Posted for "Ron Laughlin" <ronlaughlin@gmail.com>:
Hi Mark,
I was also intrigued by the concept of a Mazda 3-rotor aircraft engine
conversion. I liked the power to wt. ratio and the typically
non-catastrophic failure modes. I researched the 20B rotary a little
bit as a possible engine option for my Legacy. There were some issues
however that concerned me enough that I abandoned the idea and haven't
tried to keep up with newer developments. I was wondering what your
opinions and/or solutions might be for my concerns:
1) The propeller reduction drive (RD). These have historically been an
Achilles' heal for automotive conversions. As I understand it (or
perhaps mis-understand it) rotary engines produce some really
impressive power pulses which regularly destroyed early RD designs. Do
you know of an RD, that is available now, that has several thousand
flight hours of reliable performance mated to a 3-rotor, 300 hp
engine?
2) Un-muffled rotary engines produce a LOT of noise. Actually, it is
more like a painful hellish howl (to my ears). Do you intend to run a
muffler of some sort? If so, where do you plan to mount it and how
heavy do you think it might be? A turbo would be helpful here but that
adds another layer of complexity.
3) Their exhaust manifolds get significantly hotter than a
"traditional" aircraft engine. IIRC, Mazda had trouble with the heat
from the exhaust system on the Lemans racer igniting the the car's
plastic body panels. Are you planning to use shields to protect the
cowling and engine accessories from the additional heat?
4) Like 2-cycle engines, they require oil in the fuel for lubrication.
Will you be using a separate oil tank and automatically injecting the
oil into the fuel system or pouring it into the fuel tank? It seems to
me that either method could introduce more possible failure modes.
Thanks,
--
Ron Laughlin
Legacy N44L (res)
[I won't steal Mark's thunder here but I would like to comment on your items 3 & 4 (I'm a rotary guy too, only mine is a 2-rotor 13B. I also run the FlyRotary mail list, the red-headed step-sister of the LML <g>).... the EGTs on the rotary run about the same as what you see for TITs on a TSIO550... 1650-1750F. Part of the reason that causes these rather high temperatures has to do with the shape of the rotary's combusion chamber, which is constantly changing throughout the cycle. During the compression phase the combustion chamber is a crescent-shaped sliver and the fuel/air mixture is pretty uniformly distributed, from the center of the cresecent all the way out to its tips. During the power phase that chamber gets "thicker" and more closely approximates a semi-circular shape with a convex base, and the combustion event does its best to fill the entire space. As the rotor enters the exhaust phase it turns back into that crescent-shaped sliver again and purges the chamber. but a small amount of unburned fuel that was trapped at the tips of the crescent gets dumped into the exhaust, raising the EGT. A modification that has been successfully implemented by a number of racers, (and something that I've done to my engine as well,) is to ceramic coat the faces of the rotors... this keeps more heat in the combustion chamber, assists the combustion event to more thoroughly burn the fuel, and consequently lowers EGTs. An ancillary benefit is that less heat is transferred to the oil (which provides fully 1/3 of the engine's cooling) so the heat rejection load is lowered as well. It's one of those win-win situations.
As for adding oil to the fuel, it isn't absolutely necessary, as the Mazda rotary is equipped with a separate set of injectors (it would be better to call them "dribblers") that put a very small amount of oil in the way of the spinning rotor, which then picks it up and uses it to keep the apex seals lubricated. The reason that most of the aviation folks (as well as most rotary racers) have chosen to eliminate this device and simply mix 2-stroke oil with their fuel is that it keeps the engine lubricating oil away from the combustion event, which reduces fouling and maintains the crankcase oil at a more constant level. The typical ratio used is about 1oz per gallon, or 128:1. As for it causing an additional failure mode, I suppose it could if the operator failed to add it, but this is another of those checklist items, and once you're aware of the requirement it would take a pretty boneheaded move to not include it during a typical refueling. Some folks have added oil injection, similar to that seen on 2-stroke motorcycles, but that does add another system and an associated failure mode... much easier to simply put it in the tank and add fuel, nothing there to break. Besides, there are folks who recommend adding Marvel Mystery Oil to their fuel tanks for their TSIO-550s for similar reasons. Why they continue to use something that doesn't burn cleanly has always eluded me, although I suspect it's more due to the "appearance" of adding 2-stroke oil thereby making one's airplane appear a bit more closely related to a chain saw than a bona-fide flying machine. MMO has machismo, 2-stroke oil doesn't. Go figure <G>.
<Marv>
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