In-line engines are a type of internal-combustion engines, where the cylinders are arranged in a single row. The crankshaft is placed perpendicular to the pistons and all the cylinder heads move in the same direction, rotating the crankshaft. They come in four, six and eight cylinder configurations and are used in automobiles and air crafts.

a) In-line engines are very easy to build because the cylinders and the crankshaft can be fixed in the same casting. Straight engines have smaller physical dimensions compared to other engines and thus, installing them becomes easier. They also have a support bearing between each piston.

a) It is the most common 4-cylinder configuration seen in automobiles, whereas engines like straight-five have evolved into v5 engines. Some companies that have used straight engines in the cars are, Acura, Audi, Mercedes-Benz, Toyota, Volkswagen, Volvo BMW, Chevrolet, etc.

a) In-line-4
An inline-4 engine has 4 cylinders. An important characteristic of in-line four engines is the use of balancing shafts. During rotation, however, improperly arranged cylinders create an imbalance, which leads to undesirable vibration. This vibration increases with RPM. A balance shaft reduces this by nullifying the imbalance.

b) Straight-five
A straight-five engine has five cylinders, with two heads moving in one and the other three moving in the opposite direction. The straight-5 engine is a mix of both a straight 4 and straight six. It is longer in size and costs more to manufacture than the straight-4, but is comparatively much smoother.

c) Straight-six
Having six cylinders, the straight-six has been used by BMW, Chrysler, Chevrolet, etc. In straight four and straight six engines, due to the even number of cylinders, there has to be secondary vibration. While it is corrected in straight-4 by using a balancing shaft, straight does not use any. The secondary vibration is controlled by using crankshaft dampers.

d) Straight-eight
The straight-eight engine has eight cylinders in a straight line. It naturally has inherent primary and secondary balance, thus the crankshaft has no primary or secondary forces acting upon it. However, there is torsional vibration, and a harmonic damper is needed to cancel it. The straight six also has inbuilt balancing features, but the straight-eight has more strokes per revolution and the result is smoother functioning.

Balance shafts: In inline-4 engines, due to the asymmetry of the design, they have an inherent second order vibration. This cannot be overcome, no matter how well balanced the components are. To overcome this, the balancing shaft is used, which rotates in the opposite direction to that of the engine. This is commonly used in inline-4 engines and used in inline-6.

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