Four spark plugs, eight pistons, one stroke: how the INNengine E-REX promises to be different – Jalopnik

You’ve seen a lot of weird engines on Jalopnik, from Trojan’s strange two-stroke four-cylinder cube with wishbone connecting rods to the Napier Deltic triangle-18 diesel, but you’ve never encountered a single-stroke engine. Technically, you still won’t even after reading about the INNengine e-Rex, but we’ll get to that in a moment.

The company INNengine, based in Granada, Spain, has created two adorably small, strange little engines: the e-Rex and the Rex-B. Inside the e-Rex you’ll find a situation similar to the aforementioned Napier Deltic, in that it has twice as many pistons (eight) as cylinders (four). These pistons move towards each other to create compression and, as they move away from each other, they reveal ports in the cylinder walls for intake and exhaust. If this looks suspiciously like a two-stroke engine, then yes, don’t get ahead of yourself.

Each set of pistons sits on a rotating disk in the shape of a kind of wavy floor of a Tilt-A-Whirl. Car and driver legend and quantum-brained engineer Csaba Csere calls this thing a pressure plate, so I’ll do that too, although ‘axial cams’ are apparently acceptable too. While the e-Rex uses two of these plates for two sets of pistons, the Rex-B uses a single plate and a single set of four pistons in four fixed-head cylinders.

Now about INNengine’s patented “1Stroke” technology. This engine is not a one-stroke engine. It tackles combustion and exhaust in one stroke, then intake and compression in another stroke. For the math enthusiasts among us, you’ll notice that this equates to two strokes. The reason INNengine calls this two-stroke engine a single-stroke engine, or ‘1Stroke’, is that the company wants to distance this design from the traditional stigma of dirty two-stroke engines that burn oil along with the fuel, which the e-Rex and Rex-B do not do.

And the two pressure plates can move out of phase with each other to change where the pistons are in relation to each other. In other words, it can change compression ratios on the fly without the complexity of Nissan’s fault-prone variable-compression engine.

At 500 cubic inches and weighing 85 pounds, this engine is minuscule (the company is working on a 700cc version), but INNengine claims it delivers 120 horsepower at atmospheric pressure. For testing purposes, INNengine plugged an e-Rex into a Mazda Miata. It sounds pretty neat:

If you read through the comments you will see that many people write that there is clearly a supercharger, so saying that the e-Rex makes 120 hp purely through natural aspiration is nonsense. Well, not necessarily. This is a two-stroke engine after all, so the supercharger is probably there to clean up. It’s the same concept as the Detroit Diesel 71 series bus and truck engines, whose exhaust fans also don’t add power.

In scavenging, two-stroke engines bring in fresh air for a new combustion event and expel the exhaust gases from the latter. Where a blower/supercharger comes in handy is that it helps fill the cylinders with new air to push out spent gases and prevent them from sticking around for another combustion cycle. It does not compress the air in the chamber to increase cylinder pressure, but allows the engine to breathe easily.

Although INNengine has put an e-Rex in a Miata for testing, the company sees it more as part of a range extension system where it would essentially be a battery charger. According to the company, emissions and efficiency are comparable to four-stroke engines, yet with the power density of a two-stroke engine. In addition, the variable compression allows different tuning states for different fuels.

What about this “1Stroke” problem? If the e-Rex were a true one-stroke engine, it would have to produce power every time the piston moves from top dead center to bottom dead center and vice versa. To put it in traditional engine terms, a single stroke would produce power for each 180 degrees of crankshaft rotation, with the sucking, squeezing, popping and blowing essentially happening all at once.

Could a clever designer figure out how to create a second combustion chamber with a plug under a piston to drive it back to top dead center? Sure, that’s how AMPERe’s prototype single-stroke engines work, but that’s not what’s going on with the INN engine. Calling the e-Rex a single-stroke engine is a pragmatic choice, similar to how some racing series call Mazda’s “1.3-liter” rotary engine a 2.6-liter.