Frequency is the number of waves generated by the exhaust stroke at a given rpm. The positive pressure is the exhaust gas and the negative pressure is the space between the pressure waves. The amplitude of the wave contains positive and negative pressure. The pressure wave has three properties - amplitude, frequency, and resonance.Īmplitude is a pressure wave generated by the speed and compression of the exhaust gas as it is driven by the piston or rotor into the manifold and pipe. Exhaust 101Įach exhaust stroke of a piston or Wankel engine produces an exhaust pressure wave in the manifold and exhaust pipe. It will also try to show the correlation of a scope trace to scan data. The following is an attempt to explain the what, where and how. Using a scope can be useful if you know: what mechanical action generates the signal where the signal is generated and how the signal input affects the operation of a system. This can range from difficult to impossible.
The hard part about using a scope is finding a connector to make an observation. The scope is a last chance to find a problem like this because the controller didn’t see the interfering signal. If the engine controller is the final decision point for engine operation and the oscilloscope is an observer of one or more inputs, what is the diagnostic function of the oscilloscope? The simple answer is to find an anomaly in a signal trace, such as electromagnetic interference from a spark plug wire that would appear as a distortion of the sensor’s low voltage trace. The controller also uses as many as nine other inputs, such as rpm and throttle position, to affect fuel trim.
If the engine controller knows what cylinder is on exhaust stroke and which manifold contains the pressure wave, the controller will respond to the input from that oxygen sensor and change fuel trim. The engine controller is like the referee in that it continuously monitors and controls engine operation. If the referee didn’t see it, he can’t call it. Sometimes you miss the action and want an instant replay.
You get to see the action, but can do nothing about it as it happens. Watching an oxygen sensor output on an oscilloscope is like watching a sporting event on television.
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