A car engine needs to breathe in air, burn fuel, and push exhaust gases out in a carefully timed cycle. The camshaft is the rotating shaft that controls this breathing by opening and closing the intake and exhaust valves. Its lobes press on valve parts at precise moments so each cylinder gets air and fuel when it needs them.
Good valve timing helps an engine make power, run smoothly, and use fuel efficiently.
In a four-stroke engine, the piston moves through intake, compression, power, and exhaust strokes while the crankshaft turns twice. The camshaft usually turns once for every two crankshaft rotations, so the valves match the slower four-stroke cycle. A timing belt or timing chain keeps the camshaft synchronized with the crankshaft.
If this timing is wrong, the engine can lose power, misfire, or in some engines suffer serious valve and piston damage.
Key Facts
- Four-stroke cycle: intake, compression, power, exhaust.
- Camshaft speed in a four-stroke engine: camshaft rpm = crankshaft rpm / 2.
- The intake valve opens to let the air-fuel mixture or air enter the cylinder.
- The exhaust valve opens to let burned gases leave the cylinder.
- Valve lift is the maximum distance a valve opens from its seat.
- Engine speed relation: rpm = revolutions per minute, so 3000 rpm means 3000 crankshaft turns each minute.
Vocabulary
- Camshaft
- A rotating shaft with shaped lobes that open and close engine valves at timed moments.
- Cam lobe
- The raised part of a camshaft that pushes on a valve mechanism to open a valve.
- Valve timing
- The schedule of when intake and exhaust valves open and close compared with piston position and crankshaft angle.
- Timing belt or chain
- A belt or chain that links the crankshaft to the camshaft so their rotation stays synchronized.
- Valve spring
- A spring that closes the valve after the cam lobe stops pushing it open.
Common Mistakes to Avoid
- Thinking the camshaft and crankshaft spin at the same speed. In a four-stroke engine, the camshaft turns at half the crankshaft speed because one full valve cycle takes two crankshaft revolutions.
- Mixing up intake and exhaust valve jobs. The intake valve lets fresh air or air-fuel mixture enter, while the exhaust valve releases burned gases after combustion.
- Assuming valves open only when the piston is exactly at the top or bottom. Real engines often open and close valves slightly before or after these positions to improve airflow.
- Ignoring the timing belt or chain when diagnosing valve problems. A slipped or broken timing drive can put the camshaft out of phase with the crankshaft and cause poor running or engine damage.
Practice Questions
- 1 A four-stroke engine is running at 2400 rpm at the crankshaft. What is the camshaft speed in rpm?
- 2 A crankshaft turns 120 revolutions in 3 seconds. What is the crankshaft rpm, and what is the camshaft rpm in a four-stroke engine?
- 3 Explain why an intake valve should be closed during the compression stroke and what could happen if it stayed open too long.