Is the Future Here?

For ages one of the most common laments in aviation is we’re still flying around behind ancient engines. LyConsaurs they’re called, and when in the world are we going to move on to something designed after the demise of vacuum tube computing?

Maybe, I’m beginning to think, we already have.

No, I haven’t been smoking a strangely long pipe or just returned from a self-realization retreat, but I have been doing something eerily similar: writing the Engine Buyer’s Guide found elsewhere in this version of the KITPLANES® thicket. At times compiling such a story does approach a hallucinatory exercise, what with all the numbers glazing over. But in those moments of clarity it became obvious there are many more than just the familiar horizontally opposed powerplants in play. Even after discounting fringe players—the jets and rotaries—there are many more engines in the Buyer’s Guide than what we’re used to seeing at the typical Cessna-Piper-RV airport. Down at the flugplatz it’s nothing but the familiar air-cooled Flat Earth Society, with maybe the odd radial classic and a few Rotaxes. Meanwhile, in the Buyer’s Guide there are inlines and vees along with plenty of liquid-cooled options.

What gives?

Seems while we weren’t looking, a new breed of engines arrived. These engines are hardly in mass circulation, but they do exist, they are being fitted to airframes, they are flying and a core group of pilots are getting friendly with them. You could also say they’re modern and, depending on your Nosferatu quotient, you might postulate they are some large portion of the future.

We could start by defining “modern,” albeit in massively truncated terms suitable to this short column. Skipping the chalk talk as to why, two important characteristics of a modern airplane engine are a combustion chamber (and ports, valves and other breathing parts) designed in, oh, the 1990s or later and, to a lesser extent, liquid (water) cooling. The combustion chamber—essentially the cylinder head and piston crown—is important because it is key to establishing the engine’s need for anti-knock capability from the fuel, along with setting fuel efficiency and power output fundamentals.

Incredible knowledge about what goes on in combustion chambers has been gained by the auto industry since the 1970s. Driven by the demand for power, fuel economy and especially to reduce emissions, precisely what transpires to the air/fuel charge as it enters and responds to piston motion is today a universe ahead of what legacy aircraft engine designers gave us. Ironically, much of this fundamental combustion theory was pioneered during the big engine peak of WW-II when empires depended on it. But the turbojet ended serious piston engine development for 30 years until the auto industry took up the cause. Today, piston engine progress has come from carmakers—at least when they’ve not been distracted by electric motors and fuel cells.

As for water cooling, we could debate to a bloody draw which is better for a flying engine. Both air and water cooling have great benefits, but as engines get more efficient, and thus inevitably smaller displacement, the advantage tips to water cooling. Water cooling’s denser medium and greater thermal stability is the practical winner as power climbs. Ask Porsche, a company steeped in air cooling, which made the transition to water cooling decades ago. Or look at Rotax, which goes to the trouble to water cool the cylinder heads while leaving the cylinders air cooled.

There’s also an even more fundamental discussion regarding if smaller, higher-rpm modern engines are all that better than big, slow-turning direct drives, but we can share that beer another afternoon.

For now we can expect more contemporary general aviation engines to have sophisticated combustion chambers and possibly water cooling. To these mechanical characteristics we can add digital control of the fuel and spark as they can maximize the benefits of a good combustion chamber. So far, such engines have come from two sources: all-new aviation engines or automobile conversions. The all-new engines are in their own money class, one we mere mortals need not worry about because of the incredibly high costs. Examples are the Jet A-burning Continentals derived from Mercedes diesel car engines, the R.E.D. V-12 and EPS Graflight V-8 (wherever it may be). Fabulous engines all, but too far over the financial horizon for enthusiast flying. Look for them in military drones and business aircraft long before they ever trickle down to chasing pancakes.

What About Auto Conversions?

A smattering of domestic V-8 efforts have made or are making the rounds. A generation ago Geschwender-prepped Chevys and Fords successfully powered ag applicators; more recently Robinson turned Chevy’s LS series into a fine seaplane engine, AutoPSRU’s is zeroing in on RV-10 Chevy applications and Corsair Engine Technologies has been getting noticed for their Chevy-into-a-Cessna-172 experimentation.

Smaller auto engines are found all over the sectional but from our Buyer’s Guide, AeroMomentum and Viking are established firms putting forth some sort of auto engine.

All of these engines utilize a propeller speed reduction unit (PSRU), or gearbox for short. Now, gearboxes are a whole ’nother ball of engineering and manufacturing wax, but they are not impossible. Closer design and manufacturing tolerances and smoother (less misfiring) engines as found in cars contribute to successful PSRUs. Such gearboxes seem a necessity when working with inherently smaller displacement, higher-rpm engines.

Of course the new crop of engines have their share of questions. Cost alternates between a big advantage or an insurmountable problem. Minuscule production runs don’t build faith in reliability. Propeller, gearbox and long-term vibration effects are a few of the unknowns, and the hobby financial profile of some of the cottage companies involved may not engender confidence. But the fact is a measurable number of these engines are being sold and flown, especially in smaller aircraft. Fleet hours for these engines, still vanishingly small compared to the established aero engines, are slowly accumulating and so far we haven’t heard of many problems.

In short, there’s not much to suggest the latest round of auto conversions are failures. Time is the only measure, but so far the incentives of lower purchasing, running and overhaul costs are enticing the sportier, more Experimental builders and it seems to be working for them.

At the same time the dream of purpose-built, clean-sheet-design aircraft engines is proving fiscally challenging outside of the market stratosphere. The R.E.D. V-12 is likely the best example. It has successfully provided 500 hp to a good number of deep-pocket prototypes and proof-of-concept aircraft, along with repowering Yak and other certified airframes. But all of these have been initiatives priced well above mainstream Experimentals such as the typical RV builder might squeeze together. The Jet A Continentals have likewise flown just fine but are simply too expensive for regular people to buy. And unfortunately for Continental, this was also the case with their water-cooled Voyager series back in the ’80s. In short, the next generation of advanced aero engines for sport flying is not coming from Williamsport or Mobile.

The main issue, as always, has been scale. There just aren’t enough aircraft piston engine sales to justify what it costs to design, produce and—the big one—certify them today. It’s been that way for many years and is only getting worse as the GA fleet stagnates and the U.S. industrial base trudges along somewhere between off- and on-shoring itself.

Which points to auto conversions as the one possible source of modern engines for sport—and dare we say—general aviation aircraft. A huge problem is that we haven’t delivered much technical progress for decades, so now we must leap a chasm to make a meaningful move. Yes, the engines already exist from the auto industry. But the PSRU and other bits to adapt them to aircraft—the parts supply, maintenance and overhaul capabilities, not to mention the market and regulatory/legal acceptance required to bring any source of new engines to the general aviation mainstream—is asking a lot.

Then there is integrating new engine shapes and water cooling into what has been a horizontally opposed, air-cooled world for ages. At least that is something we Experimental airplane builders can have some fun doing.

But for now, there is movement to more contemporary engines by builders of the lighter Experimental aircraft. Fascinatingly, price seems the main driver as the repurposed auto engines often show a cost advantage. That’s something helped by the recent big increases in legacy airplane engine pricing as well as something to consider for your next build or repowering.