The Weird Engine I Can’t Stop Thinking About
A conceptual engine that lives somewhere between clever and questionable.
After spending way too much time writing about other people’s strange engines, I realized something dangerous: I have one of my own. It’s been rattling around in my head for years — a design that sits somewhere between “this might actually work” and “this is how you end up on a watchlist for experimental combustion.”
So here it is: my personal contribution to the Weird Engine Hall of Fame.
The Concept in One Sentence
A two‑stroke, diesel, supercharged, scavenged, constant‑speed engine designed solely as a range‑extender generator for a series plug‑in hybrid.
In other words: a tiny, angry, extremely efficient metal box whose only job is to sit in its happy place and make electricity.
Why Two‑Stroke Diesel?
Because two‑stroke diesels are little monsters of efficiency and power density. They fire every revolution, not every other one. They don’t waste time on strokes that don’t produce work. They’re compact, torquey, and brutally effective when you can control their environment.
The problem, historically, has been emissions. Traditional two‑strokes tend to:
- short‑circuit fresh air straight out the exhaust
- leave behind unburned hydrocarbons
- mix oil with the charge (in spark‑ignited designs)
- make EPA officials break out in hives
But that’s where the rest of the design comes in.
Supercharging + Scavenging: The Cleanup Crew
The idea is simple: use a positive‑displacement supercharger (think Roots, Lysholm, or even an electric blower) to force fresh air into the cylinder during the scavenging phase. Meanwhile, the exhaust port is still open — so the incoming air pushes the burned gases out.
Done right, this gives you:
- cleaner combustion (less leftover exhaust)
- lower emissions (less short‑circuiting)
- better cylinder filling (more oxygen = more efficient burn)
- cooler combustion temperatures (goodbye NOx spikes)
It’s basically the two‑stroke version of “open the windows and turn on a fan.”
And since this is a diesel, there’s no fuel‑air mixture to lose out the exhaust — just air. Fuel only arrives when the ports are closed and the piston is on its way up.
Why It Works Best as a Range‑Extender
Here’s the trick: this engine is not meant to drive wheels. It’s not meant to idle. It’s not meant to rev. It’s not meant to respond to your right foot at all.
It has one job:
Run at its single most efficient RPM and load, forever, while turning a generator.
That’s it. No throttle swings. No transient emissions. No lugging. No screaming. Just a steady, optimized hum at the exact point where the BSFC chart hits its lowest valley.
This is where weird engines shine — when you don’t ask them to do anything except the one thing they’re good at.
Why Constant‑Speed Operation Solves Everything
Running at a fixed RPM means:
- Emissions control becomes trivial. Aftertreatment loves steady flow.
- Noise control becomes easy. You can tune the entire exhaust for one frequency.
- Vibration is predictable. Balance shafts, mounts, and dampers can be optimized.
- Fueling is simple. No transient compensation, no throttle lag.
- Supercharger load is constant. No boost spikes or dips.
It’s like building a tiny power plant instead of a car engine.
Potential Layout
Here’s how I picture it:
- Single‑cylinder or twin‑cylinder two‑stroke diesel
- Uniflow scavenging (ports at the bottom, poppet exhaust valve at the top)
- Roots or screw supercharger for scavenging air
- Small turbocharger for energy recovery (optional but fun)
- Common‑rail direct injection for precise fueling
- Water‑cooled for packaging and noise control
- Integrated generator on the crankshaft
Think of it as a baby version of the massive marine two‑stroke diesels — except instead of pushing a container ship, it’s quietly feeding electrons to a battery pack.
Why This Might Actually Work
Because the problems that killed two‑stroke diesels in cars don’t matter here:
- Emissions? Solved by constant speed + clean scavenging + aftertreatment.
- Noise? It’s in a hybrid enclosure with insulation.
- Throttle response? Doesn’t matter — it never changes speed.
- Oil consumption? Modern lubrication + uniflow design fixes most of it.
- Complexity? It’s simpler than a four‑stroke with turbos and VVT.
And the payoff is huge:
- High efficiency at the exact point you care about
- Small size compared to a four‑stroke diesel
- High power density for quick battery charging
- Fewer moving parts than a conventional engine
It’s weird, but it’s the right kind of weird.
Would I Build It?
Honestly? Yes. If I had a 3D printer, a small lathe, and a garage with good ventilation, I’d be elbows‑deep in aluminum chips by now.
It’s the kind of engine that makes you think, “This shouldn’t work,” followed immediately by, “But what if it does?”
And that’s the whole point of weird engines: they’re optimism made mechanical.
Final Thought
This isn’t a proposal for mass production. It’s not a startup pitch. It’s just a fun, slightly unhinged idea that lives in the same mental neighborhood as the Tri‑Dyne, the Deltic, the Gnome rotary, and all the other beautifully impractical machines humans have dreamed up.
But if someone handed me a prototype tomorrow?
I’d absolutely fire it up.
Bonus Thought: Two Even Weirder Variants (Plus One Truly Unhinged One)
If I ever wanted to make this little range‑extender even stranger — and arguably better — there are a few alternate layouts that keep sneaking into my brain.
1. The Opposed‑Piston Version
This is the “what if we just removed the cylinder head entirely?” option. Two pistons, two crankshafts, perfect uniflow scavenging, no valves, no cams, no head gasket, and a longer effective expansion stroke. It’s cleaner, simpler, and thermally happier than the single‑piston layout.
For a constant‑speed genset, the usual synchronization headaches basically disappear — you phase the cranks once, lock them in, and let the whole thing hum forever at its happiest RPM.
2. The Free‑Piston Version
This is the “delete the crankshaft” option. A free‑piston two‑stroke diesel uses a linear alternator wrapped around the piston’s motion, so the piston itself becomes the generator. No rotating assembly, no bearings to baby, no phasing gears — just a controlled bounce between two combustion chambers.
Normally, free‑piston engines are hard to control. But a range‑extender that only ever runs at one power level is the easiest possible use case. Set the spring and damping characteristics, tune the injection timing, and let physics do the rest.
3. The Micro‑Turbine Version
This one isn’t a free‑piston engine at all — it’s the “let the exhaust do the work” variant. Instead of a piston‑mounted alternator, the combustion gases drive a tiny turbine that spins a generator. Think miniature APU: simple, compact, and happiest at a single RPM.
It’s less efficient than the piston versions, but it’s incredibly smooth, has almost no vibration, and sounds like you’ve hidden a small jet engine under the floor.
Would any of these be sensible to build? Probably not. Would they be incredible? Absolutely.
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