Drive system including an engine and a turbo-charger
OF THE INVENTION 
     The invention relates to a drive system including a combustion engine and a turbo-charger especially an exhaust gas turbo-charger coupled to the combustion engine of the drive system. 
     DESCRIPTION OF THE PRIOR ART 
     It is known to provide exhaust gas turbo-chargers with slide or sleeve bearings and such slide bearings are lubricated with oil from the lubricating oil circulating circuit of the combustion engine. Due to the fact that the temperature in the exhaust gas turbo-charger is high, especially in the turbine section of the turbo-charger, the conventional lubrication of the sleeve bearings has the disadvantage that the lubricating oil must be capable of retaining its lubricating ability even at these high exhaust gas discharge temperatures. In this respect problems have been encountered, especially with the sleeve bearing next to the turbine section of the turbo-charger. Due to the high temperature of the lubricating oil in the sleeve bearing next to the turbine section, it is necessary to provide conventionally an oil cooler which is larger than a respective oil cooler for a piston engine without the turbo-charger. Nevertheless less, a more rapid deterioration of the lubricating oil is encountered and the oil consumption is increased. 
     It is even possible that a substantial overheating of the turbine bearing of the turbo-charger causes coking with the result that the lubricating oil is rapidly contaminated resulting in damage to engine components which depend for their proper operation on a cooling oil circulating circuit which circulates clean cooling oil. Especially after the engine shut-down there is a danger of damage to the turbo-charger bearings which is especially pronounced for the sleeve bearing on the turbine side and coke formation also takes place because the turbine which is still hot, can overheat the bearing when the flow of cooling oil has stopped with the shut-down of the engine. 
     It is also known to cool the housing portions of the turbo-charger near the turbine with cooling water from the engine cooling water circulating circuit in an effort to prevent an overheating of the sleeve bearing on the turbine side of the turbo-charger. This type of cooling of the turbine housing portion of the turbo-charger is not very efficient and additionally, it is technically involved and expensive because the cooling oil circulating circuit and the cooling water circulating circuit of the combustion engine must be extended to flow through the exhaust gas turbo-charger. 
     It is generally known to lubricate sleeve bearings with water rather than with oil when these sleeve bearings are used, for example, in pumps that are working in the vicinity of water or even under water, or which are used for pumping water. In such a situation, the use of a lubricating medium other than water would cause sealing problems between the lubricating medium and the water. Similarly, in connection with sleeve bearings used in drive systems with combustion engines, such sleeve bearings have always been lubricated by oil for an analogous reason, namely to reduce sealing problems. 
     OBJECTS OF THE INVENTION 
     In view of the foregoing it is the aim of the invention to achieve the following objects singly or in combination: 
     to improve a turbo-charger, especially an exhaust gas turbo-charger of a combustion engine drive system, in such a manner that an effective lubrication and an effective cooling is achieved simultaneously for the turbo-charger and for the combustion engine by structurally simple and inexpensive means; 
     to avoid the problems of the prior art, especially the coking problem; 
     to use ceramic materials for the sleeve bearings; and 
     to integrate the cooling system of the exhaust gas turbo-charger of a drive system with the cooling water circulating circuit of the combustion engine of the drive system. 
     SUMMARY OF THE INVENTION 
     According to the invention a drive system with a combustion engine also as an exhaust gas turbo-charger equipped with water lubricated sleeve bearings. Preferably, the water supply for the turbo-charger sleeve bearings is connected to the cooling water circulating circuit of the combustion engine of the drive system. 
     The invention achieves substantial structural simplifications. The involved oil supply to the bearings of the turbo-charger is completely avoided. The water supply is used simultaneously for the bearing lubrication and for the cooling, not only of the bearings, but of the entire turbo-charger and of the combustion engine. 
     The use of water instead of oil as a lubricating medium has the advantage that substantially smaller frictional forces are involved in the bearings because the water has a smaller viscosity than the oil. This feature is especially advantageous in exhaust gas turbo-chargers running at high r.p.m.s. Further, the pressure of the lubricating water is sufficiently high to avoid the formation of steam bubbles. 
     The use of ceramic materials according to the invention for the sleeve or slide bearings has the advantage that these materials are corrosion resistant even under water and have simultaneously good slide characteristics. It may be sufficient under certain circumstances to make the stationary components of the sleeve bearings of ceramic materials. However, the stationary and the rotating components of the sleeve bearings may be made of ceramic materials or only the rotating components may be made of such materials. In the latter instance, the shaft ends extending into the sleeves of the bearing may carry a ceramic bushing that rotates with the shaft or the ceramic material may be applied to the shaft ends by a spraying operation. 
     The arrangement of a pump wheel on the turbo-charger shaft for circulating the entire cooling water of the system has the advantage that a separate cooling water circulating pump for the engine is obviated and that it is highly desirable to circulate the cooling water in accordance with the instantaneously prevailing load conditions. This is achieved by driving the pump wheel by the turbo-charger shaft. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein: 
     FIG. 1 is a sectional view through a turbo-charger according to the invention; 
     FIG. 2 is an enlarged view of one sleeve bearing with a ceramic stationary sleeve and a ceramic bushing on the shaft end; and 
     FIG. 3 is a view similar to that of FIG. 2 with a ceramic coating on the shaft end. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BEST MODE OF THE INVENTION 
     FIG. 1 shows an exhaust gas turbo-charger 1 illustrated in a partial axial section and connected to a combustion engine 25 having a fresh combustion air inlet 26 and an exhaust gas outlet 27. A radial turbine 2 in a housing section 7a of the housing 7 is connected to the exhaust gas outlet 27 through an exhaust gas pipe 9. Thus, exhaust gases are discharged through the space 16 inside the turbine housing section 7a and through the turbine exhaust port 8. Fresh air passes into an inlet port 10 of a compressor housing section 7b housing a radial compressor 3 for passing fresh air through the compressor space 17 and through a combustion air conduit or pipe 11 into the combustion air inlet port 26 of the engine 25. 
     Both, the radial turbine 2 and the radial compressor 3 are mounted on a common shaft 4 which in turn is supported in the housing 7 by sleeve bearings 5 and 6. A cooling and lubricating water supply pipe 12 is connected through the housing 7 directly into the sleeve bearing 5. Similarly, a cooling and lubricating water supply pipe 13 is connected through the housing 7 directly into the sleeve bearing 6. These pipes 12 and 13 are connected to the cooling water circulating circuit of the engine 25. The cooling water passes through the bearing and the respective bearing gap 23, 24 into the housing 7. Slide ring seals 14 and 15 keep the water in the housing 7 and out of the turbine space 16 as well as out of the compressor space 17. 
     A water pumping wheel 18 is rigidly mounted on the common shaft 4 in the space 19 of the housing 7 for pumping cooling water out of the space 19 through a conduit 20 into the cooling water circulating system of the engine 25. Additional cooling water may be supplied through an inlet pipe 21 connected to the housing 7 and to the cooling water circulating system of the engine. Thus, the bearings 5 and 6 as well as the housing 7 are efficiently cooled while the bearings are simultaneously lubricated by the same water. 
     The bearing 5 has a bearing sleeve 22&#39; and the bearing 6 has a bearing sleeve 22, both of which are preferably made of ceramic material, for example, non-oxidic ceramics such as silicon carbide (SiC) or silicon nitride (Si 3  N 4 ). Other suitable ceramic materials are completely or partially stabilized oxidic ceramics such as aluminum oxide (Al 2  O 3 ) or zirconium oxide (ZrO 2 ) or any combination of these materials. 
     FIG. 2 shows a bearing 28 in which the stationary bearing sleeve 29 and a rotating bearing bushing 30 are both made of ceramic materials. The bushing 30 is secured to the end 31 of the shaft 4 to rotate with the shaft. 
     FIG. 3 shows a bearing 32 in which the bearing sleeve 29 is the same as in FIG. 2. However, the shaft end 31 in FIG. 3 is provided with a ceramic coating 33 rather than with a ceramic bushing. The ceramic coating may be applied, for example, by a spraying procedure. 
     Although the invention has been described with reference to specific example embodiments, it will be appreciated, that it is intended to cover all modifications and equivalents within the scope of the appended claims.