Abstract: Ions are emitted (14) into the gas stream (15) of a gas turbine engine at a point with respect to the gas stream (15), in comparison with the position of an ion-responsive electrostatic probe (24), so that there is a discernible, reliable difference in the ion impingement on the electrostatic probe (24) during normal gas flow (FIGS. 3 and 4) in contrast with that which occurs during the abnormal gas flow attendant a surge (FIG. 5) thereby to provide early and sustained detection of stall and/or surge conditions in the engine, even in the absence of flame from the combustor (17).

Abstract: A method and apparatus are provided for indicating an operating characteristic of a periodically combusting internal combustion engine, such as a diesel engine, having exhaust gas issuing from one or more combustion chambers in a periodic manner through a duct. The particulate in the exhaust gas stream are electrically charged and of the same polarity, and are grouped in packets associated with the periodic combustion in the respective chambers. An electrically conductive, passive electrode, preferably annular in shape, is disposed such that most or all of the exhaust gas flows therethrough so as to electrostatically sense, principally by induced image charge, the passage of the respective packets of charged particles. The electrode is electrically isolated from the duct.