Internal combustion reciprocating engines (Diesel and Otto cycle), most common prime movers in use, are responsible for substantial percentage of total hydrocarbon fuels expenditure and environment pollution. Their overall efficiency, i.e. the percentage of the energy contained in the fuel converted into the useful work, is in the range of 25-54%, wherein the upper bound of this range is approached only by large stationary or marine ultra long stroke crosshead Diesel engines, (which in fact are compound heat machines comprising Diesel engine, turbocharger, supercharging air cooler and sometimes auxiliary power turbine), and average Diesel efficiency is merely ˜40%, a poor figure in comparison with 70-75% overall efficiency originally assumed by its inventor in late 19th century. It is well known that thermal efficiency of Diesel cycle rises as the compression ratio, maximum combustion pressure (now not exceeding 200 bar in special purpose engines and 140-153 bar in large stationary or marine two stroke Diesel engines, again much lower than 250 bar originally assumed by Rudolf Diesel), and the ratio ρ of thermal energy released in isochoric combustion process to thermal energy released in isobaric combustion process rise, reaching 70-75% for compression ratio 25-30 and ρ=1, but this method for improving overall efficiency of real Diesel engines is obstructed by friction losses rapidly rising with loads of engine's mechanism. Moreover, engine mechanism's strength becomes a concern in ultra high compression ratio Diesel engines, with bearings being the weakest element of engine's structure.
The compression stroke of conventional Diesel engine is responsible for large mechanical energy consumption, which substantially diminishes engine's power density, and for diminished thermal and overall engine's efficiency. Thus internal combustion engines utilizing thermodynamic cycle with substantial combustion-driven increase of pressure are of particular interest, as those engines are capable of achieving high power density and very high thermal efficiency, and thus low specific fuel consumption. These engines are mainly homogeneous charge compression ignition (HCCI) and detonation engines working on stoichiometric fuel/air mixture. However, HCCI and detonation cycles utilizing stoichiometric fuel/air mixture develop extremely high maximum pressures and gradients of pressure (understood as a function of angle of rotation of engine's shaft), which can destroy the engine. Thus engine's strength, and particularly bearings capability to withstand extremely high specific loads and their ability to last for an economically reasonable period becomes a serious concern. Some engine's elements, e.g. crankshaft and connecting rod, can be made stronger just by increasing their transverse cross section. However, increased diameter of crankshaft journals causes points on the circumference of the journal travel at increased tangential velocity, which substantially increases bearing's loads. Moreover, the mass forces are increased, which additionally increases bearings loads. All these render the engine bearings prone to defects, and increased friction losses could nullify gains in thermal efficiency.
Bearings used in internal combustion engines are mainly plain (or rather fluid dynamic or hydrostatic) bearings and, at a much smaller scale, rolling bearings. Fluid bearings have important advantages over rolling bearings. The fluid bearing is just two smooth surfaces usually made by anti-friction metal, sometimes supplemented by seals to keep in the working fluid. Therefore they can be relatively cheap compared to rolling bearings with a similar load rating. Moreover most fluid bearings require little or no maintenance. Pumped hydrostatic bearing designs retain low friction down to zero speed and need not suffer start/stop wear. Fluid bearings, when run within a rated loads, generally have very low friction, far better than mechanical bearings, and often inherently add significant damping, which helps attenuate resonances at the gyroscopic frequencies of journal bearings. An important advantage of plain bearings in engines application is that they work well while subjected to shock loads met with in internal combustion engines, assuming these shock loads are not very high. Yet another adventage of fluid bearings is that they are typically quieter and smoother than rolling-element bearings.
On the other hand, plain bearings in engine application have serious disadvantages and limitations. First of all, their ability to withstand shock loads does not extend over extreme values met with in ultra high compression ratio Diesel engines, as well as HCCI and detonation engines working on stoichiometric mixture. Second, in this application, fluid bearings are subject to severe wear during start-up and shutdown, and substantial wear is also caused by hard combustion contaminants that bridge the oil film. Third, friction losses and therefore power consumption are typically higher than rolling bearings due to microscopic roughness of bearing surfaces. Fourth, fluid bearings can catastrophically seize under shock situations. Fifth, the half frequency whirl met with in fluid bearings is the bearing instability that generates eccentric precession which can lead to poor performance or life.
In contrast, a conventional rolling bearing requires many high-precision rollers and the inner and outer races are often complex shapes, making them difficult to manufacture and rising the first cost of a machine. Its shock resistance and ability to dampen vibrations are usually lesser then fluid bearings. Moreover, conventional rolling-element bearings usually have shorter life and require regular maintenance.
However rolling element bearings have also important advantages over plain ones. They are much less prone to defect when operated at high peripheral speeds, and their ability to bear high loads is greater. Moreover, rolling element bearings work much better under start-up and shutdown conditions, and generally work well in non-ideal conditions.
Under these circumstances, it is a very natural idea to merge plain (or rather hydrostatic) and rolling element bearing advantages into one design. The idea is also known from the prior art. Namely, the U.S. Pat. No. 4,605,317 to Mr. Francesco Bonaccorso, full text of which is incorporated herein by reference, discloses a bearing for supporting a load by the reaction force of a pressurized fluid, by the reaction force of a plurality of rollers in the absence of the pressurized fluid, or by a combined reaction force of the pressurized fluid and the bearing rollers. This bearing seems well suited for application in very high combustion pressure engines, although I am not aware of any experimental data supporting this opinion. However Bonaccorso bearing is a passive bearing, i.e. a bearing that is unable to dynamically compensate for impending shaft motion leading to rubbing of rotating and stationary elements. To be more precise, there is no active fluid handling system that can respond to rapidly changing loads of the shaft. Consequently, the bearing would work in the hybrid “pressurized fluid-rolling element”-mode of operation rather than in the preferable pure “pressurized fluid” mode of operation when loaded by cyclic shock loads typical for ultra high pressure internal combustion engines. Moreover, this bearing lacks simplicity of plain bearings.
The idea of an active fluid bearing is also present in the prior art. For example the U.S. Pat. No. 5,769,545 to Mr. Mr. Bently and Grant, full text of which is incorporated herein by reference, discloses a method and apparatus for determining shaft's position in a bearing and for providing pressurized fluid so as to restore coaxial position of the shaft and the circumscribing bearing bush. Unfortunately the method of Bently and Grant calls for excessively complicated equipment, and their fluid handling apparatus is likely to be slow in response to rapidly changing forces loading the shaft.
The U.S. Pat. No. 2,578,713 to Martellotti teaches a fluid bearing with valve-enhanced fluid flow differential applied to opposite fluid pockets encircling a shaft. However, in this design the valves are directly controlled by shaft's radial movement, which is certainly to small to effectively command the valves.
Recently invented Bently Pressurized Bearing (see http://www.bpb-co.com/), unlike conventional hydrodynamic bearings, uses full 360° lubrication and a fluid externally pressurized enough to force the lubricant to flow primarily along the shaft (forming an inherently stable axial support wedge), rather than the tendency of conventional low-pressure designs to pull fluid into rotational motion around the shaft (forming a circumferential support wedge which promotes instability). This technology seems to solve instability problems met with in ordinary hydrodynamic pressure and to be very promising for application in turbomachinery and other machines not subjected to very high shock loads. However, Bently bearing is a passive one and its ability to bear extremely high shock loads is a suspect.
It is clear that complicated and expensive bearing system would be acceptable only in highly loaded expensive machines destined for sustained operation, wherein friction losses are responsible for substantial portion of overall operating costs, and large stationary and marine engines are definitely machines of this type.
As is well known, large stationary and marine engines are destined for operating primarily at nominal (design) loads, and running costs related to operating such engines at nominal loads constitute a vast majority of overall running costs. Thus effectiveness of bearing system when the engine is operated at nominal loads is of greatest importance, while its performance at partial loads and start-up and shutdown conditions is much less significant for overall running costs.
However, marine engines must operate also at partial loads and at start-up and shutdown conditions, and such conditions are responsible for considerable wear of plain bearings of contemporary engines. Thus good performance of engine's bearing at partial loads and start-up and shut down conditions is also important for engine's durability, and therefore for overall life cost of the power plant.
An internal combustion engine, when operated at design conditions, is generally subject to predetermined cyclic loads. This fact allows for designing effective fluid handling system for generating in-bearing forces counteracting mass and gas forces.