Here's how long-time Slashdot reader Baron_Yam summarizes a radically new tiny-but-powerful "opposed-piston engine" created by INNengine of Granada, Spain. "500cc, 120 horsepower, under 40 kilograms (85 pounds). No cylinder head in the motor, no camshaft, no crankshaft, no valves, and no oil mixed in with the fuel."

The company calls it "a single-stroke combustion cycle," though the engine itself still has a compression stroke and an exhaust stroke, reports The Drive:
Despite having four cylinder banks, the INNengine (depending on its configuration) actually has eight pistons. This is because the engine is an opposed-piston motor, meaning that each piston's compression stroke is performed against a second piston placed in the same cylinder bank rather than a static cylinder head. It still only has four combustion chambers, though, which means it sounds similar to a four-cylinder engine... The mechanical configuration also allows for better engine balance. That means typical drawbacks of an internal combustion motor (often referred to as noise, vibration, and harshness) are minimalized. Once combustion happens, the piston is pushed back against the plate and forces the plate to rotate. This motion is synced between each half of the motor via a shared shaft — meaning, no extra timing components...

Is it likely that we'll see INNengine's combustion tech powering the wheels of a car? Probably not, at least not directly hooked up to a gearbox. The Mazda featured in INNengine's demo video was a great concept, but the company seems to be instead targeting the EV market as a range extender, especially since that's the way the industry is ultimately headed.

If the tech had debuted a few decades ago or more, perhaps there would have been a chance of adoption in the main market (cue Felix Wankel's notorious rotary). But messing with perfection in this day and age, especially as combustion tech could be on the way out, seems a bit unlikely to take off. That's why a range extender would appear to be the most logical path forward for this tech, especially if we want more lightweight, cost-effective EVs.