Engine Valve Train Systems and Camshaft Drive Mechanisms

Engine Distribution System Fundamentals

The distribution system is the set of elements that perform the opening and closing of the valves. These valves control the entry and removal of gas in the cylinder, making possible the realization of the four strokes according to the distribution chart.

Key Components of the Valve Train

• Camshaft: Controls valve timing.
• Gears and Transmission System: Transfers motion from the crankshaft.
• Valves, Guides, Seats, and Springs: Control gas flow and ensure proper sealing.
• Pushers, Rockers, and Valve Adjustment System: Transmit motion to the valves.

Types of Valve Actuation Systems

Overhead Valve (OHV) Design

In the OHV system, the valves are located in the cylinder head, but the camshaft is located in the engine block. This results in a very short distance between the crankshaft and camshaft gears. This design is falling into disuse because cylinder filling at high RPM is not very efficient.

Overhead Camshaft (OHC and DOHC)

These systems feature the camshaft(s) located in the cylinder head.

• OHC (Single Overhead Camshaft): Uses one camshaft to operate both intake and exhaust valves.
• DOHC (Double Overhead Camshaft): Uses two camshafts; typically one dedicated to intake valves and one dedicated to exhaust valves.

Benefits of OHC/DOHC Systems

• Valve control is more direct and allows for more controlled opening and closing.
• Although more complex in design, they are generally more effective for modern engine performance.

Camshaft Drive Mechanisms

In the four-stroke cycle, the crankshaft completes two revolutions, the camshaft completes one revolution, and the valves are actuated once.

Timing Drive Methods

• Gears (Wheels): Used primarily in OHV systems only if the distance between the crankshaft and camshaft is too large, requiring an intermediate wheel. This method is currently underutilized.
• Roller Chain: This system uses two wheels with carved teeth dragged by a roller chain (which may be single or double). It is a robust transmission system with long-term durability, but it has the disadvantage of potentially lengthening the timing date over time and generating noise.
• Timing Belt: This is the most commonly used system today. It must be replaced periodically (typically every 100,000 to 130,000 km). It is provided with a turnbuckle (tensioner) and is protected in a casing to prevent damage from external elements. The timing belt is the most common mounting system for OHC engines due to its economic cost and absence of noise.

Roller Chain System Details

• Tensor (Tensioner): This component keeps the chain taut and offsets the effects of wear. The force is transmitted from the crankshaft to the camshaft via the taut side of the chain. The tensor is placed on the slack side, depending on the motor's rotation direction.
• Distribution Carter (Casing): This casing extends into the crankcase and contains engine oil. It is most often used with OHV systems, which require a long chain, often built without an intermediate pinion.

Valve Operating Conditions and Stress

Valves must withstand extreme conditions during engine operation.

Thermal Stress and Cooling

Valves are cooled primarily by the fresh gas inlet. Exhaust valves can exceed 800 degrees Celsius, while intake valves can exceed 400 degrees Celsius. The heat is dissipated mostly through the valve seat in the cylinder head and the rest through the valve stem and guide.

Mechanical Stress and Wear

In addition to withstanding high temperatures, the valve head also supports the chemical corrosion risk associated with combustion. Wear and tear can cause compression leaks that reduce overall engine performance.