RPM

In modern engines, usually a crankshaft sensor is present which generates a periodic signal with a number of cycles per revolution of the engine. The RPM I/O converts this signal into revolutions per minute. The engine speed can be calculated multiple times per revolution. Because of this, variations in the engine speed during a revolution can be seen.

In a typical crankshaft signal, gaps because of "missing" pulses are present. The signal can for example consist of three times eighteen cycles of a sine and a two cycle gap per revolution, which results in sixty cycles per revolution. The gaps are detected internally by the RPM I/O and the number of RPMs is only detected between each cycle of the sine wave.

Hint: For reliable determination of the RPMs, the sample frequency of the instrument must be 10 kS/s or higher.

RPM with crankshaft signal

In the figure you can see a crankshaft signal of a truck during start-up (graph 1), which is converted to the engine speed with an RPM I/O (graph 2). Graph 3 is again the crankshaft signal, zoomed in at a gap in the signal.

Hint: To view the number of revolutions per minute as a number, enable the cursor readout of the graph or use a Meter.

Hint: The RPM I/O can also be used to determine the frequency of a signal. Set the Pulses per revolution to 60 and the Unit to Hz.

Properties

To control the behavior of the RPM I/O, several properties are available. These can be accessed through a popup menu which is shown when the I/O is right clicked.

Maximum RPM

The property Maximum RPM sets the upper limit of the output range of the RPM I/O. Several default values are available, as well as a user defined setting.

Pulses per revolution

The Pulses per revolution property must be filled with the number of periods that the crankshaft signal of the used engine produces per revolution. This number must included the "missing" pulses in the gap(s). Several commonly used values can be selected from, a user defined setting is available as well. The default setting is 60 pulses per revolution.

Mid level

To detect the pulses in the signal, the RPM I/O uses a voltage level to compare the input signal with. The property Mid level sets this level. Some commonly used values can be selected from, as well as a user defined setting.

Hysteresis

For proper detection of the pulses in the signal, the RPM I/O can also use a hysteresis around the Mid level. The property Hysteresis sets it. Some commonly used values can be selected from, as well as a user defined setting.

Ignore periods before gap

In a typical crankshaft signal, gaps because of "missing" pulses are present. In some crankshaft signals, the signal shape at the start of the gap can cause the RPM I/O to make a wrong rpm determination, resulting in a strange peak or dip in the rpm graph. The property Ignore periods before gap tells the RPM I/O to ignore periods before the gap, giving a more stable rpm determination. Several default values can be selected from, as well as a user defined setting. The default setting is 0.

Ignore periods after gap

In a typical crankshaft signal, gaps because of "missing" pulses are present. In some crankshaft signals, the signal shape at the end of the gap can cause the RPM I/O to make a wrong rpm determination, resulting in a strange peak or dip in the rpm graph. The property Ignore periods after gap tells the RPM I/O to ignore periods after the gap, giving a more stable rpm determination. Several default values can be selected from, as well as a user defined setting. The default setting is 0.