GB2179705A

GB2179705A - Piston and cylinder arrangement in stirling engine

Info

Publication number: GB2179705A
Application number: GB08620160A
Authority: GB
Inventors: Andreas Strohmer
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1985-08-22
Filing date: 1986-08-19
Publication date: 1987-03-11
Also published as: DE3530000C2; FR2586454A1; GB2179705B; IT1197054B; IT8621381A0; GB8620160D0; DE3530000A1; IT8621381A1

Abstract

A free piston Stirling cycle engine in which at least one of the pistons, preferably the displacement piston (2) has a piston jacket stepped in diameter a number of times and fitted in a generally gastight manner into the cylinder (1) and forming in conjunction with corresponding steps in the wall of the cylinder two annular spaces (3,4) acting as gas springs, the interior of the piston having a cavity connected via channels 8 with the annular spaces (3,4) between the piston and the cylinder. The cavity in the interior of the piston being subdivided by a gastight partition wall (7) into two chambers (5,6) each chamber (5,6) being connected with one of the two annular spaces (3,4). In order to adapt the average gas spring pressure to that of the cycle, the chambers 5 and 6 communicate with channels 13 via borings 10, 11 and groove 12, each time the piston passes through a central position. <IMAGE>

Description

SPECIFICATION Free piston Stirling engine This invention relates to a free piston Stirling engine.

With free piston engines operating on the Stirling principle which may be designed, for example, as linear generators or heat pumps, kinematic.positive control of the displacement piston and working piston, for example via a rhomboid gearing or a swash plate, is dispensed with so that these machines can be sealed more simply, easily and satisfactorily than other types of construction operating on the Stirling principle. The displacement and the working pistons thus form spring/mass systems mutually interacting with and subject to the cycle gas forces, frictional forces, forces resulting from the input or output of power and also intentional elastic forces.The sum of all these forces is intended to cause the pistons to perform oscillatory movements in the direction of their longitudinal axis about a generally fixed central position and without coming in contact with each other or making impact on stop means forming part of the housing. In addition, the movements of the two pistons are to be so adapted to each other whereby they provide maximum accuracy in the phase displacement required between the displacement and the working piston in the Stirling cycle. In this process the said elastic forces are generated by mechanical springs or more frequently by gas springs.

As regards the initiation of the movement (starting operation) and the maintenance of the movement the displacement piston presents a particular problem because the upper and the lower sides are subject to at least approximately equal pressure at any given moment (with cyclical fluctuations corresponding to the course taken by the cycle).

DE A 25 24 479 discloses a free piston machine of the same kind as the present invention with the main object of the construction being to ensure that the movement of the displacement piston, particularly the phase and position, will be as close as possible to the theoretical optimum. For this purpose the displacement piston is provided with a piston rod which extends into a hollow space in the working piston and which is provided therein with an auxiliary piston. The working piston has a stepped piston sleeve interacting with a stepped recess in the cylinder, in addition to which the auxiliary piston interacts with the stepped recess in the cylinder, the stepped hollow space in the working piston have central zones of greater diameter and end zones in the form of annular spaces, each of which is sealed off from the relevant piston and which act as gas springs.One outer and one inner annular space in each case are interconnected by gas channels in the working piston, so that when the working piston approaches the dead centres the auxiliary piston and the displacement piston connected with it will be accelerated in the opposite direction by a pressure impulse. This solution has the disadvantage that the piston rod with auxiliary piston perceptibly increases the mass of the displacement piston and thus the over-all mass wherein the sealing of the piston rod in the working piston results in additional friction losses, and the processing of the numerous stepped surfaces and sealing surfaces is expensive and subject to relatively wide tolerances thus leading to increased losses from friction and leakage.

Free piston machines are also known in which only one gas spring is provided in the interior of the displacement piston. The spring position being between a guide bar integral with the housing and a pot-shaped recess in the lower side of the displacement piston.

This simple gas spring has different characteristics of tension and pressure, in addition to involving the risk of shifting the axial piston.

Owing to the small spring surface, high spring pressures are required leading to increased risk of leakage or friction losses. The tolerance in the sequence of movements from the housing via the guide bar as far as the sealing surface in the interior of the piston or from the housing via the piston as far as the sealing surface increases the difficulty of sealing off the gas spring.

This invention seeks to provide in a free piston Stirling cycle engine a simpler and lighter piston mounting system which will be free of leakage and thus involve lower losses.

According to this invention there is provided a free piston Stirling cycle engine in which at least one of the pistons preferably the displacement piston, has a piston liner or jacket stepped in diameter a number of times and fitted in a largely gastight manner into a cylinder and forming in conjunction with corresponding steps in the wall of the cylinder two annular spaces acting as gas springs, the interior of the piston having a cavity connected through channels with annular spaces between the piston and the cylinder, wherein the cavity in the interior of the piston is subdivided by a gastight partition wall into two chambers, each chamber being connected with a respective one of the two annular spaces.

If the hollow space inside the piston is included in the gas spring container it is possible to operate with very limited pressure values. Although this requires extra large operative areas for the gas spring in order to obtain the required resilience the gas spring leakage losses are reduced, since the sealing gap lengths (periphery of piston/cylinder sealing surface on the maximum step in each case) only increase in direct proportion to the exter nal gas spring diameter, whereas the pressure amplitudes decrease with the square of the latter. This also reduces the energy required for driving the piston. The displacement piston is therefore particularly suitable for the measures prescribed by the invention, because in view of its function it is usually constructed as a thin-walled body of ample volume.The absence of a guide bar for the displacement piston enables weight to be saved and the construction to be simplified.

Preferably the piston liner and the cylinder are formed with channels which on each passage of the piston through an approximately central position between dead centres forms a momentary communication betwen the two chambers within the piston and the cycle gas container.

The measure prescribed above serves to ensure that the average internal pressure of the piston (equal to the average gas spring pressure) will be continuously adapted to the process gas pressure. This not only protects the thin-walled, pressure-sensitive displacement piston but also reduces the flow through the gas spring sealing gap to a minimum.

In an embodiment the borings lead from the cavity in the piston into an external annular groove of the piston liner. Alternatively the borings lead through the cylinder into an annular groove in the internal wall of the cylinder.

These features are intended to prevent the pressure equalization borings in the piston and cylinder from becoming closed up when the piston rotates about its axis in the cylinder and the corresponding borings are no longer properly aligned with each other.

The accompanying drawing illustrates an embodiment of the invention by way of example.

In the drawings: Figure 1 shows a schematic longitudinal section through the upper zone of a cylinder and a displacement piston of a free piston engine, and Figure 2 shows a qualitative graph of pressure as a function of distance travelled for the gas springs and the displacement piston.

As shown in Fig. 1, the piston 2, which is stepped a number of times in diameter, is positioned within cylinder 1 which is also stepped so as to create annular spaces 3 and 4 acting as gas springs. The upper annular space 3 is volumetrically connected by borings 8 with the upper chamber 5 in the interior of the piston, while the lower annular space 4 is connected by borings 9 with the lower chamber 6. The gas tight partition wall 7 is positioned between the two chambers 5 and 6, so that the spaces 3, 5 and 8 together form the upper gas spring housing and the spaces 4, 6 and 9 the lower gas spring housing. The spring surfaces are formed by radial annular surfaces between the maximum and minimum diameter of each of the annular spaces 3 and 4.Against the cycle gas spaces above and below the piston 2 the gas springs are sealed off by suitable piston/cylinder fits, possibly with additional sealing elements (piston rings or the like). The requirement of low friction preclude any absolutely complete sealing. For the movement of the cycle gas, channels 13 are integrated into the cylinder 1 and also accommodate the heat exchange elements and the heat storage elements 14,15 and 16. In a free piston engine these elements would comprise the heater 14, the regenerator 15 and the radiator 16. The schematic diagram in Fig.

1 does not show the true position and size of these components. Fig. 1 shows the displacement piston in an intermediate position between dead centres. In this position the two chambers 5 and 6 are connected via radial borings 10 and 11 in the piston 2 and cylinder 1 with the cycle gas (channels 13), as a result of which the average gas spring pressure is adapted to the cycle gas pressure each time the piston passes through the central position. if the borings 10 and 11, due to rotation of the piston 2 and the cylinder 1, fail to align, the passage of gas through the continuous annular groove 12 in the cylinder is nevertheless ensured. This annular groove may also be positioned in the piston jacket. Owing to the slanting position selected for the partition walls 7 some of the borings 10 lead into the upper chamber 7 and the others into the lower chamber 6.For reasons of clarity the zone of the working piston has not been included in the diagram in Fig. 1.

Fig. 2 is a qualitative graph showing the course taken by the spring pressure p as a function of the piston stroke s for both gas springs. The values smax (+) and Smax () corresponding to the maximum piston stroke starting from the central position s=O. In these expressions (+) denotes a piston movement upwards while (-) denotes a movement downwards. Owing to the very limited pressure amplitudes Ap, resulting from the ample volumes provided for the springs, the characteristics between the two dead centres are almost linear. The average spring pressure p and s=O corresponds with the relevant cycle gas pressure. By giving the upper and the lower spring a different characteristic for the spring container and the spring surface the piston movement can be influenced in a prescribed manner as regards frequency, amplitude and shift tendency.

Claims

1. A free piston Stirling cycle engine in which at least one of the pistons preferably the displacement piston, has a piston liner or jacket stepped in diameter a number of times and fitted in a largely gastight manner into a cylinder and forming in conjunction with corresponding steps in the wall of the cylinder two annular spaces acting as gas springs, the interior of the piston having a cavity connected through channels with annular spaces between the piston and the cylinder, wherein the cavity in the interior of the piston is subdivided by a gastight partition wall into two chambers, each chamber being connected with a respective one of the two annular spaces.

2. An engine in accordance with Claim 1, wherein the piston liner and the cylinder are formed with channels which on each passage of the piston through an approximately central position between dead centres forms a momentary communication between the two chambers within the piston and the cycle gas container.

3. An engine in accordance with Claims 2, wherein the channels in the piston and in the cylinder are formed as radial borings in a plane perpendicular to the axis of the piston and the cylinder respectively.

4. An engine in accordance with Claim 3, wherein the borings lead from the cavity in the piston into an external annular groove of the piston liner.

5. An engine in accordance with Claim 3, wherein the borings lead through the cylinder into an annular groove in the internal wall of the cylinder.

6. An engine in accordance with any one of Claims 3 to 5, wherein the gastight partition wall in the cavity in the piston slants with respect to the axis of the piston and intersects the plane of the radial borings in the interior of the piston in such a way that some of the borings lead into the upper chamber and the rest of the borings lead into the lower chamber of the cavity in the piston.

7. An engine in accordance with Claim 6, wherein the gastight partition wall is constructed as a flat plate with an elliptical external contour.

8. A free piston Stirling cycle engine as described herein and exemplified with reference to the drawings.