Vehicle engine operation

1. A system for detecting an operating state of a vehicle engine, the system comprising:

an input configured to couple in a signal from a power line of a vehicle engine;

a low-pass filter configured to filter the signal received from the power line to remove transient noise components of the signal which exceed 1 kHz, the transient noise components being induced on the power line by signalling activity on a control area network of the vehicle;

a first detector configured to receive the filtered signal and detect when transient noise in the filtered signal rises above a first voltage threshold, the first voltage threshold being set at a voltage level that is lower than the voltage level of transient noise associated with operation of a high current electrical ignition system for starting the vehicle engine, and generate a first detection signal in response thereto;

a second detector configured to receive the filtered signal and detect when the transient noise in the filtered signal falls below a second voltage threshold, which is lower than the first voltage threshold and indicates that the engine has been switched off, and generate a second detection signal in response thereto; and

a circuit configured to generate a first output signal in response to the first detection signal and a second output signal in response to the second detection signal.

2. A system according to claim 1 , wherein the first threshold is at a level that is higher than the level of transient noise associated with the signalling activity on a controller area network of the vehicle when the engine is running.

3. A system according to claim 1 , wherein the second threshold is at a level that is lower than the level of transient noise associated with the signalling activity on a controller area network when the vehicle engine is switched off.

4. A system according to claim 1 , wherein the input is configured to reject a DC component of the signal received from the power line.

5. A system according to claim 1 , wherein the input rejects noise frequencies that exceed frequencies associated with alternator operation.

6. A system according to claim 1 , wherein the first detector comprises a low-gain signal path and the second detector comprises a high-gain signal path.

7. A system according to claim 6 , wherein the low-gain signal path includes a first threshold detector and the high-gain signal path includes a second threshold detector, wherein the first threshold detector and the second threshold detector operate within the same voltage range.

8. A security system for a vehicle comprising:

an input configured to couple in a signal from a power line of a vehicle engine;

a low-pass filter configured to filter the signal received from the power line to remove transient noise components of the signal which exceed 1 KHz, the transient noise components being induced on the power line by signaling activity on a control area network of the vehicle;

a first detector configured to receive the filtered signal and detect when transient noise in the filtered signal rises above a first voltage threshold, the first voltage threshold being set at a voltage level that is lower than the voltage level of transient noise associated with operation of a high current electrical ignition system for starting the vehicle engine, and generate a first detection signal in response thereto;

a second detector configured to receive the filtered signal and detect when the transient noise in the filtered signal falls below a second voltage threshold, which is lower than the first voltage threshold and indicates that the engine has been switched off, and generate a second detection signal in response thereto; and

a circuit configured to generate a first output signal in response to the first detection signal and a second output signal in response to the second detection signal;

wherein the security system is configured to disarm on the basis of the first output signal and to arm on the basis of the second output signal.

9. A security system according to claim 8 , comprising a movement detector, which is arranged to trigger an alarm signal if a respective vehicle is moved while the system is armed.

10. A security system according to claim 9 , wherein the alarm signal is communicated to a detector, which is remote from the vehicle.

11. A non-transitory computer-readable storage medium comprising computer-executable instructions which, when executed by a processor, cause a computing device to perform a method of detecting an operating state of a vehicle engine, the method comprising:

receiving a signal from a power line of a vehicle engine;

filtering the signal received from the power line to remove transient noise components of the signal which exceed 1 kHz, the transient noise components being induced on the power line by signalling activity on a control area network of the vehicle;

detecting when transient noise in the filtered signal rises above a first voltage threshold, the first voltage threshold being set at a voltage level that is lower than the voltage level of transient noise associated with operation of a high current electrical ignition system for starting the vehicle engine, and generating a first detection signal in response thereto;

detecting when the transient noise in the filtered signal falls below a second voltage threshold, which is lower than the first voltage threshold and indicates that the engine has been switched off, and generating a second detection signal in response thereto; and

generating a first output signal in response to the first detection signal and a second output signal in response to the second detection signal.