Dossier No.4: Engine Speed Switch

The simplest way to convert a bus or truck from diesel to LPG or CNG fuel is by mixing the LPG or CNG to the air intake, adding a throttle valve and limiting the diesel injection pump. This is often referred to as a Dual Fuel System.

The small amount of diesel fuel and the high compression ratio ensures ignition. Engine power is chiefly supplied by the LPG or CNG supply. Then, what is needed is a simple electronic controller to manage the fuel mix. The controller:

  1. Opens the fuel tank solenoid when the engine is running.
  2. Starts adding fuel as the engine revs above a minimum RPM
  3. Cuts fuel when the engine RPM hits a maximum

The system is currently used in countries with limited oil or monetary reserves. It is required to be easy to install, diagnose, maintain and repair.

Feature list:

  • Detect RPM by mounting magnetic pickup above flywheel.
  • Convert pulses to RPM by selecting number of teeth on flywheel.
  • All set points are in 1V/1000RPM.
  • Three high current relay outputs.
  • Protect all inputs and outputs.
  • Must be simple, easy to understand, easy to use, easy to repair.
  • Discrete electronics, no microcontrollers.
  • Simple and robust circuitry that can be repaired with commonly available components.
  • By design, the circuit does not require calibration.
  • Analog frequency to voltage converter.
  • Voltage comparators with hysteresis.
  • Trimpots to select RPM setpoints.
  • Polyfuse protection against short circuit or overloads.

Technology used:

  • Discrete electronics, no microcontrollers.
  • Simple and robust circuitry that can be repaired with commonly available components.
  • By design, the circuit does not require calibration.
  • Analog frequency to voltage converter.
  • Voltage comparators with hysteresis.
  • Trimpots to select RPM setpoints.
  • Polyfuse protection against short circuit or overloads.
  • Hardware designed in Altium Designer.