From the firewall aft, there are few differences between the standard avgas Maule M-7 and the new Diesel M-9. Adapting the diesel does demand different instrumentation and controls and larger fuel filler openings atop the wings to accommodate the larger jet fuel nozzles. The panel includes changeouts of several gauges: manifold pressure, tachometer, oil pressure and temp, cylinder head temps, turbine inlet temp and fuel level (graduated in pounds rather than gallons).
The SMA diesel utilizes an engine control unit with a full FADEC system to maximize performance and efficiency, so in essence, the only operational engine control is the throttle. The ECU automatically maintains rpm at 2,200.
Typical of diesels, engine start requires the pilot to activate the glow plugs, one per cylinder, and initiate the start when the annunciators indicate the plugs are up to temp. Ground operation is conventional Maule M-7, with no surprises, but slightly better over-the-nose visibility by reason of the sloping cowling.
The sensations of takeoff aren’t that different from what you’d experience in a standard Maule, provided you ignore the manifold pressure gauge. Takeoff power is about 90 inches of manifold pressure (yes, we said 90 inches, and no, don’t expect to see any Maules running the pylons at Reno), purely a function of the diesel’s turbocharger.
Climb is similar to that of a standard Maule at sea level, but the diesel airplane gradually pulls away from the avgas model as the ground falls away, owing to the airplane’s turbocharger. Above 5,000 feet, the diesel version will easily leave the avgas Maule behind.
The SMA’s engine control unit automatically manages rpm and mixture, so the only adjustment necessary for cruise is a reduction of manifold pressure, and that’s where the diesel’s advantage becomes most obvious. The almost universal availability of jet fuel is one benefit of the diesel engine, but another is a specific fuel consumption (SFC) that’s only about two-thirds that of the best piston engines.
Using the Maule M-7’s standard O-540 or IO-540 as the logical examples, avgas SFC works out to about 0.48 pounds/hp/hour at full power, 0.44 pounds at 65%. Converted to diesel power, the same airplane scores more like 0.35 pounds/hp/hour at all power settings. In more familiar terms, that means the SMA-powered Maule will cruise along at 75% power on about 8.5 gph, whereas the stock airplane will burn 11 to 12 gph.
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