Background

My friend bought this wonderful engine from UMS_technologies

It's a 7 cylinders star engine which uses "glow fuel".

The fuel ignites when it comes in contact with the heating element of the glow plug. Between strokes of the engine, the wire remains hot, continuing to glow partly due to thermal inertia, but largely due to the catalytic combustion reaction of methanol remaining on the platinum filament. This keeps the filament hot, allowing it to ignite the next charge, thus sustaining the power cycle.

A 7 cylinders engine is thus equiped with seven glow plugs (one per cylinder)

To start the engine the glow plugs must be heated, then the motor starts, then the heaters can be removed.

In real life diffuclties may occur, the motor starts properly and keeps running at high speed. But when reducing the throttle, temperature at the heating element decreases and may not be sufficient to ignite to next charge... and one cylinder stops... Engine is still running but the rotation is less powerful, noisy and un balanced...

To prevent this behavior an idea would be to keep the heating elements switched on external battery (glowing).

This of course works, but the current into the glow plug is quite high (4A at 1.5V) so that the battery into the plane would be really big (and thus heavy)...

This project is an attempt to power the heating elements only when the engine is at low speed (low throttle).

Electronics schematics

Schematics could not be more simple:

  • an ESP32 will take care to read the throttle stick coming form the radio receiver and to drive the spark plugs drivers
  • spark drivers are TP22996 Ti chips they are dual chanels load switches
    • Number of channels 2
    • Vin (min) 0.6V
    • Vin (max)  5.5V
    • Imax 4A

It was quite difficult to find a proper driver for this application as we needed both high current and low voltage (4A at 1.5V). This TP22996 seems to be perfect !

PCB

I have designed a nice and compact PCB allowing tofit into an RC plane cockpit.

The PCB was kindly sponsored by PCBWay and is as usual of excellent quality.

You can order it here: PCBWay shared project. It's cheap, delivered very fast and so professional looking!

You may notice that the footprint for the TP22996 are SOIC with a pitch of .3mm. They are very tiny and may require some skills to be properly soldered.

And here is the "unboxing" of the PCB and the stencil for solder paste reflow :

I must say that teh stencil packaging is "impressive" fully enclosed into a strong aluminium frame. Very professionnal