QJMotor patent in the US: why a single-cylinder is disguised as a V-twin

• The US patent office published the application on 27 August 2026
• Two cylinder bores at an 80-degree angle, but only one combustion chamber
• Dummy piston without piston rings and with a through-bore for air, acting as a balancing mass

The basic idea behind the patent sounds contradictory at first: an engine with two cylinders, a crankshaft, two connecting rods and two pistons that is nevertheless a single-cylinder. That is exactly what the document describes which the United States Patent and Trademark Office published at the end of August 2026. The first cylinder works as usual with intake, compression, ignition and exhaust. The second cylinder stays cold. Its piston does move along, but it compresses no air and contributes nothing to power output. Its sole purpose is to balance out the vibrations of the working piston. The idea is not new, and it is not the first time it has been reported on either. What is new is that QJMotor is now explicitly protecting it for the US market.

How does the dummy piston in the QJMotor single-cylinder work?

The dummy piston sits in a second bore in the crankcase and hangs from its own connecting rod on the same crank pin as the working piston. It moves up and down purely through the rotation of the crankshaft and acts as a counterweight to the real piston.

So that this piston does not pump any air, it has a bore running all the way through from top to bottom. It can therefore move freely inside the cylinder without building up pressure or performing any thermodynamic work. The dummy piston also carries no piston rings. It does not have to seal a combustion chamber, it only has to be guided cleanly. Doing without the rings noticeably reduces friction and drag. It is precisely this friction that the patent names as the main weakness of earlier attempts using the same principle.

Both connecting rods share the same crank pin, which keeps the whole assembly compact. Technically the result is a V-twin, just with a single working cylinder. The dummy cylinder and its moving parts take over the job of a balancer shaft, one that does not rotate but instead runs back and forth.

Why does the dummy cylinder sit in a separate insert?

The balancing cylinder is not built directly into the housing but into an insert of its own, which is bolted to the crankcase via a flange. This component, referred to in the patent as a connecting base, makes it possible to tilt the cylinder relative to the housing.

That is the actual trick of the design. The crankcase itself is laid out for 80 degrees. The piston axes, however, can sit at up to 90 degrees to each other in order to optimise the balancing effect of the dummy piston. This is possible because the outer diameter of the balancing cylinder is smaller than the narrowest point of the bore it sits in. It is therefore not a tight, sealed press fit. That leaves enough room to tip the dummy cylinder away from the geometry of the original block and open up the effective angle between the two cylinder axes.

Cooling fins sit all around the insert. The dummy cylinder does not benefit from the airflow through intake and exhaust that helps cool a normal cylinder. Without fins of its own, heat could build up there. Earlier reports on the Chinese application also mentioned a noticeably smaller bore for the balancing cylinder compared to the working cylinder, with a piston specifically designed as a balancing mass.

What advantage does QJMotor expect from the concept?

The stated aim is a single-cylinder with the smoothness of an engine with a balancer shaft, but at lower cost and with less effort in production. The patent explicitly talks about keeping the production costs of small engines under control while still meeting modern requirements for low vibration.

It becomes cheaper because the design builds on existing housing tooling and on an architecture that is laid out for a V-twin casting anyway. A separate balancer shaft with its own gear drive is no longer needed. For the rider, the result would at best be an engine with the mass balancing of a 90-degree V-twin, which is regarded as particularly smooth, without any additional shafts or weights.

Critical voices already pointed out in 2024 that modern high-performance single-cylinders such as the Ducati Hypermotard 698 Mono or the KTM 690 range deliver plenty of power with a classic balancer shaft without giving up the compact build of a single-cylinder. Why QJMotor takes the detour via a second cylinder remained open at the time. With the cost argument, the US patent now at least supplies an official justification.

Has this idea existed before?

Yes, the principle is more than 30 years old. Ducati chief engineer Massimo Bordi tried it out from 1990 during the development of the Supermono and then dropped it again.

Bordi developed a 487 cc single-cylinder with desmodromic valve gear and electronic fuel injection out of the 90-degree V-twin of the 888. His prototype kept both cylinders but used only the upright one for power. The horizontal cylinder ran without a head, its piston serving purely as a balancing mass. The concept ran as smoothly as a V-twin, but the additional drag of the dummy piston cost power. Bordi had to go back to the drawing board.

His solution was the double connecting rod, in Italian „doppia bielletta“. The dummy piston was dropped, the second connecting rod remained and was guided by a lever on the housing. That kept the engine smooth, removed the friction losses and made the engine more compact. The Supermono ended up at 549 cubic centimetres with a 100 millimetre bore and a 70 millimetre stroke and produced 76 hp (56 kW) at 10,500 rpm. It won the European Championship in 1993 and, with Robert Holden, the TT on the Isle of Man in 1995. Only 67 examples were built between 1993 and 1998.

The balancing connecting rod principle kept resurfacing later on. BMW used it from 2006 to 2019 in the Rotax-built twin of the F 700 and F 800. Suzuki filed a patent in 2019 for a single-cylinder with a balancing connecting rod, essentially the Supermono technology in a smaller housing. And since 2000 Yamaha has used a third cylinder with a connecting rod and piston as a balancing mass in the T-Max, offset by 180 degrees to the two working cylinders and without compression. QJMotor is therefore not falling back on the later connecting rod principle but on Bordi’s original dummy piston idea. The difference lies in the details: doing without piston rings is meant to eliminate exactly the friction that Ducati failed on back then.

Which motorcycle could the engine end up in?

The patent names neither a specific engine nor a displacement. The 80-degree angle described does, however, match the existing 997 cc V-twin from Morbidelli, or MBP, within the Qianjiang group.

Whether this housing actually serves as the basis is open. According to media reports, a smooth and cheaply produced 500 cc single-cylinder with around 40 hp (30 kW) would be an obvious application, for example as a high-revving platform for the A2 market. That is speculation, though, and the document itself gives nothing away on the subject. It merely states that the engine is intended for motorcycles.

What is the timeline of the patent application?

The US publication is so far the last step in a chain of applications that began in China in August 2022. A year later came a PCT application, which allows patent protection to be sought in around 150 member states at the same time.

Qianjiang filed the actual US application in 2025. The US patent office then published it on 27 August 2026. This sequence suggests that this is not a forgotten or abandoned application, but that the group is deliberately protecting the design for the US market as well. Whether a production engine for QJMotor or the group brand Benelli will ever come of it remains to be seen. Even the reports on the Chinese application from 2024 read the patent as a sign that the Chinese motorcycle industry is expanding its development work instead of limiting itself to copying existing concepts.

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