Abstract: There is described a motor vehicle having a spark ignition engine in an engine compartment and having automatic advance and retard device to advance or retard the angle of the firing of the spark relative to the top dead center of the pistons in the cylinders of the spark ignition engine. The engine has a control device including a microphone carried on a vibration damping mounting on the vehicle close to the engine to detect sounds emitted during the running of the vehicle. A frequency filter is connected to the microphone connected to the detector to permit transmission of sound only in a narrow band of sound frequencies between 4,000 and 20,000 Hz which correspond to the sound range of sounds caused by knock. The output from the filter is compared with a datum and when this exceeds such datum, the automatic advance and retard is retarded to prevent knock continuing.

Abstract: A plate bed filter is operated by allowing the filter medium, i.e. granules to pass continually from top to bottom. The filters may be placed in pairs in a horizontal duct with blanking walls to guide all the gas being filtered to pass therethrough. Alternatively the filter may be located in a vertical duct.