How do engine valves open at the right time?
A bump on a turning shaft pushes the valve open. A belt keeps that bump in step with the piston. Turn through the cycle slowly and watch how they’re linked.
Slowed down · simplified timing
Intake: OpenExhaust: Closed
Drag the slider slowly, or press Next stroke.
Stroke 1 of 4
Intake
The piston moves down. The intake cam’s bump pushes its follower down, so the intake valve opens. As the stroke ends, the spring closes it again.
A simplified engine with two camshafts on top. On each shaft, the toothed pulley and the shaped cam are separate parts that turn together.
Wait, what?
Two turns below. One turn above.
The crankshaft turns twice to get through Intake, Compression, Power and Exhaust. Each camshaft above it turns once, so each valve opens just once per cycle. That matters, because the piston passes the same height on different strokes. Its height alone can’t tell a valve when to open.
The shape does the pushing.
While the round part of the cam passes the follower, the valve stays shut. When the raised lobe comes around, it pushes the follower and valve down, opening a gap.
The spring gets squashed as the valve opens. As the lobe turns away, the spring pushes the valve shut and keeps the follower pressed against the cam. The cam pushes. The spring brings it back.
The belt keeps everything in step.
Teeth on the belt fit into teeth on the pulleys, so the belt doesn’t slip. The larger cam pulleys have twice as many teeth as the crank pulley, so they turn at half its speed. Two turns below always give one turn above. Some engines use a chain or gears instead of a belt.
Real timing has some overlap.
In a real engine, a valve can start opening a little before its stroke begins and finish closing after it ends. The intake and exhaust valves can even be open together for a moment. Our drawing keeps each valve inside its own stroke so it’s easier to follow. Real timing varies by engine.
Talk like you know your cams
Camshaft
A turning shaft with cams on it. Their shape decides when each valve opens.
“The cams turn at half the crank’s speed.”Cam lobe
The raised part of a cam that pushes a follower as it turns.
“The bump does the pushing.”Follower
The part between the cam and the valve. It rides the cam’s shape and passes the push on.
“It just rides the bump.”Timing belt
A toothed belt that links the crankshaft to the camshafts so they stay in step.
“Always in step, every turn.”Go down the rabbit hole
ReadLook closer at a cam and its followerEPI, an engine engineering firm, explains cam shapes, followers, springs and when valves open and close. It gets technical.ReadWatch the four strokes repeatNASA animates the Wright brothers’ engine. Its intake valve works differently, but you’ll see the same two-turn cycle.
TryThe piston goes down twice. Watch the valves.Press Intake, then press Power. Both times the piston is on its way down. Now look at the intake valve above it: open the first time, shut the second. The cams have turned half a turn in between.
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