Commander electronic fuel injection

[mike59bike]

Progress report on my conversion to EFI. I had a lot of problems with plugs fouling and very high fuel consumption. It turns out this was a fundamental fuelling problem and I can perhaps share my experiences with anyone thinking of converting a rotary to EFI. I use a MegaSquirt V4 system, by the way, Pico injectors into Jenvey throttle bodies.The problem arose because I followed the MS guidance on using EFI on Mazda rotary engines. Their electronics "sees" a two-rotor system as a four-cylinder four-stroke for purposes of ignition, but this is NOT the same for the injector pulses. These work off a tacho pulse, generated from a "missing tooth" 36-tooth trigger wheel. In other words, I get two inlet pulses or "squirts" per crankshaft revolution (ie one each for the two rotors). I have now realised, after a lot of head scratching, that the inlet pulses should be for THREE rotor chambers per one crankshaft revolution, not two. This means the set-up I had was over-fuelling by one-third, ie 33% rich!! Not much wonder it fouled the plugs. I have altered the injector characteristics from 330 ml/min to 440 ml/min (which has the effect of shortening the injector pulse width by the same ratio) and bingo! it runs fine. In other woerds, I am sharing one Otto cycle with three chambers instead of two. I have tried to get my head around exactly why this works, and it seems to make sense but I get lost in the maths. Any comments on this gratefully received. Mike

[Richard Negus]

Hi Mike,You may have sorted the over-fuelling, but your original understanding of induction periods is correct. There are two equally-spaced 'bangs' per complete revolution of the eccentric shaft and, correspondingly, two induction 'sucks' per revolution, one per rotor housing.If I understand correctly, you haven't actually changed injectors for ones with a higher max flow rate (440ml/min ), but have changed the ECU parameter as if you had changed. The ECU then calculates a shorter 'ON' time to compensate for the higher flow rate. The result is a lower actual flow amount through the 330 injector and hence a weaker mixture.I could, of course, have totally misunderstood what you are saying and the above paragraph is total bo££ox !R.

[mike59bike]

Hi Richard. I think you've got it spot on. According to the MegaSquirt manual, a rotary "looks" the same to an ECU as a four-cylinder four-stroke, but it does all its rated induction over 1080 degrees of eccentric shaft rotation, not 720 (ie two crankshaft revolutions). As the fuel is calculated on the complete cycle (three complete induction/exhaust cycles per rotor) this means the fuel is shared over the ratio 1080 to 720, which gives us the magic 33.3% overfuelling. I think this is because the ECU doesn't "care" what particular rotor chamber it is seeing at any time, so it can fire a spark which doesn't matter how much it is (if you see what I mean) but the fuel pulse is volume-dependent. Quad errat demonstrandum. Mike