The Weirdest New Engine Has Eight Pistons, Four Cylinders, No Crankshaft, And Is Shaped Like A Tube

The piston engine has reigned supreme in internal combustion vehicles for longer than a century. While the piston engine is not perfect, even after so long, it’s reliable, predictable, and just works. Throughout history, countless engineers have challenged the supremacy of the piston engine with gas turbines, Wankels, and even bizarre crankcase-spinning rotaries. Now, a new engine wants to enter the ring. This is the INNengine e-REX, and it’s a compact, tube-shaped engine with eight pistons, four cylinders, no crankshaft, and not even a cylinder head. This engine has even powered a Mazda MX-5 Miata already.

Countless engineers throughout automotive history have identified weak points in typical internal combustion engines. The powerplant that moves your car has a lot of moving parts, like a crankshaft, camshafts, valves, pistons, and connecting rods that can wear out over time. Then there’s thermal efficiency, where a good piston engine might see around 40 percent efficiency or so. Many of these engineers thought that piston engines weren’t powerful enough for their size, weren’t reliable enough, weren’t fuel agnostic, or polluted too much.

The world has seen valiant efforts to beat the piston engine. I have now written about seemingly countless gas turbine projects and a handful of Wankel rotary cars. There have even been derivatives of the piston engine, like the free-piston engine and the opposed-piston engine. Most of these ideas dream of a future with fewer moving parts, higher power-to-weight ratios, and compact builds.

Yet, with only some exceptions, these technologies just haven’t been able to replace what already exists in cars, planes, locomotives, and trucks. Historically, gas turbines were loud, laggy, thirsty, and expensive. Wankel engines were unreliable, thirsty, and pollute more than desired. Free-piston engines were complicated and hard to control. Some great opposed-piston engines have succeeded, like the iconic Napier Deltic and the Commer TS3. Yet, for how awesome those engines were, they were also horribly complicated.

The INNengine e-REX (above) is yet another engine that is being promoted as a better alternative to a typical piston mill, and it’s a real weirdo. The company that designed it even goes as far as to call it a “one-stroke” engine.

One Man’s Dream

The maker of the e-REX is a small company called INNengine. It was founded in 2011 in Granada, Spain, by Juan Garrido Requena, and INNengine tells its founder’s story:

1995
When he was only 9 years old, Juan Garrido Requena was taken to a karting course with his brothers. There and then, among speed wheels and screeching breaking sounds an obsession was born: one day he would make his own kart.

1998
To everyone’s surprise and many incredulous smiles, he got right to work. Any money he could make was put to the task. Rusty metal, second hand tools and reclaimed parts were acquired and manufactured turning his parents’ garage into a makeshift workshop. By the age of 12 Juan had designed & built a go-kart from scratch.

2010
Juan went on to study a Bachelor of Science in Mechanical Engineering. His obsession with engines did not dwindle during his university years but rather grew culminating with the presentation of his Senior Thesis: Garrido Engine: a new Internal Combustion Engine. Thanks to 2 angel investors, INNengine started like the quintessential start-up: with a group of friends and just a garage. The very same garage where many years before, a 12 year old built his own go-kart from a pile of metal rods.

Garrido Requena made his dream a reality in 2012 when the first prototype (above) of the INNengine came to life. The M40_2C_2R was a tiny engine with only two cylinders and a 40cc displacement, but it proved that Garrido Requena’s concept worked. He then went on to patent the design in 2013, build a second prototype in 2015, and a third prototype in 2018.

Nowadays, INNengine says it has “reinvented” the old Otto engine concept with a “revolutionary new multifuel engine which positions INNengine at a unique competitive advantage to license its patented ICE technology to the more than 226 million units/year expected by 2027.”

INNengine says that its new engine, called the e-REX, is as clean and efficient as a four-stroke engine but as power-dense as a two-stroke. It can drink pretty much any fuel or hydrogen, is supposedly a “one-stroke” engine, and INNengine wants it to power the world’s future range-extended electric vehicles.

INNengine is promising a lot here. So what is the e-REX, exactly?

Opposed-Piston Engines

The e-REX actually utilizes technologies that have already been proven in decades past. It’s an opposed-piston engine like the Commer TS3 and the Napier Deltic. While those two engines are perhaps the most famous examples of opposed-piston engines, the technology goes back well beyond a century.

Here’s what Motor Trend wrote about the beginnings of the opposed-piston engine:

The original idea dates back to 1882 and James Atkinson, who developed the Atkinson differential engine. The Atkinson engine used two pistons in a single cylinder actuated by a flywheel and connecting rods to create compression in a four-stroke cycle. The first large industrial two-stroke gas OPEs appeared in 1898 and were built by Dr. Oechelhauser in Germany. The earliest powerplant was a twin-cylinder that used one crankshaft with rods. It was a spark-ignited engine that made 600 hp. A French company, Gobron-Brillie, developed smaller versions of gas OPEs suitable for motor vehicle use around 1900. In 1904, a Gorbron-Brillie car with an OPE set a “World’s Record Speed” of more than 100 mph in the flying kilometer. The first diesel version appeared in 1907. Russian designer Raymond A. Koreyvo built a working prototype and received a patent for it in France. He then displayed it at international exhibitions, where other developers saw it and began building their own versions.

There have been many different OPEs built through the years (gas and diesel versions), but smaller versions (compact enough for a car or truck) were not at the forefront. Most companies produced large-displacement versions that were used in ships, locomotives, submarines, and airplanes. The most famous diesel OPE is the Junkers Jumo developed by Professor Hugo Junkers. The Jumo 205 was the first successful diesel aircraft engine. There were several different versions of the Jumo built (205, 206, 207, and 208) from the 1930s through World War II.

All the Jumos were vertical two-stroke, six-cylinder, 12-piston, supercharged engines that had two crankshafts and used four injection nozzles per cylinder. Two camshaft-operated injection pumps were used per cylinder to feed the fuel nozzles. Air was drawn in through the intake port located under the lower piston and spent gases exited the exhaust port located under the upper piston. This layout worked because the lower crankshaft was located 11 degrees behind the upper. By adjusting the timing, the pistons opened and closed the ports at different times to create proper scavenging in the cylinder.

Opposed-piston engines even made it over to America, where Ransom Eli Olds adapted the technology to solve the problem of early diesels being so heavy for their power output. Instead of the typical piston engine, which had its pistons firing up and down in their own cylinders, Olds made an opposed-piston engine that cut down on material by having the pistons fire horizontally.

During the 20th century, opposed-piston engines also solved another problem, and it was with the exhaust scavenging issues faced by typical two-stroke piston engines. In a two-stroke engine, intake (“scavenging”) and compression are done in one stroke, then combustion and exhaust happen on the next stroke. Unlike a four-stroke, where combustion occurs every second time the pistons reach the top of their travel, with a two-stroke you get combustion every time the pistons reach the top of the cylinder. The graphics below are from Yamaha:

In a basic, crankcase-aspirated two-stroke, the exhaust port is exposed during the power stroke, and exhaust begins to exit. As the piston continues down, the transfer port opens, which sends a fresh air-fuel charge to enter the combustion chamber, pushing remaining exhaust gases out while preparing for the next power stroke. When the piston starts heading upward again, the transfer port closes, the exhaust port closes, and the intake port opens.

Scavenging is the process by which the incoming charge of air and fuel pushes exhaust gases out.

The transfer and exhaust ports were often located across from one another, resulting in cross-flow scavenging. One of the quirks of the basic two-stroke was that the fresh air-fuel charge wasn’t perfect at scavenging, and thus, some exhaust gas would become trapped in the combustion chamber after the end of the exhaust portion of the stroke. This reduces the engine’s output. Another quirk was that some of the fresh air-fuel charge was wasted out of the exhaust during scavenging, causing higher emissions and robbing the engine of potential even further.

Opposed-piston engines were seen by some engineers as the solution to two-stroke scavenging issues because their intake and exhaust ports are located at opposite ends of any given cylinder, rather than across from each other, resulting in straight-through airflow.

A Barrel Of Power

The INNengine e-REX is also an opposed-piston engine, but it does things differently. The most immediate difference between an e-REX and other opposed-piston engines is how the pistons are arranged. There are four cylinders, which have eight pistons within them. These cylinders are arranged in a rotary pattern, sort of like the chambers of a revolver.

Hot Rod magazine explains what you’re looking at here:

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).

Since this engine combines intake and compression on one stroke and combustion and exhaust on the next stroke, it is a two-stroke engine. It’s a very complex design, but still a two-stroke at heart. But as Hot Rod noted above, it’s a modern one. As was common in two-cycle diesels, the fuel isn’t being used to lubricate the internals of this engine.

Something else that’s neat about this engine is that since it can have a variable compression ratio, it can be a multi-fuel engine. Apparently, when the e-REX is running, it’s so smooth that you can balance a coin on it.

The e-REX also has an impressive spec sheet. INNengine claims that the e-REX makes 120 HP and 180 lb-ft of twist from 700cc of displacement. The engine measures 19 inches long, is 11 inches tall, and weighs 84 pounds.

INNengine says that the e-REX is 70 percent smaller and lighter than an equivalent engine, while also having 53 percent fewer parts, no crankshaft, no valvetrain, and no cylinder head. INNengine also claims that, since this engine is so simple (in its eyes, anyway), it will even be 40 percent cheaper to make than an equivalent inline piston engine.

About That Whole ‘1Stroke’ Thing

What’s not neat is how INNengine continues to market the e-REX. Even today, the company claims it’s a “1Stroke” and “Patented 1Stroke,” but this seems to ignore how everyone else defines engine strokes. In theory, an actual one-stroke engine would mean an engine that would fire every single time the piston reaches top dead center and bottom dead center. This engine doesn’t do that. INNengine says this is a “1Stroke” engine because:

The REX-B can do this thanks to having twice as many power strokes per cylinder than a 2 stroke engine and 4x that of a 4 stroke one.

Basically, since the cylinders have two pistons that fire per engine revolution, it’s a “1Stroke.” But that doesn’t make sense. The pistons in those cylinders still work on a two-stroke principle; there are just two of them. Confusingly, elsewhere in INNengine’s marketing, it says its “1Stroke” label is just so the public doesn’t think of the e-REX as a dirty two-stroke:

We call it 1Stroke because it has none of the emission issues of the 2Stroke. There is no oil in the mix and pressurized lubricated areas are far from the combustion chamber. There’s also the fact that the piston never works as a pump (sucking air) and Variable Port Timing… and so much more.

But this is also weird because there are two-stroke engines out there that don’t mix oil into their gasoline. Honestly, INNengine should just drop the whole “1Stroke” schtick because no amount of mental gymnastics makes it true.

There’s more, too. As Hot Rod notes, since this engine is still using old-school ports rather than poppet valves, there’s still a risk of scavenging issues and pollution resulting from them. Bolting a supercharger to the engine can help ensure burned gases escape the engine, but there’s still the possibility of getting some of the fresh air-fuel charge in the exhaust. This isn’t anything new. Classic Detroit Diesel two-cycle engines employed superchargers, not to increase power, but to improve scavenging. INNengine claims the engine is capable of effective scavenging without a supercharger, but it’s unclear if the INNengine team solved the issue with unburned fuel reaching the exhaust.

Hot Rod also notes that the e-REX has other potential downsides, including potentially as much friction as a V8 engine as a result of having 24 rollers on the swashplates’ tracks. A report by Engine Labs noted a concern that the engine might deliver torque like a Wankel due to its lack of a lever effect from not having a crankshaft. Then there’s the rollers themselves, as it’s not known how long they’ll last.

The E-REX Has Powered A Miata

INNengine has proven that it has a working 500cc version of the e-REX by placing it into a Mazda MX-5 Miata NB. There’s a video of the car scooting around, and for a brief moment before some annoying music is added into the video, you can hear how the engine sounds. The soundtrack is sort of a cross between a four-cylinder inline-four and a Wankel.

Take a listen (click here if you can’t see it):

Weirdly, you can see that the INNengine guys added a supercharger to the e-REX in the Miata, but they never explain why. They still claim a 120 HP output, however. There are two possibilities here. Either the supercharger is for scavenging, which would seem to invalidate INNengine’s claim about effective scavenging without a supercharger. Or, it’s there for power. I reached out to INNengine for clarification because a supercharger isn’t a part of the marketing.

While INNengine made the e-REX power a Miata, it says the goal isn’t to have this engine powering your track car. Instead, INNengine hopes OEMs pick it up as a range extender for electric cars. The elevator pitch is that the e-REX is smaller, lighter, greener, cheaper, more powerful, and more efficient than using a typical piston engine as a range extender.

There’s also a smaller REX-B 300 version of the engine. This mill targets light aircraft and makes up to 50 HP. INNengine sees itself selling three different types of the REX-B, one that’s a direct replacement for an aviation two-stroke engine, one to be used in hybrid-electric aviation setups, and one that can be used as a range extender in electric aircraft.

INNengine also has a 125cc version of the REX-B that makes 21.5 HP. The company claims this engine makes 80 percent more power than the typical 125cc engine. This engine does exist, and was tested in a large RC plane. If this engine actually makes the advertised power, I’d love to see it in a small-bore motorcycle.

INNengine also hopes to see its engines used inside boats and as stationary generators.

Why You Can’t Buy An E-REX

Of course, there’s a reason why there are no production vehicles using an INNengine. The company says it needs investment to industrialize and then scale the e-REX and the REX-B. Once INNengine can create production versions of its engine, it wants to license the technology out to OEMs. Yes, that is a roughly similar path that led to so many of the world’s largest car companies to play with Wankel engines.

At this time, the biggest company that’s publicly interested in the technology is Horse Powertrain, which will help INNengine validate the e-REX. It’s unclear if any automakers are currently interested. As it is, some automakers are pumping the brakes on their green vehicle initiatives. Will they be interested in experimenting with an entirely new engine? We also haven’t even seen how this engine would work in its intended application as a range extender. INNengine hopes to start slinging these things to OEMs beginning in 2027, so maybe we’ll have answers to the questions posed here.

Really, all of this just means that INNengine probably has a long way to go. Maybe this will be like another Wankel, where it gets made, licensed out to some companies, and we get to see how it works out.

Regardless of where it goes, there’s no doubt that the e-REX is a weird engine by today’s standards. It’s a tiny cylinder of power that makes some incredible promises. Will it actually go into mass production? I have no idea, and I’m not holding my breath. But it is seriously cool that one man had a dream to make a futuristic engine and then actually made it happen. I love a story about people making their dreams come true.

Top graphic images: INNengine; Mazda