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Engine configuration is an engineering term for the layout of the major components of a reciprocating piston internal combustion engine. These components are the cylinders and crankshafts in particular but also, sometimes, the camshaft(s).

Many apparently 'standard' names for configurations are historic, arbitrary, or overlapping. For example, the 180° V engine is so named because the crankshaft is related to a V engine more closely than it is related to other opposed-piston engines such as the boxer. Others would consider it a flat engine because of its shape.

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.

Categorisation by piston motion

Types of engines Boxer vs Inline vs V.jpg

Engine types include:

  • Single cylinder engines
  • Inline engine designs:
    • Straight engine, with all of the cylinders placed in a single row
      • U engine, two separate straight engines with crankshafts linked by a central gear.
        • The square four is a U engine where the two straight engines have two cylinders each.
    • V engine, with two banks of cylinders at an angle, most commonly 60 or 90 degrees.
    • Flat engine, two banks of cylinders directly opposite each other on either side of the crankshaft.
    • W engine. Combination of V and straight, giving 3 banks, or two V's intertwined giving 4 banks.
    • Opposed piston engine, with multiple crankshafts, an example being:
      • Delta engines, with three banks of cylinders and three crankshafts
    • X engine.
  • Radial designs, including most:
  • Pistonless rotary engines, notably:

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 Group VR6 engine is a hybrid of the V engine and the straight engine, and can not be definitively labeled as either.

Other categorisations

By valve placement

The majority of four stroke engines have poppet valves, although some aircraft engines have sleeve valves. Valves may be located in the cylinder block (side valves), or in the cylinder head (overhead valves). Modern engines are invariably of the latter design. There may be two, three, four or five valves per cylinder, with the intake valves outnumbering the exhaust valves in case of an odd number.

By camshaft placement

Poppet valves are opened by means of a camshaft which revolves at half the crankshaft speed. This can be either chain, gear or toothed belt driven from the crankshaft, and can be located in the crankcase (where it may serve one or more banks of cylinders) or in the cylinder head.

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 in an overhead camshaft (OHC) design. There may be one or two camshafts in the cylinder head; a single camshaft design is called single overhead camshaft (SOHC). A design with two camshafts per cylinder head is called double overhead camshaft (DOHC). 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, and a V engine with two camshafts per cylinder head would be described as 2xDOHC, or informally as a 'quad cam' engine.

With overhead camshafts, the valvetrain will be shorter and lighter, as no pushrods are required. Some overhead camshaft designs still have rocker arms; this facilitates adjustment of mechanical clearances.

If there are two camshafts in the cylinder head, the cams can sometimes 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 arrangement for high performance automobile engines. It also permits the spark plug to be located in the center 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 center.

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