Inside INNengine’s One Stroke Engine

Throughout the history of the internal combustion engine, there have been many important changes and trends. Gas engines, diesel engines, two-stroke engines, forced induction engines, and so much more.

But, one thing we have yet to see is a single-stroke engine, that is, until now, thanks to a company from Spain. So, today let’s take a look at the one-stroke engine and see what it’s all about.

So, the first question is, how is this possible?

Over 150 years after the invention of the internal combustion engine, we’re just now seeing this supposed breakthrough in technology.

And well, the answer is first requires us to understand how the internal combustion engine works.

How the Standard Internal Combustion Engine Works

With the standard internal combustion engine found in just about every single road you’ll ever see, it works as a four-stroke engine.

Meaning there are four stages required to complete a full engine cycle, which are intake, compression, combustion, and exhaust.

To which the boomers will call this by its less flattering name of suck, squeeze, bang, blow.

There are obvious outliers to this premise, such as the rotary engine which doesn’t really work like that, but kind of also do.

Then, there are two-stroke engines, which operate differently depending on the two-stroke you’re talking about, but the basic idea is that instead of requiring four stages to complete an engine cycle, it only takes two.

Now this is done by completing the exhaust and intake stages at the same time.

For instance, with a small two-stroke dirt bike engine, after combustion occurs, the piston is forced downwards until an exhaust port on the cylinder wall is exposed rather than a valve on the head.

At the same time, however, fresh air is pulled or pushed into the cylinder to replace the exhaust gases that have left the cylinder.

In the case of something like a two-stroke diesel, such as a Detroit 71 series, it works similarly to a two-stroke dirt bike engine, but instead of using the underside of the piston to help push air into the cylinder, it simply uses a supercharger.

However, engines like those that Detroit used to build and offer are strictly using a supercharger for the engine to just function and not add power by creating positive cylinder pressure, meaning it’s technically considered naturally aspirated.

Two-Stroke vs Four-Stroke

Okay, so what exactly is the point of reducing the amount of strokes? What is the real benefit here? Well, there are a few.

For one, two-stroke engines typically have fewer moving parts, meaning they’re generally simpler and lighter, on top of being physically smaller.

This is why two-stroke dirt bike engines have been so popular for so long.

Really, they’re still popular today, but their heyday has passed for a variety of reasons, one of which includes government regulation of emissions outputs.

If you can use a two-stroke engine instead of a four-stroke engine, you’re effectively doubling the power since there are twice as many combustion events per one crankshaft rotation.

The math doesn’t always add up because there are many other factors to consider when trying to calculate an engine’s performance, but you get the point:

An engine with fewer strokes is simpler, smaller, lighter, and generates more power per crankshaft rotation.

How the “One Stroke” Engine Works

So that brings us to the real question at hand, what the heck is a one-stroke engine? How can you possibly have intake, compression, combustion, and exhaust all happen in a singular stroke?

The short answer is that you don’t because it’s not really possible, but I’ll explain that later in the video.

For now, let’s take a look at the inside of INNengine’s one-stroke engine to see how this works.

And to give you an idea of how different this engine is compared to your standard piston engine, the one-stroke engine lacks a crankshaft, camshaft, connecting rods, valves, and cylinder heads.

So, it doesn’t sound much like an engine at all, but I promise you, it is.

Now, the way this engine works is a scalable design, just like many engines, meaning you could add or take away cylinders or add or remove displacement; the architecture and engine geometry would stay the same.

So, to simplify the article and video, we’re going look at INNengine’s e-REX engine, but this company does offer their REX-B engine, and I’m sure they’ll eventually have smaller and larger variants of both existing engines, so I’d recommend you check that out too on their website.

Okay, onto how this thing works.

The e-REX engine we’re looking at here has four cylinders with eight pistons.

Why so many pistons? Well, this is an opposed-piston engine, meaning two pistons compress toward each other in each of the four cylinders.

Interestingly though, there are no connecting rods that you’ll find inside this engine, at least not in the traditional sense.

So, how does the power go from the pistons to anything? Well, the pistons slide on rollers that ride on a large circular plate that is lobed.

As the lobe reaches its peak, it will push the piston forwards, where the opposing piston is also being pushed forwards from its own lobed circular plate.

Once the two pistons reach top-dead-center, fuel is directly injected into the cylinder, and a spark plug ignites the compressed air-fuel mixture.

As combustion and expansion occur, both pistons experience force that pushes them away from each other, which ultimately rotates the lobed circular plate, turning all that combustion power into rotational power.

When the pistons reach the bottom of their strokes, the intake and exhaust ports are uncovered.

It’s important to note that one piston is timed to reach bottom-dead-center slightly before the other to help improve exhaust gas scavenging.

So, much like a two-stroke engine, there are no valves. Instead, the piston acts as the valve as it uncovers the ports.

Interestingly, this comes with the massive benefit of eliminating a common problem with direct-injected engines, with the valves getting covered in carbon.

Displacement for this engine is listed at 500cc with a power output of 120hp and 180lb-ft of torque.

I know the total power output is pretty low, but when you account for the displacement being only half a liter, that’s a very impressive 240hp per liter.

For reference, if something like a Coyote-powered Ford Mustang had that kind of power per liter, it would make 1200hp, but the real Mustang only makes around 500hp, and that kind of helps put into perspective the absurdity of what we’re talking about.

It’s Just a Two-Stroke Engine

Here’s the tricky part, though, if you look at the way this engine works, it’s by definition a two-stroke engine. So why the one-stroke nomenclature?

Well, as many things in life, using a different or even slightly inaccurate name or way of describing something can be pretty good for marketing.

We saw this with the liquid piston engine we covered on this channel a while back, where the engines we looked at didn’t even have liquid pistons.

And in the case of the one-stroke engine, the name is ultimately just for marketing.

At the end of the day, this should and does fall under the category of a two-stroke engine. However, the company behind it didn’t want to give it that name because two-strokes are generally associated with premixed fuel, bad emissions output, and many other stigmas surrounding two-stroke engines.

But, because the one-stroke engine we’ve been looking at doesn’t really have the negative drawbacks associated with two-stroke engines.

They believed that calling it a two-stroke engine, which is ultimately what it is, would be bad for marketing and so they decided to take a stroke out.

The Test Engine and Miata

But, unlike some of the more experimental or interesting engines we’ve looked at on this channel, the INNengine one stroke has actually been used in a real-world application that’s not something like an RC plane.

What I’m talking about is a Mazda Miata. You may have seen JZ, Ford small block, or Chevy LS swapped Miatas, but now you can officially say you’ve seen a 500cc one-stroke engine swapped Miata.

What a crazy time to be alive.

What is a little curious, however, is that this engine has a centrifugal supercharger on it, so the power figures associated with this engine are a little confusing because either this Miata has more than the claimed 120hp, or the claimed 120hp is only with the addition of forced induction.

In this application, though, this engine is absolutely perfect.

The power figure is similar to that of the standard 1.8L NB Miata engine, but since the one-stroke engine only weighs 85lbs without all the accessories, it’s a great way to drop a ton of weight off the nose of the car.

The Real World Applications

So, will we see this power the wheels on a car like their test Miata or maybe one day even a production road car? Maybe, but likely no.

The real-world use case for this type of engine will likely be as a range extender in a vehicle with an electric powertrain because, ultimately, things are headed toward going all-electric anyways.

Why fight the inevitable vehicle electrification when you can profit from it?

Seriously though, hybrid electric vehicles could clearly benefit from using a lightweight and powerful engine like this.

I mean, that’s pretty much the exact reason that Mazda brought the rotary back from the dead as a range extender.

There are also other industries, such as off-grid power units and marine, both of which can benefit from physically small and lightweight engines.

But, seeing as how many brilliant engines have come and gone without catching onto much of anything, only time will tell where or if we’ll see this used in the real world in anything other than the test Miata.

Pros and Cons

Okay, so other than the low weight and small size, what are the other benefits of using an engine design like this?

And more importantly, what are the drawbacks? No design is perfect, and there is always give and take.

Of course, since this is a new design that few people have seen and even fewer people have put their hands on, so the downsides to this engine aren’t exactly clear, and some of the problems I’m going to list are partially speculation on my part.

Okay, so the benefits other than weight and size:

For one, power can be output at both ends of the motor because it’s an opposed piston engine, and there isn’t really a front or back to the engine.

This means it could be used for multiple wheels or even multiple axles from one unit power unit.

But technically speaking, you could do the same thing with something like a V8 engine, but it would be very complicated, and frankly, the potential upside isn’t worth it at all.

Arguably the biggest benefit to this design that we haven’t covered yet is noise, vibration, and harshness.

These are key points that need to be nailed when producing road-going engines, and this engine knocks it out of the park.

All the reciprocating masses inside the engine are perfectly balanced. And because it’s an opposed piston engine, combustion forces act equally in both directions, so vibration is nearly non-existent.

There’s also the potential upside of improved thermal efficiency because the combustion presses equally on two pistons.

In theory, this should transfer more heat and energy to the pistons rather than the cylinder walls and, more specifically, the cylinder head, which generally requires cooling because of the amount of heat that the head absorbs.

And if this was to be used in a plug-in hybrid electric vehicle, having the ability to run on multiple different types of fuel would be a massive advantage, and that’s something this engine is capable of, thanks to the variable compression ratio.

Okay, so what about the downsides?

Well, with how this engine works, it uses 24 rollers on four cam tracks to operate, which could bring with it quite a bit of friction as compared to standard journal bearings with pressurized oil.

And then those rollers also present another failing point, which is the combustion forces wearing out both the roller thrust bearings prematurely, which can likely get expensive considering how many of them there are in this engine.

Arguably the biggest issue with this engine, in my opinion, is the low torque output.

You see, in a standard internal combustion engine, the crankshaft helps to multiply combustion forces thanks to its fairly large swing, but that lobed circular plate simply cannot offer that kind of leverage, and that means it can’t offer that kind of torque output.

What does this mean? It means that the one-stroke engine will likely have power delivery similar to that of a four-cylinder engine or even a rotary, which means it makes lots of power high in the rev range but not much power down low, which is a recipe for less than ideal road-going engine.

I suspect that’s part of the reason we saw their test Miata use a supercharger, but that’s just speculation on my part.

My Thoughts

So, what do I think of this engine? Well, it’s certainly interesting, but it’s quite similar in basic concept to the other opposed piston engines, such as the two you see on your screen now.

In terms of use cases, I think this could be an excellent range extender or off-grid power solution, but otherwise, I just don’t see this being used in much else. Maybe the marine industry, but even then, it might be a stretch.

In the case of a plug-in hybrid electric vehicle, a lightweight and powerful engine like this means the batteries can be quite a bit smaller, meaning there is less cobalt mining involved, less lithium mining and refining, and less battery waste.

But, the unfortunate reality in the world of internal combustion engine engineering is that new inventions are very slow to be adopted, and that goes for nearly all industries.

And understandably so.

Why would a company, say Honda or Toyota, switch to this new engine style that is relatively unproven compared to standard internal combustion engines? For them, it would be a monumental gamble for extremely minimal benefit.

So, while I hope the one-stroke engine can catch on and become a thing, I do have my speculations.

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