Stroboscopic spark advance measuring apparatus

Claim 1

Apparatus for determining the spark advance of an internal combustion engine having a timing mark for indicating a particular position of engine rotation, a spark plug, means for producing a high potential firing voltage for the spark plug including breaker points, and means for applying to the spark plug the firing voltage at a time substantially the same as that at which the points open, said apparatus comprising: a coupling circuit for producing discrete pulses at its output coincident with the application thereto of discrete input signals, a connector for coupling the input of said coupling circuit to a spark plug of the internal combustion engine whose spark advance is to be determined, means connected to said coupling circuit for generating a sawtooth voltage each cycle of which is initiated as a result of a discrete pulse from said coupling circuit resulting from the application to said plug of a firing voltage, means for maintaining the maximum amplitude of the sawtooth voltage at a constant value, an electrically energized lamp for illuminating the timing mark of the engine, adjustable means for selecting a predetermined fixed value of voltage, comprising an adjustable impedance, a manually movable element connected to said impedance for adjusting the same, and indicia means calibrated in degrees of the cycle of operation of the engine, a portion of the indicia means being positioned by said manually movable element so that said indicia means always indicates the position of the manually movable member in terms of degrees of the cycle of operating means, and voltage comparison means for comparing said fixed value of voltage with the instantaneous value of said sawtooth voltage and causing energization of said lamp each time that the instantaneous value of said sawtooth voltage bears a predetermined relation to said fixed value of voltage so that said lamp is illuminated once each cycle of said sawtooth voltage at a time which differs from the time at which the firing voltage is applied to the plug by the extenT of adjustment of said adjustable means so that the amount of the spark advance is indicated by said indicating means associated with the adjustable means without the use of any means for measuring the elapsed time between the application of the firing voltage to said plug and the time of energization of said lamp.