundamental problem with reciprocating pistons is that a significant fraction of fuel energy is spent on abrupt changes of direction, so rotary designs tend to have better thermal efficiencies. Some of these designs are best suited for large engines. For example, turbojets power large aircrafts, whereas otto cycle engines are still used in smaller aircrafts. Similarly, gas turbines with rotating vanes are used in large power plants, whereas engines with reciprocating pistons are still used in power plants for ships. The latter engines are typically heavy duty diesel engines, possibly modified to run on methane or liquefied petroleum gas (LPG), and to drive an electric generator. Thereby, these engines can run at optimum angular frequency for improved fuel economy. Further, mechanical transmission may be replaced with electric cables and motors to reduce friction losses, improve response times and enable new applications. For example, dynamic positioning would hardly be possible with an engine controlled by fuel supply and mechanical transmission.

[0014] Several designs with rotors better suited for small engines have been proposed. Most of these designs are historical curiosities, but some have valuable features. For example, US 1,953,695 discloses a rotary piston engine with several rotors of different shapes rotating in cylindrical casings. US 2,845,909 discloses a rotary piston engine in which all pistons rotate in the same direction at the same frequency within semi-cylindrical casings, and may be counterbalanced to avoid undesired vibrations. The pistons are generally cam shaped, with one side adapted to seal against the cylindrical part of the casing.

[0015] The Wankel engine was first patented in 1929, and has been developed and sold by several companies since 1957. A Wankel engine comprises a triangular rotor with curved sides that rotates eccentrically within a rotor house. Viewed along the rotational axis, the rotor house has an intake chamber on one side and a combustion chamber on the opposite side. The compressed mixture is ignited when one of the rotor sides is about midways between the intake chamber and the combustion chamber. During one rotation, pressure from combustion gases works on each of the three curved sides of the rotor to drive the rotation. Thus, the Wankel engine completes three full four stroke cycles during one revolution of the drive shaft. Compared to designs with reciprocating pistons, the Wankel engine has a high power to weight ratio, few moving parts and low cost of manufacture. Despite these benefits, only Mazda has made Wankel engines in large series.

[0016] Several problems with early Wankel engines have been resolved. For example, spring loaded rotor- tip seals have solved initial problems with wear and sealing. Other problems include poor fuel economy and a complicated exhaust system with openings in the rotor house walls that caused Mazda to halt mass production. Mazda later resumed production with an improved engine named Renesis, and again ceased production in 2012 as Renesis failed to meet Euro 5 emission standards. According to mazda.com, Mazda continues R&D to adapt its rotary engines to hydrogen fuel and for use as range extenders in electric cars.

[0017] The main shortcomings of modern Wankel engines are related to emission. Firstly, the flame front travels along the rotor side during the power stroke. In an Otto-cycle version, the required pressure at the end of the power stroke increases the risk for unburnt fuel in the combustion gases. Secondly, excess lubricant needed to seal between the rotor and house may increase the amount of unburnt HC and soot at the exhaust ports. Thirdly, uneven thermal expansion between the intake and combustion chambers may increase the need for sealing lubricant between the rotor and planes perpendicular to the rotational axis. All these factors contribute to the amount of unburnt HC and soot at the exhaust ports, and thus require a redesign or an expensive catalytic converter to comply with stringent emission standards.

[0018] US patent publication no. 2015329431 and European patent application EP3003969 disclose ceramics with mineral particles of different lengths. In a related article, Bouville et al.: "Strong, tough and stiff bioinspired ceramics from brittle constituents", Nature Materials 13, 2014, p. 508-514, the inventors report a bulk ceramic with high strength (470 MPa), high toughness (22MPa m 1/2 ) and high stiffness (290 GPa). Because only mineral constituents are needed, the ceramics retain their mechanical properties at high temperatures (600 °C).

[0019] The problem to be solved by the present invention is to provide an improved internal combustion engine that solves at least one of the problems above while retaining benefits from prior art. More particularly, high thermal efficiency and power to weight ratio, low fuel consumption, low manufacturing and maintenance costs and low emission of smog precursors are important parameters in the design and development of the invention.

SUMMARY OF THE INVENTION

[0020] This objective is achieved by a combustion engine according to claim 1. Further features and benefits appear from the dependent claims.

[0021] In particular, an internal combustion engine comprises a housing with three equal rotors. The housing comprises three straight cylinder segments with rotational axes through apices of an equilateral triangle, and wall segments connect the cylinder segments to form an inner wall with three sides. The rotors are configured to rotate with equal orientation and angular frequency about the respective rotational axes of the cylinder segments. Each rotor has an arcuate front face with a tip at each end, and the arcuate front face is adapted to rotate within an adjacent cylinder segment with a predefined clearance. A side face runs from one tip of the front face. The side face has a curve segment defined by the trajectory of a tip on an adjacent rotor during a power stroke. The rotors form a combustion chamber at each wall segment of the housing during one revolution.

[0022] The inner wall of the housing roughly resembles an equilateral triangle with rounded corners defines by the cylinder segments. The arcuate front face of each rotor is a sector of a circular cylinder with slightly less radius than the corresponding cylinder segment.

[0023] Common means for configuring the rotors to rotate with equal orientation and angular frequency include V-belts, toothed belts, chains and gears, all of which are known to those skilled in the art, e.g. for synchronising camshafts in traditional engines. [0024] In vector notation, the curved segment is defined by the trajectory of the tip of the vector P(cot) = R(cot) - D . R(cot) points from the rotational axis to a point on an arcuate front face of an adjacent rotor, D is the fixed vector from the adjacent rotor's rotational axis to the rotational axis of the present rotor, ω is an angular frequency and 0< t < T is a time from start to finish of a power stroke. This applies to rotors rotating clockwise or counter-clockwise.

[0025] The curved segment of the rotor, the wall segment and the arcuate front face of the adjacent rotor form the combustion chamber. Pressure acting on the curved segment contribute to the rotation, whereas pressure acting on the arcuate front face of the adjacent rotor causes radial forces that neither contribute to nor oppose rotation. As a large fraction of the fuel's energy converts to useful kinetic energy, not just kinetic energy, the engine has a high thermal efficiency. In particular, the engine has greater thermal efficiency than an engine with reciprocating pistons because no energy is needed for abrupt changes of direction.

[0026] In an example embodiment, the arcuate front face has an arcuate length 2^R, and the curve segment is defined by

χ(θ) = Rsin - DcosO andy(6) = Rco^ - DsinO,

where x, y is a Cartesian coordinate system with origin in the axis of rotation and y-axis through the midpoint of the arcuate front face, D is the distance between rotational axes in the equilateral triangle and Θ a parameter defining the curve segment during the power stroke.

[0027] In this embodiment, the rotors spin clockwise, and the tip of the front face slides along the curved segment of the next rotor in the clockwise direction during the power stroke. The angle φ is the angle from the rotational axis to the ends of a cylinder segment. The parameter Θ is an angle between an X-axis between the two rotational axes and the x-axis fixed to the rotor, 0 < θ < π/2 - φ. The parameter Θ corresponds to cot in the previous example. The radius of the curved segments of the housing is R + AR, where the clearance AR may be, for example, in the order of tenths of millimetres or less. Alternatively, the clearance AR may depend on a choice of lubricant. Embodiments made of steel, aluminium alloy and/or ceramic materials have different thermal expansions and different requirements for lubrication.

[0028] In some embodiments, the power stroke further extends to an end of the rotor opposite the midpoint of the front face. In these embodiments, the y-axes of the adjacent rotors are parallel to the X-axis at the end of the power stroke, and y(n/2) = -r = R— D. A longest possible power stroke may increase thermal efficiency on the expense of power density as in an Atkinson-cycle engine. The loss of power density is somewhat compensated by a 'fifth' stroke and supply of pressurised, cooled air in the Miller cycle. [0029] In some embodiments, the wall segment is inclined relative to an axis between two adjacent rotational axes. Thus, the end of one cylinder segment may extend a different angle into the rotor housing than the beginning of the next cylinder segment. For example, the need for sealing along the arcuate front faces differ as one front face moves along with a flame front, whereas the other moves into the combustion chamber.

[0030] In some embodiments, the housing comprises a channel for coolant. The coolant is typically water, and the housing wall may essentially be a double shell with water circulating between the shells. In particular, this feature removes heat from the region near the combustion chambers and minimise deformation due to thermal expansion.

[0031] Each rotor is preferably rotationally fixed to a counter weight providing a radial force of equal size and opposite direction to the radial force provided by the rotor. Using the notation above, a rotor with mass m attached to a rotor shaft exert a radial force F = mco yo on the rotor shaft. The distance yo denotes the rotor's centre of mass, and is a positive distance on the y-axis of symmetrical rotor. The force F may cause vibration, noise and extra wear on the bearings. A counter weight with mass M and centre of mass yj eliminates the vibration, noise and extra wear if the counter weight is attached to the rotor shaft and ; = -myo.

[0032] Preferred embodiments comprise a fuel injection nozzle configured to inject a fuel into the combustion chamber. An alternative would be to mix the fuel into the intake air upstream from the combustion chamber. A suitable nozzle should be able to supply fuel to achieve a near stoichiometric ratio, e.g. a little above the nominal ratio 14.7: 1 for petrol, at a rate corresponding to the angular frequency, e.g. once every few milliseconds. The stoichiometric ratio increases power density and permits using effective catalytic converters in the exhaust system. The fuel injection nozzle with associated sensors and controls is preferably of a type used for other direct injection systems and commercially available.

[0033] Preferred embodiments further comprises a spark plug configured to ignite a mixture of fuel and air in the combustion chamber. Alternatively, the engine could run on a fuel adapted to autoignite, e.g. diesel oil. However, autoignition would require a mechanism to increase the pressure and increase the demands for sealing. In addition, an increased peak temperature would increase production of undesired NO _x . Adapting the fuel and spark plug to temperature in the combustion chamber to avoid premature ignition is known in the art.

[0034] Preferably, each rotor is rotationally fixed to a rotor gear engaging a common drive gear. In practice, 'rotationally fixed' means attached to a common rotor shaft. The drive gear may simply synchronise the rotation of the rotor shafts, and the drive shaft(s) extending out of the engine may be one or more extended rotor shafts. [0035] In preferred embodiments, the drive gear is attached to a drive shaft extending out of the engine. The ratio of teeth on the rotor gears to teeth on the drive gear determine the output power and torque for a given angular frequency of the rotors.

[0036] Preferably, the engine is designed for a constant angular frequency. This removes a need for throttling fuel supply. In addition to a simplified design, there is no need for lean fuel mixtures and associated problems with modern catalytic converters, c.f. the previous reference to stoichiometric ratios. Using an appropriate gear ratio between the rotor gears and drive gear described above, the drive shaft may drive a suitable electric generator. Then, electric cables, power electronics and electro motors may replace mechanical transmissions in applications that need variable power and torque.

[0037] Air is advantageously supplied through an intake in a top cover substantially perpendicular to the rotational axes. This increases the size of the air intakes, and corresponds to the 'side intake' in some modern Wankel engines. The orientation of the engine is arbitrary. The 'top cover' is a lid that, among other functions, covers the rotors and closes the housing.

[0038] Exhaust is preferably be expelled through a cover plate essentially perpendicular to the rotational axes. The cover plate is preferably on the opposite side of the air intake to allow a flow of gases in one direction through the housing. The cover plate may cover belts, gears, counter weights etc., and may provide brackets or support for additional equipment such as the generator described above. The spinning rotors may expel combustion gases through exhaust openings in the housing wall. Alternatively, exhaust openings may be arranged in a floor of the housing, and be temporarily closed by the rotors forming a combustion chamber.

[0039] Some embodiments further comprises a compressor for increasing the pressure of the intake air. The compressor may be a supercharger driven by a rotor shaft. Alternatively, the compressor may be a turbocharger driven by the exhaust, or run by an electric motor. Either way, the compressed air should pass through an intercooler according to the Miller cycle. The benefits of a Miller-cycle engine are well known, and include improved fuel economy over an Otto-cycle engine and improved power density over an Atkinson-cycle engine.

[0040] Some embodiments comprise a circular groove for the arcuate front face. In these embodiments, the path around the front face from the combustion chamber is extended into the circular groove to reduce the pressure bleeding off over and under the adjacent rotor. The pressure bleeding off along the rotor moving with the flame front and expanding gas may be kept small by small tolerances and a long path along the respective cylinder segment of the housing. [0041] In some embodiments, the front face comprises ridges and/or grooves for inducing turbulence. In these embodiments, air may provide sufficient sealing to avoid or minimise the use of lubricant and associated problems with catalytic converters and/or emission. Such embodiments require materials that retain their mechanical properties at high temperatures, e.g. the strong and tough ceramics disclosed in US 2015329431 and briefly described above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The invention will be explained in the following detailed description with reference to the accompanying drawings, in which:

Figure 1 is a top view of an engine according to the invention,

Figure 2 is a view from the opposite side,

Figure 3 is a partially sectioned side view,

Figure 4 is a section of a cylinder with a rotor,

Figure 5 illustrates one embodiment of a front face on a rotor

Figure 6 illustrates a power stroke,

Figure 7 illustrates the geometry of the rotors and

Figure 8 is a sample plot of a parametrisation from Figure 7 DETAILED DESCRIPTION

[0043] The drawings illustrate the principle of the invention. They are not to scale, and numerous details known to the skilled person are omitted for clarity. For convenience, Fig. 1 is described as a view from 'above' a dividing plane and Fig. 2 as a view from 'below'.

However, the engine 100 can have any orientation, so Fig. 1 could equally well be a view from a 'front'. Either way, Fig. 2 is a view from the opposite side of the dividing plane.

[0044] Figure 1 shows a housing 110 with three rotors 120. The housing 110 has a double shell with a channel 111 between them for circulating water or another cooling medium. Embodiments with a single shell housing 110 are feasible.

[0045] Each rotor 120 is attached to a rotor shaft 121 that may rotate about a rotational axis. In Fig. 1, the rotors 120 are configured to rotate clockwise. Other embodiments may have rotors 120 that rotate counter-clockwise.

[0046] The rotational axes are also axes of rotational symmetry of three cylinder segments 112, and form an imaginary equilateral triangle on the dividing plane. The cylinder segments 112 are connected by wall segments 113 to form a roughly triangular inner wall with rounded corners. Specifically, each rotor 120 has an arcuate front face 122 with radius R, and the cylinder segments 112 have a slightly larger radius R + AR to permit the arcuate front faces 122 to pass. The clearance AR may be adapted for a lubricating film, which has a sealing function as well as a lubricating function. Using ceramic materials, the clearance AR at the operating temperature might be sufficiently small to eliminate the lubricating film with its associated HC-emissions and/or demands on a catalytic converter in the exhaust system.

[0047] Threaded boltholes 114 are provided to close the housing 110 with a top cover.

[0048] The rotors 120 are synchronised, e.g. by a toothed belt, a chain or the gears 124, 154 in Fig. 2, such that all three rotors 120 have identical angular orientation and identical angular velocities at all times. The skilled person is familiar with similar belts, chains or gears used to synchronise camshafts in known engines.

[0049] Each rotor 120 also has a side face 123 that extends from one tip 125 of the front face 122 around the rotor shaft 121 to a tip 125 at the opposite end of the front face 122. This side face 123 has a curved segment shaped such that a tip 125 of a cylindrical face 122 of a first rotor slides along the side face 123 of an adjacent rotor when the two rotors rotate with equal angular frequency and orientation. The tip 125 may be provided with a spring loaded seal such as those used in the rotor-tips of a Wankel engine. Alternatively, ceramic rotors might have mechanical properties at high temperatures that allow small clearances and elimination of motor oil as lubricant.

[0050] A temporary combustion chamber 130 is formed between the cylindrical face 122 of the rotor 120, part of the side face 123 of the next rotor 120 in the clockwise direction and the wall segment 113. A fuel injection nozzle 131 injects fuel directly into the combustion chamber 130, where the fuel mixes with compressed air. A variety of fuel injection nozzles with associated sensors and control circuits for direct injection engines are commercially available. An alternative where the fuel is mixed into the intake air will be described with reference to Fig. 4. Fuel should never be injected into the region 133 behind the rotors 120, as this might fill the entire housing 110 with an explosive or combustible charge.

[0051] Currently preferred embodiments comprise a spark plug 132 to ignite the charge. This reduces the compression rate needed for autoignition, and thereby the requirements for sealing. Additionally, a lower peak temperature reduces production of NO _x . As noted in the introduction, a compressor and intercooler may further reduce the charge temperature and NO _x -emission. The Miller cycle also resolves problems with 'knocking', i.e. premature ignition as fuel is injected into a hot combustion chamber. This improves fuel economy. [0052] After ignition, the expanding gases create a pressure that works on all surfaces within the combustion chamber 130. Small arrows indicate relevant forces. The first arrow indicates radial forces working on the arcuate face 122 of the rotor 120 at the left hand side. These forces are absorbed effectively by the fixed shaft 121, and do not oppose the rotation. The second arrow illustrates forces acting on the side face 123 of the next rotor 120 that causes it to rotate clockwise. This geometry combined with an effective combustion converts a large fraction of the fuel energy into useful kinetic energy, thereby improving thermal efficiency.

[0053] During the power stroke, the part of side face 123 exposed to the combustion chamber 130 gets longer, i.e. a larger area becomes exposed to the combustion gases as the chamber 130 expands and the pressure within decreases. Thus, the force acting on the side 123 decreases slowly. This feature provides the present design with a near constant torque during the power stroke.

[0054] The sealing around the combustion chamber 130 must be sufficient to withstand the pressure from expanding combustion gases. The path around the cylindrical faces 122 are relatively long, and ensures that little pressure bleeds off from the combustion chamber 130 between the rotors 120 and the cylinder walls 112. Seals over and under the rotors 120 will be further described with reference to Fig. 4.

[0055] In Fig 1, there are two restrictions on any path from the combustion chamber 130 to the intake chamber 133. The first restriction leads into a central region with a roughly triangular shape and reduces the pressure. The second restriction leads from an apex of the central area to the region 133 and causes a further pressure drop.

[0056] The rotors 120 define a second combustion chamber when they have rotated 120° from the position shown in figure 1, and a third combustion chamber when they have rotated 240° from the position shown in figure 1. The ignition and subsequent distribution of forces from the expanding gases in the second and third combustion chambers are similar to those described for the combustion chamber 130 above. Thus, each rotor 120 receives a kick from a respective combustion chamber once per revolution. Due to the coupled rotation, a kick on any rotor 120 causes a rotational shift of the associated rotor shaft 121. In other words, each rotor shaft 121 receive three nearly constant torques per revolution, and hence provides a near constant torque on an output shaft.

[0057] For example, assume that the engine 100 achieves maximum torque at 1800 rpm, i.e. close to that of a typical Roots supercharger. Then, the ripple in output power corresponds to the slowdown that occurs in 1800/60/3 of a second, i.e. in 10 ms, and decreases with increasing angular frequency. [0058] The engine 100 implements three four-stroke cycles per revolution. Each cycle comprises an intake stroke in which air is supplied to the temporary intake chamber 133, a compression stroke in which a rotor presses air into the temporary combustion chamber 130 for the power stroke described above, and an exhaust stroke expelling enough combustion gases to maintain the oxygen level in the intake chamber 133. Embodiments implementing an Atkinson and/or Miller cycle are of particular interest, but Otto and Diesel-cycle engines are not excluded.

[0059] Figure 2 shows the housing 110 from 'below', i.e. from the opposite direction as in Fig. 1. Threaded boltholes 114 illustrate that a cover plate (119, Fig. 3) is removed. In the present embodiment, each rotor shaft 121 has a counterweight 126 providing a radial force with equal size and opposite direction as the rotors 120 in Fig. 1. Specifically, a rotor 120 with mass m and angular frequency ω attached to a rotor shaft 121 exert a radial force F = mco yo on the rotor shaft. The distance yo denotes the rotor's centre of mass, and is a positive distance on a y-axis along the symmetry axis of a mirror-symmetric rotor 120. The force F may cause vibration, noise and extra wear on the bearings. A counter weight 126 with mass M and centre of mass yi eliminates the vibration, noise and extra wear if the counter weight is attached to the rotor shaft 121 and ; = -myo.

[0060] Each rotor shaft 121 is attached to a rotor gear 124, and each rotor gear 124 engages a common drive gear 154, which in turn drives a central drive shaft 150. The ratio of the rotor diameters determines the angular frequency, power and/or torque of the drive shaft 150.

Alternatively, the output drive shaft(s) might be one or more extended drive shafts 120.

However, the output angular frequency and torque would then be those of the rotors 120, and an external mechanism and less compact design might be needed for a desired output power and torque.

[0061] In Fig. 2, the rotor gears 124 have larger diameters and more teeth than the drive gear 154 such that the drive shaft 150 rotates faster. Continuing a previous example, rotor gears 124 rotating at 1800 rpm might run the drive gear 154 at 3000 or 3600 rpm suitable for generating AC at 50 or 60 Hz, respectively. Alternatively, the rotor gears 124 might have fewer teeth than the drive gear 154 to increase the torque and decrease the angular frequency of the output shaft 150. The ratio between the diameters of the rotor gears 124 and the drive gear 154 is a design issue, e.g. determined by the fuel and desired output power and torque.

[0062] Figure 3 is a partially sectioned side view of the engine 100. A top cover 115 closes the housing 110 on the side shown in Fig. 1. The top cover 115 has an air intake 116 and enables larger intake openings than, for example, the wall segments 113 in Fig. 1. [0063] Air from the air intake 116 enters the intake chamber 133, and the rotors 120 compress the air to the combustion chamber as described above. Relatively simple flapper valves may close the combustion chamber 130 during the power stroke. An Atkinson cycle require a modest compression ratio, and may be realised without great difficulty.

[0064] A recess 117 in the housing 110 has openings for the fuel injection nozzle 131 and spark plug 132 shown in Fig. 1. The openings 118 illustrate optional inlets and outlets for cooling water circulating in the optional chamber 111.

[0065] A cover plate 119 closes the bottom of housing 110, and provides brackets and other supports for equipment, e.g. the compressor 160 and a rotary machine 200.

[0066] The output shaft 150 may be connected through a mechanical transmission line to a place where a certain torque and/or power is required. Alternatively, the output shaft 150 may drive an electric generator at optimum angular frequency, and the generator may drive an electric motor through cables and power electronics. The alternative is preferred, e.g. because it improves fuel economy and eliminates friction loss in a mechanical transmission. The alternative also eliminates a need for throttling the fuel supply in order to control output power. This simplifies the control system associated with the fuel injection nozzle 131. As noted in the introduction, the lean fuel mixtures from prior art solved problems associated with throttling. Thus, the engine 100 may run at a constant optimum angular frequency with a fuel mixture close to the stoichiometric ratio. This further improves the efficiency of current three way catalytic converters. The optimum angular frequency of the rotor shafts 121 may provide an angular frequency and torque suitable for a generator through the gears 124, 154 shown in Fig. 2 and discussed above.

[0067] Some marine propulsion systems use a similar arrangement with an engine running at optimum speed, a generator, power electronics and electric motors providing power and/or torque where and when needed. This is known to improve fuel economy and to provide additional benefits. For example, dynamic positioning systems would be impossible with the latency and losses of a fuel throttled engine with a subsequent mechanical transmission.

[0068] The box 160 schematically represent the compressor and associated intercooler for a Miller-cycle engine. As noted above, a supercharger typically consumes 15-20 % of the fuel energy. An electric motor, e.g. powered by a generator on drive shaft 150 as discussed above, may run the compressor. This would reduce loss in a mechanical transmission. However, energy is necessary to compress the charge air, and some energy is removed as heat by the intercooler. Thus, the main benefits of an electric motor are simplified design and control. For example, the compressor may be mounted in any suitable place as no mechanical transmission is needed. An electric motor also enables fast and convenient switching between Atkinson and Miller cycle operation depending on power needs. Additionally, an electric motor reduces the number of movable parts, and may even be less expensive than a mechanical transmission.

[0069] The rotary machine 200 represent a pump, compressor, electric generator etc. with a limited range of loads and/or angular frequency. For example, a pump may be installed on a pipeline for transporting gas and drive an impeller via belts, chains or gears as known in the art. A similar example with a supercharger is discussed above. A split sheave with axially movable conical faces and a belt, e.g. a V-belt or a chain, may provide continuously variable power on an output. However, the generator, power electronics and electromotor may provide more accurate variable power and torque with less latency.

[0070] Figure 4 is a cross section through part of an engine 100 in which the top cover 115 is split in two parts 115 and 115b for manufacturing reasons. Specifically, the air intake 116 may be cast and/or milled as desired, and intake openings 116b may be provided with suitable valves (not shown) before the top cover 115 is clamped onto the housing 110 by means of bolts in the boltholes 114 shown in Fig. 1. Similarly, the channel 111 may be cast and/or machined before the cover plate 119 is attached by means of the boltholes 114 in Fig. 2.

[0071] As an alternative to the direct injection of fuel into the combustion chamber 130 described above, the fuel may be supplied to the intake air in the top cover 116, e.g. in the chamber above the openings 116b.

[0072] While current Atkinson and Miller-cycle engines are modified Otto-cycle engines, there is no objective reason to mimic the compression stroke or intake valve timing of an otto engine. As noted, the valve timing and associated reduced compression ratio are just selected to avoid discharging energy as pressurised gas. Electronic valves in the air supply 116 and exhaust system provides a simpler way to approach atmospheric pressure at the end of the power stroke and thereby achieve the thermal efficiency of the Atkinson and Miller cycles. Specifically, check valves in the openings 116b may control the charge pressure by a proper forward bias and prevent backflow during the power stroke. When computing the forward bias, the compression caused by the rotors 120 should be accounted for. The charge composition may be controlled by direct injection into the combustion chamber 130, i.e. by the fuel injection nozzle 131 described with reference to Fig. 1.

[0073] After the power stroke, fresh air at a pressure slightly above atmospheric pressure supplied through the openings 116b may help expelling the combustion gases, e.g. through an exhaust valve covered by the leading rotor 120 during the power stroke. Covering the exhaust valve by a rotor 120 is a practical way to reduce the requirements for the exhaust valve. [0074] In an alternative embodiment, the charge may be mixed and pressurised in the intake manifold 116, e.g. downstream from a check valve as described in the previous paragraph. In this embodiment, the openings 116b may be configured to release the charge as the passing rotors 120 form a combustion chamber 130. However, preparing the charge in the intake manifold is likely to cause a more complicated design. For example, valves in the openings 116b would be similar in both alternatives, but the latter alternative might need an extra check valve to prevent charge from entering the intake chamber 133, and perhaps extra valves to implement flushing with clean air.

[0075] Seals 127 close the gaps above and below the rotor 120, and are adapted to the thermal expansion at operating temperatures. The seals 127 may be part of the rotor 120 or separate parts. Similar seals are used in a Wankel engine or a Roots supercharger, in which a film of lubricant seals the small gap between the mechanical seal and a plate over and under the rotor. Alternatively, the rotors 120 and/or the inner walls 112 may be made of a tough ceramic or other heat resistant material, possibly with a low coefficient of thermal expansion. Such ceramic seals may not require a lubricant.

[0076] The cylindrical face 122 extends into an arcuate groove 127 in the top cover 114 and housing 110. This provides a longer path, i.e. above and below the rotor 120, for the expanding gas acting on the cylindrical face 122 during the power stroke, and may hence limit the pressure bleeding. If such a groove 120 extends around the entire rotational path, it must of course intersect with similar grooves 127 for the other two rotors. Whether the possible increased compression rate outweighs the added complexity of design and increased cost of manufacture is a design issue that must be left to the skilled person knowing the application.

[0077] Figure 5 is a front view of a rotor 120 in which the arcuate front face 122 is provided with an uneven surface 129, e.g. comprising protrusions or grooves. In embodiments without lubricant, such an uneven surface 129 causes turbulence in a thin layer of air as the front face 122 passes a cylinder segment 112. The turbulent air may replace a lubricant. An uneven surface 129 adds drag in a viscous lubricant, and is not suited for such embodiments.

[0078] Figures 6-8 further illustrate the geometry of the rotors 120 and cylinder walls 112. A first rotor 120 has an axis of mirror symmetry S, and rotates about the origin of a Cartesian coordinate system X,Y. The system X,Y is fixed relative to the housing 110, and the X-axis runs through the rotation axis of the next rotor 120b. The rotors 120, 120b have identical shapes, and rotate clockwise with equal angular orientations and angular velocities. At the start of the power stroke, a tip of the front face on rotor 120 engages the side face 123 of the next rotor 120b to form a combustion chamber, and a charge is ignited by the spark plug 132. [0079] Part of the arcuate front face 122 on the first rotor 120 is still within the cylinder segment 112 and limits the pressure bleeding back to the intake chamber. The overlapping lengths of cylinder segment 112 and front face 122 are adjusted to contain the pressure sufficiently for a desired compression ratio. The wall 113 is inclined with respect to the X- axis to illustrate an asymmetric overlap of the cylinder segments 112 surrounding the two front faces.

[0080] The next rotor 120b moves with the pressure, and opens for an exhaust port 134 at the end of the power stroke as illustrated by the dash-dot line 120c. As the exhaust opening 134 is covered by rotor 120b for the entire power stroke, the exhaust valves may be simplified, perhaps even to reed valves. In one embodiment check valves or electronically controlled valves are mounted at the exhaust ports 134 in order to prevent suction of air opposing the direction of flow of the exhaust gas

[0081] A second Cartesian coordinate system x, y has its origin in the rotational axis of the next or leading rotor 120b, and is fixed relative to the rotor 120b, not to the housing.

[0082] Fig. 7 shows a simplified version of the rotors 120, 120b in Fig. 6. The axis of rotation for the next rotor 120b is displaced by a distance D along the X-axis from the origin of the Χ,Υ-system and rotation axis of the first rotor 120. That is, D is the length of each side in the equilateral triangle mentioned with reference to Fig. 1. An angle φ denotes the length of a circular arc 122 such that the length of arc 122 is L = I R, where R is a radius slightly less than the inner radius of the cylinder segments 112 as explained above.

[0083] The axis of symmetry S and the corresponding y-axis of rotor 120b are displaced by an angle Θ from the fixed axis Y, and the tip of arrow R travels along a circular path about the origin of the X,Y -system.

[0084] The curve segment 123 in Fig. 7 represents the part of the side face 123 of the rotor 120b that engages the rotor 120 during the power stroke. In the first part of the power stroke, the apex represented by the tip of arrow R engages the curve segment 123. The cylindrical face 122 has radius R and may engage the curve segment 123 in a later part of the power stroke, e.g. as in Fig. 1. The curve segment 123 and other parts of the curved side face are mirrored about the y-axis. This is the meaning of the term 'axis of symmetry S

[0085] From Fig. 7, it is seen that the points on the curve segment 123 may be expressed in the x,y- system as: χ(θ) = Rsinc^ - DcosG (1)

y(0) = Rcosc^ - DsinG (2) where Θ is a parameter 0 < θ < π/2 - φ, and φ is a constant as explained above.

[0086] Similar parametric expressions describe an optional curve segment prolonged to the y-axis, i.e. χ(π/2) = 0 and γ(π/2) = R-D = -r as shown in Fig. 1. This makes the power stroke as long as possible, e.g. for an Atkinson or Miller cycle.

[0087] Fig. 8 is a plot of the parametric expressions (1) and (2) with D = 1.2R and φ = 45°. The units are fraction of R, i.e. corresponding to R = 1, and the parameters are selected for illustration only.

[0088] Summarised, the engine 100 may implement an Atkinson cycle for a high thermal efficiency in low-power applications. If moderate power is required, an electric motor powered by the engine may be switched on to run the compressor in a Miller cycle. In these embodiments, the thermal efficiency and fuel economy is improved relative to a Wankel engine running in an Otto cycle. The engine 100 may implement an Otto cycle with suction air intake, but this option may require a more complicated design and would reduce the thermal efficiency. A Diesel cycle is possible, but problems with NO _x emission are expected due to high peak temperatures.

[0089] The engine 100 may run on a variety of fuels including methane, petrol and alcohols made from biomass already present in the troposphere. Spark ignited versions are preferred, and the charge is preferably close to the stoichiometric ratio. Thus, a modern three way catalytic converter will be effective in the exhaust system. Improved thermal efficiency and fuel economy, few moving parts, ease of manufacture as well as low emission of NO _x and dangerous airborne particles are other benefits of the engine 100.

[0090] While the invention has been described above with reference to specific examples, the scope of the invention is defined by the accompanying claims.