Of combustion engines, the best efficiency is normally achieved with large two-stroke diesel engines, so-called crosshead engines. The net efficiency of such a turbocharged engine is about 50 %. However, significant losses are caused by the crankshaft mechanism used in the engine, including its bearings. Furthermore, the lubricating oil used for lubrication of the bearings and the piston causes significant operating costs. Also, it contaminates the exhaust gases so that crust induced by the lubricating oil significantly increases the sweeping costs of the exhaust gas boiler. Naturally, the lubricating oil is also a considerable environmental hazard. If lubrication oil were not needed, the two-stroke diesel engine, fueled by natural gas with high-pressure injection, would be relatively ecological.

It is an aim of the present invention to provide an improvement to the above-mentioned problems. The aim is achieved by the invention primarily according to the characteristics of claim 1.

The invention will be illustrated in the following description and in the appended drawings, in which Fig. 1 illustrates the structure and operating principle of a two- stroke engine according to the invention in a cross-section in the longitudinal direction, and Fig. 2 shows a schematical perspective view of an application, in which four two-stroke engines according to Fig. 1 are used as a group.

The principle of the engine according to the invention is shown in Fig. 1. The basic elements are two pistons 11 a, 11 b (placed on opposite ends of a shaft 13) coupled together and mounted on active magnetic bearings 12 (equipped with a feedback control loop). By

means of a strong, continuously controlled magnetic field, the magnetic bearings 12 keep the shaft 13 connecting the pistons 11 a, 11 b continuously on a desired line, at a precision of about 0.01 mm. As a seal 14 for the pistons 11 a, 11 b and the shaft 13 in relation to the frame structure 15, contactless labyrinth seals are used. The whole piston mechanism moves without a mechanical contact with the frame structure 15, and a lubricating oil is thus not needed.

The power is output from the engine by means of a linear generator 16.

A movable rotor 17 placed between the pistons 11 a, 11 b and connected to the shaft 13 moving with the pistons 11 a, 11 b is magnetized e. g. by means of a flexible cable 18, and the magnetization current is continuously controlled by means of a control device 28 in connection with the frame structure 15 in such a way that the generator consisting of said rotor 17 and a stator 19 surrounding the same and connected to the inner surface of the frame structure 15 generates a counterforce for the pistons 11 a, 11 b, suitable for the operation of the engine. The linear generator 16 produces an asynchronic, primarily high-frequency current which is transformed by means of a frequency transformer 20 to a synchronized alternating current suitable for the mains system.

We shall now discuss the cycle in more detail. When the upper piston 11 a of Fig. 1 moves upwards, it compresses the air in a cylinder 15a, whose temperature is increased by the effect of the compression stroke. At the same time, the piston 11 b effects the combustion stroke, i. e the combustion gas underneath it expands.

When the piston 11a is close to the upper dead centre, an exhaust valve 21 in the cylinder 15b of the lower piston 11 b will open to allow exhaust gases to flow into a turbine 23 in a turbocharger 22. After the piston 11 a has moved still a bit upwards, air ducts 24b in the cylinder 15b of the lower piston 11 b will open to allow the compressed air in the space between the piston 11b and the magnetic bearing 12b to flow into the cylinder 15b. Now, the upper piston 11a is in the upper dead centre, and its cylinder 15a is supplied by an injector valve 27a with fuel (liquid or gaseous fuel, or a mixture of gas and air, so-called pilot fuel) which is ignited in the air heated by the compression

according to the principles of the diesel engine. Now, the upper piston 11 a starts to move downwards, starting a combustion stroke, whereas in the cylinder 15b of the lower piston 11b, the exhaust valve 21 b is closed and the above-described compression stroke is started, as described above for the piston 11 a. Back-pressure valves 25 prevent the compressed air between the piston 11 and the magnetic bearing 12 from flowing back into the compressor 26 of the turbocharger 22.

The above-described working cycle is the normal working cycle of a conventional large two-stroke diesel engine, with the exception that the movement of the piston is now not controlled by a crankshaft. When the pistons 11 are moving, there is always a working cycle going on in either cylinder 15a or 15b, which secures the continuity of the process.

The compression ratio of the engine is no longer determined by the dimensions of the crankshaft but by the timing of the injection and exhaust valves and the counterforce produced by the linear generator 16.

The reciprocating motion of the pistons produces an inertial force swinging the frame structure 15 up and down. This can be compensated by mounting four units, each consisting of pistons 11 a, 11b, a shaft 13, a linear generator, and a frame structure 15, in one integrated block as shown in Fig. 2, in such a way that the shafts 13 are in parallel. The movements of the pistons are here scheduled so that when the pistons move upwards in the cylinders a and b, they always move downwards in the cylinders c and d, and vice versa. In this way, the inertial forces are compensated for, and the engine will be completely free of vibrations.

In large engines, the cooling of the piston is conventionally effected by lubricating oil. On the other hand, the need for cooling the piston is increased by the requirement of light weight, which will normally lead to the choice of a material which is poorly resistant to heat (an aluminium alloy). In this case, the piston is not subjected to equally high demands of light weight, because the reciprocating motion is implemented by means of the joint-free shaft 13 resistant to a high load. The piston

could be made of a heat-resisting, nickel-based alloy (e. g. the Inconel series) and coated with a ceramic substance. Thus, the piston is sufficiently cooled through its bottom.

In a conventional engine, oil-lubricated piston rings are used for sealing of the piston. Here, using a multi-stage labyrinth seal 14, a sufficient sealing is achieved when the clearance between the labyrinth ribs and the frame structure 15 is kept sufficiently small. For this purpose, an active clearance control can be implemented in the same way as in other applications, for example modern jet engines. Thus, sensors are continuously used to measure the clearance between the piston 11 a (lib) and the cylinder 15a (15b), to control the cooling medium circulation of the cylinders 15a, 15b in cooling ducts 28a, 28b which are placed around the cylinders 15a, 15b in the frame structure 15.

At least in some cases, the cooling of the linear generator 16 can be provided without a separate fan by means of the air flow produced by the reciprocating motion of the rotor 17.

The engine can be started, for example, by pressurized air or by using the generator 16 as a linear engine. Starting by pressurized air can be effected eg. by the following way: Pressurized air is supplied via a pressurized air duct 29 to the lower cylinder 15b so that the upper piston 11 a is lifted to the upper dead centre, and fuel is injected to the upper cylinder 15a. At the same time, the exhaust valve 21 b of the lower cylinder 15b is opened until the pressure of the lower cylinder 15a is sufficiently reduced so that the working cycle of the upper cylinder 15a can bring the piston 11 b to the upper dead centre.

Thus, fuel is injected into the lower cylinder 15b, and the operation of the engine is continued in the normal way.

As an example, the engine according to the invention will now be compared with the conventional engine. As the application, a large two- stroke engine is primarily considered, wherein the output would be in the order of 1 MW per cylinder. Compared with a conventional engine, there are much fewer parts; for example, the crankshaft, the reciprocating rod and the crank bearings are missing. Furthermore,

there is no lubricating oil system (also a turbocharger can be easily provided with magnetic bearings). It can thus be estimated that in spite of the more complex control systems and the frequency transformer, the engine according to the invention would be less expensive than a conventional engine in said size category. Thanks to the simpler structure, the mean piston speed can be higher than in the conventional engine, which will further reduce the specific investment.

The efficiency of a modern large two-stroke diesel engine is about 50 %. Because there are no bearing losses or lubricating oil losses in the engine according to the invention, the efficiency may be as high as 55%.

As the consumption of lubricating oil and its processing costs are eliminated, the operating costs are significantly reduced. Furthermore, there will be less crust in the exhaust gas boiler, as mentioned above.

As only two cylinders are coupled together in the engine of the invention, it will be easy to assemble an engine of a desired size without special planning. Naturally, the cylinder blocks can also be placed in the horizontal direction.