Alternative Aircraft Engines


Alternative engine fans are always quick to point out how fast and efficient the North American P-51 was during WW2. Being water-cooled, geared and supercharged, it was the fastest production fighter.  So why did the next 60 years of civil aviation go the other way?  The fact is, the next 15 years saw no production US airliner employ water-cooling.  You can only assume engineers saw more disadvantages to their use than air-cooled.  From 1945 till 1960, air-cooled engines ruled the roost, until displaced by turbines.  With Rolls-Royce Merlins and Allisons laying around in crates by the thousands, manufacturers elected to utilize air-cooled radials exclusively in post-war civil aviation .

Most large air-cooled radials employed reduction gearboxes to achieve high horsepower while keeping the tip speeds of the large propellers in an efficient range. By necessity, and not choice, the weight and complexity of gear reduction was incorporated into the last of the radials.  A frequent source of engine failure, the gearbox was heavy and complex. 

So is it any wonder that post-war aeronautical engineers turned to the simplicity and reliability of the direct-drive air-cooled engine for post-war general aviation use? Auto conversion enthusiasts like to poke fun at the 70 year old technology of Lycoming and Continental engines. The fact is these engines matured at a time when massive amounts of water-cooled and gear reduction technology was in favor, and it was shunned for the reliabiliity of what has become the modern light-plane engine.

In every generation of pilots are a few who think they can beat the system by installing car parts in an airplane. Lower costs and less vibration is the mantra.  Since the 20's people have tried to install automotive engines in aircraft, with varying amounts of success.  Weight, complexity and reliability have always been the downfall of every attempt. In the 20's and 30's, it was the Model A Ford. In the 60's, it was the VW and Corvair.  In the 70's, the Pinto motor was the new savior of low-cost aviation.  The 1980's brought not only a belt-drive GM V6 in favor, but even the Porsche factory tried unsucessfully to market their famous "Boxer" engine as an aircraft engine. In the 90's guys were installing Chevy V-8's and Mazda Rotarys. For the new millenium the Subaru and the Mazda seems to be taking the lead in auto conversions.

A company in ******* sells a good looking, professional adaptation of the Subaru engine for aircraft use. Definitely the best attempt to date at a viable aircraft alternative engine, the Subaru suffers from many of the same problems of it's predecessors. While very smooth operationally, it weighs 30-50 pounds more than a Lycoming of comparable horsepower. Having tried to market a 4 cylinder 160 hp engine, it has been discontinued in favor of a 6 cylinder version, seemingly because it was not making advertised horsepower.

The horsepower shortfall of the 4 cylinder Subaru, is in part due to what appears to be too low a ratio of the gearbox.  At 1.82, it turns considerably slower than other designs.  While this limits the amount of horsepower that can be produced by the small 150 cu in dispacement motor, it is not the only cause.

A wet-sump reduction gearbox looses approximately 7% of available horsepower through friction and lubricant drag. Frictional and pumping losses of running an auto engine at nearly twice it's designed operating rpm cause losses on the order of 8%. The largest offender seems to be the cooling drag losses associated with running poorly placed automotive radiators in an airstream 6 times faster than their design application.  Cooling drag accounts for the balance of the efficiency losses that don't seem to be readily recognized by the Subaru community, losses that seem to cause the 4 cylinder Subaru to produce about 110 hp.

Current Subaru conversions utilize only one ignition system. While the electronic ignition system is quite reliable, it goes against the long-standing FAA requirement for 2 spark plugs per cylinder, as well as redundant ignition systems.

Subaru conversions also require constant speed props to compensate for their inate inability to power the prop through the aircraft speed regime.  This should come as no surprise, as modern car engines require 4-6 gears to produce roadability in a car. Why would anybody expect one gear ratio to do the job in an RV? Detractors like to disparage Lycoming's ploddingly low RPM, but the fact is large pistons on a long stroke are what light aircraft require to operate efficiently.

In the new milleniun, the Ford, Corvair and Chevy are out, Subaru is the "engine de jour".  However there is a "new" problem.  The new breed of auto adapters is discovering what designers have long known.  Gear reduction is a black art plagued with gremlins.  Seems the interaction of the power strokes with the prop sets up rather nasty torsional resonant vibrations , causing short service life and the occasional failure.  A Subaru converter recently grounded all of his legacy gearboxes as of Dec 31, 2007 because of safety concerns. 

The other current contender for the most complicated and troublesome aircraft installation is the Mazda rotary.  It not only features the weight and complexities of liquid cooling, but suffers from a loud and very hot exhaust (spelled "low efficiency"). A recent flyoff at Van's between two Mazda powered RV-8's and two Lycoming 8's showed the Mazda aircraft to be at least 25% less fuel efficient.  This engine is so incredibly inefficicient, that many proponents seem to feel compelled to throw in the added weight and complexity of a turbocharger. And to make the turbo work, we must include the complexity of an electrically operated scavange oil pump.

Speaking of electric pumps, all recent auto installations I am aware of require full-time electric high-pressure fuel pumps to operate, as they do in cars. This makes the aircraft highly electrically dependent, unlike certified engines which can run indefinately in the event of a charging system failure.  And who hasn't had an alternator failure in their car or airplane?

So if smoothness is your only design criteria, the auto conversion may be what you are looking for. If your requirements include simplicity, reliability and efficiency, stick with a certified direct-drive, air-cooled aircraft engine. 

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