Operation of an engine

1. A method of determining the pressure of a gas within an engine relative to a pre-defined pressure of the gas, comprising the steps of:

(a) measuring the power factor of electricity generated by the engine;

(b) comparing the measured power factor with a power factor determined to correspond to the power factor of electricity generated by the engine when operating at the pre-defined pressure; and

(c) determining whether the measured power factor is less than the determined power factor, to thereby determine the pressure of the gas relative to a pre-defined pressure.

2. A method of operating an engine containing a working gas comprising the steps of:

(a) measuring repeatedly the power factor of electricity generated by the engine when running;

(b) comparing measured power factors to a pre-defined power factor determined to correspond to the power factor of electricity generated by the engine when operating such that the working gas within the engine is at a pre-defined pressure; and

(c) producing an alarm when a measured power factor is found to be less than the determined power factor.

3. A method of operating an engine according to claim 2 , wherein the engine is a Stirling engine.

4. A method of operating an according to claim 3 , wherein the engine forms part of a domestic combined heat and power unit.

5. A method according to claim 2 wherein the engine is connectable to an electrical grid and step (b) of the method comprises comparing power factors measured when the engine was isolated from the electrical grid.

6. A method according to claim 5 , wherein step (b) of the method comprises comparing power factors measured during start-up of the engine.

7. A method according to claim 2 , wherein step (b) comprises comparing the measured power factor with a pair of determined power factors and step (c) comprises producing an alarm if a measured power factor is found to be less than the higher of the determined power factors and shutting down the engine if a measured power factor is found to be less than the lower of the determined power factors.

8. A method according to claim 2 , wherein the determined power factor or factors is/are determined empirically.

9. A method of operating an engine containing a working gas comprising the steps of:

(a) measuring repeatedly the power factor of electricity generated by the engine when running;

(b) storing the measured power factors;

(c) analysing at least some of the stored power factors to identify any variation across the power factors; and

(d) producing an alarm when a variation beyond an acceptable limit is identified.

10. The method of claim 9 , further comprising shutting down the engine when the variation is beyond the acceptable limit.

11. The method of claim 9 , further comprising distinguishing between a gradual variation and an abrupt variation, and providing an alarm when a gradual variation is identified and providing an alarm and shutting down the engine when an abrupt variation is identified.

12. An engine unit comprising:

an engine containing a working gas;

a power monitor operable to produce a power factor signal representative of the power factor of electricity generated by the engine;

control means configured to receive the power factor signal; and

an alarm;

wherein the control means is operable:

to use the power factor signal to determine whether the power factor of the engine is less than a pre-determined power factor that corresponds to the power factor of electricity generated by the engine running with the working gas at a pre-defined pressure; and

to operate the alarm if the power factor is determined to be less than the pre-determined power factor.

13. An engine unit comprising:

an engine containing a working gas;

a power monitor operable to produce a power factor signal representative of the power factor of electricity generated by the engine;

control means configured to receive the power factor signal; and

an alarm;

wherein the control means is operable:

to store the measured power factors;

to analyse at least some of the stored power factors to identify any variation across the power factors; and

to produce an alarm when a variation beyond an acceptable limit is identified.

14. A processor programmed to perform the following steps:

to receive from a power monitor a power factor signal representative of a power factor of electricity generated by an engine containing a working gas;

to use the power factor signal to determine whether the power factor of the engine is less than a pre-determined power factor value stored in memory that corresponds to the power factor of electricity generated by the engine running with the working gas at a pre-defined pressure; and

to create an alarm if the power factor is determined to be less than the pre-determined power factor.

15. A processor programmed to perform the following steps:

to receive repeatedly from a power monitor a power factor signal representative of a power factor of electricity generated by an engine containing a working gas;

to store the measured power factors in a memory;

to analyse at least some of the stored power factors to identify any variation across the power factors;

to compare any variation found with an acceptable limit stored in a memory; and

to create an alarm when the compared variation is found to exceed the acceptable limit.

16. A computer-readable medium encoded with a program comprising computer program instructions that, when loaded into a processor, cause it to perform the steps of:

receiving from a power monitor a power factor signal representative of a power factor of electricity generated by an engine containing a working gas;

using the power factor signal to determine whether the power factor of the engine is less than a pre-determined power factor value stored in memory that corresponds to the power factor of electricity generated by the engine running with the working gas at a pre-defined pressure; and

creating an alarm if the power factor is determined to be less than the pre-determined power factor.

17. A computer-readable medium encoded with a program comprising computer program instructions that, when loaded into a processor, cause it to perform the steps of:

receiving repeatedly from a power monitor a power factor signal representative of a power factor of electricity generated by an engine containing a working gas;

storing the measured power factors in a memory;

analyzing at least some of the stored power factors to identify any variation across the power factors;

comparing any variation found with an acceptable limit stored in a memory; and

creating an alarm when the compared variation is found to exceed the acceptable limit.