151 Engine Service and Repair Technician Handbook Cylinder Head Components The cylinder head is bolted to the top of the cylinder block and provides a path for the air-fuel mixture to enter the cylinder (intake port), and a path for spent gases to exit the cylinder (exhaust port). It also controls the timing of the opening and closing of the ports, and houses the fuel injectors and spark plugs. Cylinder head components and their functions are: Intake Port – The intake port is the passageway through which fuel and air enter the cylinder. Exhaust Port – The exhaust port is the passageway through which exhaust gases exit the cylinder.
Intake Valve – When closed, the intake valve seals the passageway between the intake port and the cylinder. When opened by the rotating cam, air and fuel are able to enter the cylinder. Exhaust Valve – When closed, the exhaust valve seals the passageway between the exhaust port and the cylinder. When opened by the rotating cam, exhaust gasses are able to exit the cylinder. Fuel Injector – Just prior to the intake stroke, this high pressure valve opens and sprays fuel into the intake port. Spark Plug – At the appropriate moment, the spark plug ignites the air-fuel mixture and the resulting expansion of hot gases powers the engine.
Camshafts and Cams – The rotating cams on the camshafts control the timing of intake and exhaust valve opening and closing. In the dual overhead cam (DOHC) engine design, one camshaft controls the intake valves and the other camshaft controls the exhaust valves. 8 Technical Training