Motorcycle engines are, in essence, pretty easy and simple to understand, right? I mean, there can only be so many different types of engines out there, from single-cylinder mills to the ubiquitous parallel and V-twin motors.

Of course, we can’t forget about the buttery smooth triples and howling in-line fours. There are also a few horizontally opposed (boxer) and six-cylinder mills from some manufacturers out there. But that’s about it. 

Well, as it would turn out, motorcycle engines aren’t as clear-cut as we’d like them to be. Take, for example, QJ Motor and its oddball engine that isn’t exactly sure whether it wants to be a single-cylinder or a V-twin. As you can see in the image above, the engine sort of looks like a V-twin, but you’ll notice that the rear cylinder is much shorter and narrower than the front cylinder. But why?

Well, it would seem that the rear cylinder serves no other purpose than that of a counterbalancer. Counterbalancers aren't uncommon, as single-cylinder engines are inherently buzzy, and can lead to quite a lot of vibrations all across the rev range. The idea of having a faux cylinder, however, is odd but isn’t original to QJ Motor either.

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The Ducati Supermono was touted as a bike that revolutionized the single-cylinder engine. 

Ducati dabbled in the concept with its 1990s Supermono racer, whose engine was essentially a Desmoquattro V-twin with its rear cylinder lopped off. The final design, however, ditched the rear cylinder, but kept the connecting rod attached to a weighted linkage to replicate the mass of the rear piston. The additional mass gave the engine the balance it needed to rev higher, thereby producing more power.

As for QJ Motor’s design, it seems to be employing an entirely inactive rear cylinder, complete with a piston and connecting rod. There’s even a huge hole in the piston to prevent it from compressing air, and the dummy cylinder has a much smaller bore than the front cylinder. So in theory, this engine would have the same mechanical balance as a 90-degree V-twin, an engine hailed as the gold standard when it comes to performance-oriented V-twins.

The rear cylinder has a smaller bore, and the piston has a hole to prevent it from compressing air

The rear cylinder has a smaller bore, and the piston has a hole to prevent it from compressing air

Those of you who own Ducatis must be grinning now, and for good reason. Anyone who’s ridden any L-twin Ducati would know just how smooth yet guttural the power delivery of these engines are. Even Suzuki SV650 enthusiasts can attest to this. Today’s crop of 270-degree V-twins have been engineered precisely to emulate the feel and power delivery of a 90-degree V-twin; a testament to the latter’s legendary status in the world of motorcycle engines.

But—and that’s a big but—counterbalancers exist for a reason, and we’re seeing some thoroughly high-performance single-cylinder engines, like the new Ducati Hypermotard 698 Mono and KTM’s entire 690 lineup, produce impressive amounts of power without sacrificing the compact packaging of a thumper. So the question remains: why would QJ Motor go through the hassle of developing an engine like this? Is it for efficiency, for performance, or simply so the engine looks like a V-twin—you know, some sort of moto cosplay?

 

We went on and on among ourselves as to why this could be the case, and we ultimately decided to ask engine expert and friend to RideApart's editors Hank O’Hop over at The Drive to weigh in on this curious case.

Hank explained that stuff like this is pretty common in racing, wherein four-cylinder engines found in race cars are often just V8s cut in half. This is because it’s much easier to just deactivate a bunch of cylinders than to engineer what’s basically an entirely new engine. But in QJ Motor’s case, it doesn’t seem like this new engine is based on an existing one, but rather, it’s being developed from the ground up to be this way.

Another theory proposed by Hank is that the dummy rear cylinder was engineered with a smaller bore so as to be more cost-effective—QJ Motor could just stamp out a solid slug for less, and call it a day. Furthermore, the smaller bore is engineered to offset any harmonic issues presented by an inactive full-size piston, similar to the imbalance you feel when you pull the spark plug out of one of the cylinders of your engine and start it up.

At the end of the day, we still can’t wrap our heads around why QJ Motor would develop an engine like this. It’ll surely be interesting to see what motorcycle this oddball engine would end up in. Chances are it’ll be quite the interesting machine, and sit in a category all its own.

Do you have any theories as to why QJ would make an engine like this? Let us know in the comments below.

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