On Monday, 3 September 2018 09:01:07 UTC+10, John Seaborn (A8) wrote:
Mike, great background and information on the small turbine world. Thank you for the in-depth and knowledgeable post. When you mention that JetCat uses an internal igniter rather than the glow plug can you explain this a bit? What is the difference and, theoretically speaking, could other jets use an igniter? Thanks again for posting.

John Seaborn

Hi John,

Thanks.

The AMT and PSR (AMT derived) engines and most other small turbines used to use a model airplane glow plug with a turn or so teased out into the breeze to ignite the propane starting gas. There was a delicate balance between getting the thing to light off by making the glowplug hot enough and glowplug life. It was difficult to know if you were going to get one start or 100 starts from a glowplug. Doesn't matter when launching a model airplane or drone. From all accounts though the gas start system was pretty good. I designed a spark unit using a modified model airplane spark plug which worked well and then kero start became available. Good for military drone operators as you can't carry pressurised inflammable gas on a C-130 etc.
The AMT/M&D igniter is a silicon nitride rod with a heating coil inside it. It is a commercial part used in diesel fuel heaters etc. Fuel gets dribbled around the outside of it under no pressure so you get a vapor barrier which prevents all the fuel from heating and excess unburnt fuel falls in to the engine causing hot starts (auto shut down) or excess fuel being blown out the back of the engine. At least it isn't prone to burning out.
I've never managed to find an image of the current JetCat igniter, not for lack of trying. I suspect it is a flat heated strip with fuel sprayed on to it as droplets which would seem much better.
One thing I have thought of - the M&D system seems to use a pretty standard K2 12 volt nominal LiFePO4 battery. AMT specify a 4 cell LiPo pack which goes up to 16+ volts when fully charged. I think a 5 cell LiFePo4 would be better to get the heater hotter and spin the starter faster. I'm experimenting with this and a way of reducing the kero flow rate into the igniter while still having the main fuel pump start. There is only one pump which requires a certain minimum voltage to start but then produces too much flow rate into the igniter resulting in excess fuel.
Mike