Double Overhead Cam Engines
A double overhead cam engine has two cams per head. So inline engines have two cams, and V engines have four. Usually, double overhead cams are used on engines with four or more valves per cylinder. A single camshaft simply cannot fit enough cam lobes to actuate all of those valves.
The main reason to use double overhead cams is to allow for more intake and exhaust valves. More valves mean that intake and exhaust gases can flow more freely because there are more openings for them to flow through. This increases the power of the engine. You may have also heard of Twincam engines. This is the same as a double overhead cam engine because the engine has dual overhead cams just like in a double overhead cam engine.
Single Overhead Cam Engines
Dual Overhead Cam Engine This arrangement denotes an engine with one cam per head. So if the engine is an inline 4-cylinder or inline 6-cylinder, it will have one cam. If it's a V-6 or V-8, it will have two cams (one for each head).
The cam actuates rocker arms that press down on the valves causing them to open. Springs then return the valves to their closed position again. These springs have to be very strong because at high engine speeds, the valves are pushed down very quickly, and it is the springs that keep the valves in contact with the rocker arms. If the springs were not strong enough, the valves might come away from the rocker arms and snap back. This is an undesirable situation that would result in extra wear on the cams and rocker arms.
On single and double overhead cam engines, the cams are driven by the crankshaft, via either a belt or chain called the timing belt or timing chain. These belts and chains need to be replaced or adjusted at regular intervals. If a timing belt breaks, the cam will stop spinning and the piston could hit the open valves.
An engine configuration in which all of the valves are located in the cylinder head and the camshaft is located in the cylinder block. The camshaft operates the valves via lifters and pushrods.