In a piston engine , a piston rod connects a piston to the crosshead and thus to the connecting rod that drives the crankshaft or ( for steam engines ) the driving wheel.
Internal combustion engines , and in particular all current automobile engines, generally do not have piston rods. Instead they use trunk pistons , where the piston and crosshead are combined and so a rod is not needed between them. The term piston rod has been used as a synonym for ‘connecting rod’ in the context of these engines.
Engines with crossheads have piston rods. These include most steam engines and some large marine diesel engines .
Earlier single-acting beam engines , such as the Newcomens , had a single power stroke that worked downwards. He used an iron chain instead of a piston rod. It can transmit a tensile force, but not a compressive force that pushes upward. The piston in the cylinder was sealed around its rim but the top of the cylinder was open. To reduce the cost of a longer forged chain, a rudimentary piston rod was used.
Watt ‘s development of the steam engine introduced an enclosed upper cylinder. This now requires a stuffing box to seal around the piston rod, and to make the piston rod smooth and precise cylindrical. At this time the engines were still single-acting and the rod was still operating only under tension.
Later developments, also by Watt, produced a double-acting cylinder . The piston rod now had to transmit the pushing and pulling force alternately. The typical use of a steam engine of an enclosed cylinder, almost always double-acting, relies on the stuffing box and therefore the piston rod.
Piston rods are usually attached to the crosshead and piston to a transverse slot and a tapered key or gib. Moving this key sideways strengthens the attachment. The use of a transverse key allows for relatively easy dismantling for maintenance. Some smaller pistons are retained by a threaded piston rod and a larger nut, although the crosshead almost always uses a key. Because the precise length of the piston rod is critical to timing the engine’s valvegear, the attachment tightens the piston to a certain step or location in the piston rod and the length (and valve timing) is not adjustable. This length requires precision of the manufactured rod, but unlike the rod that drives the valve, does not need to be adjusted even more precisely by a fitter during the build of the engine.
Early steam engineers were confident that horizontal cylinders would wear out excessively due to the weight of the pistons mounted on the bore of the cylinders. Vertical cylinders remained a favorite for a few years. When horizontal cylinders were adopted, one measure used to carry the weight of the piston was to have an extended or second piston rod on the other side of the piston. It emerged through a second stuffing box and in rare cases ( illustration ) was supported by a second crosshead. Enclosing the entire tail rod in a sealed cover does not require stuffing box and any leakage of steam. However this also posed a risk that condensed water could accumulate in the casing, causing hydraulic lock and mechanical damage.
Trunk engines were compact steam engines , developed for propulsion of ships. They reduced their overall length by placing the crosshead with the piston in the form of a hollow tubular trunk. Thus there was no longer a need for a piston rod. The connecting rod and its gudden pin were installed inside this trunk . Trunk engines had pistons of much larger diameter and so the old issue of supporting the weight of the pistons and avoiding cylinder wear was again a concern. The trunk engine used the second trunk as a tail rod to support its larger pistons.
Single-acting steam engine
Some late steam engines, mostly high-speed steam engines , were also single acting. These also used trunk pistons, but in a new form. These engines were vertical and used a simple piston design, with no separate trunk, but the sides of an extended piston as the crosshead bearing surface.
This trunk piston design would become almost universal for internal combustion engines.
Almost all steam locomotives use a similar design to the cylinder, with the crosshead slide adjacent to the cylinders and a short piston rod between them.
In some cases, tail rods were also used. These were rare in the United Kingdom, but relatively common in Europe, at least for large locomotives.
Internal combustion (piston) engines operate at a higher speed than steam engines. Thus they benefit from a low reciprocating mass and a short, rigid connection between the piston and the connecting rod. Double-acting engines are also very rare, although they used piston rods. Thus the lightweight trunk piston of the high-speed steam engine was soon adopted and is still in service today. Its major changes have been in the adoption of aluminum alloys to make its sealing rings and pistons lighter.
Some large slow-speed diesel engines, especially for ships, use crossheads and piston rods. Medium and high speed diesel engines operate at faster speeds and therefore use lighter-weight trunk pistons.