 DOHC
The puzzle of the letters is generally solved with 'Double Overhead Camshaft'. Anyone who is satisfied with this will not even get half of the importance of the four letters.
Yes, the first appearance of dual overhead camshafts, attributed to the Swiss Ernest Henry, was at a Peugeot R4 racing engine. The technology was later extended to normal sports engines too.
In case of engine control, the overall concept must always be considered. After all, at least in racing, for which DOHC was first used, it's about the rapid entry and exit of gases.
The aerodynamics in the engine, so to speak. You have to ask yourself how the valves are literally positioned, certainly not straight and in a row, where they would be easily accessible for a camshaft.
One would like the combustion chamber to be spherical, which causes problems with the piston. Then at least like a hemisphere, which in turn is prevented by the valve plates, which are usually quite flat.
The only option is to arrange the valves completely in a V shape. Yes, that works, but how can you actuate it? BMW did it very successfully with an underneath camshaft in the 1937 328, but it was a complicated design.
Please keep in mind that an engine control works differently than a crank mechanism, namely that almost the entire mechanism has to be pushed back by the valve spring when it closes.
If it has a large mass, then the valve spring must also be strong. But that means more friction, possibly wear and higher power loss. So it makes sense to choose the shortest path between the activating cam and the valve.
That's why the first choice is the bucket tappet between the cam and valve. However, it has the disadvantage that the plunger is not particularly stable laterally, as the cam not only presses it downwards, but also constantly
presses it against the one wall of its guidance.
This in turn is avoided by (roller) rocker or drag levers. With hydraulic tappets they have the further advantage that, with their greater weight, they can be arranged at the pivot point, i.e. they are not part of the moving masses.
| Racing engines often do not have hydraulic valve lifters |
However, the space required in the cylinder head is larger, impossible for the five-valve engines that were built for a short time and is also a problem with direct injection. So set everything to zero and define DOHC as engine
control with two overhead camshafts with direct valve control.
And another special feature: there were, again for a short time, two lower camshafts and, in a VR engine from VW, there were even three camshafts in one cylinder head.
| 
