Speed Measurement (RPM Measurement)
Speed measurement is the determination of how many revolutions per minute the engine shaft turns. Non-contact methods are the magnetic pickup, the Hall sensor, the alternator W terminal, and the optical or laser tachometer. Each method has a different uncertainty; before making adjustments the accuracy of the measurement must be known.
The three most used routes in the field are these:
- Handheld optical/laser tachometer: a reflective strip is stuck on the pulley and one pulse per revolution is read. It gives a fast and independent reference.
- Alternator frequency: on a 4-pole alternator, measuring 50 Hz means 1500 rpm. It is an indirect but reliable measurement taken on the electrical side.
- Flywheel teeth + magnetic pickup: the method used in permanent installations.
Always take the measurement in steady state and on a warmed-up engine; also take two values, one unloaded and one loaded. The difference between them gives the droop characteristic of the engine.
In an electronic throttle kit installation, speed measurement is a verification tool: when teaching the upper limit on the GS740-3P and when storing the work speed on the GS740, you can only know how many rpm the point you stored actually is through an independent measurement.
Context in which this term is used: Electronic speed and throttle control on diesel engines