Crank pin

In a reciprocating engine, the crank pins are the bearing journals of the big end bearings, at the opposite ends of the connecting rods to the pistons. If the engine has a crankshaft, then the crank pins are the journals of the off-centre bearings of the crankshaft. In a beam engine the single crank pin is mounted on the flywheel; In a steam locomotive the crank pins are often mounted directly on the driving wheels.

Big end bearings are commonly plain bearings, but less commonly may be roller bearings, see crankshaft.

In a multi-cylinder engine, a crank pin can serve one or many cylinders, for example:

* In a straight engine each crank pin normally serves only one cylinder.

* In a V engine each crank pin usually serves two cylinders, one in each cylinder bank.

* In a radial engine each crank pin serves an entire row of cylinders.



Big end design

There are three common configurations of big end bearing:

* If a crank pin serves only one cylinder, then the big end is a relatively simple design, accommodating only one connecting rod. This design is the cheapest to produce, and is used in:
o All single cylinder engines.
o Most straight engines.
o All boxer engines.
o Some V-twin engines.

* If a crank pin serves more than one cylinder, then the corresponding cylinders may have an offset, to simplify the design of the big end bearing. This design is used in:
o Most V engines.
o Multiple row radial engines.

* If more than one cylinder is served by a single crank pin but there is no offset, then some or all of the connecting rods must be forked at the big end. This design in theory provides better engine balance than designs with an offset, but at the cost of considerable extra complexity and cost in both design and manufacture, and either more weight or closer manufacturing tolerances or both to achieve the same strength and reliability. Any extra weight added to the big end itself also carries a penalty of adding vibration and reducing balance. As the number of cylinders grows, the effect of the offset on balance becomes less important, and forked connecting rods become less common. They are mainly used in:
o Single-row radial engines.
o Some V-twin engines, notably including motorcycle engines.

Reciprocating engine

Reciprocating engine

A reciprocating engine, also often known as a piston engine, is an engine that uses one or more pistons to convert pressure into a rotating motion.

The reciprocating engine was first developed as the now largely obsolete steam engine during the eighteenth century, followed by the Stirling engine in the nineteenth. Today the most common form of reciprocating engines is the internal combustion engine which uses the combustion of petrol, diesel fuel, oil or natural gas inside the cylinder to provide pressure.

There may be one or more pistons. Each piston is inside a cylinder, into which a gas is introduced, either already hot and under pressure, or heated inside the cylinder by ignition of a fuel air mixture. The hot gases expand, pushing the piston to the bottom of the cylinder. The piston is returned to the cylinder top (Top dead centre) either by a flywheel or the power from other pistons connected to the same shaft. In most types the expanded or "exhausted" gases are removed from the cylinder by this stroke.

The linear movement of the piston is converted to a circular movement via a connecting rod and a crankshaft or by a swashplate. A flywheel is often used to ensure smooth rotation. The more cylinders a piston engine has, the more vibration-free (smooth) it can run. The higher the combined piston displacement volume it has the more power it is capable of producing.

It is common for such engines to be classified by the number and alignment of cylinders and the total volume of displacement of gas by the pistons moving in the cylinders. Single- and two-cylinder engines are common in smaller vehicles such as motorcycles. Automobiles typically have between four and eight, while locomotives, and ships may have a dozen cylinders or more.

Cylinders may be aligned in line, in a V configuration, opposite each other , or radially around the crankshaft. Opposed piston engines put 2 pistons working at opposite ends of the same cylinder and this has been extended into triangular arrangements such as the Napier Deltic. Some designs have set the cylinders in motion around the shaft, see the Rotary engine.

Reciprocating engines that are powered by compressed air, steam or other hot gasses are still used in some applications such as to drive many modern torpedoes. In most cases the gas, like that produced by high test peroxide or Otto fuel II, is pressurised without the need of combustion and therfore oxygen. This allows propulsion under water for considerable periods of time and over significant distances. e.g. see Mark 46 torpedo.

In most applications of steam power today , the piston engine has been replaced by the more efficient turbine.