Figure 2: Double overhead camshafts (DOHC) driven by chain (Alfa-Romeo).
The overhead location of the camshaft (OHC) is known since the first internal combustion engines and it has always been the standard for aircraft and racing engines. It became more and more widespread in the Sixties and Seventies on the engines of mass production cars because the direct actuation of the valves by elimination of the pushrods and rockers confer many advantages:
- Reduction of the valve train inertia which allows higher revs.
- More precise actuation due to the elimination of the deflection of the intermediate parts.
- Reduction in the friction losses due to the possibility to use less strong valve springs.
- The cylinder head side is cleared of the pushrods, which constitute a hindrance to the free design of the intake and/or exhaust ducts.
In 4-stroke engines, the complete cycle requires two crankshaft revolutions for a single camshaft revolution. Consequently, the camshaft is driven at half engine speed. A single camshaft by cylinder row may be sufficient, but the current trend is to use two of them, that is to say one for the inlet valves actuation and the other for the actuation of the exhaust ones. The angular setting (timing) of the camshaft(s) compared to the crankshaft is an operation which requires a great attention because the valves must be closed in the vicinity of the top dead center (TDC) otherwise the pistons may collide with them, which causes an immediate breakage and catastrophic engine failure. In this respect, the timing belts must be periodically changed according to recommendations of the manufacturer because their rupture causes the same kind of breakdowns. They were adopted for the following reasons:
- Their lower cost compared to a chain or gear drive.
- They are quiet.
- They absorb the torsion vibrations.
Another way of limiting the vibrations of torsion transmitted to the camshaft consists to locate the camshaft drive at the rear of the engine and thus benefit from the regularity of rotation which the proximity of the flywheel provides.
The cam follower can be a lifter, a rocker or a lever. Some thermal expansion clearance (valve lash) is necessary so that the valve head, while seating properly on its seat, can completely close the exhaust or inlet duct and is able the cool down. This clearance must be checked once in a way, although it is remarkably stable on most current engines. Hydraulic lifters which compensate for the clearance at each stroke were already installed on most American cars at the end of the Fifties. They ensure a quiet operation of the valve train and exempt it of any maintenance. Their disadvantages are slightly higher weight, cost and a somewhat diminished reliability. If the cam is a lever, the hydraulic lifter becomes a motionless base and the disadvantage of its inertia is removed. The European and Japanese manufacturers nowadays usually also use hydraulic lifters, which is a positive development because the lash adjustment of the 32 valves of V8 would be a substantial task.