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.
Understanding Automotive Technology: The Camshaft and Valves
A camshaft does not usually touch every valve directly. Its lobes may push on lifters, pushrods, rocker arms, or followers. These parts make up the valvetrain.
In an overhead cam engine, the camshaft sits in the cylinder head near the valves. In an older pushrod engine, it sits lower in the engine block. A lobe has a round base section and a raised nose.
The base circle lets a valve stay closed. As the raised part turns toward the valve mechanism, it moves the valve down against its spring. Once the lobe turns away, the spring closes the valve tightly against its seat.
Valve opening is not as simple as opening exactly when a piston starts moving. Air has mass, so it needs time to begin flowing. Engineers often open the intake valve slightly before the piston starts its downward intake movement.
They may keep it open briefly after the piston begins rising on compression. The fast-moving incoming air can still fill the cylinder during that short time. The exhaust valve can open before the piston reaches the bottom of its power movement, releasing pressure early so exhaust gases leave more easily.
Near the change between exhaust and intake, both valves may be open for a short time. This is called valve overlap. It can improve cylinder clearing at higher engine speeds, though too much overlap can make low speed running rough.
Cam design affects an engine's character. Higher valve lift creates a larger opening for gases to pass through. Longer valve duration keeps a valve open for more of the camshaft rotation.
These changes can help an engine move more air at high speed, which can increase power. They can reduce smoothness or efficiency at low speed. Many modern engines use variable valve timing.
Oil pressure and computer controlled devices adjust the camshaft position while the engine runs. Some systems can change valve lift or duration too. The engine can then use calmer timing while idling and stronger airflow when the driver accelerates.
Valve clearance is an important service detail on engines that require adjustment. A small gap is needed because metal parts expand as the engine gets hot. Too much clearance can cause a ticking sound and reduce how far the valve opens.
Too little clearance can stop a valve from fully closing. A valve that cannot close loses compression and may overheat because it cannot transfer heat into its seat. Timing belts, chains, tensioners, and guides need attention as well.
A worn chain can stretch, while a failed belt can allow the crankshaft and camshaft to lose their correct relationship. When studying this system, trace one cylinder through a complete cycle and focus on which valve is seated, which valve is moving, and why airflow needs to start before or continue after a piston reaches a turning point.
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.