Because four strokes take two full crankshaft revolutions, the camshaft that opens each valve exactly once per cycle must turn at exactly half the crankshaft\’s speed — never the same speed.
Intake, compression, power and exhaust each take one piston pass (a half-revolution); four halves make two revolutions, but each valve still only needs to open once, so the cam driving it is geared or chained down 2:1 from the crank.
Only on a four-stroke. A two-stroke fires every revolution, so any camshaft it has runs at crank speed. And on a variable-valve-timing engine the ratio holds while the phase does not — the cam is still at half speed but no longer at the timing you assumed.
Estimate with the rule, then check it against the calculator that models it properly.
Open 2 & 4-Stroke Engine Simulator →Because four strokes take two full crankshaft revolutions, the camshaft that opens each valve exactly once per cycle must turn at exactly half the crankshaft\’s speed — never the same speed. Intake, compression, power and exhaust each take one piston pass (a half-revolution); four halves make two revolutions, but each valve still only needs to open once, so the cam driving it is geared or chained down 2:1 from the crank.