Understanding the Operation of a Tachometer

A tachometer is a device that measures the rotational speed of an engine, typically expressed in revolutions per minute (RPM). It is widely used in vehicles such as cars, boats, and airplanes to monitor engine performance.

The most common forms of tachometers use either an analog dial or a digital LCD or LED screen display. These devices are usually located next to the speedometer, connected via a cable to the engine's distributor or through a teleflex cable, much like a speedometer.

How a Tachometer Measures RPM

One method of measuring RPM involves a rotating cable connecting the distributor to the tachometer. This is a popular setup in classic vehicles such as the author's 1964 Corvette. The rotating cable sends signals from the distributor to the tachometer, which then displays the RPM.

Modern Tachometer Technologies

Another method, often used in modern bikes, employs a Hall effect sensor. This sensor detects the presence of a magnetic field. A small magnet is attached to a spoke of the front wheel, while the sensor is mounted on one of the fork arms. As the wheel rotates and the magnet passes the sensor, a small spike of voltage is generated.

This voltage spike is transmitted by either wires or wirelessly to a microcomputer in the display unit. The computer counts the number of voltage spikes per unit of time and translates this into a speed reading. If the circumference of the wheel is known, each rotation can be converted into distance traveled, similar to an odometer function.

Contacting Type Tachometers

Contacting type tachometers utilize small AC or DC generators to translate rotational speed into voltage. In a simple setup, a coil is attached to a shaft, which can rotate between two permanent magnets. As the tachometer is attached to a rotating shaft, the coil rotates within the magnetic field, producing a voltage according to Faraday's law. This produced voltage is proportional to the speed of the coil or the shaft, and it is calibrated in terms of RPM. In a similar setup, a permanent magnet can rotate instead of the coil, generating an AC voltage with the same process.

Conclusion

The operation of a tachometer can be as simple as a rotating cable connecting a distributor to the gauge or as complex as a Hall effect sensor detecting magnetic field changes. Both methods provide valuable information about engine speed, contributing to the efficient and safe operation of vehicles and machinery.