Key highlights
- QJMotor has patented a single-cylinder engine with a second dummy piston
- The dummy piston works as a counterweight to reduce vibrations
- The design could offer a simpler and cheaper alternative to a conventional balancer shaft
QJMotor has patented an unusual single-cylinder engine that looks like a V-twin. The patent describes a second cylinder that does not produce power but instead uses a dummy piston to counter the vibrations generated by the combustion cylinder.
What’s unusual?
Single-cylinder engines are inherently prone to vibration, particularly as displacement and performance increase. Manufacturers generally address this using counterbalancer shafts. QJMotor’s patented system takes a different approach by using a second piston as the balancing mass.
The patent shows two cylinder bores positioned at an 80-degree angle. The larger cylinder houses the conventional combustion piston, while the smaller cylinder contains the dummy piston. Both pistons are connected to the same crankshaft through connecting rods sharing a common crankpin.
The dummy cylinder does not function as a combustion chamber. It has an air vent, allowing the piston to move freely without compressing air. As the crankshaft rotates, the dummy piston moves up and down to counteract the movement of the working piston and reduce vibration.
Interestingly, QJMotor has also addressed one of the biggest drawbacks of this concept. The dummy piston does not use piston rings, since it does not need to seal a combustion chamber. This reduces friction and drag compared with earlier versions of the idea.
The cylinders are positioned at 80 degrees, while the pistons effectively operate at a 90-degree angle. QJMotor achieves this by mounting the balancing cylinder as a separate component on the crankcase. The dummy cylinder also gets cooling fins to manage heat buildup.
The patent’s stated objective is to provide the vibration-control benefits of a balancer shaft while reducing mechanical complexity and production costs. However, there is no confirmation of a production motorcycle using this technology.
Final thoughts
The idea may look strange on paper, but it addresses a very real limitation of large single-cylinder engines. If QJMotor can make the system cheaper and lighter than a conventional balancer shaft, it could become an interesting solution for future single-cylinder motorcycles. For now, though, it remains a patent rather than a confirmed production technology.
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