Internal combustion engines, and in particular all current automobile engines, do not generally have piston rods. Instead they use trunk pistons, where the piston and crosshead are combined and so do not need a rod between them. The term piston rod has been used as a synonym for 'connecting rod' in the context of these engines.[citation needed]
The first single-actingbeam engines, such as Newcomen's, had a single power stroke acting downwards. Rather than a piston rod, they used an iron chain. This could transmit a tensile force, but not a compression force pushing upwards. The piston was sealed in the cylinder around its rim but the top of the cylinder was open. A rudimentary piston rod was used, simply to reduce the cost of a long forged chain.
Watt's development of the steam engine introduced an enclosed upper cylinder. This now required a stuffing box to seal around the piston rod, and for the piston rod to be machined smooth and accurately cylindrical. The engines were still single-acting at this time and the rod was still only acting in tension.
Later developments, also by Watt, produced a double-acting cylinder. The piston rod now had to transmit a force alternately pushing and pulling. The steam engine's general use of an enclosed cylinder, nearly always double-acting, made it dependent on the stuffing box and so the piston rod.
Attachment
Piston rods are usually attached to the crosshead and the piston by a transverse slot and a tapered key or gib. Driving this key sideways tightens the attachment. Using a transverse key allows relatively easy dismantling for maintenance. Some smaller pistons are retained by a threaded piston rod and a large nut, although the crosshead nearly always uses a key.
As the precise length of the piston rod is important for timing the engine's valvegear, the attachment tightens the piston down onto a fixed step or location in the piston rod and the length (and valve timing) is not adjustable. This length requires the precision of the manufactured rod but, unlike the rod driving the valves, does not need to be adjusted even more precisely by a fitter during the engine's erection.
Tail rods
Early steam engineers were convinced that horizontal cylinders would suffer excessive wear, owing to the weight of the piston riding on the bore of the cylinder. Vertical cylinders remained a favourite for some years. When horizontal cylinders were adopted, one measure used to carry the piston weight was to have an extended or second piston rod on the other side of the piston. This emerged through a second stuffing box and in rare cases (illus.) was supported by a second crosshead.
Enclosing the entire tail rod in a sealed casing avoided the need for a stuffing box and any leakage of steam. However this also gave a risk that condensed water could accumulate in the casing, leading to hydraulic lock and mechanical damage.
Trunk engines
Trunk engines were compact steam engines, developed for the propulsion of ships. They reduced their overall length by placing the crosshead alongside the piston in the form of a hollow tubular trunk. There was thus no longer any need for a piston rod. The connecting rod and its gudgeon pin were mounted inside this trunk. Trunk engines had extremely large diameter pistons and so the old issue of supporting piston weight and avoiding cylinder wear was again a concern. Trunk engines used a second trunk as a tail rod to support their large pistons.
Single-acting steam engines
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 using the sides of an elongated piston as the crosshead bearing surface.
This trunk piston design would become almost universal for internal combustion engines.
Steam locomotives
Small steam locomotive, with piston rod visible between the cylinder (green, left) and the crosshead (right). Note the small brass lubricator (the leftmost), whose function is to lubricate the piston rod and stuffing box, through a short oil pipe
Almost all steam locomotives have used a similar design of cylinder, with a crosshead slide mounted adjacent to the cylinder 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.
Diesel engines
Large diesel engines, crosshead type on the left, with piston rod (9) and trunk piston type on the right.
Internal combustion (piston) engines operate at higher speeds than steam engines. They thus benefit from reduced reciprocating mass and from a shorter, stiffer linkage between the piston and the connecting rod. Double-acting engines are also very rare, although these did use piston rods. The high-speed steam engine's lightweight trunk piston was thus soon adopted and remains in service today. Its major changes have been in its sealing rings and the adoption of aluminium alloys to make the pistons lighter.
Some large slow-speed Diesel engines, particularly those for ships, use crossheads and piston rods. Medium- and high-speed diesel engines operate at faster speeds and so use lighter-weight trunk pistons.
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder.
A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types. The main types are: the internal combustion engine, used extensively in motor vehicles; the steam engine, the mainstay of the Industrial Revolution; and the niche application Stirling engine. Internal combustion engines are further classified in two ways: either a spark-ignition (SI) engine, where the spark plug initiates the combustion; or a compression-ignition (CI) engine, where the air within the cylinder is compressed, thus heating it, so that the heated air ignites fuel that is injected then or earlier.
A crosshead is a mechanism used as part of the slider-crank linkages of long reciprocating engines and reciprocating compressors to eliminate sideways pressure on the piston. Also, the crosshead enables the connecting rod to freely move outside the cylinder. Because of the very small bore-to-stroke ratio on such engines, the connecting rod would hit the cylinder walls and block the engine from rotating if the piston was attached directly to the connecting rod like on trunk engines. Therefore, the longitudinal dimension of the crosshead must be matched to the stroke of the engine.
An opposed-piston engine is a piston engine in which each cylinder has a piston at both ends, and no cylinder head.
A pumpjack is the overground drive for a reciprocating piston pump in an oil well.
The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle. It can also serve as a reversing gear. It is sometimes referred to as the "motion".
A connecting rod, also called a con rod, is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the crankshaft. The connecting rod is required to transmit the compressive and tensile forces from the piston, and rotate at both ends.
The Wärtsilä RT-flex96C is a two-stroke turbocharged low-speed diesel engine designed by the Finnish manufacturer Wärtsilä. It is designed for large container ships that run on heavy fuel oil. Its largest 14-cylinder version is 13.5 metres (44 ft) high, 26.59 m (87 ft) long, weighs over 2,300 tons, and produces 80,080 kW (107,390 hp). The engine is the largest reciprocating engine in the world.
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Engine balance refers to those factors in the design, production, engine tuning, maintenance and the operation of an engine that benefit from being balanced. Major considerations are:
A jackshaft is an intermediate shaft used to transfer power from a powered shaft such as the output shaft of an engine or motor to driven shafts such as the drive axles of a locomotive. As applied to railroad locomotives in the 19th and 20th centuries, jackshafts were typically in line with the drive axles of locomotives and connected to them by side rods. In general, each drive axle on a locomotive is free to move about one inch (2.5 cm) vertically relative to the frame, with the locomotive weight carried on springs. This means that if the engine, motor or transmission is rigidly attached to the locomotive frame, it cannot be rigidly connected to the axle. This problem can be solved by mounting the jackshaft on unsprung bearings and using side-rods or chain drives.
In rail terminology, the hammer blow is a vertical force which alternately adds to and subtracts from the locomotive's weight on a wheel. It is transferred to the track by the driving wheels of many steam locomotives. It is an out-of-balance force on the wheel. It is the result of a compromise when a locomotive's wheels are unbalanced to off-set horizontal reciprocating masses, such as connecting rods and pistons, to improve the ride. The hammer blow may cause damage to the locomotive and track if the wheel/rail force is high enough. 'Dynamic augment' is the US term for the same force.
A marine steam engine is a steam engine that is used to power a ship or boat. This article deals mainly with marine steam engines of the reciprocating type, which were in use from the inception of the steamboat in the early 19th century to their last years of large-scale manufacture during World War II. Reciprocating steam engines were progressively replaced in marine applications during the 20th century by steam turbines and marine diesel engines.
The 5AT Advanced Technology steam locomotive is a conceptual design conceived by the British engineer David Wardale, and first described in his definitive work on modern steam, The Red Devil and Other Tales from the Age of Steam.
Internal combustion engines come in a wide variety of types, but have certain family resemblances, and thus share many common types of components.
A steam motor is a form of steam engine used for light locomotives. They represented one of the final developments of the steam locomotive, in the final decades of the widespread use of steam power.
Reciprocating engine cylinders are often classified by whether they are single- or double-acting, depending on how the working fluid acts on the piston.
A return connecting rod, return piston rod or double piston rod engine or back-acting engine is a particular layout for a steam engine.
High-speed steam engines were one of the final developments of the stationary steam engine. They ran at a high speed, of several hundred rpm, which was needed by tasks such as electricity generation.
The Willans engine or central valve engine was a high-speed stationary steam engine used mainly for electricity generation around the start of the 20th century.
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