WO2007003926A1

WO2007003926A1 - Crankshaft assembly

Info

Publication number: WO2007003926A1
Application number: PCT/GB2006/002452
Authority: WO
Inventors: Per D. Sollie; Lars Kjosbakken; Sven Erik Fossum; Eldar Onsoyen; Sverre Johansen
Original assignee: Disenco Limited
Priority date: 2005-07-01
Filing date: 2006-06-30
Publication date: 2007-01-11
Also published as: GB0513589D0

Abstract

A crankshaft assembly (4) in a Stirling engine (1), which assembly consists of an oscillating assembly (7), connecting rods, displacer piston, displacer rod, power piston, cross heads, crankshaft (8) main bearings (9), power big end bearings (10), displacer big end bearing (11), bearing retainers (12), flywheel (13), generator rotor; and in which there is an offset longitudinal bore (8.1) in the crankshaft communicating with a radial bore (8.2) to promote circulation of gas within the crankcase

Description

CRANKSHAJFT ASSEMBLY Technical Field of the Invention

The invention relates to a crankshaft assembly, particularly for a Stirling engine.

Background to the Invention

Stirling engines offer advantages of multi-fuel capabilities (geothermal, solar, bio-, fossil- and nuclear fuel), very low NO_x and HC emissions when burning fossil fuels, very high total efficiency (particularly when used with CHP) and very low • maintenance intervals compared to internal combustion engines.

The principle of operation of a Stirling engine can be described with reference to FIG. 1. A displacer (a) and power piston (b) reciprocate within a cylinder with a fixed charge of working gas (e.g. air, nitrogen, helium or hydrogen). The displacer and power piston are connected to a crankshaft (c) via crossheads, connecting rods (d) and wristpins. As the displacer (a) reciprocates, it displaces the working gas (usually nitrogen or helium in production engines) through the heater head tubes (e), regenerator (f) and cooler (g) that are placed in the hot and cold portions of the engine. The displacer (a) and power piston (b) have different phase angles so that more work is put into the power piston during the expansion stroke, when most of the gas is in the hot space, than the work the piston returns to the gas a cycle later to compress cold gas back to the hot part of the engine. The net surplus of expansion work over compression work is^" extracted as useful work by the power piston, which in turn is transferred to the crankshaft (c) with its outgoing shaft. All external heat is supplied at the heater head (e) and rejected in the cooler (g). The regenerator (f) absorbs heat from the working gas as the gas moves from the hot end to the cold end. It returns the stored heat to the working gas when the gas is pushed from the cold end to the hot end. One can say that the regenerator acts as a "thermal dynamic sponge".

There exist several types of Stirling engines; α-, β- and γ-type. In addition there are engines with oil lubrication and non-lubricated (or 'lubricated for life') engines. Next, there are engines that are hermetically sealed and ones that have a so-called "atmospheric" crankcase whereas there is a need for a dynamic seal between the oil lubricated crankshaft assembly, displacer rod, crosshead and power piston rings.

The seal requires oil in order to avoid any gas leakage from the Stirling process and to lubricate and cool the seal. The seal is necessary to avoid oil contamination in the hot gas circuit of the Stirling process, which would be detrimental to the function of the regenerator, cooler and heater tubes. In addition, another dynamic seal is usually necessary if the Stirling unit has an outgoing shaft from the crankcase. However, this seal usually doesn't pose a problem if the crankcase isn't pressurised.

Previous Stirling engines using dynamic seals (such as the V- 160 and^-161) must have an extra container (pressure vessel) containing helium or hydrogen to control and also reduce the Stirling cycle pressure when the engine is not in operation. This also entails extra piping, fittings, pump and controls that increase complexity of the engine and production costs.

U.S. Pat. No. 4,765,138 discloses a Stirling engine with a pressurised crankcase to eliminate the piston rods, rod seals, crossheads and lubrication system. However, this solution just moves the problem to the crankshaft where pressure seals and end caps are required. If the working gas is helium it is almost impossible to attain a leak proof seal (for both dynamic and static conditions, and with no lubricating oil) where the power take off shaft penetrates the pressurised crankcase. Said solution also requires that the crankshaft be cooled to prevent deterioration of the connecting rod big end bearings. An external cooling circuit would be required which would involve another seal through the pressurised crankcase, thereby further increasing production costs and including a potential leakage source. In addition, the crankshaft serves as an outer race for the piston connecting rod roller bearing, so requiring the crankshaft bearing surfaces to be hardened and ground which again adds to production cost and complexity.

JP 2001099508 depicts a gas compressor/expander with crankshaft, connecting rods, pistons, crankcase, oil sump and flywheel with an oil pump function. During operation, oscillating and rotating parts within the compressor/expander can be lubricated with oil that flows through a bore located on the central axis of the crankshaft.

JP 1151709 describes an internal combustion engine with an oil sump that forces lubricating oil through a balancing shaft that is in communicates with the engine's crankshaft. The oil can then flow from the crankshaft through a central bore to a flywheel that is installed outside the crankshaft assembly. In this case the flywheel acts as an external cooler.

Both JP publications are specifically directed towards oil lubricated gas compressor/expanders or oil lubricated internal combustion engines, and are not related towards pressurised Stirling engines with enhanced gas circulation. These arrangements are complicated, require several bores and passages in order to function, and can be costly to manufacture. In order to achieve efficient internal cooling and circulation of the working gas within the Stirling engine, there is a need to simplify the internal design. Prior versions of hermetically sealed Stirling engines lack efficient means to circulate the working gas.

It is an object of the present invention to provide a hermetically sealed crankcase having a crankshaft assembly that is capable of circulating fluid within the crankcase. It is another object of the present invention to provide a Stirling engine with such a crankshaft assembly.

Disclosure of the Invention

The invention provides a crankshaft assembly for use within a hermetically sealed crankcase in which there is a flywheel set on the crankshaft, and in which the crankshaft has a longitudinal bore placed offset with respect to the central axis of the crankshaft, and a radial bore substantially perpendicular to and communicating with that longitudinal bore, and the flywheel has a generally corresponding radial bore communicating with the radial bore in the crankshaft, whereby to create a passageway for a working fluid between a point on the crankshaft and a point on or near to the periphery of the flywheel.

In one preferred form, the longitudinal bore is open at one end of the crankshaft, whereby to create a passageway between that end of the crankshaft and the point at or near the periphery of the flywheel. In this form it is further preferred that the longitudinal bore is open at both ends of the crankshaft.

In an alternative preferred form, the longitudinal bore is closed at both ends of the crankshaft, and there is a further radial bore in the crankshaft communicating with the longitudinal bore, whereby to create a passageway between the further radial bore and the point at or near the periphery of the flywheel.

In either of these last mentioned preferred forms, there may be a further radial bore in the crankshaft communicating with the longitudinal bore, whereby to create a plurality of passageways the interior of the crankcase and the longitudinal bore.

It is generally preferred that the radial bore aforesaid in the crankshaft is concentric with the radial bore in the flywheel.

In an alternative form, there may be a discrete pipe between the longitudinal bore in the crankshaft and an outer bore at or near the periphery of the flywheel. The invention includes a crankshaft as described above, in combination with a hermetically sealed crankcase for a Stirling engine.

Brief Description of the Drawings

FIG. 1 shows a simplified Stirling engine FIG. 2 is a cross section of a Stirling engine.

FIG. 3 is a perspective view of the crankshaft assembly. FIG. 4 is a plan view of the crankshaft assembly. FIG. 5 is an exploded view of the crankshaft assembly. FIG. 6a is a cross sectional view and FIG. 6b is a plan view of the crankshaft assembly.

Detailed Description

FIG. 2 is a cross section of a hermetically sealed β-type Stirling engine 1. A non- lubricated hermetically sealed β-type (or commonly called displacer type) Stirling engine has a power piston and displacer piston coaxially disposed within the same cylinder. In addition, all rotating components including generator rotor and the generator stator are placed within the pressurised container; in this case covered by the crankcase 2, generator housing 3, bottom Hd 5, flywheel cover 6 and hot end comprising cooler, regenerator, heater head and burner. (For clarity reasons the cooler, regenerator, heater head, burner and its accessories are not shown.) This version of engine is a very compact type of design, especially compared to the V- type version mentioned previously in the description of prior art.

A hermetically sealed β-type Stirling engine has a helium gas reservoir contained within the crankcase 2. This results in the crankcase being pressurised to the mean cycle pressure.

During operation, the working gas (in this instance helium; however hydrogen and nitrogen and even air are also commonly known working gases) will become quite hot. It is not unusual to see helium gas temperatures within the Stirling engine crankcase in the region of 100°C. Therefore there is a need to find means to cool the working gas. For this reason it is common to install cooling shrouds on the outer surfaces of the crankcase. In this case a cooling shroud 3.1 is placed around the generator housing 3 and another cooling shroud 6.1 (not shown in detail), is placed on the surface of the flywheel cover 6. By doing so it has been observed that the thermodynamic stability of the Stirling engine is much better. Cooling of the generator housing can be especially critical if the temperature of the stator windings gets too high. This can be detrimental to the generator performance and also, in the long run, can cause breakdown (embrittlement) within the insulation of the generator windings.

Traditionally, the working gas is cooled by convection against the parts of the crankcase that are cooled by means of e.g. shrouds. Said shrouds have cooling water circulating at a temperature around 60 - 75°C. The working gas within the Stirling engine crankcase is circulated within the crankcase by means of the bottom parts of the oscillating components such as crossheads and power piston. In addition, the rotational movement of the connecting rods and flywheels can contribute to some sort of turbulence and disturbance so to create a certain movement of the working gas. This will help the working gas within the crankcase to dissipate its heat against the cooled parts of the crankcase.

Following the invention, to improve the heat exchange between the working gas (in this case helium) and the cooled portions of the crankcase, a form of centrifugal pump is utilized in order to enhance the movement of the working gas.

This solution lies within the crankshaft assembly 4. Drilling a hole through a length of the crankshaft and a radial hole through the flywheel, and connecting these two holes together creates a simple centrifugal pump. During operation of the Stirling engine when the crankshaft rotates, a pressure difference will be apparent across these bores. The pressure difference will result in a flow of the working gas through the crankshaft bore and flywheel bore thus creating an additional movement and circulation of the working gas within the closed crankcase volume. By doing so, additional cooling of the working gas is achieved. Thus, improved cooling of the helium is attained without any additional cost of adding accessories such as pumps, fans and drives.

Details of the crankshaft bore will be clearly described in the text for FIG. 6. Details of the flywheel bore are illustrated in detail in FIGs. 3 and 5.

FIG. 3 is a perspective view of the crankshaft assembly 4. The crankshaft assembly comprises the following components; the main component being the crankshaft 8, main big end bearings 9, power piston bearings 10, displacer piston bearing 11, flywheel 13 and generator rotor 14 with locknut 14.1. A radially drilled hole 13.1 in the flywheel is also shown in this view. Said drilled hole 13.1 lines up with and is in communication with the longitudinal drilled hole in the crankshaft. This is described in further detail in FIG. 5.

FIG. 4 is a plan view of the crankshaft assembly 4. All components have been previously described with respect to FIG. 3 except for a bearing retainer 12 that contains the main bearings. In order to simplify and speed up the mounting of the crankshaft assembly 4 within the crankcase 2, the crankshaft assembly ,4 is fully assembled with all components as per FIG. 4. Thereafter, the complete crankshaft assembly 4 is located in the crankcase 2 with the front end entering first (i.e. where the reference line is pointing at the crankshaft 8).

FIG. 5 is an exploded view of the crankshaft assembly 4. The oscillating assembly 7 (shown in FIG 2) has been omitted for clarity. Most components have been "pulled off in order to reveal radial bore 8.2 in the crankshaft 8. The radial bore 8.2 on the crankshaft 8 will line up concentrically with the radial bore 13.1 on the flywheel 13. The positioning of said concentric holes can be by utilizing a keyway that is milled on the crankshaft and a keyway 13.2 that is milled within the flywheel bore with corresponding angles.

The main big end bearings 9, power piston bearings 10 and displacer piston bearing 11 have both inner and outer races. Because of this the crankshaft doesn't have to be hardened and precision ground in order to accommodate for the placement of the bearings.

Fastening of the generator rotor 14 is by means of generator rotor fastening nut 14.1. Said nut exerts an axial force against the generator rotor that locks it in position against the flywheel 13.

FIG. 6 is a cross sectional view and a plan view of the crankshaft 8. The cross sectional view visibly shows a longitudinal bore 8.1 and the radial bore 8.2 within the crankshaft (8). The longitudinal bore 8.1 is drilled to a given length L. The length L corresponds to the position of the flywheel 13 and its corresponding bore 13.1. The bore diameter is chosen so that it doesn't significantly diminish the crankshaft's strength, deflection and fatigue properties. In addition, the bore diameters 8.1 and 8.2 are calculated to an extent in order to minimise the pressure drop across the length of the passage. As shown particularly in Fig 6a, placement of the bore 8.1 is offset with respect to the crankshaft's centre axis (C.L.). This offset depends upon the crankshaft's throw and journal diameters. The radial bore 8.2 is in general perpendicular to the longitudinal bore 8.1 and intersects with said bore in order to form a continuous passage for the flow of the working gas (in this case helium).

A keyway 8.3 is shown in the plan view (Fig 6b) of the crankshaft 8. As mentioned in the description of FIG. 5, said keyway is used to position and line up the radial bore 13.1 in the flywheel and the radial bore 8.2 in the crankshaft 8. The keyway 8.3 is located and milled on the flywheel journal 8.4. As mentioned previously, the crankshaft does not have to be hardened ør precision ground. This is due to the fact that the ball bearings have their own inner races that are pressed directly on to the main and connecting rod journals. Traditional shaft materials for the fabrication of the crankshaft can be utilized e.g. AISI 4140 or ENl 9T.

Claims

1. A crankshaft assembly for use within a hermetically sealed crankcase (2) in which there is a flywheel (13) set on the crankshaft, and in which the crankshaft has a longitudinal bore (8.1) placed offset with respect to the central axis of the crankshaft (C.L.), and a radial bore (8.2) substantially perpendicular to and communicating with that longitudinal bore, and the flywheel has a generally corresponding radial bore (13.1) communicating with the radial bore (8.2) in the crankshaft, whereby to create a passageway for a working fluid between a point on the crankshaft and a point on or near to the periphery of the flywheel.

2. A crankshaft assembly as claimed in claim 1, in which the longitudinal bore is open at one end of the crankshaft, whereby to create a passageway between that end of the crankshaft and the point at or near the periphery of the flywheel.

3. A crankshaft assembly as claimed in claim 2, in which the longitudinal bore is open at both ends of the crankshaft.

4. A crankshaft assembly as claimed in claim 1, in which the longitudinal bore is closed at both ends of the crankshaft, and there is a further radial bore in the crankshaft communicating with the longitudinal bore, whereby to create a passageway between the further radial bore and the point at or near the periphery of the flywheel.

5. A crankshaft as claimed in claim 4, in which there is a further radial bore in the crankshaft communicating with the longitudinal bore, whereby to create a plurality of passageways the interior of the crankcase and the longitudinal bore.

6. A crankshaft as claimed in any one of the preceding claims, in which the radial bore aforesaid in the crankshaft is concentric with the radial bore in the flywheel.

7. A crankshaft as claimed in claim 1, in which there is a discrete pipe between the longitudinal bore in the crankshaft and an outer bore at or near the periphery of the flywheel.

8. A crankshaft as claimed in any one of the preceding claims, in combination with a hermetically sealed crankcase for a Stirling engine.