For more than a hundred years, the internal combustion engine has been refined, optimized, turbocharged, hybridized, and electronically managed, but never truly questioned. We kept the same basic geometry and simply piled solutions on top of it. Valves became variable. Crankshafts got lighter. Balance shafts multiplied. Software grew smarter to compensate for mechanical compromise.
And then a small Spanish engineering company quietly asked an uncomfortable question:
What if the crankshaft itself is the problem?
That question led to the INNengine e-Rex, an engine marketed, controversially, as a one-stroke engine. The name sounds like marketing nonsense at first glance, and in a strict thermodynamic sense, it is. But dismissing it on that basis would miss something far more interesting. Behind the branding is a genuinely radical rethinking of how combustion force is converted into motion, and why internal combustion engines have been inefficient, heavy, and vibration-prone for so long.
This is not a story about breaking the laws of physics. It is a story about rearranging them.
Why the “One-Stroke” name exists at all
To understand why INNengine even uses the term one-stroke, you have to forget definitions for a moment and think in terms of time and torque delivery.
A conventional four-stroke engine produces one power event every two crankshaft revolutions. A two-stroke engine produces one power event per revolution. The e-Rex, in its current configuration, produces four power events per revolution. Not because it invented a new thermodynamic cycle, but because its geometry allows multiple combustion events to overlap in a way that feels unusually continuous.
From the driver’s seat, or from a generator’s perspective, the result is torque delivery that feels closer to a smooth turbine than a pulsing piston engine. That sensation is what INNengine is branding as “one-stroke.” It’s not academically correct, but it communicates the effect better than the chemistry.
Crucially, this engine avoids the fatal flaw that killed classic two-strokes. It does not mix oil with fuel. Lubrication is fully separated, emissions are tightly controlled, and combustion is managed via modern direct injection. This is not nostalgia. It is a clean-sheet design.
The moment everything changes: Removing the crankshaft
Every internal combustion engine you’ve ever seen relies on a crankshaft. It’s so fundamental that we rarely question it. Pistons move up and down, connecting rods push at an angle, and the crankshaft turns that chaos into rotation.
But that angled push is the source of one of the engine’s greatest inefficiencies: side-loading. As combustion pressure builds, the piston is forced sideways into the cylinder wall, creating friction, heat, and wear. Modern engines spend enormous effort minimizing the damage caused by this geometry, without ever eliminating it.
While Porsche’s patented W12 engine layout reimagines cylinder packaging within traditional architecture, INNengine goes further, questioning whether the crankshaft makes sense.
INNengine’s solution was not to improve the crankshaft. It was to delete it.
In its place is a sinusoidal axial cam, often described as a wave plate. Instead of pistons pushing on rods, each piston carries a roller that rides along this wave-shaped track. When combustion occurs, the pistons move linearly and push directly against the slopes of the cam. That interaction forces the cam and the central shaft to rotate.
What is important here is alignment. The force of combustion stays almost perfectly in line with the piston’s movement, the side-loading largely disappears, the friction drops, and heat losses fall, which means that the engine does not need to fight itself to stay alive. This single geometric change explains most of what makes the e-Rex feel different.
Opposed pistons and the death of the cylinder head
The e-Rex does not have cylinder heads. At all.
Instead, each cylinder contains two pistons facing one another. When the air-fuel mixture ignites, the pistons are pushed apart. No exhaust valves are glowing red-hot, no overhead camshafts, no valvetrain mass slamming open and shut thousands of times per minute.
Intake and exhaust are handled through ports in the cylinder walls, timed precisely by piston position. This design has existed before, especially in diesel engines, but pairing it with an axial cam drive changes everything about how smooth and compact the engine can be.
Because the pistons move in opposite directions with equal mass, their forces cancel out. The engine is inherently balanced. No balance shafts, no counterweights, and no elaborate engine mounts trying to hide vibration that shouldn’t exist in the first place.
This matters far more than it sounds like it does.
A small engine that behaves like a big one
On paper, the e-Rex looks like a physics prank. The current 700cc prototype weighs around 38 kilograms (84 pounds) and targets an output of 142 horsepower with roughly 180 lb-ft of torque. Those are numbers you would normally associate with a naturally aspirated two-liter engine, except that engine would weigh three times as much and occupy far more space.
But the real advantage is not peak power. It is power density and packaging freedom.
Because the e-Rex is short, cylindrical, and compact, it can be mounted low and far back in a chassis. INNengine proved this by installing one into a first-generation Mazda Miata. The engine sat so low and rearward that it fundamentally altered the car’s center of gravity, freeing space and improving balance in ways traditional engines simply can’t.
This is where the e-Rex starts to make sense as more than an engineering curiosity.
Variable compression without mechanical gymnastics
One of the quiet triumphs of the e-Rex is how it handles variable compression ratio. Most attempts at VCR rely on intricate linkages or movable engine structures, solutions that add weight, complexity, and long-term durability concerns.
INNengine’s approach is almost annoyingly elegant.
The engine uses two axial cams, one at each end. By rotating them slightly out of phase using an ECU-controlled actuator, the engine changes how close the opposed pistons come to one another at peak compression. No additional valvetrain and no exotic linkages, just geometry.
Radical geometry is not new to engine development; Ferrari’s experimental oval piston V12 design attempted to bend conventional piston logic similarly, stretching the definition of what a “cylinder” even means.
This allows the engine to raise compression during light-load cruising for efficiency, then lower it under heavy load to prevent knock. More importantly, it allows the same engine to adapt to different fuels, including gasoline, hydrogen, and synthetic e-fuels, without fundamental redesign.
Why this engine makes sense as a range extender
If you are waiting for the e-Rex to power the next generation of sports cars, you may be disappointed. That is not where it fits best.
Where it does fit almost perfectly is as an EV range extender.
In that role, an engine does not need a wide RPM range or dramatic throttle response. It needs to run smoothly, efficiently, and unobtrusively at a steady operating point. Vibration becomes the enemy. Noise becomes the enemy. Packaging becomes critical.
The e-Rex excels precisely where conventional engines struggle. Its inherent balance means you do not feel it start, its compact size means it does not dominate the vehicle layout, and its efficiency potential means it can sip fuel while keeping a battery charged.
This is internal combustion, not as a drivetrain, but as an appliance.
Hydrogen, e-Fuels, and the long game
INNengine is working with the University of Valencia to optimize the e-Rex for hydrogen combustion. That might sound like a footnote, but it is a logical extension of the design.
Hydrogen burns hot and fast, and exhaust valves hate it. Not only does the e-Rex not have any exhaust valves, but the compression can be adjusted dynamically, and the combustion can be precisely controlled. Pair that with synthetic fuels, and suddenly internal combustion looks less like a dead end and more like a specialized tool.
This does not mean hydrogen ICEs will dominate the future, but it means the conversation is not over yet.
The catch (because there is always one)
If the e-Rex seems too good to be true, that is because adoption is about more than physics.
Meeting Euro 7 emissions standards is brutally difficult, especially for unconventional architectures. Manufacturing axial cam systems at scale demands extreme precision. The automotive industry is risk-averse at a moment when most investment is flowing into batteries, not pistons.
INNengine has reached Technology Readiness Level 6, which means working prototypes exist in real-world conditions, but mass production is still a climb.
This is not vaporware. But it is not a revolution that happens overnight.
Why the e-Rex still matters
The INNengine e-Rex does not save the internal combustion engine. It redefines its role.
Instead of being a massive, vibrating, multi-purpose compromise, combustion becomes small, smooth, specialized, and efficient. It complements electric drivetrains instead of fighting them, and instead of growing more complex, it becomes simpler by design.
For enthusiasts, it is proof that mechanical innovation is not dead. For engineers, it is a reminder that sometimes the biggest gains do not come from better software or tighter tolerances, but from questioning assumptions so old we stopped seeing them.
And for internal combustion, it may be the most compelling argument yet for a future that is quieter, cleaner, and still very much alive.
