For many automobile engines, the term ´block' is interchangeable with engine in this context, for example V block and V engine can often be used interchangeably in American English. This is because the most common forms are all based on a combined engine block and crankcase that are milled from a single piece of cast metal. The locations of the major components are largely determined by the shape of this one component.
The standard names for some configurations are historic, arbitrary, or both, with some overlap. For example, the cylinder banks of a 180° V engine do not in any way form a V, but it is regarded as a V engine because of its crankshaft and big end configuration, which result in performance characteristics similar to a V engine. But it is also considered a flat engine because of its shape. On the other hand, some engines which have none of the typical V engine crankshaft design features and consequent performance characteristics are also regarded as V engines, purely because of their shape. Similarly, the Volkswagen VR6 engine is a hybrid of the V engine and the straight engine, and can not be definitively labeled as either. The names W engine and rotary engine have each been used for several unconnected designs. The H-4 and H-6 engines produced by Subaru are not H engines at all, but boxer engines.
If the camshaft is located in the crankcase, a valve train of pushrods and rocker arms will be required to operate overhead valves. Mechanically simpler are side valves, where the valve stems rested directly on the camshaft. However, this gives poor gas flows within the cylinder head as well as heat problems and fell out of favor for automobile use, see flathead engine.
The majority of modern automobile engines place the camshaft on the cylinder head. There may be one or two camshafts in the cylinder head; a single camshaft design is called OHC or SOHC for (Single) OverHead Cam. A design with two camshafts per cylinder head is called DOHC for Double OverHead Cam. Note that the camshafts are counted per cylinder head, so a V engine with one camshaft in each of its two cylinder heads is still an SOHC design. See overhead camshaft.
With overhead cams, the valvetrain will be shorter and lighter, as no pushrods are required. Some single camshaft designs still have rocker arms; this facilates adustment of mechanical clearances.
If there are two camshafts in the cylinder head, the cams normally bear directly on cam followers on the valve stems. This is the usual arrangement for a four-valves-per-cylinder design. This latter arrangement is the most inertia free, allows the most unimpeded gas flows in the engine and is the usual arrengement for high performance automobile engines. It also permits the spark plug to be located in the centre of the cylinder head, which promotes better combustion characteristics. Very large engines (e.g. marine engines) can have either extra camshafts or extra lobes on the camshaft to enable the engine to run in either direction.
A disadvantage of overhead cams is that a much longer chain (or belt) is needed to drive the cams than with a camshaft located in the cylinder block, usually a tensioner is also needed. A break in the belt may destroy the engine if pistons touch open valves at top dead centre.
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