Reading engine speed

Engine speed information is required to calculate engine torque and to plot the right RPM scale on dyno report. There are some situations where engine speed doesn’t have constant relation with dyno speed and it cannot be calculated with simple ratio and OBD interface is not available. To get correct engine speed reading in these situations, the controller is equipped with specialized Engine speed input, that allows direct readout of the engine speed. The following sensors can be connected to this input:

Variable reluctance sensor, VR sensor

+12V / +5V
SIG+First VR sensor wire
SIG-Second VR sensor wire

Hall type sensor or other digital signal

+12V / +5VSensor power supply
SIG+Sensor signal
SIG-Reference voltage (must be between 0V and 4.5V)
GND-Sensor return ground

When hall type sensor is used, it is required to connect some external resistors. R1 is usually required for sensors with open collector output. R2 and R3 create a voltage divider that supply a reference voltage to S- input.

Injector, ignition coil

+12V / +5VVoltage divider supply
SIG+Ignition or injector signal
SIG-Reference voltage (must be between 0V and 4.5V)
GND-Sensor return ground

In case of signal from ignition coil low voltage side, it is required to add an extra R1 resistor in series to protect the controller from high voltage spike.

PEREK PES1 photoelectric sensor

The photoelectric sensor has the same output and connection as mentioned above hall sensor. It can be used with a bright marker placed on a part rotating in sync with the engine.

photoelectric speed sensor wheel speed
Development demo of PES1 sensor operation with white tape marker placed on electric drill

PEREK CS1/CS2 clamp inductive sensor

CS1/CS2 is an inductive sensor designed to read engine speed signal from ignition coil wires (high or low voltage).

+12V / +5VSensor power supplyWhite wire
SIG+Sensor signalGreen wire
SIG-Reference voltageBrown wire
GND-Sensor return groundBrown wire

Any sensor different from mentioned above

+12V / +5VPower supply output to be used by the sensor or reference voltage divider
SIG+Input signal
SIG-Reference voltage input (must be between 0V and 4.5V)
GND-Return ground for sensor or voltage divider

Many sensors not mentioned above could be connected to the Engine speed input. Input circuit analyzes voltage between S+ and S- terminals. The speed signal is triggered when S+ potential drops below S- potential. Circuit rearm voltage is set to 30% of S+/S- peak voltage. When the rearm voltage is crossed, the circuit gets armed to generate the next speed signal.

After a typical 85ms (45ms – 140ms) timeout, the adaptive arming threshold is reset to 30mV.

To connect any sensor, a circuit must be created which ensures that S+ potential will drop below S- potential every time that speed signal generation is required. If a fixed S- reference is used, it must be between 0V and 4.5V.

A custom reference voltage strategy can be implemented by connecting the engine speed input S- pin to the analog output S pin, and programming the analog output to provide the voltage calculated by your strategy.

Measurement methodology

The primary way to make measurement using the signal on the Engine speed input is to select eng. speed in as a source of engine speed and set signal multiplier and divider based on used signal source.

Signal sourceMultiplierDivider
Photoelectric sensor with marker on crankshaft
or pickup on ignition coil in wasted spark configuration
or any configuration with one signal per engine revolution
Photoelectric sensor with marker on camshaft
or pickup on ignition coil in full sequence ignition
or pickup on injector wire in full sequence injection
or any configuration with one signal per 4 stroke engine cycle
Signal borrowed from engine trigger wheel1number of teeth per crankshaft revolution
60-2 wheel -> divider = 58
36-1 wheel -> divider = 35
Photoelectric sensor with marker on vehicle wheelCalculate multiplier from gearbox and final gear ratios1

Sometimes the engine speed signal source doesn’t give a good signal in the whole range of engine speeds, or the signal is missing on coastdown. This can be a common case with signal sourced from ignition coils. The faulty signal will corrupt the measurement when it occurs. To make measurement in this case, first we need to use eng. speed in option when the signal is clear and correct. Next we switch the source to calculated option. The software automatically sets the calculated speed ratio from the one that was present before mode switching. The measurement is made with calculated option.