Piston unit for an internal combustion engine

Abstract: A piston unit for an internal combustion engine, especially for a large diesel engine, comprising at least three main parts (1-3) located in successive order in the direction of a longitudinal axis of the piston unit and connected to each other. The piston unit includes a uniform upper part (3), which defines, when installed within a cylinder of the engine, a combustion chamber from the side of the piston and which is fixed inside the piston at its central region to a middle part (2) of the piston unit, preferably by means of a screw (4) or the like. At least the main part of, preferably all of, the piston ring grooves (7) are arranged on a middle part (2). In addition the upper part (1) is selected to be of a material with better heat resistance than that of the middle part (2) and of a lower part (1).

Description: This invention relates to a piston unit for an internal combustion engine, especially, but not exclusively, for a large diesel engine, the piston unit being of the kind comprising a number of main parts located in successive order in the direction of the longitudinal axis of the piston unit and connected to each other. In this specification the term "large diesel engine" refers to such an engine that may be used, for example, as the main propulsion engine or auxiliary engine of a ship or in a power plant for the production of electricity and/or heat energy. A piston of an internal combustion engine transfers the energy released through the burning of the fuel via a piston pin to a connecting rod and further to a crank mechanism. The piston seals against the walls of the cylinder by means of piston rings so that the transfer of energy occurs without substantial energy losses. The upper part of the piston defines a part of the combustion chamber in the cylinder and is subject to substantial thermic stresses. The walls of the piston which extend in the longitudinal direction of the cylinder below the piston rings guide the movements of the piston and serve as lubricating surfaces. In pressure charged engines the pistons are, without exception, also provided with oil cooling passages. In modern heavily loaded diesel engines so called combined or composite pistons are often used with the lower part or skirt of the piston being manufactured of spheroidal graphite cast iron or aluminum. In this case the upper part of the piston may be forged from steel so that its loading properties are improved in comparison with an entirely cast piston. A composite piston is conveniently assembled by attaching the upper and lower parts together through one or more screw joints. Typically the number of screws used can be from 1 to 6 depending on the manufacture. Pistons in heavy duty diesel engines are subject to high mechanical and thermic stresses. The highest allowable load capacity of a piston is indeed often a restraint for increasing the effect and/or the temperature of the combustion process of an engine. The increase of the process temperature serves its purpose for instance in diesel power plants, in which the thermic energy of the exhaust gases is availed of, and in engines, in which the operation of a catalytic converter is endeavoured to be improved in connection with starting and/or under partial load operation. Quenched and tempered steel is often used as the material of the upper part of a piston due to the manufacturing technique. However the strength of quenched and tempered steels at increased temperatures is rather limited. In addition the heat expansion of such material may cause further problems with the connection surfaces of the piston, since deformations cause changes in the distribution of tension in the contact surfaces and, thus, in the tension fields being formed. Each piston construction has an allowed field of deformation of its own, which does not permit any additional increases in its temperature. It is possible to take account of thermal expansion by increasing cold clearance in the radial direction. However this has its own limitations since a large clearance can cause additional deposits of carbon to be formed on the crown of the piston during cold starting causing wear of the cylinder in subsequent operation. Thermal expansion also causes bending of the fixing screws thereby decreasing the reliability of the joint. However, these phenomena need not be taken account of in constructions where the upper part is fixed to the lower part only by means of a single, centrally located screw. When a piston is subjected to increased temperatures, lubricating oil burns down on the inner surfaces of the piston resulting in a decreased cooling effect and also a deterioration in the lubricating quality of the lubricating oil. In addition, in heavy oil operated engines, raised temperatures increase the risk of hot corrosion. One way of reducing the thermic stresses in the upper part of a piston is to coat the piston, at least on its side or surface defining in part the combustion chamber, with a thermally insulating coating, for example of zircon oxide. However in this case the reliability of the coating has been a problem. FR-A-1337311 and US-A-3323503 show multi-part piston units. In these known piston units, upper and middle piston parts are not supported against each other substantially only in a longitudinal direction. These known piston units are complicated with regard both to their manufacture and assembly. An aim of the invention is to accomplish a new construction for a piston unit which provides better possibilities than before to take account of high process temperatures, and enable use of even higher process temperatures, especially in diesel engines, but from which the drawbacks of the known solutions described above have essentially been eliminated. Another aim of the invention is to provide a solution, which is advantageous from the viewpoint of manufacturing technique and reliable as to is construction. A further aim is to reduce heat losses occurring through the upper part of a piston. A still further aim is to provide a piston unit the construction of which allows easy assembly. The aims of the invention can be met in a piston unit as claimed in the ensuing claim 1. Thus, the invention is based on the idea of assembling a piston made of separate parts in a certain way so that each piston part serves the operational requirements and conditions of the piston part in question as well as possible. Since the mass of the upper part is relatively small, it is generally sufficient to fix the upper part only at its central region. It is then possible to avoid unfavourable deformations resulting from the use of several fixing screws. Depending on the selected materials, however, a number of auxiliary screws may be utilized, when necessary, in order to ensure that the upper part keeps tightly fixed to the lower construction of the piston unit. If auxiliary screws are employed, however, a substantially smaller prestressing force is generally utilized for the auxiliary screws than for the fixing screw in the central region. The middle part, on the other hand, is with advantage connected to the lower part of the piston unit by means of four or more screws or the like, since the combined mass of the upper and middle parts is substantially greater. The middle part and the lower part of the piston are suitably provided with passages and/or bores for circulation of lubricating and/or cooling medium. Since the material of the upper part is preferably selected to be heat resistant, it is preferably uncooled or connected only to a minor degree to the cooling system of the lower parts of the piston in order to increase the process temperature and to diminish heat losses. In order to further reduce heat losses, contact between the upper and middle parts is minimized by arranging for a number of cavities to be formed between the upper part and the middle part. In addition a number of grooves is arranged on one or both of the contacting surfaces between the upper and middle parts. In practice the connection surface between the upper and middle parts can with advantage comprise contact surfaces limited in the radial direction of the piston and extending generally circumferentially, e.g. concentrically with the periphery thereof. The proposed piston construction results in a substantial increase in the surface temperature of the piston located on the side of the combustion chamber, whereby the upper part is with advantage made of heat resistant steel. In case the temperature grows substantially higher than in conventional pistons, the upper part can be made of heat resisting alloy material having a relatively low thermal coefficient or linear expansion (so-called low expansion alloys), for example from 5.10 -6 K -1 to 8.10 -6 K -1 , which is about 30 to 50 % lower than for quenched and tempered steel. Such materials are known per se and are commercially available. Materials which are especially suitable for the upper part are composite materials which typically comprise from about 20 to 30 % of nickel. Depending on the material the upper part can be made by forging, casting or through powder metallurgy. The middle part is preferably made of surface hardened steel bearing in mind the requirement for durability of the piston ring grooves. The lower part can typically be made, in a manner known as such, of spheroidal graphite cast iron or of aluminum. An embodiment of the invention will now be described, by way of example only with particular reference to the accompanying drawings, in which:- Figure 1 is a longitudinal section of an embodiment of a piston unit according to the invention; and Figure 2 is a view from above of the piston unit of Figure 1. A piston unit shown in the drawings includes a first or lower part 1, a second or middle part 2 and a uniform third or upper part 3 arranged coaxially with the longitudinal axis of the piston unit. The middle part 2 and the upper part 3 are fixed to each other by means of a screw bolt 4 located in a central region of the piston. The middle part 2 is fixed to the lower part 1 by means of four screw bolts 5 (only one of which can be seen in Figure 1). A construction of this kind provides possibilities to select the material for each piston part independently of each other to conform to the operation and specific requirements and conditions of each piston part in a way serving its purpose as well as possible. From the viewpoint of keeping the upper part 3 tightly pressed to the lower construction of the piston unit, auxiliary screws (not shown) may be used which would be located closer to the periphery of the piston than the preferably coaxially positioned screw bolt 4. The prestressing force of such auxiliary screws, however, is essentially less than that of the screw bolt 4. As can be clearly seen in Figure 1 the middle part 2 has a substantially flat upper surface which is contacted by the lower surface of the upper part 3 at central and peripheral regions and also in an intermediate region. The intermediate and peripheral regions have annular surface contact with the middle part 2 and are arranged concentrically with respect to each other and the longitudinal axis of the piston unit. The screw bolt 4 has a threaded end which is in screw threaded engagement with a bore in the central region of the upper part 3. The bolt 4 further has a shank which passes through an aperture in the middle part 2 and a head. A sleeve is arranged between the head of the screw bolt 4 and the middle part 2 for transmitting force when the screw bolt is tightened. The lower part 1 includes a connecting rod boss 6. In addition the lower part 1 and the middle part 2 together define a passage 8 which is part of a cooling system. The cooling system can include, when necessary, a number of passages and bores arranged in a known manner in the lower part 1 and/or in the middle part 2 and which, for clarity, have not been shown in the drawings. The lower part can with advantage be of spheroidal graphite cast iron or aluminum. The middle part 2 is provided with piston ring grooves 7 and accordingly is preferably made of surface hardened steel. When installed within a cylinder (not shown) of an engine, the upper surface of the upper part 3 limits and, hence, defines one end of a combustion chamber of the cylinder and accordingly it is, with advantage, made or heat resistant material, e.g. heat resistant steel material or the like. The upper part 3 is also provided with cavities 9, which limit the area of contact surfaces 10 between the upper part 3 and the middle part 2 to the minimum for decreasing heat losses. This, for its part, makes it possible to increase the process temperature in the combustion chamber of a cylinder and thus improves the efficiency ratio of the burning process of an engine and the possible recovery of heat energy from the exhaust gases of an engine. The effect is increased by grooves 11, which decrease further the direct contact surface. The reference numerals 12 and 13 indicate guide pins, which guide the separate parts to correct position relative to each other during assembly of the piston unit. The invention is not limited to the embodiments disclosed, but several variations thereof are feasible, including variations which have features equivalent to, but not necessarily literally within the meaning of, features in any of the ensuing claims.

Claims: A piston unit for an internal combustion engine, especially for a large diesel engine, comprising at least three main parts (1-3) which are located in successive order in the direction of the longitudinal axis of the piston unit, which are connected to each other and which comprise an upper part (3) having a central region and a peripheral region, a middle part (2) having a substantially flat upper surface lying in a plane perpendicular to the longitudinal axis of the piston unit and a lower part (1), the upper part (3) being of a material with greater heat resistance than that of said lower part (2) and limiting, when installed in a cylinder of the engine, one end of a combustion chamber, fastening means (4) comprising a screw bolt connecting said central region of said upper part (3) to said middle part (2) of the piston unit, and piston ring grooves (7) at least mainly arranged on said middle part (2), characterised in that the upper part (3) is of a material with greater heat resistance than that of said middle part (2), extends across the full extent of the piston unit and has a lower surface which contacts said flat upper surface of the middle part (2) at said central and peripheral regions, and in that the lower surface of said upper part is formed between the central and peripheral regions of the latter with recesses (9) having interior surfaces spaced from the flat upper surface of the middle part (2). A piston unit according to claim 1, characterised in that said screw bolt (4) extends along the longitudinal axis of the piston unit. A piston unit according to claim 2, characterised in that said screw bolt (4) has a threaded end portion in threaded engagement with a threaded bore in the central region of said upper part (3), a shank passing through an aperture in the middle part (2) and a head spaced from the middle part (2), and in that the fastening means further comprises a sleeve in force transmitting relationship between the head of the bolt and said middle part (2). A piston unit according to any one of claims 1 to 3, characterised in that the middle part (2) is connected to the lower part (1) of the piston unit by means of four or more screws (5) or the like. A piston unit according to any one of the preceding claims, characterised in that the middle part (2) and the lower part (1) are provided with passages (8) and/or bores for circulation of lubricating and/or cooling medium. A piston unit according to any of the preceding claims, characterised in that the lower surface of the upper part (3) and/or the upper surface of the middle part (2) is/are provided with grooves (11) where regions of the two surfaces are in contact with each other. A piston unit according to any of the preceding claims, characterised in that said lower surface of the upper part (3), at the peripheral region of the latter, contacts the upper surface of the middle part (2) over a first annular surface region. A piston unit according to claim 7, characterised in that said lower surface of the upper part (3) contacts the upper surface of the middle part (2) over a second annular surface concentric with the first annular surface. A piston unit according to any of the preceding claims, characterised in that the upper part (3) is made of heat resistant steel. A piston unit according to any of claims 1 to 8, characterised in that the upper part (3) is made of a heat resisting alloy material, the thermal expansion coefficient of which is relatively low, e.g. from 5.10 -6 K -1 to 8.10 -6 K -1 . A piston unit according to any of the preceding claims, characterised in that the middle part (2) is made of surface hardened steel. A piston unit according to any of the preceding claims, characterised in that the lower part (1) is made of spheroidal graphite cast iron or of aluminum.

Inventors: AHLROTH TRYGVE, HALLA-AHO PASI, SILVONEN AULIS, AHLROTH, TRYGVE, HALLA-AHO, PASI, SILVONEN, AULIS
Applicants: WAERTSILAE FINLAND OY
Application year: 1996
Main classification: F02F3
IPC codes: F02B3, F02B23, F02F3