Amongst the internal combustion engine components, some need to be in or attached to the upper part of the cylinders. Such components are primarily the intake and exhaust valves and the spark plugs. A typical cylinder head is a block of metal sitting atop the cylinders, with valves and spark plugs for each cylinder built-in and sealed against the main engine body that contains the cylinders. In this way, the cylinder head contains the top sections of the cylinders. The coolant ducts are also contained within the cylinder head in water-cooled engines.
This engine head design became common in early engines due to its simplicity. It featured the valves themselves built on sides of the cylinders rather than in the top section, with the lower part of the head containing chambers for the valves to rise into to enable intake and exhaust. The head was essentially a single slab of metal without any mechanical parts, which made its manufacturing and assembly easy. This design also offered simpler and better cooling mechanisms, but eventually lost popularity due to severe performance limitations as the airflow required a 90-degree turn to enter the combustion chamber resulting in inefficient compression and poor combustion. Similarly, another drawback is the complicated exhaust path, leading to overheating of the engine by keeping the exhaust gases within it for a longer duration.
These engine head designs are found in engines with cylinder blocks containing the camshaft. The head features intake and spark mechanisms as well as the valves, which are actuated for intake and exhaust using mechanical pushrods. This design helps meet some of the limitations of the flathead design, resulting in better performance while keeping the engine compact enough. The complexity of the drive timing system is also quite low due to the camshaft being located close to the crankshaft, a small chain—or more efficiently—a direct gear mechanism connecting them.
As featured on the Datsuns engine comparison, these heads—as the name suggests—feature an embedded camshaft and are thus more complex than the types previously discussed. However, this eliminates the use of pushrods to actuate valves as the camshaft is located directly above the valves and can actuate them directly. These heads come in two variants: one for single overhead camshaft (SOHC) engines, featuring one camshaft built into the head, and another for double overhead camshaft (DOHC) engines with two camshafts in the head. In the latter version, one of the camshafts is responsible for controlling the inlet valves while the other one operates the exhaust valves. OHC heads also feature multiple valves per cylinder and modern OHC engine heads feature integrated variable valve timing systems for improved performance.
