The cylinder head is the top section of many internal combustion engines, bolted above the engine block and sealed by the head gasket. It forms the roof of the combustion chamber, where air, fuel, spark, and pressure come together to make power. Because it controls breathing, sealing, cooling, and lubrication, the cylinder head has a major effect on engine performance and reliability.
Students can think of it as the upper control center of the engine.
Key Facts
- The cylinder head bolts to the engine block and seals the combustion chamber with a head gasket.
- Intake valves let the air fuel mixture or fresh air enter, while exhaust valves let burned gases leave.
- In a four stroke engine, one full cycle takes 720° of crankshaft rotation.
- For a four stroke engine, camshaft speed = crankshaft speed / 2.
- Compression ratio = total cylinder volume / clearance volume.
- Torque from one cylinder increases when combustion pressure acts on the piston: F = P A.
Vocabulary
- Cylinder head
- The upper engine part that contains valves, ports, passages, and part or all of the combustion chamber.
- Combustion chamber
- The space where compressed air and fuel burn to produce hot expanding gas.
- Camshaft
- A rotating shaft with lobes that open and close engine valves at the correct times.
- Valve
- A movable part that opens or seals an intake or exhaust passage in the cylinder head.
- Head gasket
- A thin sealing layer between the cylinder head and engine block that keeps compression, coolant, and oil separated.
Common Mistakes to Avoid
- Confusing the cylinder head with the engine block, which is wrong because the block holds the cylinders and pistons while the head controls airflow, sealing, and combustion above them.
- Assuming both valves are open at the same time during most of the cycle, which is wrong because valve timing is carefully controlled and most strokes need one or both valves closed.
- Ignoring coolant and oil passages, which is wrong because the head must remove heat and lubricate moving parts or it can warp, wear, or fail.
- Thinking a head gasket only prevents oil leaks, which is wrong because it also seals combustion pressure and keeps coolant, oil, and cylinder gases from mixing.
Practice Questions
- 1 A four stroke engine is running at 3000 rpm. What is the camshaft speed in rpm?
- 2 A cylinder has a total volume of 500 cm3 at bottom dead center and a clearance volume of 50 cm3 at top dead center. What is its compression ratio?
- 3 Explain why a cracked cylinder head or failed head gasket can cause overheating, loss of power, and white exhaust smoke.