What are the classifications of valve trains?
2 Answers
Engine valve trains come in various forms, with the main differences lying in the arrangement and number of valves, as well as the layout and driving method of the camshaft. Below are the detailed explanations: Based on the valve arrangement, valve trains can be classified into overhead valve (OHV) trains and side valve (SV) trains. According to the camshaft layout, valve trains can be divided into three types: overhead camshaft (OHC), mid-mounted camshaft, and underhead camshaft. All three can be used in overhead valve trains, while side valve trains can only use underhead camshafts. Based on the driving method between the crankshaft and the camshaft, valve trains can be categorized into gear-driven, chain-driven, and toothed belt-driven (timing belt) types. In terms of the number and arrangement of valves, valve trains can be classified into two-valve and multi-valve trains. Early engines typically used two valves per cylinder—one intake valve and one exhaust valve. Currently, multi-valve configurations per cylinder, such as three-valve, four-valve, and five-valve setups, are commonly used in passenger car engines. The multi-valve structure significantly increases the cross-sectional area of the engine's intake and exhaust passages, greatly improving the engine's volumetric efficiency, thereby enhancing both power and fuel economy.
The classification of valve trains is primarily based on the position of the camshaft. There is the overhead valve (OHV) pushrod type, where the camshaft is located on the side of the cylinder and uses pushrods to actuate the valves. This type is commonly found in older V8 engines; it has a simple structure but lower efficiency. The overhead camshaft (OHC) type is further divided into single overhead camshaft (SOHC) and double overhead camshaft (DOHC). The camshaft is directly mounted on the cylinder head. For example, the Honda Civic uses SOHC, while the Toyota Camry uses DOHC. The driving mechanisms vary, with chain-driven systems being more durable, while belt-driven systems require regular replacement. From my experience in car repair, OHC engines tend to be quieter than OHV engines and offer more precise valve control, improving combustion efficiency. Nowadays, mainstream vehicles mostly use OHC systems, often combined with hydraulic lifters for maintenance-free operation. However, aging chains can cause noise and should be inspected promptly. The differences in classification affect engine lifespan, making it a factor worth considering when choosing a car.
