To hell with Atkinson, Miller, and Otto combustion cycles, along with all of their two-stroke and four-stroke engines. We’re downsizing to a single-stroke configuration today… or at least that’s what we’ve been told by the cats over at INNengine.

Based in Granada, Spain, INNengine is the brainchild of inventor Juan Garrido. A man whose passions mirror his business motto, as he remains: “Committed to bringing revolutionary ideas to combustion engines and transportation.”

But unlike so many other prototypes that fizzle and fade due to being little more than vaporware, INNengine has already been producing its powerplants in limited quantities and testing them. While a “700cc e-REX” motor and a much smaller, “125cc REX-B” variant are listed on the INNengine company profile webpage, it is the 500cc “M500” version that wound up in a Mazda Miata that truly caught our attention.

Weighing little more than 35 kilograms (77 pounds), with a 40cm (15.75 inches) maximum footprint down its longest dimension, this patented “1-stroke engine” powered the Mazda roadster to the tune of 120 horsepower without the assistance of a turbo, blower, or EV battery. Photo Credit: INNengine

Unlike most combustion engines, this design does not rely upon a crankshaft or cylinder heads for operational purposes, but instead relies upon eight opposed pistons that utilize “cam-track” rollers to create power. Sound familiar? We see some of you aviation engine nerds nodding in the back. We also see those of you who are shaking your heads in disapproval and don’t worry, we understand your skepticism.

While there is a lot to admire in these crafty little contraptions, and we recognize that they have indeed been proven to work, there is a lot to be wary of as well. So much still has yet to be answered, both in regard to power production and reliability, as well as maintenance and the matter of mass production. Some of which we will expound upon here in just a sec.

With up to 70% fewer components in its construction than a traditional engine, this motor can run on any kind of fuel, including hydrogen and potentially wood pellets! Photo Credit: INNengine

 The Fateful Eight

On paper, INNengine claims that its engine is a single-stroke design, even though many are calling it a trick 2-stroke motor. The combustion stroke just overlaps with the exhaust stroke and is then proceeded by an intake stroke that is timed to coincide with the compression stroke. To achieve this symbiotic dance, the motor must rely upon direct injection, and keep the oil quarantined beneath the piston so that it can’t be combusted.

With its eight opposed pistons firing efficiently, there is no need to deal with an overhead combustion chamber, and instead, the engine merely reabsorbs the energy as heat. This results in more combustion energy being converted into torque and thus increasing efficiency while reducing emissions.

INNengine insists on calling this engine a “1-stroke” because it has none of the emission issues of a 2-stroke, with zero oil being thrown in the mix, with pressurized lubricated areas being far removed from the combustion chamber. Photo Credit: INNengine

From what we’ve been able to gather from various tech videos and explanations on the INNengine website, it is the combustion side of the detonation process that forces the piston down onto those wave-like cam tracks. As it does so, the piston causes the track to rotate.

For longevity’s sake, those cam-tracks within the motor are made from a specialized form of tempered steel, and being that there are two of them, they both come affixed to a shared center shaft. With all eight pistons pumping and spinning the shaft via their respective tracks, a single source of torque is produced at both ends of the shaft, which in theory could permit the implementation of dual drivetrains, one on each end.

Since this version of the INNengine does not have a cylinder head, nor anything resembling camshafts or valves, there is no need for timing chains or timing belts. This not only allows the engine to be mounted in tight areas, but it also allows you to slap a drivetrain on both ends of the engine.  Photo Credit: INNengine

We Don’t Need No Stinkin’ Cams, Cranks or Valves!

Wait, so how does all of this work without the help of cams or a valvetrain? Apparently, INNengine utilizes a series of intake and exhaust ports that can be opened and closed via a piston stroke, thus providing further support for the argument that this is indeed a fancy, fuel-injected two-stroke engine.

While we’re on the topic of fuel and spark, note that it is within the center of each set of opposed pistons, that the injector and its applied spark plug make the magic happen. Being that the combustion of the air/fuel mixture produces enough pressure to push the piston outward, the intake and exhaust ports are subsequently exposed during each stroke. To achieve a firing order equilibrium, four pistons down one side are assigned intake duty, allowing the other side to handle all things exhaust.

To prevent exhaust gasses from escaping via the intake, a whole lot of scavenging must first occur. As the exhaust port opens, it allows the pressurized gasses to vent outward from the combustion chamber prior to the intake port opening. Being that this entire internal operation is highly pressurized, exhaust fumes are vented almost instantly, leaving little more than a vacuum behind for the next stroke to fill.

Instead of both intake and exhaust valves opening simultaneously, the exhaust side likely opens first to prevent exhaust blowback from occurring within the intake. Photo Credit: INNengine

That said, the vacuum caused by the exhaust ventilation cycle is more than likely quite low in pressure when compared to the intake side of the scenario. Atmospheric pressure is a hell of a thing, and when applied to this kind of engine, can create a very dense intake charge.

This velocity likely plays a crucial role within the INNengine’s cycling process, as it allows the upward-sloped part of the cam-track the nudge it needs to push all of the pistons back into place. This, in turn, closes the intake and exhaust ports, the injectors start spraying fuel into the combustion chamber, and the entire air compression process starts all over again.

The INNengine is a very simple design, in that it does not have cams or valves, but relies upon direct injection. This means zero deposit accumulation on intake or exhaust valves, but all of the benefits of direct injection. Photo Credit: INNengine

The Cynics Have Spoken, and Dammit if We Aren’t Skeptical 

Despite appearing pretty damn brilliant at first glance, and being proven to actually work, the INNengine is still just a prototype. And as with most prototypes, there are more questions and concerns surrounding its design than reassurances.

For instance, what about the load-bearing surface interface between the piston roller and cam plate? Doesn’t this look like a prime area for premature failure? Reciprocating engine components require insane amounts of reinforcement and lubrication, and a single roller definitely isn’t going to cut the kimchi after 100,000-plus miles.

Photo Credit: INNengine

Additionally, this engine doesn’t have a crankshaft, so it can’t take advantage of the lever effect that provides typical piston-powered engines the torque they require. To do so, this motor will need to rev like crazy to make any substantial power, which comes with all manner of reliability risks. Power could be improved with forced induction, but again, that comes with its own dangers.

And while the engine is its self-balancing act, and does not create any vibration, long-term reliability remains a genuine concern, especially if detonation problems or pinging occur. So yeah… I guess we’ll just have to wait and see if this little oddball of an engine has staying power, or if it will fizzle and fade like so many clever automotive advancements that have come before it. 

Photo Credit: INNengine