While it is true that automotive engines have evolved considerably in the past 50 years, most of  the development has been towards the ancillary mechanisms and attributes (fuel delivery, emissions, efficiency, etc.), not toward the basic functionality of the engine itself. Recently (last 20 years) and with broadly applied computational analytical and design tools, engine development has produced designs that are better optimized for their applications. Automobile engines are optimized for high peak horsepower and  low average horsepower. A 200 Hp car engine spends it's average life at ~25 Hp (~15% max output). Aircraft engines spend their lives at >65% of their maximum output. Big difference.

The common misconception is that a modern automotive engine with all its advanced fuel and ignition management systems would make an ideal aircraft engine. Unfortunately, engines, like dentures, all look more or less the same but you wouldn't want to use one that wasn't designed for a specific installation. Judging by the numerous failed attempts to bring automotive technology directly to airplanes, one is left with the impression that it is simply a bad idea.  Airplanes need airplane technology.

There are the exceptions that give us hope and the most notable exception is the Teichert diesel. Why has this engine failed where so many others have failed. Its a diesel, and diesel engines are designed, due to high peek combustion pressures, for higher loading. In order to have reasonable fatigue life the engine components must be designed with higher margins and can therefore operate at higher average loads.

Diesel aircraft engines are nothing new, In the late '20s and early '30s the Germans were flying 700 Hp Junkers Jumo opposed piston diesel engines across the Atlantic to South America.

Aircraft rotary engines are noting new either. Curtis Wright had an extensive rotary engine development program in the '70s culminating with the SCORE (Stratified Charge Omnivorous (fuel) Rotary Engine) that would burn almost any hydrocarbon fuel. The design rights to these engines was sold to John Deere in the '80s and JD eventually re-sold the rights.

I have some experience with rotary engines, having built several that competed in endurance racing including the 24 Hours of Daytona. I was the first in IMSA to apply ceramic coatings to the rotor face to reduce thermal conductivity. This engine set the GTU qualifying record but was not run during the race due to concerns over the longevity of the coating and the resulting catastrophic consequences of coating ablation.

Some corrections about rotary engines. They are NOT harder on a power train than piston engines for two reasons, they have a longer power stroke angular duration than the piston and they produce a true sinusoid output. Piston engines, because of the connecting rod, have a nasty little wiggle in the piston acceleration that introduces torsional harmonics into the reduction gearing. Rotaries are more robust in general, primarily due to a the lower part count, but are particularly fragile when pushed beyond their thermal limits. Coolant overheating causes trochoid housing axial shrinkage and oil overheating causes rapid bearing failure. Synthetic lubricating oils, with their higher operating temperatures are a must.  Rotor neutation can occur due to to the asymmetrical torque of the stator gear but this only becomes a problem at high rpms. But I digress....

It's the details, stupid!

There is no fundamental engine configuration that can overcome a poor installation. The vast majority of engine related failures are the result of poor engine installation design and execution, poor maintenance or blatantly ignoring the signs of an imminent mechanical catastrophe. Debating the relative merits of rotary versus piston may be entertaining but is dwarfed by the greater issue of poor application and its friend, poor testing. Most people who attempt their own engine installation are blissfully ignorant of the magnitude of the problem. Many aspiring engine systems designers "think" they know the scope of what they are attempting when they are in fact ignorant of their own ignorance.

Fortunately , when I was building my IV-P in the early '90s I was smart enough to know how much I didn't know and limited my engine experimenting to adapting a Lycoming TIO-540 to the IV-P. I dyno tested the complete engine and performed extensive systems testing prior to first flight.  Since that first flight I have only had to fix a cracked intake runner, cracked turbo bracket, designed a higher capacity dual turbo scavenge pump and performed standard maintenance. Over a thousand hours on the tack and several race wins later the old 540 still has good compression.

"Modern" aircraft engines have been around for a long time. They are simple to install and operate and they are well proven.  Like the DC-3, they were designed in a time where conservative margins were the rule and their longevity in the marketplace is a testimony to their suitability.

I am not arguing against experimenting and development. I am just advising that the challenges are considerable. Those that chose to develop their own installations will be standing on the shoulders of giants... Dead giants.

The best experimental aircraft engine, for me, is an aircraft engine.

Regards
Brent Regan