Some engines are weird because they were built.
The Tri‑Dyne rotary is weird because it never was — and yet it still managed to lodge itself permanently in my brain after I saw a diagram of it in an old Encyclopaedia Britannica. (That article had an excellent diagram, but you can see the basic idea from the cover of the July, 1969 Popular Science magazine — link courtesy of books.google.com — and that cover was the inspiration of the Gemini AI Nano-Banana generated image above — very similar to the diagram in the encyclopedia.)
I remember staring at that diagram way longer than any normal child should stare at a hypothetical combustion chamber; in fact, I once used it in a project in high school for "what I want to be when I grow up," or something like that (the "what" was "automotive engineer" — I did work in the reactor department, mechanical division, onboard a Nimitz-class US Navy Nuclear Aircraft Carrier, but then I switched back to my computer days, and I've been doing that ever since).
The Tri‑Dyne was one of those “future of engines” concepts that lived entirely in beautifully rendered cutaway drawings. It promised all the elegance of a rotary without the Wankel’s triangle‑shaped chaos:
- three oscillating vanes
- three combustion chambers
- three power pulses per revolution
- and theoretically better sealing, efficiency, and sanity
It looked like someone took the Wankel, removed the Dorito, and replaced it with a mechanical flower that opened and closed in perfect synchronization.
It was mesmerizing.
It was elegant.
It was also, as far as I can tell, never actually built — which is probably why the diagrams looked so clean. Real engines leak. Real engines get hot. Real engines warp, seize, and fling metal shrapnel into the nearest wall. The Tri‑Dyne lived in a world where none of that existed. (Actually, to be fair, some of them only threaten to do that — usually the ones that are in vehicles I own — but the Tri‑Dyne never even got the chance; maybe we should rename it "Try-Dyne" since it never actually "did".)
But the idea?
The idea was intoxicating.
It was the kind of engine that made 12‑year‑old me think, “I’m going to build one of these someday,” and adult me think, “Ah. That’s why nobody built one of these.” (Actually, I'd still like to take a stab at building that thing someday... maybe I can get a 3D printer and build a prototype sometime.)
Still, I love it.
It’s the perfect example of the rotary dream: the belief that if we just rearrange geometry cleverly enough, we can escape pistons forever.
Spoiler: we cannot.
But I respect the optimism.
Why the Tri-Dyne Never Made It
After reading the Popular Science article (linked above) and talking it through with Copilot, the reason the Tri‑Dyne never reached production finally clicked. This thing supposedly made 90 hp from 350 cc — roughly 21 cubic inches, or more than 4 hp per ci. As wild as that sounds, two‑stroke race bikes of the late ’60s were already in that neighborhood.
But, for all its brilliance, the Tri-Dyne lived in the narrow space between “ingenious” and “impossible.” On paper, it solved every problem the Wankel struggled with: no apex seals, no eccentric shaft, no rubbing contact, no oil burning, and a power-to-weight ratio that bordered on science fiction. In the lab, it even ran — three prototypes survived long enough for Lycoming and the British Army to take a look. But the same features that made the Tri-Dyne so seductive also made it fundamentally unmanufacturable.
The entire design depends on 0.004-inch clearances between three independently rotating parts, each with its own cavities, lobes, and internal cooling passages. That’s turbine-class precision, but unlike a turbine, the Tri-Dyne’s rotors don’t live in a predictable thermal environment. Combustion chambers move. Pressure spikes vary cycle-to-cycle. The housing is water-cooled, the rotors are oil-cooled, and every surface expands at a different rate. Even a small temperature gradient can close that 0.004-inch gap, and when nothing is supposed to touch, “touching” doesn’t mean wear — it means seizure.
Marshall tried to avoid seals entirely by using labyrinth geometry on the rotor tips and dimpled plates on the flanks. That works beautifully in a gas turbine, where the flow is continuous and the pressure ratios are modest. In a 350 cc internal-combustion engine with violent pressure pulses and constantly shifting chambers, a labyrinth seal becomes a polite suggestion. Leakage goes up, efficiency collapses, and the whole gas-flow choreography — the barrier valve, the communicating pipe, the balance tube, the split-second scavenging event — falls out of sync.
Then there’s the ignition system. At 12,000 rpm, the Tri-Dyne needs 36,000 sparks per minute, which Marshall originally handled with two distributors. Lycoming replaced that with a rotating internal electrode system that fired through the shaft. It worked, technically, but it was the kind of solution that tells you the underlying architecture is fighting you.
And that’s the theme: the Tri-Dyne worked only when everything was perfect — perfect machining, perfect timing, perfect thermal behavior, perfect gas flow. Engines that survive the real world don’t depend on perfection. They depend on forgiveness. The Wankel struggled because its apex seals wore out slowly. The Tri-Dyne would have failed because its clearances would have disappeared suddenly.
Lycoming and the British Army both tested it, and then the trail goes cold. No patents, no prototypes, no follow-up announcements. In the engine world, silence is its own verdict.
The Tri-Dyne wasn’t a bad idea. It was a beautiful one — the kind of elegant, over-clever, wildly optimistic machine that could only come from a single engineer working in a home workshop. It promised the moon, and in a frictionless universe it might have delivered. But in this universe, metal expands, tolerances drift, combustion is chaotic, and engines have to survive more than a test stand. The Tri-Dyne never made it because it asked the real world to behave like a blueprint, and the real world refused.
Here Are Some Other Wild Engines From Across the Years
Some of these were built. Some were prototypes. Some were sketches in a notebook that somehow escaped into the public consciousness. All of them share the same energy: “What if we ignored common sense and tried something fun?”
The Wankel Rotary: The Triangle That Couldn’t Stop Leaking
Let’s start with the classic: the Wankel rotary engine, the only engine in history that looks like it was designed by someone who lost a bet.
Instead of pistons, it uses a spinning Dorito.
Instead of reliability, it uses hope.
Mazda spent decades trying to convince the world that this was the future, and to be fair, the rotary does have some advantages:
- smooth power delivery
- high revs
- compact size
- sounds like a swarm of angry bees trapped in a metal blender
But then there are the disadvantages:
- apex seals (if you know, you know)
- fuel economy that makes V8s look responsible
- emissions that could blot out the sun
- a constant, lingering smell of “something is burning and I don’t know what”
It’s the perfect engine for people who say things like, “I don’t need reliability, I need vibes.”
The Free‑Piston Engine: What If We Just… Didn’t Connect Anything?
Most engines have pistons connected to a crankshaft.
The free‑piston engine said:
“What if we didn’t?”
In a free‑piston design, the pistons just bounce back and forth like caffeinated toddlers on a trampoline. No crankshaft. No connecting rods. No rotational output unless you bolt on some kind of generator or air pump.
It’s basically an engine that refuses to commit.
Engineers love it because it’s efficient.
Mechanics hate it because it’s confusing.
Everyone else hates it because it looks like a physics demo that escaped the lab.
The Opposed‑Piston Engine: Two Pistons, One Hole, Zero Shame
This one is actually clever — two pistons facing each other in the same cylinder, moving inward to compress the charge and outward to exhaust it.
It’s efficient, powerful, and mechanically elegant.
But also:
- it requires two crankshafts
- or one crankshaft and a bunch of gears
- or no crankshaft and a free‑piston setup (see above, chaos ensues)
It’s the engine equivalent of a polyamorous relationship chart: technically functional, but you need a diagram to understand who’s connected to what.
The Napier Deltic: A Love Letter to Over‑Engineering
Imagine three opposed‑piston engines arranged in a triangle.
Now connect them with crankshafts.
Now synchronize all of them with gears.
Now put the whole thing in a train or a torpedo boat and pray.
Congratulations, you’ve built the Napier Deltic, an engine so complicated that even the people who maintained it probably had to lie down afterward.
It made a ton of power.
It sounded incredible.
It also looked like something you’d find in a museum exhibit titled “When Engineers Had Too Much Authority.”
The Bourke Engine: The Eternal Kickstarter Project
The Bourke engine promised:
- insane efficiency
- ultra‑low emissions
- almost no moving parts
- near‑zero friction
- the power of a small sun
What it delivered:
- prototypes
- diagrams
- more prototypes
- more diagrams
- a cult following
- and absolutely no mass‑produced engines
It’s the “perpetual motion machine” of internal combustion — not because it violates physics, but because it violates the part of your brain that says, “This seems practical.”
The Rotating‑Block Engine: When Someone Decided the Block Should Spin Too
Most engines spin a crankshaft. Some spin camshafts. A few spin balance shafts. The rotating‑block engine looked at all of that and said:
“What if we spun the entire block?”
This design — sometimes called a barrel engine or axial rotating‑block engine — arranged pistons radially around a cylindrical block that rotated as a single piece. The pistons moved in and out while the whole barrel spun around the crankshaft axis like a mechanical roulette wheel.
And unlike a lot of the engines on this list, this one was actually built. Multiple times. Early aviation experimented with rotating cylinders. Modern startups built rotating‑block prototypes for generators and compressors. A few of them even ran surprisingly well, assuming your definition of “well” includes “vibrates like a washing machine full of bricks.”
On paper, it offered:
- excellent cooling (the whole block is a fan)
- high power density
- beautiful mechanical symmetry
In practice, it offered:
- gyroscopic forces strong enough to influence steering
- maintenance nightmares
- the constant fear that the entire rotating assembly might decide to leave the building
It’s the kind of engine that makes you think, “This is brilliant,” followed immediately by, “This is why engineers need adult supervision.”
The Gnome Rotary: When the Entire Engine Became the Propeller
If the rotating‑block engine made you nervous, the Gnome rotary will make you question early aviation entirely. This was an engine where the crankshaft stayed still and the entire engine spun around it, cylinders and all, bolted directly to the propeller.
Yes, really.
Early aircraft needed lots of power with very little weight, and someone at Gnome apparently said, “What if we just spin the whole thing?” And then they did. The result was an engine that acted like a giant mechanical flywheel strapped to the front of a canvas biplane.
On paper, it offered:
- fantastic cooling (the cylinders are literally windmilling themselves)
- excellent power‑to‑weight ratio
- simple construction
In practice, it offered:
- gyroscopic forces so strong they affected turning direction
- fuel consumption that bordered on performance art
- castor‑oil lubrication that pilots inhaled whether they wanted to or not
- the constant fear that the engine might decide to become a centrifuge grenade
And yet, it powered some of the most iconic early aircraft — the Sopwith Camel, the Fokker Eindecker, and basically any plane that looked like it was made of canvas, piano wire, and optimism.
It’s one of the few engines in history where the correct maintenance advice was probably, “Don’t stand too close.”
The Gerotor: The Weird Rotary Mechanism That Actually Worked
Not technically an engine, but it absolutely deserves a spot in the Weird Rotary Hall of Fame. A gerotor is a beautifully simple internal‑gear pump made of two oddly shaped gears:
- an inner gear with n lobes
- an outer gear with n+1 lobes
The inner gear spins inside the outer one, slightly off‑center, creating expanding and contracting chambers that pump fluid with surprising smoothness. It looks like a Wankel rotor and a fidget spinner had a child who grew up to be a hydraulic engineer.
Unlike many of the engines on this list, the gerotor wasn’t a prototype or a fever dream — it became wildly successful. You’ll find it in:
- engine oil pumps
- hydraulic motors
- superchargers
- industrial pumps
It’s elegant, compact, efficient, and almost impossible to kill. In a post full of engines that barely worked or shouldn’t have worked, the gerotor is the rare rotary idea that actually delivered.
Sometimes the weird ones win.
The Chrysler Turbine Car: When Your Daily Driver Was Basically a Jet
On paper, the Chrysler Turbine Car sounds like a prank: a family car powered by a gas turbine that could run on almost anything vaguely flammable. Gasoline, diesel, kerosene, tequila, perfume, peanut oil — if it poured, they probably tried it.
Chrysler actually built a small fleet of these and handed them out to regular people as part of a public test program. Imagine your neighbor borrowing your car and coming back with, “Yeah, I topped it off with leftover cooking oil and some aftershave. Seems fine.”
On paper, it offered:
- smooth, vibration‑free operation
- multi‑fuel capability
- a sound somewhere between “jet spool‑up” and “angry vacuum cleaner”
In practice, it offered:
- terrible fuel economy at low speeds
- slow throttle response (turbines do not “blip”)
- emissions that regulators did not find charming
It was one of the rare weird engines that actually made it into people’s driveways — and then, just as quickly, disappeared back into the museum wing labeled “We Probably Couldn’t Get Away With This Now.”
The Swashplate Engine: The Mechanical Octopus With a Tilted Heart
The swashplate engine is what you get when you look at a normal crankshaft and say, “Too linear. Needs more chaos.” Instead of pistons pushing on a crank, they push on a tilted, rotating plate — a swashplate — that wobbles its way into rotational motion.
Picture a bunch of pistons arranged in a circle, all poking at a slanted disc that spins like a drunk vinyl record. It’s compact, it’s clever, and it looks like a mechanical octopus trying to escape.
On paper, it offered:
- high power density
- fewer major rotating parts
- a very satisfying animation in engineering textbooks
In practice, it offered:
- complex bearing loads
- weird wear patterns
- the kind of geometry that makes machinists sigh heavily
It’s one of those designs that makes you think, “This is brilliant,” followed quickly by, “I do not want to be the one maintaining it.”
The Eisenhuth Compound: Three Cylinders, One Big Idea, Questionable Results
Back in the early 1900s, when internal combustion was still in its “throw ideas at the wall and see what explodes” phase, the Eisenhuth Compound engine showed up with a bold plan. It used two conventional cylinders whose exhaust was routed into a third, larger “atmospheric” cylinder to squeeze a little more work out of the leftover gases.
In theory, it was an early attempt at energy recovery — a sort of proto‑turbocharging without the turbo. In practice, it was more like three engines duct‑taped together and told to share.
On paper, it offered:
- better efficiency from the same fuel
- more power without a bigger primary engine
- a very impressive sales pitch
In practice, it offered:
- complex plumbing
- more parts to break
- the realization that sometimes “good enough” is better than “clever”
It’s a perfect example of that era’s optimism: if two cylinders are good, three interconnected ones must be better… right?
The Ford No‑Crankshaft Tractor Engine: Two Pistons, One Cylinder, and a Turbine Walk Into a Bar…
Then there’s the experimental Ford engine that feels like it was designed on a dare. As beautifully documented by The Autopian, this thing had:
- no crankshaft
- two pistons sharing one cylinder
- a turbine to extract power
- and it was mounted in a tractor, because of course it was
It was basically a free‑piston engine feeding a turbine, wrapped in agricultural sheetmetal. The pistons bounced back and forth, compressing and combusting, and the resulting gas flow spun a turbine instead of turning a crank.
On paper, it offered:
- high efficiency
- fewer traditional moving parts
- a great way to confuse anyone looking under the hood
In practice, it offered:
- control challenges (free pistons are not known for their chill)
- complex starting and load management
- a level of experimental weirdness that probably scared the product planners
It’s the kind of engine that makes perfect sense in a lab, looks amazing in a cutaway, and then quietly disappears when someone asks, “So how do we explain this to farmers?”
The Real Question: Why Do We Love These?
Because weird engines are the purest form of human optimism.
They say:
- “What if we tried something different?”
- “What if the rules are optional?”
- “What if triangles are pistons?”
- “What if we removed the crankshaft and just… believed?”
Most of these engines didn’t win.
Some barely worked.
A few were brilliant but doomed by timing, emissions laws, or the fact that they required a maintenance schedule written by NASA.
But they all share one thing:
they’re honest attempts to make the world more interesting.
And in a universe full of boring, efficient, reliable engines that do exactly what they’re supposed to, I’ll always have a soft spot for the ones that tried something beautifully stupid.
And if all this mechanical chaos has you wondering what kind of weird engine I would design, well… I did (at least in my head). It’s a two‑stroke diesel supercharged scavenging genset meant for a series hybrid, and it’s every bit as questionable as it sounds. Here’s the deep dive.
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