How Do Engine Valves Work?

The four-stroke internal combustion engine relies on the precise timing and operation of intake and exhaust valves to control the flow of the air-fuel mixture into the cylinder and the exhaust gases out of the cylinder. Understanding how engine valves work is key to appreciating the elegant sophistication of the four-stroke engine.

The Four-Stroke Cycle

The four-stroke cycle consists of four distinct processes – the intake stroke, the compression stroke, the power stroke, and the exhaust stroke.

• During the intake stroke, the intake valve opens to allow the air-fuel mixture to enter the combustion chamber. The piston moves downward, drawing in the air-fuel mixture.
• In the compression stroke, both valves are closed and the piston moves upward, compressing the air-fuel mixture. Just before the piston reaches the top, the spark plug fires, igniting the mixture. The exploding mixture drives the piston downward in the power stroke, generating power.
• Finally, in the exhaust stroke, the exhaust valve opens to allow the spent gases to escape as the piston moves upward again. This completes the four-stroke cycle.

Valve Timing

The valves must be timed to open and close at precise points in the four-stroke cycle. The intake valve opens slightly before the intake stroke starts and closes shortly after the stroke ends. This allows maximum flow of the air-fuel mixture into the cylinder.

Similarly, the exhaust valve opens slightly before the exhaust stroke starts and closes shortly after it ends, enabling complete scavenging of the spent gases. The valves are closed during the compression and power strokes to harness the power of the combustion.

Valve Actuation

The camshaft and crankshaft work together to open and close the valves at the right moments. The crankshaft rotates twice for every rotation of the camshaft. The cam lobes on the camshaft push the valve stems down to open the valves against the force of the valve springs.

There are several designs for actuating the valves:

• Overhead camshaft engines have the camshaft in the cylinder head opening the valves directly.
• In pushrod engines, long pushrods transfer the cam motion to rocker arms that open the valves.
• Desmodromic valves use a positive opening and closing mechanism instead of springs.
• Pneumatic valves use compressed air to open the valves.

Regardless of the exact design, the coordinated motions of the camshaft and crankshaft time the valve events to the four-stroke cycle.

Valve Designs

While most engines use poppet valves, some other valve designs have been tried:

• Sleeve valves have ports covered and uncovered by a sliding cylinder surrounding the piston.
• Rotary valves rotate to align openings with the intake and exhaust ports.

Poppet valves have proven to be the best compromise between cost, reliability, and performance. They come in two main types – intake and exhaust valves. Exhaust valves generally are made from high-temperature alloys like Nimonic 80A to withstand the hottest exhaust gases. Intake valves operate at lower temperatures allowing the use of cheaper alloys. The size and flow characteristics are tailored to the engine parameters.

Valve Timing and Lift Effects

The timing refers to when the valves open and close with respect to the pistons position. It is measured as degrees of crank angle relative to top dead center. Advancing or retarding the timing affects the power output.

The lift refers to how much the valve opens. More lift allows more flow but also requires higher spring pressures. Variable valve timing and lift technology allows optimization of these parameters across engine speeds for improved performance.

What are the parts of the engine valve? 

The main parts of an engine valve are the head, face, stem, keeper, seal, seat, and springs. The head and face seal against the seat to control flow. The stem connects to the actuator. The keeper and seals hold the valve in place. The springs provide closing force.

In summary

Engine valves are precisely timed portals that control the vital flow of gases into and out of the combustion chamber. Their timing and design are critical to generating the power that drives the world forward.