A double-cylinder refers to a mechanical system that consists of two cylinders working together. This design is often used in engines, where two cylinders can improve efficiency and power output. By having two cylinders, the engine can produce more torque and smoother operation compared to a single-cylinder engine. In various applications, such as hydraulic systems or pneumatic devices, double-cylinder setups can provide balanced force and stability. This configuration allows for better control and can enhance the performance of machines, making them more effective in tasks like lifting or moving heavy objects.