A knock detector for an internal combustion engine includes a pair of optical fiber thermometers that sense variations in heat flow within an engine cylinder. Each optical fiber thermometer includes a black body emitter disposed inside the cylinder which is heated by combustion gases to provide an optical output. The optical output is converted to an electrical signal which is analyzed to determine the relative time of arrival of a flame front at each thermometer, from which may be determined the speed of travel of the flame front between the thermometers and thereby whether a knock condition exists.