Patent Application
20170350345
This application claims priority to German Patent Applications DE 10 2016 209 651.6, filed Jun. 2, 2016, and DE 10 2016 224 280.6, filed on Dec. 6, 2016, the contents of both of which are incorporated herein by reference in their entireties.
The present invention relates to a piston of an internal combustion engine having a piston shaft and a piston head. The invention further relates to an internal combustion engine having at least one such piston.
In current, supercharged diesel engines, as a result of the very high specific power levels of over 60 kW per litre cubic capacity, there is often a powerful thermal loading of the pistons of the internal combustion engine and in particular a piston base. The piston base in this instance faces the combustion chamber with the combustion chamber bowl thereof and consequently has to withstand the highest thermal loading. In order to be able to operate such an internal combustion engine or such a diesel engine in the long term, it is necessary to carry out a cooling of the piston which in particular reduces the thermal loading of the piston, in particular in the piston head thereof, and furthermore to prevent coking of oil which takes up the lubrication of the piston in a cylinder, in particular in an annular groove which is adjacent to a top land.
This problem is solved according to the invention by the subject-matter of independent claim 1. The dependent claims relate to advantageous embodiments.
The present invention is based on the general notion of constructing a shaft form of a piston of an internal combustion engine in such a manner that it has an increased abutment face against a cylinder wall or a cylinder liner which is arranged in an associated cylinder and thereby an improved heat transfer and also improved cooling of the piston can be achieved. The piston according to the invention has in this instance the said piston shaft and a piston head, in which a closed cooling channel with a cooling medium arranged therein is provided.
A combustion chamber bowl is further arranged in the piston head. According to the invention, the piston shaft now has a spherical and at the same time round cross-sectional shape which differs significantly from the spherical and oval cross-sectional shapes previously known from the prior art, and wherein a deviation from the roundness with respect to a piston diameter is less than 0.5 per thousand. The term “spherical” in this instance is intended to mean that the piston is constructed along the piston axis thereof in the manner of a barrel, that is to say, a diameter of the piston in the region of the piston head and in the region at a lower end of the piston shaft is smaller than therebetween. The deviation of the roundness should in this instance always be considered in a plane transverse relative to the piston axis. Over the height, therefore, the radii differ as a result of the convexity. The spherical construction enables in this instance a round sliding of the shaft wall on the cylinder or on the cylinder liner during the change of abutment of the piston. As a result of the spherically round embodiment of the shaft wall, according to the invention the face which is in abutment with the cylinder or the cylinder liner increases and consequently also the possibility of heat transfer from the piston to the cylinder. Tests have in this instance already shown that, as a result of the spherical and round embodiment of the piston shaft according to the invention and the consequently improved heat transfer, a significant temperature reduction can be achieved in the piston head.
In an advantageous development of the solution according to the invention, a thermally conductive coating is arranged on a shaft face of the piston. Via such a thermally conductive coating which has, for example, an increased graphite proportion and consequently an improved thermal conductivity, an additionally improved heat transfer from the piston to the cylinder and consequently an improved cooling of the piston can be achieved.
In an advantageous development of the solution according to the invention, the cooling channel is expanded radially outwards in the region of a piston base in the direction of a top land. The top land extends from the piston base as far as the first annular groove in order to receive a piston ring. As a result of the expansion of the cooling channel as provided for according to the invention in a radial direction outwards in the direction of the top land, the temperature in the first annular groove can be reduced by up to 10 K, whereby in particular the problem of oil carbon formation in the said first annular groove can be prevented, but at least greatly reduced. Additionally or alternatively, the cooling channel can also be expanded in the region of a piston base in the direction of the combustion chamber bowl, that is to say, radially inwards. It is also thereby possible to obtain improved cooling of the piston.
In another advantageous development of the solution according to the invention, the piston is constructed in two parts with an upper portion and a lower portion which is connected thereto, in particular welded thereto, wherein the cooling channel is formed partially in the upper portion and partially in the lower portion. Such a multi-component piston affords in this instance the possibility of expanding the cooling channel downwards in the direction of the shaft by means of milling and/or bores and thereby achieving improved heat discharge of the cooling medium which is thrown back and forth in the closed cooling channel during operation in the direction of the piston shaft. If the cooling channel is, for example, expanded in the direction of the piston shaft by means of milling, it has, on an inner wall which faces a lower piston side, an undulating shape which leads to an increased surface and consequently also to an improved heat transfer. In addition to such a hollow milling, which also brings about the undulating shape of the cooling channel base as a result of the process, additional bores which extend significantly deeper into the piston shaft and thereby bring about a further improved heat discharge may be provided.
In an advantageous development of the solution according to the invention, ribs which protrude from the lower piston side are arranged in the region of the cooling channel at a lower piston side. These ribs preferably extend only over the region between inner shaft walls and the connection thereof to a piston base and at the same time have several functions: on the one hand, as a result of such ribs, the surface increases by at least 1.2 to 2 times, whereby a heat transfer to the oil which is injected from below is also increased and thereby the heat discharge and on the whole the cooling of the piston can be improved. On the other hand, the ribs guide the injected oil over a centre axis towards the opposing side. In addition, with such pistons, the injection nozzle for the oil can be positioned in an oblique manner, whereby an impact location of the oil jet moves depending on the piston position between the top dead centre and the bottom dead centre and thereby brings about a particularly uniform cooling. As a result of open cooling channels, this cannot be implemented in such a manner because the oil jet always has to be directed onto a supply hole of the open cooling channel in order to always be able to inject sufficient oil into the cooling channel.
Advantageously, the ribs are produced by means of stamping/forging. The production of the ribs and the recesses which are arranged therebetween can consequently be produced without significant additional expenditure when the piston is produced, for which a stamping or forging die simply has to be adapted accordingly.
In another advantageous embodiment of the solution according to the invention, the ribs extend substantially in a radial direction with respect to a piston axis, wherein there may additionally or alternatively be provision for the recesses which are described above to be arranged between the ribs and wherein a volume of the ribs which protrude from the lower piston side corresponds to the volume of the recesses which are stamped in the lower piston side. A volume compensation between recesses and ribs takes place during stamping or forging of the ribs only locally by means of flowing of the material, whereby only a very small loading or no additional loading at all is produced for the forging tool and the service-life of the tool is not influenced or is influenced only in an insignificantly negative manner.
There is preferably used as cooling medium, for example, sodium and/or potassium, wherein there are also considered in particular admixtures thereof which become liquid, for example, at −12° C. and during operation of the internal combustion engine are shaken back and forth by the back-and-forth movement of the piston and thereby absorb heat from the piston base and discharge it into the piston shaft. Alternatively, water can also be used as a cooling medium. Water affords the advantage that it is very cost-effective and a far less complex filling installation can be used for it. Furthermore, it is available everywhere and does not pose any risk to humans and the environment. The operating principle has a similar basis in this instance to a heatpipe with which it is possible to transmit large quantities of heat. Such a “heatpipe” uses the evaporation and condensation enthalpy of the cooling medium (operating medium). The water evaporates in the upper region of the cooling channel which faces the piston base and the bowl wall and condenses in the lower portion of the cooling channel, where the heat is discharged, for example, to the piston shaft. As a result of the pressures which become increasingly high as the temperature of the cooling medium rises, a correspondingly constructed closure element should be used, for example, a Konig Expander, which withstands pressures of up to 350 bar. Furthermore, attention must be paid to the filling quantity since water in comparison with sodium/potassium is a worse heat conductor and the evaporation and condensation enthalpy is the only important aspect. In order where possible not to impede the transport of heat through the water, it is therefore advantageous if there is substantially only so much water available in the cooling channel that the maximum energy which is introduced into the piston during a work cycle evaporates the majority of the water present to the greatest possible extent. A filling quantity typically of from 0.01% to 10% of the volume of the cooling channel should accordingly already be sufficient to transport the heat from the hot locations of the piston into colder regions. The function of this method is in this instance connected with the physical properties of water, according to which, during transition from the liquid phase into the gas phase, heat is absorbed and, vice versa when the water vapour is condensed, heat is discharged to the environment. The function is accordingly limited in an upward direction to a maximum temperature of 374° C. (critical temperature) since above the critical temperature there occurs no phase jump. In a downward direction, the melting point of the water 0° C. has a limiting action. It has been found that in particular for steel pistons during operation of the engine, this temperature range is not left. Typically, temperatures from 100 to 300° C. are observed. The extent of the expansion of the cooling channel under pressure naturally has to be taken into account during the configuration which may lead to greater wall thicknesses in the region of the cooling channel. The pressure varies in this instance typically between 50 to 100 bar at a maximum, depending on the respective engine concept. At high specific power levels, it has been found that, as a result of the addition of salt or highly thermally conductive powders (for example, based on copper, aluminium, silicon carbide or low-melting metals such as tin, an SnBi-eutectic, bismuth or gallium), the boiling power of the water is significantly increased and the film boiling which otherwise occurs from a heat flow density of approximately 1000 kW/m2 can be displaced to higher heat flow densities.
Other important features and advantages of the invention will be appreciated from the dependent claims, the drawings and the associated description of the Figures with reference to the drawings.
Of course, the features which are mentioned above and those which will be explained below can be used not only in the combination set out in each case, but also in other combinations or alone without departing from the scope of the present invention.
Preferred embodiments of the invention are illustrated in the drawings and are explained in greater detail in the following description, wherein identical reference numerals relate to identical or similar or functionally identical components.
In the schematic drawings:
According to
If the left-hand illustration in
As a result of the spherical round cross-sectional shape of the piston shaft 3 according to the invention, it is not only possible to achieve improved sliding of the piston shaft 3 on a cylinder wall 10 (cf.
If
If
Additionally or alternatively, there may be arranged at a lower piston side 22 (cf.
Preferably, for example, sodium and/or potassium is used as a cooling medium 6 in the cooling channel 5, wherein there are also considered in particular admixtures thereof which become liquid, for example, at −12° C. and during operation of the internal combustion engine 2 are shaken back and forth by the back-and-forth movement of the piston 1 and thereby absorb heat from the piston base 13 and discharge it to the piston shaft 3. Alternatively, water can also be used as a cooling medium 6. Water affords the advantage that it is very cost-effective and a far less complex filling installation can be used for it. Furthermore, it is available everywhere and does not pose any risk to humans and the environment. The operating principle is in this instance based on the use of evaporation and condensation enthalpy of the cooling medium 6. The water evaporates in the upper region of the cooling channel 5 which faces the piston base 13 and the combustion bowl 7 and condenses in the lower portion of the cooling channel 5, where the heat is discharged, for example, to the piston shaft 3. The operating principle functions in this instance in a similar manner to a heatpipe with which large quantities of heat can be transferred. Such a “heatpipe” uses the evaporation and condensation enthalpy of the cooling medium (operating medium). When water is used as a cooling medium 6, precise attention must be paid to the filling quantity since water in comparison with sodium/potassium is a worse heat conductor and the evaporation and condensation enthalpy is the only important aspect. In order, where possible, not to impede the transport of heat through the water, it is therefore advantageous if there is substantially only so much water available in the cooling channel 5 that the maximum energy which is introduced into the piston 1 during an operating cycle evaporates the majority of the water present to the greatest possible extent. A filling quantity typically of from 0.01% to 10% of the volume of the cooling channel 5 should accordingly already be sufficient to transport the heat from the hot locations of the piston 1 into colder regions. The function of this method is in this instance connected with the physical properties of water, according to which, during the transition from the liquid phase into the gas phase, heat is absorbed and, vice versa when the water vapour is condensed, heat is discharged to the environment. The function is accordingly limited in an upward direction to a maximum temperature of 374° C. (critical temperature) since above the critical temperature, there occurs no phase jump. In a downward direction, the melting point of the water at 0° C. has a limiting action. It has been found that in particular for steel pistons during operation of the engine, this temperature range is not left. Typically, temperatures from 100 to 300° C. are observed. The extent of the expansion of the cooling channel 5 under pressure naturally has to be taken into account during the configuration which may lead to greater wall thicknesses in the region of the cooling channel 5. The pressure varies in this instance typically between 50 and 100 bar, depending on the respective engine concept.
At high specific power levels, it has additionally been found that as a result of the addition of salt or highly thermally conductive powders (for example, based on copper, aluminium or silicon carbide or low-melting metals, such as tin, an SnBi-eutectic, bismuth or gallium), the boiling power of the water is significantly increased and the film boiling which otherwise occurs from a heat flow density of approximately 1000 kW/m2 can be displaced to higher heat flow densities.
When
With the piston 1 according to the invention and the spherically round cross-sectional shape thereof, in the region of the piston shaft an abutment face against a cylinder liner 11 or a cylinder wall 10 of the internal combustion engine can be increased, whereby improved heat transfer and consequently also improved cooling of the piston 1 can be achieved, which is a great advantage, in particular for highly supercharged diesel engines with a specific power of over 60 kW per litre cubic capacity.
In addition to the spherically round cross-sectional shape of the piston shaft 3, other measures which promote the cooling of the piston 1, such as, for example, the ribs 25, the bores 20, the expansions 15 can be applied cumulatively or alternatively.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2016 209 651.6 | Jun 2016 | DE | national |
| 10 2016 224 280.6 | Dec 2016 | DE | national |
