A viable alternative to avgas has arrived
If you fly a typical general-aviation airplane, you probably can’t imagine a world without avgas. I fly a Mooney with a four-cylinder, 200 hp Lycoming, and there’s currently no alternative engine available. For me and for thousands of other aircraft owners, the thought of avgas becoming obsolete is simply inconceivable.
Diesels have been on the periphery of aviation for decades. A quartet of 1,100 hp Daimler-Benz diesels powered the Hindenburg, but there’s been little diesel engine development for fixed-wing aircraft. Diesels can burn practically any combustible liquid—obviously diesel fuel, jet fuel, avgas or mogas, and even pure kerosene (though not very efficiently).
The dominant aviation fuel these days is Jet A, and diesels run just fine on it. Several companies have taken the lead in developing diesel engines for aircraft. One of the oldest is France’s SMA, now a subsidiary of the French jet engine conglomerate SNECMA. SMA began developing an aviation diesel engine in 1998 and earned FAA certification for its first product in May 2005.
That first product is the SMA SR305-230, a 230 hp, four-cylinder, turbo diesel powerplant that’s been installed and flight-tested in a totally refurbished 1980 Cessna Skylane. I recently spent a day flying that airplane with Rick Rossner, president of Tule River Aero-Industries in Porterville, Calif. Tule River Aero has been modifying airplanes for seven years, primarily the pressurized Riley Skymaster, so they know what’s involved in a complete engine makeover. In this case, Tule has transformed a fairly pedestrian 182 into the standout, SMA-diesel-powered Cessna you see on these pages.
SMA is currently pursuing STCs for seven aircraft models, including the Piper Dakota and Seneca, Socata Trinidad and Cessna Skyhawk. The company is also developing six-cylinder, 300 to 350 hp models for possible use on large singles and medium twins. (There’s currently an experimental, diesel-powered Duke flying in Europe.)
One look at the Diesel Skylane’s cowl is enough to convince you that there’s something quite different going on forward of the firewall. In fact, everything forward of the firewall is different. The conversion kit has new engine and prop parts and accessories. Even the battery is updated, and the landing light has been removed from the cowling and mounted on the wing, similar to the design on the new-generation Skylanes.
The SMA engine is air- and oil-cooled, and the combination of induction and cooling air demands four large inlets. The three-blade prop is an MT or optional Hartzell specifically designed for the diesel engine; it’s intended to provide maximum torque at the constant 2,200 rpm for climb and cruise.
SMA boasts a parts count that’s roughly 70% less than the comparable O-470U Continental. That should guarantee lower maintenance costs. TBO is currently set at 2,000 hours. Rossner reports that the company is hard at work on a higher TBO, and SMA hopes to have its TBO upgraded to 3,000 hours by the year’s end.
From the firewall aft, there are few differences other than engine instruments and controls and bigger 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 temperature, cylinder head temps, turbine inlet temp and fuel level (graduated in pounds rather than gallons).
The SMA diesel utilizes an engine control unit (ECU) with a full FADEC system to maximize performance and efficiency, so in essence, the only operational engine control is the throttle. There’s a prop control on Hartzell-equipped airplanes, but its sole purpose is to cycle fresh oil into the governor during the run-up prior to takeoff. The ECU automatically maintains rpm at 2,200.