The valve which allows mixture into the cylinder is the inlet valve; the one through which the spent gases escape is the exhaust valve. They are designed to open and close at precise moments, to allow the engine to run efficiently at all speeds.
The operation is controlled by pear-shaped lobes, called cams, on a rotating shaft, the camshaft, driven by a chain, a belt, or a set of gears from the crankshaft.
Where the camshaft is mounted in the engine block, small metal cylinders tappets sit in channels above each cam, and from the tappets metal pushrod extend up into the cylinder head. The top of each pushrod meets a rocker arm which bears against the stem of a valve, which is held in a raised (closed) position by a strong coiled spring the valve spring.
As the pushrod rises on the cam it pivots the rocker arm, which pushes the valve down (open) against the pressure of its spring. As the cam lobe rotates further, the valve spring acts to close the valve. This is called an overhead-valve (OHV) system.
Some engines have no pushrods; the valves are operated more directly by single or double camshafts in the cylinder head itself the overhead-cam system.
As there are fewer moving parts between the camshaft and the valve, the overhead-cam (OHC) method is more efficient and produces more power for a given engine capacity than an engine with pushrods, because it can operate at higher speeds. With either system, there must be some free play in the operating gear, so that the valve can still close completely when parts have expanded through heat.
A pre-set gap tappet clearance is essential between the valve stem and the rocker arm or cam, to allow for expansion. Tappet clearances vary widely on different cars, and faulty adjustment can have serious effects.
If the gap is too large, the valves open late and close early, reducing power and increasing engine noise.
Too small a clearance prevents the valves from closing properly, with a consequent loss of compression.
Some engines have self-adjusting tappets, which are hydraulically operated by the engine oil pressure.
The overhead valve system (OHV) system, operated by pushrods, has the crankshaft adjacent and parallel to the crankshaft in the cylinder block.
As the crankshaft rotates, each valve is opened by means of a tappet, pushrod and rocker arm. The valve is closed by spring pressure.
The camshaft drive-chain sprocket has twice as many teeth as the crankshaft sprocket, so that the camshaft rotates at half engine speed.
An overhead-cam (OHC) engine needs fewer parts to operate the valves. The cams act directly on bucket tappets or on short levers - known as fingers - which in turn act directly on the valve stems.
The system dispenses with the extra weight and mechanical complexity of pushrods and rocker arms.
A long chain is frequently used to drive the camshaft from a sprocket on the crankshaft, but such a long chain tends to 'whip'. The problem is overcome in some designs by fitting intermediate sprockets and two shorter drive chains, kept under tension.
FAQs
The valves are controlled by a camshaft which, at the correct time, pushes each valve open - either directly or through a linkage. The valves must be synchronised with the piston so that they open and close at the correct moment on the piston's stroke.
What closes and opens the valves? ›
The camshaft, driven by the engine's crankshaft, is equipped with lobes that push against the valves' valve stems, causing them to open and close. The camshaft's design determines the precise timing of valve opening and closing, which influences the engine's power, torque, and efficiency.
What component opens and closes the valves of the engine? ›
The camshaft is a mechanical component of an internal combustion engine. It opens and closes the inlet and exhaust valves of the engine at the right time, with the exact stroke and in a precisely defined sequence.
How do you tell if engine valves are open or closed? ›
In order to tell if the valve is open or closed, the general rule is, if the handle is switched 90deg to the flow, it is off. As you can see from the diagram above, if the handle is in line with the flow path, it is on.
On which strokes do the valves open and close? ›
In the theoretical 4-stroke process (Figure 4), the exhaust valve opens fully at the start of the exhaust stroke and closes fully at the end of the exhaust stroke. Similarly, the intake valve opens at the start and closes at the end of the intake stroke.
How do the valves work in an engine? ›
Intake or exhaust valves
The up and down motion of the valve opens and closes the engine's combustion chamber. Intake valves open and close the fresh air intake (or air/fuel mixture intake, depending on the engine). Exhaust valves open and close the outlet for exhaust gases.
What part opens the valves? ›
Engine valves are operated by the rotation of a camshaft that has eccentric, egg-shaped lobes on it. As those cam lobes rotate, the high part of their egg shape presses on the valve, moving it into an open position.
What moves the valves in an engine? ›
The Engine's camshaft drives the motion of the engine valves. From the rotation of the camshaft, the linear motion of the valve is created by a series of lobes or cams, situated in the camshaft.
How do you fix a bad engine valve? ›
A burnt valve cannot be fixed—it must be replaced.
In some cases, a machine shop may be able to replace the valve and recondition the cylinder head for you.
What holds the valves in place in an engine? ›
A valve spring is placed around the stem of a valve and held in place by a retainer. Its main job is to control the entire valvetrain, ensuring that the proper amount of spring pressure is applied consistently to prevent valve bounce.
The camshaft, which opens and closes the valves, is driven from the crankshaft by a chain drive or gears on the front end of the engine.
What damaged engine valves? ›
Over time, the constant flexing can lead to metal fatigue, cracking and valve failure. Valve-related failures are often due to factors such as detonation, poor quality or defective parts, over-revving the engine, or not setting or maintaining proper valve lash.
Are all valves closed at TDC? ›
The valves must be more-or-less closed when the piston is at TDC to avoid contact and the resulting expensive disaster.
What are the 4 strokes of an engine? ›
Four Stroke Cycle Engines. A four-stroke cycle engine is an internal combustion engine that utilizes four distinct piston strokes (intake, compression, power, and exhaust) to complete one operating cycle.
Can intake and exhaust valves open at the same time? ›
As the engine cycles, there is a period when both the intake and exhaust valves are open at the same time. This valve timing is known as "overlap." Think of this as the exhaust and intake cycles overlapping each other.
What is the most common valve failure that affects compression? ›
A burned valve is the most common type of failure, and as it gets burned, compression is lost around the valve seat. The engine will misfire when it is the turn of that valve's cylinder to have it's power stroke. The other type of failure is if the valve stem breaks in two, or the valve keeper lets go.
Which valves are open at TDC? ›
The piston is now at TDC, both the intake and exhaust valves are partially open. As the piston travels back down the cylinder, the exhaust valve goes fully shut and the intake valve goes fully open and starts to shut. Compression stroke.
Are any valves open during compression stroke? ›
The compression stroke is an upstroke, meaning it occurs from BDC to TDC. For it to compress the air/fuel mixture, the intake and exhaust valves must be closed.
