Internal Combustion Engines (ICE)

Canberra UAV’s petrol powered tricopter. See blog post here

Internal combustion engines can increase flight times but care must be taken to handle the extra complexity and increased vibration.

What to Buy

Connection and Configuration

The engine’s ignition power switch and (optional) starter should be connected to the autopilot’s servo outputs, Ignition and Starter (see ICE section in Autopilot Output Functions).

  • Set ICE_ENABLE = 1 to enable the ICE feature (you may need to reload parameters after setting this in order to see below parameters)
  • Set ICE_START_CHAN to the channel number corresponding to a switch on the transmitter which will be used to start the engine

These parameters may also need to be adjusted:

  • Set ICE_PWM_STRT_ON is the PWM value sent to the starter to start the engine
  • Set ICE_STARTER_TIME is the time (in seconds) that the starter motor should run to start the engine
  • Set ICE_PWM_IGN_ON is the PWM value sent to the ignition power switch when the engine should be running
  • Set ICE_PWM_IGN_OFF is the PWM value sent to the ignition power switch when the engine should be stopped

If using an on-board starter, it is important to configure an RPM sensor for the engine. This will allow the ArduPilot to detect an in-flight engine failure and attempt to restart the engine. ArduPilot supports generic pulse tachometers connected to GPIO pins (such as the Aux servo pins on a PixHawk or Cube). A tachometer may be made using a simple hall effect switch IC. Alternatively, some ignition modules support a tachometer output that can be connected directly to the GPIO pins. Desert Aircraft Electronic Ignition V2 modules support tachometer output on the signal pin of the power input connector. Note that when connecting an RPM sensor to an AUX pin, it is important to make sure that this pin is not configured to output a PWM value. On a PixHawk or Cube, Aux 5 and 6 are configured to not output PWM by default. If you need to use a different Aux pin you may need to adjust BRD_PWM_COUNT.

In order to configure an RPM sensor, the following parameters must be set:

  • Set RPM_TYPE to 2 for a standard GPIO input pin.
  • Set RPM_PIN to the appropriate value for the auxiliary pin you are using.
  • Set the remaining ‘RPM_*’ parameters as appropriate for your system.
  • Set ICE_RPM_CHAN to 1.

Throttle control for an ICE engine is similar to controlling a standard brushless ESC. The throttle servo can be connected to any servo output with a SERVOX_FUNCTION set to 70 (Channel 3 is configured this way by default). It is important to set the Min and Max PWM values on this servo output to be within the mechanical limits of your throttle assembly (using SEVOX_MIN and SERVOX_MAX). While doing this, also verify that the servo moves in the correct direction with respect to a manual throttle input. Note that the throttle servo will not move unless the vehicle is armed. It is recommended to arm the vehicle with ignition power disconnected in order to test the throttle servo.

After configuring the limits of your throttle servo, the following parameters must be set:

  • Set THR_MIN to the desired setting at idle. This will be found empirically during engine testing.
  • Set THR_SLEWRATE to a value appropriate for your engine. 20%/s is a good starting point.
  • Set THR_MAX if you would like to prevent your engine from reaching full throttle.

If you are using a quadplane and would like the ICE engine to be disabled during a VTOL descent to reduce the risk of prop strikes, set Q_LAND_ICE_CUT to 1.

Advanced Starter Configuration

A variety of parameters are available for configuring the engine start routine. The auto-start functionality will attempt to start the engine any time the vehicle is armed, the engine is enabled, and the measured RPM is below the ICE_RPM_THRESH. If the engine is not successfully started within a configurable amount of time, the program will wait for a configurable delay before attempting to start again. It is important to remember that the starter will run in pulses. DO NOT approach the engine between failed start attempts as the starter will attempt to start again if the engine is still enabled.

  • ICE_START_PCT overrides the throttle setting during start.
  • ICE_START_TIME controls the maximum amount of time the starter will run in each start attempt.
  • ICE_START_DELAY sets a delay between start attempts. This can be useful when your starter has a limited duty cycle.
  • ICE_RPM_THRESH sets the minimum RPM reading for the engine to be considered running. This should be set to a value below your idle RPM.

Starting and Stopping the Motor

When using the ArduPilot ICE library to control an engine, the engine can be enabled or disabled using an RC switch, via MAVLink command, or via a mission command. Typically, a 3 position switch on an RC transmitter is used to control the engine. This switch has the following three positions:

  • Low: Force the engine to be disabled. This ignores MAVLink commands and mission items that attempt to control the engine’s state.
  • Mid: Keep the current state of the engine, but allow MAVLink commands and mission items to change the state of the engine.
  • High: Force the engine to be enabled. This ignores MAVLink commands and mission items that attempt to control the engine’s state.

If an ICE_START_CHAN is not configured, the behavior will be the same as when the switch is in the middle position.

To start the motor with RC control:

  • Arm the vehicle
  • Raise the transmitter’s starter switch to start the motor
  • If the engine does not include a starter motor, use a hand-held starter motor to start it

To stop the motor with RC control:

  • Lower the transmitter’s starter switch
  • Disarm the vehicle

Quadplanes can also automatically stop the engine once the final phase of a VTOL landing is entered by setting the Q_LAND_ICE_CUT parameter to 1.

Note

The engine can be started and stopped in autonomous missions by using the DO_ENGINE_CONTROL mission command. This useful preceding NAV_VTOL_TAKEOFF or NAV_VTOL_LAND to start or stop the engine to prevent prop strikes while landing in wind with a low clearance prop. See Mission Commands section.

Vibration isolation

The high vibration from internal combustion engines means that vibration dampening is critical. It is often necessary to mount the autopilot on an plate with self adhesive lead weights added to increase its mass.