How Do Tachometers Work?

by Whitney Arana
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Tachometers, in their most basic forms, are devices that measure the speed of an object. Most commonly, they measure the rotation of a mechanism, like the engine shaft in a car. Traditionally, tachometers are dials with a needle pointing to the current speed in RPMs (revolutions per minute). However, with the onset of new reading systems, the use of digital tachometers has risen sharply.


The dial tells the driver the tachometer's reading. In a car, it is located on the dashboard. The instrument itself measures the RPMs of the engine drive shaft. The device is necessary in order to regulate how hard the engine is being worked. The way in which the measurements themselves are taken, though, can vary.


Engines with ignition systems usually utilize a small generator attached to the engine drive shaft. In this case, the tachometer is actually a voltage meter, meaning that it counts the pulsations of voltage in the ignition system. The output voltage is proportional to the shaft's speed so measuring voltage is converted into an accurate measurement in RPMs.

The voltage is generated via a permanent magnet on the shaft. There is a toothed wheel made of iron, which becomes magnetized as the magnet passes the teeth. Then, as the magnet rotates away from the teeth, the wheel becomes de-magnetized. As these changes occur, an electric field forms around the permanent magnet. This field affects the electric charges in a wire coil that surrounds the magnet, generating electricity. As the tooth approaches the magnet, the current flows one way in the coil. As the tooth moves away from the magnet, the coil's current switches direction. The tachometer reads the frequency with which the coil's current changes direction.

Additionally, if the engine turns more quickly, the change in the magnetic field becomes more radical, generating higher voltage. The tachometer also uses this information to inform its reading.


A simpler--yet less common--method is to measure the rate at which sparks are released into the engine's cylinders. This is obviously only useful in a gasoline engine, which uses spark plugs to provide the explosive heat energy that moves the vehicle.


A newer version that is quickly gaining popularity due to convenience and accuracy is the laser tachometer. This type requires no physical contact between the tachometer and the engine shaft. Basically, it beams infrared light at the shaft. One place on the rotating shaft is reflective. The tachometer measures the rate at which the light is reflected back onto the tachometer.

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