[0001] The invention relates to a double acting opposed-piston engine comprising at least two pistons arranged in a cylinder and moving counter to one another, the pistons being arranged to compress air into a combustion chamber between the pistons, and means for feeding air and fuel into the combustion chamber between the pistons and for conveying exhaust gases away from the combustion chamber located between the pistons.

[0002] The double acting opposed-piston engine is a very old invention. At its simplest, the double acting opposed-piston engine is a two-stroke valveless internal-combustion engine wherein air is fed into a cylinder and exhaust gases are removed from the cylinder simultaneously.

[0003] The principle of the opposed-piston engine has been known already since the 19 ^th century. An example of this is a solution described in CH Patent Specification No. 20385.

[0004] The opposed-piston engine was intensively developed in the 1920's and 1930's, when solutions developed by a German Hugo Junkers in particular, e.g. Jumo 205, were widely known engines. Junkers engines were used e.g. in airliners for long-distance flights. Examples of such airliners include the then extremely important long-distance mail planes that were able to fly very long distances nonstop, crossing the Atlantic, for instance. In those days, no other engine could compare to such performance. In addition to Junkers' proprietary airplane types, Junkers engines were also used in aircraft by other known manufacturers of the time. Examples of such manufacturers include Blohm & Voss as well as Dornier.

[0005] An example of opposed-piston engines developed by Hugo Junkers is a solution described in US Patent Specification No. 2 031 318.

[0006] The main advantage of the opposed-piston engine in aircraft use in the 920's and 1930's was their good fuel economy and simple structure. A disadvantage was the size and weight of the engine, since those engines were rather large and heavy. All other engines suffered from the same disadvantage in the 1920's and 1930's, for that matter. In any case, in those days the opposed-piston engine was a highly advantageous solution even in aeroplanes since the Junkers engines, for example, could produce powers of almost 900 horsepower, which was quite a good power level then. The large size of the engine meant that any aircraft using the engine in question was in- evitably large, particularly when in practice the number of engines was two, three, or four. However, the large size and weight of the aircraft was not such a grave drawback in the long-distance mail planes, particularly when these planes most often were waterplanes, in which case the long take-off area required by the large and heavy aeroplane presented no problems. The development of other engine types in the late 1930's and after in practice put an end to the use of the opposed-piston engines in aircraft use. Most recently, however, opposed-piston engine projects have re-emerged with the purpose of developing such an engine for aircraft use as well, for a so-called general aviation use in particular. Such projects have resurged in England in particular.

[0007] The opposed-piston engine has been widely used also in other fields, e.g. in lorry or truck use; examples include a TS3 engine, manufactured by a British factory called Commer, which was a commercial and widely used product. Leyland, another British company, manufactured at least an engine L60, which was employed in military use as a tank engine, for instance. The British companies Rolls Royce and Napier also once manufactured opposed-piston engines for military use in particular. Engine manufacturers in the Soviet Union also manufactured opposed-piston engines for military use. These engines were particularly used as power sources for tanks.

[0008] Examples of fields wherein opposed-piston engines have been used as a power source also include boats, ships, and trains. These have been manufactured by the above-mentioned British companies, for instance.

[0009] Decades of development have revealed that the opposed- piston engine is probably most suitable for high power classes, for which the current piston engine structure is not very suitable. The current piston engine in the high power classes inevitably leads to a large size and heavy structure. For example, the weight of a mere crankshaft of the most powerful (about 80 Mw) piston engine on the market is about 300 tons, the total weight of the engine being about 2300 tons. The engine is about 27 m long and about 13.5 m high. In this respect, the opposed-piston engine is better.

[0010] An object of the invention is to provide an engine which enables prior art drawbacks to be eliminated. This is achieved by an opposed- piston engine according to the invention. The opposed-piston engine according to the invention is characterized in that the opposed-piston engine comprises a basic unit in which the pistons moving counter to one another are formed by means of two piston units, each piston unit comprising a first piston part and a second piston part having the shape of a section of a circle and being arranged with respect to one another at 180 degrees from each other, the piston units being arranged to reciprocate about a common axle in a longitudinal direction of the cylinder so that the piston parts reciprocate along a path in a direction of the circumference of a circle so that between aligned sides of the piston parts of both piston units, four combustion chambers are formed into which air and fuel are arranged to be fed and from which exhaust gases are arranged to be removed.

[0011] An advantage of the invention is that as compared with existing piston engines, the engine is lightweight and small. A further advantage is a simple structure and, furthermore, modifiability in an advantageous manner, i.e. few simple basic structures enable a set of engines covering a wide power range to be achieved.

[0012] In the following, the invention will be explained in closer detail by means of an exemplary embodiment described in the attached drawing, in which

Figure 1 is a schematic view of a piston unit used in the invention, Figure 2 is a schematic view of a basic unit to be used in the invention,

Figure 3 is a schematic view of an embodiment of a complete basic unit,

Figures 4 to 7 schematically show an operation principle of the basic unit,

Figure 8 schematically shows opposite forces affecting the piston units, and

Figure 9 schematically shows a difference in size between a conventional 80 Mw engine and a similarly powered engine according to the invention.

[0013] The double acting opposed-piston engine according to the invention is primarily meant for a power range of 500 kw to 100 Mw. According to analyses conducted, the invention is the more competitive the more powerful the engine in question. Really great in space, weight and costs are achieved in the 1 Mw power class already. Also, the greatest demand on the market lies in the high power classes. The lower-power engines are under se- vere competition and emission regulations are the strictest within the range of 50 to 500 kw, and will become even stricter.

[0014] The engine according to the invention is valveless and extremely simple in its structure. Manufacturing costs are clearly lower than in the existing high-powered engines. The saving in weight and space is considerable over the prior art, and the difference becomes greater in connection with higher power classes.

[0015] The engine according to the invention may preferably be applied e.g. to natural gas or fuel oil operated power plants, marine engines, machinery, lorries, diesel locomotives, military vehicles, etc. Due to high efficiency and ease of use, fuel oil or natural gas operated piston engines are rapidly becoming more common in less than 50 Mw electric power plants. However, the engine according to the invention is by no means restricted to the above- mentioned fields only but, naturally, the engine according to the invention may be employed in any appropriate power source use.

[0016] The engine according to the invention being lightweight and small as compared with the existing high-powered engines, in many applications it may replace a gas turbine because of its clearly higher efficiency. Namely, it is to be noted that a piston engine always has a higher efficiency than a gas turbine. However, an open gas turbine is small and lightweight so, owing to these properties, it often replaces an existing piston engine irrespective of the fact that the efficiency of the gas turbine is lower than that of the piston engine.

[0017] In practice, the structure of the engine according to the invention is modular, i.e. the structure of a basic unit of the engine according to the invention is designed to enable the basic unit to be used for readily building differently powered engines by connecting basic units in succession on one crankshaft. When an engine as short as possible is desired, it is possible to connect two basic units in parallel and a required number one after the other on one crankshaft. This is enabled by the basic unit unit structure, mass balance, the fact that the basic unit exhibits no mass oscillation as well as the fact that the engine comprises no valve gear. In practice, in view of the above, what is required is only few differently sized basic units which enable a perfectly powered engine set to be built in an advantageous manner within a desired power range. [0018] The engine according to the invention operates on an opposed-piston principle widely known per se. According to the basic principle of an opposed-piston engine, two pistons move counter to one another, compressing air into a combustion chamber provided between the pistons. Thus, the structure requires no cylinder heads necessary in a conventional piston engine at all. Consequently, since the engine according to the invention includes no cylinder heads, heat losses are considerably reduced as compared with a normal engine. The total need for cooling in the opposed-piston engine is clearly smaller than that in a piston engine with a normal structure. Thus, the invention requires a smaller cooling battery than a conventional piston engine of a similar size.

[0019] The critical advantage of the opposed-piston engine over other two-stroke engines is cylinder scavenging. In a conventional two-stroke engine, filling the cylinder with air and removal of exhaust gas take place simultaneously. Normally, gas circulation is implemented by blowing air through apertures provided at a lower part of the cylinder while exhaust gas is simultaneously being removed from the other side, through apertures in the lower part of the cylinder. In such a case, air and exhaust gas are highly mixed with one another. In fact, it is impossible by this method to provide the cylinder with completely clean air within the time available. A severe drawback of residual exhaust gas, i.e. an exhaust gas portion remaining in the cylinder, is that it raises the temperature of the filling prior to compression, which decreases the efficiency, lowers the maximum engine revolution rate and causes nitrogen oxide emissions (high combustion temperature).

[0020] In the opposed-piston engine, air flows in from one end of a cylinder passage while exhaust gas is discharged from an opposite end, in which case cylinder scavenging is highly efficient. The mixing of air and exhaust gas in the cylinder of the opposed-piston engine is low, and the amount of air to be blown in is only slightly larger than a theoretical value. In fact, the basic principle of the opposed-piston engine is the only well-working valveless solution, and it enables the competitiveness of medium-speed and high-speed two-stoke diesel engines with four-stroke engines. The maximum revolution rate of the opposed-piston engine may be increased as compared with an ordinary two-stroke engine. This results from the better scavenging of the opposed-piston engine. [0021] Two-stroke diesel engines with valves are also manufactured to some extent. Generally speaking, however, the valve gear is expensive, space-consuming and susceptible to faults. If, for one reason or another, a valve gear is used, a four-stroke principle has usually been found more sensible to use.

[0022] As stated above, the opposed-piston principle as a basic principle has been known for a long time already. It is to be noted, however, that when implemented in a conventional manner, the engine requires the use of two crankshafts, making the engine large and complex. One of the gravest difficulties was how to interconnect the two crankshafts at a power take-off. The conventional opposed-piston engines are still being intensively studied, and engines are at least to some degree manufactured in practice even today.

[0023] The engine according to the invention operates on an opposed-piston principle. This means that the pistons move counter to one another, and fuel is injected into a combustion chamber provided between the pistons, each piston working simultaneously. No cylinder heads are provided, and heat losses are smaller than in a normal engine.

[0024] Cylinder scavenging, i.e. gas exchange, takes place by feeding air through inlet ports into the cylinder, whereby exhaust gas is simultaneously removed through outlet ports. The engine is thus a two-stroke one. The amount of air to be fed is always larger than the necessary theoretical amount since the air and the exhaust gas always mix with one another to some degree.

[0025] In the following, reference is made to Figures 1 to 7.

[0026] The fundamental idea underlying the opposed-piston engine according to the invention is that the opposed-piston engine comprises a basic unit 2 wherein pistons moving counter to one another are formed by means of two piston units 1. The piston unit 1 is schematically shown in Figure 1 while the basic unit is shown in Figure 2.

[0027] Each piston unit 1 comprises a first and a second piston part 1A, 1 D and 1 B, 1 C, respectively, having the shape of a section of a circle and being arranged with respect to one another at 180 degrees from each other. The piston units are arranged to reciprocate about a common axle N located in a longitudinal direction of the cylinder 3 so that the piston parts 1A, 1 D; 1 B, 1 C reciprocate along a path in a direction of the circumference of a circle so that between aligned sides of the piston parts of both piston units, four combustion chambers 4 are formed into which air and fuel are arranged to be fed and from which exhaust gases are arranged to be removed. Means used for feeding the fuel are schematically shown in Figures 4 to 7 by reference number 5.

[0028] Figures 1 and 2 further show that each piston unit 1 of the basic unit 2 is connected by a 180-degree phase shift to a common crankshaft 6. The piston parts of each piston unit 1 are connected to the crankshaft by means of a crank mechanism of their own. In the example of the figures, the crank mechanism comprises a connecting rod 7 and a torque arm 8.

[0029] In the opposed-piston engine according to the invention, the piston parts 1 A, 1 D; 1 B, 1 C of both piston units 1 of the basic unit 2 are double acting piston parts. Air and fuel are arranged to be fed into combustion chambers 4 being formed between piston parts moving towards one another while exhaust gases are arranged to be removed from combustion chambers 4 being formed between two piston parts moving away from one another. The air feed into the combustion chamber and removal of exhaust gases from the combustion chamber are schematically shown in Figures 4 to 7 by means of arrows. The fuel is fed into the combustion chamber at the end of the compression phase by the means 5, as disclosed above. The piston units are arranged on a common crankshaft so that the sides of the piston parts align as viewed in the direction of the circumference of the cylinder.

[0030] In practice, the air feed into the combustion chamber requires means for implementing air feed. In practice, a supercharger may be used as the aforementioned means. Preferably, e.g. a turbocharger may be used as the supercharger. In addition to the turbocharger, an engine with varying revolutions per minute requires a mechanical blower, which may be a Roots supercharger, for instance. In constant speed machines, a turbocharger alone will suffice. The means used for feeding air are schematically shown in Figures 4 and 6 by reference number 9.

[0031] The basic unit 2 of the opposed-piston engine according to the invention comprises two opposed piston units 1 , a crank mechanism, i.e. a torque arm 8 and a connecting rod 7, a crankshaft 6, a circular cylinder 3, and four fuel jets 5 (diesel process). The basic unit 2 is provided with two air inlet ports and two exhaust gas outlet ports, as shown in Figures 4 to 7.

[0032] As shown above, the piston unit 1 comprises two double acting piston parts 1A, 1 D and 1 B, 1 C, correspondingly. One revolution yields the power of eight piston parts, which is received by two bearings of the crankshaft two times during the revolution. Thus, the power is quite considerable with respect to the size of the engine. The engine is very compact with no unused space. The engine is thus both small and lightweight. The engine has very few moving parts as compared with a conventional engine. Figure 3 schematically shows a sketch illustrating what an embodiment of an opposed-piston engine according to the invention could look like.

[0033] In the engine according to the invention, the pistons are not in a rotating motion but the piston parts reciprocate. There are two reasons for this. First, the surface velocity of seals of the piston parts can be made low, multiplying the service life of the seals. A second, even more important reason is that the combustion process is given more time. A good efficiency requires combustion in a constant volume. In practice this is never achieved, but if the combustion volume increases greatly during combustion, the efficiency drops dramatically. A rotary piston engine, i.e. the Wankel engine, for instance, failed exactly because of these two factors.

[0034] The symmetrical structure of the basic unit 2 also results in an increase in pressure taking place simultaneously on both sides of the basic unit between two piston units, e.g. between the piston parts 1A, 1 B and between 1 C and 1 D, correspondingly, as schematically shown in Figure 8. In such a case, the bearing between two piston units is hardly subjected to forces, and the piston unit is hardly subjected to twisting forces. Only torque occurs, the forces caused thereby being directed at main bearings and the crankshaft.

[0035] In the engine according to the invention, frictional losses caused by the piston seals and the bearings are also clearly smaller than in a normal engine with respect to the power produced. The basic unit 2 comprises only four or five high-velocity bearings (the bearings of the crankshaft).

[0036] The structure of the engine according to the invention includes one extremely important factor as far as the manufacture of high- powered engines is concerned. As the diameter of a piston increases, the force directed at the crankshaft increases in proportion to the square of the diameter. In the largest engines, for instance, the diameter of the piston is 95 cm and the maximum pressure is of the order of 150 bar. In such a case, the . crankshaft is subjected to a force of about one million kilopond. Naturally, this leads to grave difficulties in the bearing of the crankshaft and unproportionately increases the weight of the crankshaft. [0037] In the engine according to the invention, power is transferred to the crankshaft 6 through the torque arm 8. By increasing the length of the torque arm 8, the force being directed at the crankshaft and the bearings may be decreased directly proportionately to the length of the torque arm. This seemingly small factor may prove to be highly important already in a basic unit of the order of 1 Mw, and more important in larger basic units. In such a case, bearing friction is also considerably reduced.

[0038] Today, the highest efficiencies in combustion engines are achieved by two-stroke low-speed diesel engines. These engines have no valves, and the gas exchange takes place conventionally by feeding air from the lower part of the cylinder and removing exhaust gas from the lower part of the cylinder. In such a case, the mixing of exhaust gas and air is high and the amount of air blown into the cylinder is much larger than a theoretical value. This, again, decreases the revolutions per minute and lowers the temperature of the exhaust gas entering the turbocharger. The power required by the turbo- charger is also high. In the engine according to the invention, the revolutions per minute may be increased without compromising the efficiency as compared with a normal two-stroke engine. Also, the turbocharger requires less power, which contributes to increasing the efficiency.

[0039] A strength of the basic unit 2 used in the engine according to the invention is that owing to the symmetrical structure, no mass vibration occurs. This is also a huge advantage when manufacturing engines consisting of several basic units, since no need exists to take mass vibrations into account while building different engine variations from the basic units. This is not possible in connection with a normal piston engine.

[0040] A further advantage of the engine according to the invention is the absence of valve gear. The aforementioned advantage is very significant when building differently powered engines, since each variation requires a valve gear of its own size. In this connection it is to be noted that the valve gear poses one of the most difficult challenges to the manufacture of durable engines. In the engine according to the invention, lubrication of the crankshaft and other bearings is carried out in a manner similar to that used in connection with normal piston engines. The cylinder is lubricated through a wall of the cylinder directly by means of so-called dimension lubrication. This is a widely- used lubrication manner in large diesel engines, for instance. [0041] Piston cooling is always necessary when the diameter of a piston is more than 120 mm. In normal engines, piston cooling is very difficult when the diameter of a piston is large. In the largest conventional piston engines, piston cooling requires highly complex and expensive equipment. In the engine according to the invention, the piston is double acting and the thermal stress directed at the piston is double as compared with a normal engine, which is why the pistons always have to be cooled. However, the pistons are considerably simpler to cool than those in an ordinary engine. Preferably, the cooling may be implemented e.g. by oil to be fed to the pistons through a centre axle and returned through a hollow hub of the centre axle.

[0042] In practice, the piston parts of the engine according to the invention are not angular as in the accompanying schematic views, but rounded at their ends in order to make them easier to seal. The sealing of the piston is slightly different from that used in a normal engine, but the sealing is not technically difficult since it is to be noted that the low average speed of the piston already mentioned above makes the sealing easier.

[0043] In all power classes, the engine according to the invention is lighter and smaller in its external volume than the existing piston engines. However, the difference from the ordinary piston engines becomes incredibly great at the high-power end of the power range, as shown in Figure 9. Figure 9 schematically shows a size of a conventional 80 Mw piston engine as a rectangular graph and a size of a similarly powered engine according to the invention by means of a rectangle marked with diagonal lines. According to rough calculations, the external volume and weight of the engine according to the invention are about five times smaller as compared with ordinary low-speed two- stroke engines. The size difference becomes smaller when considering medium-speed four-stroke engines, still remaining more than double, however. In high-speed engines the difference is about double.

[0044] Today, the highest efficiencies (about 50%) are achieved by low-speed two-stroke diesel engines. In power plant and marine engines, efficiency is perhaps the most important factor. The efficiency is influenced by many different factors, such as frictions of piston seals and bearings, revolutions per minute, combustion velocity, heat losses, pressure level (turbo- charger pressure), etc. In the engine according to the invention, the frictional losses of both the bearing seals and the piston seals (piston curl) are always smaller than in normal engines. This also applies to heat losses. Consequent- ly, in the low-speed engines according to the invention, an even higher efficiency is achieved than in the existing low-speed two-stroke engines. As compared with the medium-speed and high-speed diesel engines, the efficiency is at least equally high.

[0045] Nowadays large diesel power stations are often combined plants. They employ a steam turbine for utilizing waste heat from exhaust gas. Usually a small organic turbine installation is used. Here, the circulating agent is an organic gas (rather than water). It is then possible to increase a total efficiency by 4 to 8%. Due to lack of space, such a combined principle is not used on ships. The small-volume and lightweight engine according to the invention enables a combined plant to be used in large ships as well. In such a case, the total efficiency would increase to almost 60%, which is the same as that in the existing combined gas turbine/steam plants of more than 500 Mw.

[0046] The modular structure of the engine according to the invention allows for a manufacturing technique which is completely different from that used in connection with the existing high-powered engines. It is feasible that the entire high power range could be managed with three differently powered basic units, for instance by basic units of 300 kw, 1 Mw and 4 Mw. By interconnecting units of 300 kw, the engines could be within a power range of 500 kw to 2 Mw. By units of 1 Mw, 2 Mw to 12 Mw. By units of 4 Mw, 2 Mw to 100 Mw. In practice, this also means that the crankshafts of each basic unit would be interconnected with the crankshaft of the following unit. In practice, many different solutions are provided for interconnecting the crankshafts. The aforementioned values are only rough estimates and in practice require a more accurate analysis, but in principle the aforementioned values describe the situation quite well. In addition to what has been disclosed above, it is to be noted that the valveless structure of the opposed-piston engine according to the invention enables such a manufacturing principle.

[0047] Naturally, the simplicity and low weight of the engine according to the invention contribute to reducing the price of the engine, but the greatest savings would result from the manufacturing technique. In connection with high-powered engines, the runs are never large, so a few basic units will have to suffice to provide all engines within the power range needed in order for the manufacturing costs to be low. The invention enables this to be achieved in an advantageous manner. [0048] Natural gas and heavy fuel oil operated piston motors are becoming more and more common in less than 50 Mw power plants in the developing countries in particular. This area in particular presents strong growth potential. Owing to the moderate price, compact size and high efficiency, the engine according to the invention is highly suitable particularly for the power class in question. The small size and low price enable it to be used also in higher power classes, up to 100 Mv (possibly even more) since an ordinary piston engine has an about 25% higher efficiency than a gas turbine in this power class. If the waste heat from exhaust gas is utilized in an organic steam plant, the invention enables an about 60% total efficiency to be achieved by means of a combined plant of more than 50 Mw. A corresponding efficiency is achieved only by a combined gas turbine/steam plant of more than 500 Mw. Other uses also exist wherein the engine may replace the gas turbine.

[0049] Nothing in the operating principle of the engine according to the invention is unclear to an expert. The basic principle of the operation is the old opposed-piston principle. In the invention, however, the old basic principle is solved in a novel manner, in which case all its previous drawbacks are eliminated. The end result is a novel, compact, inexpensive and simple construction of great potency.

[0050] As disclosed above, at its most advantageous the invention is in the high power range since within this power range the engine according to the invention is at its most competitive. However, the invention is by no means restricted to the high power range but the engine according to the invention is also suitable for a less than 500 Kw power range, particularly when a small size is to produce a lot of power. The invention is by no means restricted to any particular fuel, either, but the invention may also, petrol-driven, be applied e.g. when direct injection is used, wherein fuel is injected directly into the cylinder, etc.