This is the D-Motor LF26, a 2.7 liter (they seem to have trouble with displacements corresponding to model numbers, the first-gen LF26 was a 2.5 liter, and the upcoming third-gen will be a 2.9 liter) flat-4 aero engine.

It’s aimed at light sport aircraft, making 91.8 hp @ 3000 max power, 88.8 hp @ 2800 max continuous power, on just about whatever fuel you can throw at it. Their biggest competition would be the Rotax 912 family, a 1.2 or 1.35 liter engine with watercooled heads, aircooled cylinders, at 80 or 100 hp @ 5800 depending on displacement - note that the higher RPM of the Rotax requires reduction gearing for the propeller. The Rotax is lighter in carbureted versions, but the D-Motor engine is fuel-injected, and the fuel-injected Rotax 912 iS (also 100 hp @ 5800) is heavier.

There’s also a flat-6 variant that’s under development - basically the same thing, but two more cylinders.

There’s a couple things, though, that are odd about this one. First off, note that there’s no cooling fins - this one’s entirely watercooled. Typically, airplane engines are aircooled for weight and simplicity reasons (and the Rotax 912 family only watercools the heads), but there are reasons to watercool - better capability to handle varying power levels without having to worry about thermal shock, for one. And, if you could get watercooling in, that can help stabilize the combustion behavior.

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But, that’s not what’s really weird - the really weird part is that it’s a flathead - aviation engines are usually overhead valve with pushrods. Apparently, they decided that the low RPM operation meant that the negatives of a flathead (poor breathing, poor combustion chamber shape) were reduced, and it greatly reduced weight and size. Reduced weight meant that there was the ability to watercool it without excessive weight, getting the benefits of watercooling (and in turn reducing more of the flathead’s negatives), and reduced size meant that they could get more displacement in a small enough package to fit into tiny LSAs (enabling decent power at low RPM).

Fuel consumption looks like it’s a bit higher than the Rotax 912 iS, but significantly lower than the carbureted 912s. (And, simplicity is sometimes worth spending some fuel on to ensure reliability - look at all the huge displacement pushrod aircooled flat-4s in the aero space.) Looks like price is right in between the carbed and the injected Rotax engines. It’s certainly interesting to see a new flathead existing in 2017, though, that’s for sure.

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Now, I should probably work on that state of flat-4s in cars article...