INNengine e

Jun 09, 2023

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Any oddball engine is clickbait crack for mechanical nerds, and the most compelling ones purport to introduce an entirely new combustion concept. "Not a four-stroke, not a two-stroke, a revolutionary one-stroke engine!" Well, that claim is utter hokum here. The combustion cycle is textbook two-stroke. Can we believe the other claims Granada, Spain-based INNengine is making for its e-Rex "Patented 1 Stroke" engine? Or the claimed output of 120 hp and 180 lb-ft, from a tiny 19-inch-long, 11-inch-tall, 84-pound engine that generates little or no vibration while acing its emissions tests? Here's how the radical new engine concept is designed to work.

This is basically a four-cylinder, opposed-piston engine, similar in basic concept to the EcoMotors OPOC and Achates Power opposed-piston engines. It trades off the challenges and mass of two crankshafts for the increased friction and reduced torque generating nature of a pair of interlinked swashplates (INNengine calls them "cam tracks") capable of delivering power at either end (or both) of the engine.

Each piston completes one back-and-forth motion in half an engine revolution. That means it executes both its compression/combustion, and exhaust/intake strokes twice per revolution, and by INNengine's logic, two-strokes times half a revolution equals "Patented 1Stroke." The name was chosen to distance the INNengine e-Rex from the challenges that have always dogged two-stroke engines.

Instead of connecting rods attaching to a crankshaft, each piston moves up and down on a little piston-shaped cart of sorts. These feature two wide rollers that convey the combustion energy to the cam-track, and a third narrower one that's trapped beneath a smaller diameter cam track that helps finish pulling the pistons apart after the combustion energy has exited the exhaust ports. The company claims the exhaust rushes out with sufficient force to create a vacuum capable of drawing in atmospheric intake charge, though supercharging is also possible.

Each main cam track is keyed to the common output shaft by means of an angled gear. Moving collars that engage these gears can change the timing of the left and right cam track, altering the intake/exhaust timing gap by up to 12.8 degrees, and thereby altering the compression ratio. This would be most useful in applications where the INNengine is used as a primary drive motor, pairing low compression (9.1:1) with super- or turbocharging, and higher compression (16.7:1) with low- or no boost operation for peak efficiency.

The oily blue smoke we associate with old Lawn Boy mowers, dirt-bikes, and Trabants came from mixing lubricating oil with the intake air to lubricate the (usually roller-type) crankshaft bearings. The smoke problem was largely been solved years ago by segregating the crankshaft lubrication from the intake and exhaust systems, and by employing direct fuel injection (this engine does both).

Unlike Orbital two-strokes and others that utilize normal poppet valves, this one still uses cylinder ports near the bottom of each pistons' stroke. That leaves very little time for the spent combustion gasses to exit and the fresh intake charge to enter, which is why turbo- or supercharging is often used. Even without forced induction, there's basically no way to ensure oxygen doesn't exit with the exhaust, rendering the traditional three-way gasoline engine catalytic converter ineffective.

Lean-NOx adsorber "trap" style catalysts like those used on diesels are costly, temperamental, and often require a separate fluid to periodically purge and regenerate them. A proposed solution to the above problem is operating the engine on hydrogen—a fuel that produces no hydrocarbons—and because an opposed-piston engine's combustion chamber remains cooler, far less NOx is generated in the first place. But unless INNengine has figured out some means of lubricating the piston rings that doesn't involve a hydrocarbon-based oil, trace hydrocarbon-based CO2 and other emissions will be emitted even when running on hydrogen.

The above benefits make the INNengine e-Rex engine close to ideal for use as a range-extender. Such applications would likely require neither turbocharging nor variable compression, while the small size and inherent smoothness are ideally suited to the silent nature of an EV.

The INNengine team mounted a 500cc e-Rex engine in an NB-generation Mazda Miata and demonstrated it driving along (sharing no footage of full-throttle acceleration). This one was presumably gas-powered, and although the video claims the 120-hp output the website associates with an atmospheric e-Rex, there's obviously a centrifugal supercharger mounted to the engine.

The sad truth facing the proponents of any engine this far outside the mainstream is that no matter how compelling the technology, the industry is not currently tooled up to make any of it and convincing it to do so with full electrification looming seems exceedingly unlikely.

These applications prize the INNengine e-Rex 1Stroke's superpowers (small, light, smooth) far greater than even a PHEV ever could. Additionally, as long as attention is paid to oiling system pickups, the engine shouldn't care which end is up, making it ideal for aerobatic applications.