What is the second variation of 4-stroke engines aside from Vertical?

The second variation of 4-stroke engines, aside from the vertical configuration, is the horizontal configuration. In horizontal four-stroke engines, the cylinders are laid out horizontally, allowing the engine to have a lower center of gravity. This orientation can offer advantages in various applications, such as improved stability and better weight distribution, which can be particularly useful in certain vehicle designs and equipment.

Horizontal engines are prevalent in motorcycles, some outboard motors, and various types of machinery where space efficiency and balance are essential. By having the cylinders positioned horizontally, these engines can often achieve a more compact design, making them suitable for situations where vertical space is limited.

The other options, while they may relate to different aspects of engine design or applications, do not represent a recognized configuration of four-stroke engines in the same manner that vertical and horizontal do. Heavy duty refers more to the durability and application rather than a physical configuration, reverse does not denote a standard engine layout, and brake-blade-clutch seems more relevant to specific mechanical components or system designs rather than a classification of engine type.