Why Do Valves Open Early and Close Late?

[TheoMarie]

Intake valves open early to allow sufficient air to enter the cylinder. Exhaust valves close late to ensure that exhaust gases are expelled as much as possible after the power stroke. Below is relevant knowledge about valves: Increasing Intake Volume: The purpose of early intake valve opening is to achieve a larger valve opening at the start of intake, reducing intake resistance and ensuring smooth airflow. The purpose of late intake valve closing is to maximize the use of airflow inertia to increase intake volume. Reducing Residual Exhaust Gas: The purpose of early exhaust valve opening is to utilize the higher cylinder pressure during the initial exhaust phase for free exhaust, thereby reducing the resistance of forced exhaust and the power consumed during exhaust. The purpose of late exhaust valve closing is to fully utilize the inertia of exhaust gas flow to minimize residual exhaust gas.

[ManuelDella]

During engine operation, the early opening and delayed closing of valves primarily aim to improve volumetric efficiency, allowing more air to be drawn into the cylinder. I've seen too many cars suffer from insufficient power due to poor design. By opening the intake valve slightly earlier—before the piston reaches bottom dead center—fresh air begins entering sooner. Delaying the closing ensures additional air intake as the piston moves upward, promoting more complete combustion. It's like taking an extra breath during a sprint on a racetrack, providing extra energy, especially noticeable at low RPMs where engine torque improves significantly, resulting in smoother driving. If valve timing is too rigid, insufficient air intake leads to incomplete fuel combustion, increasing fuel consumption and producing dirtier exhaust. Having tested various engines, I can confirm that this mechanism, combined with variable valve technology, adapts to different driving conditions, making daily driving both fuel-efficient and quiet. Overall, this represents fundamental modern automotive design wisdom, eliminating that jerky sensation—particularly noticeable on mountain roads or in stop-and-go traffic.

[LaLuna]

I often race on the track and find that early valve opening and delayed closure are crucial for enhancing high-speed performance. The intake valve opens early, allowing air to start entering as the piston moves downward; it closes late, continuing to intake air as the piston pushes upward, maximizing oxygen intake. This directly boosts combustion power, enabling the engine to deliver stronger bursts of power at high speeds and more linear acceleration. Without this, like in some older cars, the engine roars without moving at startup, and fuel consumption skyrockets. A well-designed valve timing reduces gas resistance, making piston movement smoother and driving feel as effortless as gliding. It also works in harmony with automatic transmission systems, ensuring timely power response whether cruising at high speeds or climbing at low speeds. In everyday city driving, this mechanism keeps the car quieter and more reliable, avoiding the embarrassment of sudden stalling.

[VonNaomi]

Valve timing with early opening and delayed closing is designed to optimize engine efficiency, ensuring fuel economy during driving. The intake valve opens before the piston reaches top dead center, allowing more time for air intake as it approaches bottom dead center; delaying the closure enables the piston to draw in more air during its upward stroke. This increases oxygen intake, promoting complete fuel combustion and delivering stronger horsepower. After years of driving family cars, I've noticed that without this design, the vehicle struggles at low speeds in urban areas, with sluggish throttle response and significant fuel wastage. Proper valve timing ensures smooth transitions between idling and acceleration while reducing emissions for better environmental performance. The key lies in preventing reverse gas flow within cylinders, enhancing daily driving comfort. Regular valve timing inspections and keeping components free from dust buildup are crucial for maintaining vehicle longevity.