|
(11) | EP 0 074 156 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
|
|
|
|
|||||||||||||||||||||||||||
| (54) | Piston in an internal combustion engine |
| (57) The invention concerns pistons of the liquid-cooled type, and provides an advantageous
alternative to the known types of oil or water-cooled pistons. The invention is characterised in that the liquid (A) is sealingly enclosed within a sealed cooling space or chamber provided within the piston (4) so as to form a free surface, in that a heat-exchanger (3) for effecting heat transfer between said liquid and oil (c) supplied to the head exchanger is associated with said sealed space, and in that the oil within said heat-exchanger is arranged to be fed and discharged through oil passageways (21,11) connected to the exterior of the piston. Preferably, the liquid (A) is a liquid having a boiling point of 200°C or lower at atmospheric pressure, and air, or an inert gas (B) is sealingly enclosed within said sealed space. |
[0001] The present invention relates to pistons for internal combustion engines, and is particularly concerned with the cooling performance of such pistons.
[0002] Figure 1 is a cross-section view showing a known type of oil-cooled piston used in a large diesel engine. In this figure, 01 is a piston rod, 02 a cooling oil feed pipe, 03 a piston cooling inner housing and 04 is a piston crown. Cooling oil fed through a passageway 021 in the oil feed pipe 02 within the piston rod 01 enters through an oil passageway 031 within the inner housing 03 into a toroidal cooling chamber 041 formed by the piston crown 04 and the inner housing 03, and passes through a plurality of holes 042 which rise from the chamber 041 into another ring shaped chamber 043 formed partly in the inner housing and the junction of the crown and sides of the piston,where it continues the cooling process. After the oil has cooled this region of the piston, it gathers at the centre of the piston crown and cools its upper surface, then drains through an oil passageway 032 within the inner housing 03, and is discharged via an oil discharge passageway 011 within the piston rod into a crank case (not shown) or to a system external to the engine.
[0003] However, this arrangement suffers from the following disadvantage :
Although the oil-cooling system is satisfactory in that it provides a simple internal structure for the engine, oil has a lower heat transfer capability as compared to water, and hence in the case of a high output engine with a high thermal load, in order to sufficiently reduce the surface temperature of the piston crown,it is necessary to restrict the thickness of the crown. However, this causes the temperature of the sides of the cooling surface to rise and leads to a significant deterioration of the coolinq oil where carbonaceous materials may be deposited on the side of the cooling surface which can result in a reduction of the cooling effect.
[0004] Figure 2 is a cross-sectional view showing a known type of water-cooled piston. Whilst the structure is somewhat similar to that shown in Figure 1, because water is used as a cooling medium, it requires a more complex system. In this Figure 101 is a piston rod, 102 and 103 are cooling water inlet and outlet pipes, respectively, which consist of so-called telescopic tubes, 104 is an inner housing, and 105 is a piston crown. Externally introduced cooling water passes through a double-tube arrangement (not shown) to the cooling water inlet pipe 102 in Figure 2, thence it passes through the inner housing to toroidal chambers 151 and 152. Thereafter it passes into another toroidal chamber 154 where it cools the outer regions of the piston crown and the piston ring grooves as it passes through a large number of cooling holes 153, which connect the toroidal chambers 151, 152 and 154. The cooling water then gathers at the centre where it cools the upper surface of the piston crown and drains through a passageway 142 within the inner housing, and is discharged via the outlet pipe 103 and through a double-tube arranqement (not shown) exterior to the engine.
[0005] The disadvantages of this arrangement are as follows :
Despite the improvement in the cooling effect due to the greater heat transfer capability of water as a cooling medium, the use of sliding telescopic tubes present sealing problems. Since it is not possible to seal such sliding tubes completely, the cooling water becomes contaminated with products of combustion which are gradually deposited on the inner surface of the cooling chambers, where it acts as an insulating medium. Furthermore, as these combustion products are dissolved in the cooling water, it gradually becomes acidic, and therefore, corrosion is liable to occur on cooling surfaces inside the piston. As a result, the fatigue strength of the piston may be reduced, which could lead to cracking or damage to the piston.
[0006] An object of the present invention is to provide a liquid-cooled type piston whose cooling properties are as good as those of the water-cooled type, yet does not suffer from the problems of corrosion fatigue and the limited life of the telescopic tube packing.
[0007] According to this invention, such a liquid cooled type piston is characterised in that the liquid is sealingly enclosed within a sealed cooling space provided within the piston so as to form a free surface, in that a heat-exchanger for effecting heat transfer between said liquid and oil supplied to the heat exchanger is associated with said sealed space, and in that the oil within said heat-exchanger is arranged to be fed and discharqed through oil passageways connected to the exterior of the piston. Thus, in accordance with the invention, liquid, preferably having a boiling point less than 200°C is introduced into a sealed space within the piston where it forms a free surface, whilst its main body is in contact with a heat-exchanger where heat is transferred between said liquid and oil, which is supplied throuqh oil passages from a source exterior to the piston.
[0008] The present invention has wide application to medium and large size diesel engines, other internal combustion engines and reciprocating machines such as air compressors and the like.
[0009] In the following, one preferred embodiment of the present invention will be explained with reference to the accompanying drawings, in which :-
Figure 1 is a cross-sectional view showing a known type of oil-cooled piston.
Figure 2 is a cross-sectional view showing a known type of water-cooled piston, and
Figure 3 is a cross-sectional view showing a piston accordinq to one preferred embodiment of the present invention.
[0010] Referring to Figure 3, in this preferred embodiment 1 is a piston rod, 4 a piston crown fastened to the piston rod 1 by means of bolts 22, whilst 23 is a sealed space bounded by the cooling surface 4a of the piston crown 4 and the upper surface of the piston rod 1, and 3 is a heat-exchanger located above the piston rod 1. The heat-exchanger 3 is disposed so that its heat transfer surface is exposed in the sealed chamber 23, and it is fastened to the piston rod 1 by means of a bolt 24, Also, 2 is a coolant oil pipe, 21 a cooling oil inlet passageway formed within the cooling oil pipe 2 and 11 is a cooling oil outlet annulus formed by the periphery of the cooling oil pipe 2 and the axial bore of the piston rod 1, the cooling oil inlet passageway 21 and the outlet annulus 11 being connected to the heat-exchanger 3.
[0011] A quantity of liquid A (preferably a neutral liquid such as water) having a boiling point of 200°C or lower at ambient pressure is sealed into the chamber 23 jointly with a quantity of air or inert gas (e.g. nitrogen, argon, helium, etc.) B above its free surface 20.
[0013] As the engine performs its rotary motion the piston is subjected to a vertical reciprocating motion, which causes the liquid A sealed within the chamber 23 to be violently agitated inside the piston. Consequently, the underside of the piston crown is continuouslv washed by the liquid A and heat is transferred in the process. Once the boiling point of the liquid A has been reached, the rate of heat transfer from the piston crown to the liquid increases by a factor of 10 or more.
[0014] The heat received by the liquid A in the above-described manner is continuously transferred to a supply of cooling oil C in the heat-exchanger 3, and the heated oil is then discharged through the outlet passageway 11 into a crank case (not shown), or to the exterior of the engine.
[0015] In the above-described case, the following advantaqes are obtained :-
In the case of the piston according to the present invention, since the medium that directly cools the piston is a liquid having a boiling point of 200°C or lower, the convection heat transfer rate is high, and moreover, once the temperature of the liquid rises above boiling point, the heat transfer rate rises by a factor of 10 or more (called the "nucleate boiling region"). This ensures that the temperature of the sides of the sealed chamber within the piston does not greatly exceed the boiling point of the liquid.
[0016] Furthermore, because in the case of the piston accordinq to the present invention, the coolant is sealed within the chamber, there is no possibility of combustion products mixing with the coolant, and moreover, if an inert qas is also sealed with the coolant, corrosion of the underside of the piston crown is eliminated.
[0017] Still further, since the heat is eventually carried away by the oil, the previously described advantaqe of the oil-cooled type of piston is retained, in that the internal structure is simple and there is no danger of water leakage and the like; yet since the oil is not in direct contact with the high-temperature surface, there is little likelihood that the oil will deteriorate.
[0018] Thereby, the reliability and life of a piston in the thermally loaded condition are greatly improved, whilst at the same time the reliability of the cooling system per se is also improved.
[0019] The reason why a coolant having a boiling point of 200°C or less at ambient pressure is preferred is because the piston ring groove temperature is limited to about 2000C to provide satisfactory lubrication of the piston rings; the lubricating properties of lubricating oil are degraded if this limit is exceeded.