large crosshead piston engine
The invention relates to a large crosshead piston engine, particularly a two-stroke ship propulsion engine, having a bedplate in which the main bearings for the crankshaft are mounted, at least one cylinder section and a frame box positioned between the bedplate and the cylinder section and carrying guide planes extending in the direction of height of the frame box for the crosshead of each cylinder, and stay bolts clamping the cylinder section, frame box and bedplate together and extending inside the frame box on the outside of the adjacent guide planes.
The frame box mounted on the bedplate envelopes the connecting rod and the crosshead with the required space for their movements, and it is usually provided with a transverse stiffening carrying the guide planes for the slide shoes of the crosshead. The frame box is mounted on top of the bedplate and carries on its top a cylinder section with cylinder liners and cylinder covers. The pistons positioned in the cylinder liners are connected with the crankshaft via the piston rods, the crossheads and the connecting rods. The combustion pressure in the combustion chambers of the cylinders is transmitted from the pistons to the crankshaft and has the effect that in the structure connecting the cylinder cover with the main bearings, tensile forces occur corresponding to the co pressive forces on the piston. In order to prevent these tensile forces from opening the horizontal joints between the bedplate, the frame box and the cylinder section, the stay bolts associated with each cylinder clamp the cylinder section to the bedplate with a force larger than the largest tensile force.
The stay bolts usually extend from above the cylinder section down to a fastening in the bedplate,
normally at the lower side thereof. In the longitudinal direction of the engine, the stay bolts are positioned at the separations between the cylinders and at the ends of the engine. In the known crosshead engines, the stay bolts extend in parallel with the longitudinal median plane of the associated cylinder, and at each cylinder separation the stay bolts are arranged in pairs in the transverse direction of the engine at a mutual distance allowing space for the main bearings and the cylinders. German patent publication No. AS 1 258 185 describes a V-engine in which the stay bolts associated with each cylinder form a large angle with the longi¬ tudinal median plane of the associated cylinder in order to create such a large free space above the main bearings that the crankshaft may be dismantled and taken out through openings in the end wall of the frame box without loosening the stay bolts and without separating the frame box from the bedplate. The stay bolts cross over the longitudinal median plane of the engine, which is quite impermissible in a crosshead engine, where the crosshead has to be able to reciprocate unimpededly over the height of the frame box. The cylinder section has a large transverse width, and there seems to be no actual structural connection between the location of the upper ends of the stay bolts and the design of the cylinder section. In consequence of the large angle of inclination of the stay bolts, special inclined surfaces have been formed in the cylinder section and the bedplate, against which surfaces the bolt heads are tightened. Such inclined surfaces are costly to manufac¬ ture in large engines, where the dimensions and mass of the bedplate are so large that it is difficult to machine it.
The purpose of the invention is to improve the transmission of the tensile forces from the cylinder
cover to the stay bolts in order to reduce the stress level in the cylinder section and/or to reduce its outer dimensions and mass.
With this object in view, the large crosshead engine according to the invention mentioned in the introduction is characterized in that in a manner known per se the longitudinal axes of the stay bolts form an angle (β) with the longitudinal median plane of the associated cylinder so that the distance between the stay bolts and the median plane is larger at the main bearings than at the cylinder section, and that the angle (β) is at least 0.5° and at the most 6°, preferab¬ ly at the most 5°.
The development of large crosshead piston engines goes in the direction of steadily increasing cylinder pressures to obtain better fuel economy and a larger output per cylinder. The high cylinder pressures have the effect that the diameter of the main bearings determines the required distance between each pair of stay bolts. The position of the stay bolts in relation to the median plane renders it possible to design the engine with large main bearings without at the same time using an unnecessarily wide cylinder section.
Compared to the known engine design with parallel stay bolts, the engine according to the invention may have a more narrow cylinder section, which partly reduces the weight of and the manufacturing costs for the cylinder section, partly yields an advantageous increase of the natural frequency for transverse vibrations of the cylinder section. With the invention, the transmission of the vertical tensile forces through the cylinder section and the stay bolts to the bedplate produced by the combustion becomes more direct, and the transmission path becomes shorter, especially in the cylinder section. The angling of the stay bolts thus
causes a substantial reduction of the stresses in the cylinder section. The lower stress load on the structure of the cylinder section may, according to desire, be used either to increase the reliability of the engine or to reduce the amount of material in the cylinder section and thus the cost of its manufacture. The advantageous effects of the invention may also be combined so as to both increase the reliability of the engine and reduce its manufacturing cost. The lower weight and reduced top width of the engine are particu¬ larly advantageous when the engine is used as the propulsion engine of a ship, as in that case the engine takes up both space and buoyancy which may be used for other purposes. As the width of the cylinder section determines the top width of the frame box, the invention provides the further advantage that also the weight and dimension of the frame box in the transverse direction may be reduced. Preferably, the angle between the longitudinal axis of the stay bolts and the median plane of the engine is 5° at the most, as this relatively small angle permits the contact faces for the stay bolt heads formed on the cylinder section and/or the bedplate to extend at right angles to the median plane of the cylinder. This has the advantage as concerns machining that the contact faces may be machined by means of the usual cutting tools used for fine machining of the other surfaces extending at right angles to the median plane, such as the upward contact face for the cylinder cover on the cylinder section.
A wedge-shaped washer may be arranged between the stay bolt head and its contact face to compensate for the angling of the stay bolt head in relation to the contact face. If the above angle is smaller than about
6° , the frictional force between the bolt head and the contact face is larger than the component of the bolt force in the plane of the contact face, which prevents lateral movement of the bolt head. By limiting the above angle to 5°, it is suitably ensured that abnormally large vibrations cannot lead to lateral movement of the stay bolt in relation to the stay bolt hole.
Preferably, all stay bolts form substantially the same angle with the median plane, as this simplifies the manufacture of the engine, but it is possible to let one or more associated pairs of stay bolts extend at a larger angle than the remaining stay bolts to allow space at selected locations for especially large engine components in the area around the crankshaft. For example, the aft-most pair of stay bolts may extend at a larger angle to allow space for the thrust bearing of the engine.
An example of an embodiment of the invention will now be described in further detail below with reference to the schematic drawing showing a cross-sectional view along the longitudinal axis of a cylinder of an engine according to the invention.
For the sake of clarity, the drawing only shows the main components of the stationary structure of a large two-stroke crosshead engine generally designated 1. The engine has a bedplate 2 with main bearings 3 for the crankshaft. An A-shaped frame box 4 is mounted on top of the bedplate, and a cylinder section 5 is mounted on top of the A-shaped frame box. The cylinder section 5 may consist of one unit extending in the longitudinal direction of the engine, or, as shown, of separate units for each engine cylinder. The separate units are bolted together at longitudinal flange portions 6.
Between each cylinder, the A-shaped frame box is provided with a transverse stiffening 7 carrying guide
planes 8 for the crosshead. Each cylinder has a cylinder liner, not shown, which is inserted in a central hole 9 in the cylinder section, and a cylinder cover, also not shown, which is clamped down against the top of the cylinder liner by means of bolts screwed into threaded holes in the upper side of the cylinder section 5. Via a piston rod, the crosshead and the connecting rod, a piston inserted in each cylinder liner is connected with the crankshaft of the engine which is journalled in the main bearings 3. During the working stroke of the piston, the combustion pressure in the combustion chamber influences the piston with a downward force and the cylinder liner with an upward force, which is transmitted to the cylinder section 5 via the clamping bolts. The force on the piston is transmitted through the connecting rod to the crankshaft, and via the crankshaft and the main bearings 3, the vertical component of the force in the connecting rod influences the bedplate 2 with a downward force which, together with the upward pull of the cylinder cover in the cylinder section 5, attempts to pull the cylinder section away from the bedplate. To counteract this, the cylinder section 5, the A-shaped frame box 4 and the bedplate 2 are clamped together by means of stay bolts 10 which abut on an upward surface on the cylinder section at the top and are fixed in the bedplate 2 at the bottom.
The stay bolts extend obliquely so that they form an angle β with the longitudinal median plane 11 of the engine. Thus, the distance between the stay bolts increases in the downward direction, which permits the A-shaped frame box and the cylinder section to be designed with a smaller width than in an engine with vertical stay bolts. In the drawing, the broken line indicates the width which the A-shaped frame box and the
cylinder section would have if the stay bolts were vertical. It may be seen that the obliquely extending stay bolts render it possible to reduce the half width of the cylinder section by the distance a. The smaller dimensions of the A-shaped frame box 4 and the cylinder section 5 result in a substantial reduction of the engine weight. Furthermore, a substantial advantage is that the upper ends of the stay bolts are clamped against the cylinder section at a shorter distance from the median plane of the engine, which plane, in the embodiment shown, coincides with the median plane 11 of the cylinder, which results in a more direct trans¬ mission of the tensile forces from the cylinder cover to the stay bolts. Thus it is possible to reduce the amount of material in the cylinder section 5 beyond the reduction obtained in consequence of the smaller width of the cylinder section.
At the top, each stay bolt carries a bolt head 12 with a downward surface intended for abutment on a corresponding upward contact face on the cylinder section 5. If the angle between the two contact faces is smaller than 5-6", it is possible to absorb the angular difference in a wedge-shaped washer arranged between the two surfaces without the bolt head 12 with associated washer tending to move in the transverse direction of the cylinder. It is, of course, also possible to form the contact face on the cylinder section so that it is at right angles to the longi¬ tudinal axis of the stay bolt, and in that case, the wedge-shaped washer is dispensable.
The lower ends of the stay bolts are fixed in the bedplate 2, for example by means of bolt heads 13 contacting downward contact faces on the bedplate 2. The associated contact faces at the lower end of the stay
bolt may be designed in the same manner as the contact faces at its upper end.
The engine may, for example, be a two-stroke ship propulsion engine or a stationary engine used as a drive engine in a power-producing plant.