Measurement of Engine Speed by the Analysis of Vibration

Modern automotive engine design is making the measurement of engine speed increasingly difficult, due to problems with access for the sensors and, hence, forms the motivation for this investigation. Some manufacturers provide a dedicated pin/socket for engine speed, but this is clearly vehicle specific and may not always be accessible.

A novel method is described for the measurement of the speed of a reciprocating internal combustion engine, based on the analysis of the vibration of the engine block using an accelerometer. The Power Spectral Density (PSD) derived from the accelerometer signal contains a dominant peak of which engine speed is a multiple, and it is possible to track this peak over the engine speed range. Whilst initially developed for diesel engine application, the range of application has been extended to petrol engines, and hence the device can be truly described as a universal tachometer.