Analysis of the free-space acoustic signature of a BBMF Rolls-Royce Merlin engine from 842 rpm to 2740 rpm

A technique has been developed to analyse the free-space acoustic signature of the Rolls-Royce Merlin engine in order to provide diagnostic information relating to its performance. The work is motivated by a need to maintain the reliability and maximise the lifetime of this historic engine which is an important component of the aircraft operated by the UK Battle of Britain Memorial Flight (BBMF). For convenience, the data were extracted from the soundtrack of a video recording made whilst the aircraft was undergoing a full-power ground-run. The aim of the work is to generate information relating the individual firings of each cylinder and this is presented as a multi-level colour image. Here, the ignition time history for two revolutions (six ignitions) is shown vertically and corresponds to the bank of exhaust ports facing the microphone. Subsequent similar histories are presented along the horizontal time axis. The results are sensitive to engine speed variations, and a correlation between successive vertical columns is carried out to quantify this drift and adjust the data accordingly. Seven engine speeds from 842 to 2740 rpm are examined. The processed data illustrate that some speeds exhibit well-defined, single ignitions whereas others show varying degrees of after-firing type effects. At the higher engine speeds, it would seem that the cylinder ignition amplitudes become much more irregular, and the results for 2740 rpm indicate a breakdown in engine performance. The coefficient of variation (Cv) has been demonstrated as a measure of these factors. Finally, a technique has been proposed and demonstrated for the identification of individual cylinders in the firing sequence. The data acquisition process is relatively straight-forward and should not demand excessive time to cover a full range of engine speeds. With further investigations, it could be possible to readily relate the imaging results to specific engine faults.