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A Stirling engine (1) includes a reciprocating piston which drives an alternator to provide an electrical output (7). The current signal (7) is measured by a current monitor (60) and a fast Fourier Transform value is produced. The FFT of the current signal (7) has been found to be directly related t

A Stirling engine (1) includes a reciprocating piston which drives an alternator to provide an electrical output (7). The current signal (7) is measured by a current monitor (60) and a fast Fourier Transform value is produced. The FFT of the current signal (7) has been found to be directly related to the stability of operation of the Stirling engine. As harmonic peaks in the FFT increase, this indicates that the stroke length of the piston is approaching or exceeding a safe maximum. Correction action, such as a reduction in heat to the Stirling engine, can be taken in consequence. A variety of stroke length detectors (110) are also disclosed, for use in combination with, or separately from, the FFT analysis of the current signal (7). Optical and mechanical switches and analogue sensors are disclosed, along with an accelerometer mounted upon the casing of the Stirling engine.

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The invention claimed is: 1. A domestic combined heat and power, dchp, unit comprising a Stirling engine having an engine head and a burner to input heat energy into the engine head to drive a reciprocating member; a combustible fuel supply; a generator for producing an electrical power output; and

The invention claimed is: 1. A domestic combined heat and power, dchp, unit comprising a Stirling engine having an engine head and a burner to input heat energy into the engine head to drive a reciprocating member; a combustible fuel supply; a generator for producing an electrical power output; and an instability detection apparatus for detecting instabilities of the Stirling engine, the apparatus being arranged to receive a signal from the generator to carry out a Fourier Transform on that signal, to monitor the Fourier Transform so as to identify the presence of artefacts within the Fourier Transform which are indicative of existing or developing instabilities within the engine, and in response to the identified presence of artifacts, to implement action on the Stirling engine to correct the instabilities. 2. The dchp unit of claim 1, wherein the instability detection apparatus is further arranged to receive a current signal from the generator, and to carry out a Fourier Transform of that current signal. 3. The dchp unit of claim 1, wherein the instability detection apparatus is further configured to identify the present of non-resonant harmonic frequencies in the Fourier Transform. 4. The of claim 1, wherein the instability detection apparatus is further arranged to monitor the Fourier Transform so as to identify a change in the number, and/or magnitude of the said artefacts within the Fourier Transform over time. 5. The dchp unit of claim 1, further comprising a heat output. 6. The dchp unit of claim 5, wherein the generator further comprises an alternator driven by the reciprocating member. 7. The dchp unit as claimed in claim 5, further comprising a controller arranged to receive an output from the instability detection apparatus indicative of the presence of the said artefacts in the Fourier Transform, the controller being configured to control the dchp unit to reduce the presence of instabilities when the said output of the apparatus indicates the presence of the artefacts in the Fourier Transform. 8. The dchp unit as claimed in claim 7, in which the controller is arranged to adjust the burner so as in turn to control the heat energy into the Stirling engine when the output received from the instability detection apparatus indicates the presence of artefacts in the Fourier Transform. 9. The dchp unit according to claim 1, further comprising: a stroke length detector arranged to provide a stroke length detection output; and a system controller arranged to receive as a first input from the instability detection apparatus, an indication of the presence of artefacts in the Fourier Transform of the generator electrical power output, to receive as a second input from the stroke length indicator the said stroke length detection output; and to control the Stirling engine so as to reduce the stroke length of the reciprocating member in dependence upon the first input thereto, the second input thereto, or both. 10. The dchp unit of claim 9, wherein the system controller is further arranged to control the burner so as to reduce the input heat energy to the engine head, whereby to reduce the stroke length of the reciprocating member, in dependence upon the said first and/or second inputs to the system controller. 11. The dchp unit of claim 9, wherein the system controller is further arranged to control the Stirling engine so as to stop the reciprocating member, in dependence upon the said first and/or second inputs to the system controller. 12. The dchp unit of claim 9, wherein the stroke length detector includes a switch operable when the reciprocating member approaches or exceeds an overstroke condition, to provide a stroke length detection output. 13. The dchp unit of claim 12, wherein the optical switch comprises means for generating a light beam, a means for receiving the light beam, the optical switch being arranged to provide the said stroke length detection output when a component that reciprocates directly or indirectly with the reciprocating member cuts the said light beam of the optical switch as a consequence of an approaching or exceeded overstroke condition. 14. A method of detecting instabilities of a Stirling engine of a domestic combined heat and power dchp unit, the dchp unit having a combustible fuel supply, being capable of generating a heat output, and having a generator for producing an electrical power output, the Stirling engine having an engine head and a burner to input heat energy into the engine head to drive a reciprocating member, the method comprising: receiving a signal from the generator of the dchp unit; carrying out a Fourier Transform on that signal; monitoring the Fourier Transform so as to identify the presence of artefacts within the Fourier Transform which are indicative of existing or developing instabilities in the Stirling engine; and responding to the identified presence of artefacts by implementing action on the Stirling engine to correct the instabilities. 15. The method of claim 14, wherein the step of receiving a signal from the generator comprises receiving a current signal therefrom. 16. The method of claim 14, wherein the step of monitoring the Fourier Transform comprises identifying non-resonant, harmonic peaks within the Fourier Transform. 17. The method of claim 14, further comprising monitoring the Fourier Transform of the generator signal, over a period of time; and identifying an increase in the number and/or amplitude of artefacts in the Fourier Transform over that time period, indicative of an increase in the instabilities in the dchp. 18. An instability detection apparatus for detecting instabilities within an engine of a domestic combined heat and power, dchp, unit, the dchp unit having a combustible fuel supply and a generator for producing an electrical power output, the apparatus being arranged to receive a signal from the generator of the dchp, to carry out a Fourier Transform on that signal, and to monitor the Fourier Transform so as to identify the presence of artefacts within the Fourier Transform which are indicative of existing or developing instabilities within the dchp unit, the instability detection apparatus further comprising: a dchp unit having: an engine with a reciprocating member; a combustible fuel supply; a heat output; and a generator for generating an electrical power output; a stroke length detector arranged to provide a stroke length detection output; and a system controller arranged to receive as a first input from the instability detection apparatus, an indication of the presence of artefacts in the Fourier Transform of the generator output, to receive as a second input from the stroke length indicator the said stroke length detection output; and to control the dchp unit so as to reduce the stroke length of the reciprocating member in dependence upon the first input thereto, the second input thereto, or both. 19. The apparatus of claim 18, wherein the engine of the dchp unit further comprises a burner to input heat energy into the engine head to drive the reciprocating member, the system controller being arranged to control the burner so as to reduce the input heat energy to the engine head, whereby to reduce the stroke length of the reciprocating member, in dependence upon the said first and/or second inputs to the system controller. 20. The apparatus of claim 18, wherein the system controller is arranged to control the engine. 21. The apparatus of claim 18, wherein the stroke length detector includes a switch operable when the reciprocating member approaches or exceeds an overstroke condition, to provide a stroke length detection output. 22. The apparatus of claim 21, wherein the switch is an optical switch comprising means for generating a light beam, a means for receiving the light beam, the optical switch being arranged to provide the said stroke length detection output when a component that reciprocates directly or indirectly with the reciprocating member cuts the said light beam of the optical switch as a consequence of an approaching or exceeded overstroke condition. 23. An instability detection apparatus for detecting instabilities within an engine of a domestic combined heat and power unit, the dchp unit having a combustible fuel supply and a generator for producing an electrical power output, the apparatus being arranged to receive a signal from the generator of the dchp, to carry out a Fourier Transform on that signal, and to monitor the Fourier Transform so as to identify the presence of artefacts within the Fourier Transform which are indicative of existing or developing instabilities within the dchp unit, the instability detection apparatus further comprising: a dchp unit having a combustible fuel supply; a generator for producing an electrical power output; a heat output; and a controller arranged to receive an output from the instability detection apparatus indicative of the presence of the said artefacts in the Fourier Transform, the controller being configured to control the dchp engine to reduce the presence of instabilities when the said output of the apparatus indicates the presence of the artefacts in the Fourier Transform. 24. The apparatus of claim 23, wherein the engine is a Stirling engine having a burner to input heat energy into the engine head to drive a reciprocating member and the generator further comprises an alternator driven by the said reciprocating member, and further wherein the controller is arranged to adjust the burner so as in turn to control the heat energy into the Stirling engine when the output received from the instability detection apparatus indicates the presence of artefacts in the Fourier Transform.