Inline is a particular configuration for an internal combustion engine, in which the cylinders are arranged in a row (in line). It is sometimes referred to as a 'linear' engine in parts of Europe. Typically, the cylinders are arranged so that the pistons travel vertically, and the crankshaft is mounted beneath the cylinders. This is a simple layout with several advantages.

It's simple to manufacture and maintain within tolerances. The identical alignment of the cylinders makes the design, manufacturing and maintenance of the rotating components easier. The engine is easier to balance, and the design makes it easier to avoid nasty harmonics causing problems. Making engines bigger can be done by simply adding more cylinders to the design and 'stretching' the engine, making appropriate adjustments to cylinder firing sequence and timing. Since the tops of the cylinders, and hence the valves, are all aligned, it makes actuating them with a single cam (for all, or per type) simple.

Some disadvantages do exist, however. The arrangement means that increasing the number of cylinders requires allocating greater space to the engine, as it will grow in one direction. The linear placement of the valve systems means that space on the engine head is used somewhat inefficiently. The larger size of the engine block means coolant and lubricant may have longer galleries to gravel through. Inline engines are much more difficult to make air cooled. In smaller cars, the longer engine block means that the engine cannot be mounted transversely.

Inline engines tend to accept performance modifications quite well, as their simpler mechanics and design mean that increasing engine output can be done with less risk of overload or of interference. The solid, oblong nature of the engine block, with no cylinder 'angled' to another, means a consistant mass of metal in between, which is structurally safer.

Some other engine types include the V-type engine, the boxer engine, the radial engine and the rotary.

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