An oscillating cylinder steam engine (also known as a wobbler in the US) is a simple steam-engine design (proposed by
William Murdoch
William Murdoch (sometimes spelled Murdock) (21 August 1754 – 15 November 1839) was a Scottish engineer and inventor.
Murdoch was employed by the firm of Boulton & Watt and worked for them in Cornwall, as a steam engine erector for ten yea ...
at the end of 18th century) that requires no
valve gear
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 ...
. Instead the
cylinder
A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base.
A cylinder may also be defined as an infin ...
rocks, or oscillates, as the
crank moves the piston, pivoting in the mounting
trunnion
A trunnion (from Old French "''trognon''", trunk) is a cylindrical protrusion used as a mounting or pivoting point. First associated with cannons, they are an important military development.
Alternatively, a trunnion is a shaft that positions a ...
so that ports in the cylinder line up with ports in a fixed port face alternately to direct steam into or out of the cylinder.
Oscillating cylinder
steam engines are now mainly used in toys and models but, in the past, have been used in full-size working engines, mainly on
ships and small
stationary engines. They have the advantage of simplicity and, therefore, low manufacturing costs. They also tend to be more compact than other types of cylinder of the same capacity, which makes them advantageous for use in ships.
Operation
alt=oscillating cylinder diagram, Operation of a simple oscillating cylinder steam engine
The
steam
Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization ...
needs to be fed into the end of the cylinder at just the right time in the cycle to push the
piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tig ...
in the correct direction. In the other direction, the steam needs to be allowed to escape from the cylinder. As the
crankshaft
A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecting ...
rotates, the
piston rod
In a piston engine, a piston rod joins a piston to the crosshead and thus to the connecting rod that drives the crankshaft or (for steam locomotives) the driving wheels.
Internal combustion engines, and in particular all current automobile engin ...
moves up and down (or side to side in the case of a vertical cylinder) as well as in and out. Because the piston rod is rigid and the piston itself is long relative to its diameter, this causes the cylinder to rock, or "oscillate" on its special mounting (
trunnion
A trunnion (from Old French "''trognon''", trunk) is a cylindrical protrusion used as a mounting or pivoting point. First associated with cannons, they are an important military development.
Alternatively, a trunnion is a shaft that positions a ...
). In the design usually found in a toy or model engine, a hole in the side of the cylinder (one at each end for a
double-acting cylinder
In mechanical engineering, the cylinders of reciprocating engines are often classified by whether they are single- or double-acting, depending on how the working fluid acts on the piston.
Single-acting
A single-acting cylinder in a reciproc ...
) and a pair of holes in the port block are arranged so that this rocking motion lines up the holes at the correct times, allowing steam to enter the cylinder in one direction and to escape into the atmosphere or
condenser in the other direction.
[Roly Williams (2003-2010) ''Live Steam Toys - A Users Guide'', published by the author]
In full-size engines, the steam and exhaust ports are usually built into the pivot (
trunnion
A trunnion (from Old French "''trognon''", trunk) is a cylindrical protrusion used as a mounting or pivoting point. First associated with cannons, they are an important military development.
Alternatively, a trunnion is a shaft that positions a ...
) mounting. However, separate valves may be provided, controlled by the oscillating motion. This allows the cutoff timing to be varied to enable expansive working, as in, for example, the engine in the paddle ship PD Krippen. Alternatively, expansive working can also be provided by means of
Woolf compounding, just as compactly if the two expansion stages are provided on either side of a single piston within a single cylinder, all arranged as a
trunk engine
Trunk may refer to:
Biology
* Trunk (anatomy), synonym for torso
* Trunk (botany), a tree's central superstructure
* Trunk of corpus callosum, in neuroanatomy
* Elephant trunk, the proboscis of an elephant
Computing
* Trunk (software), in re ...
(however, an oscillating cylinder engine does not benefit from the advantage of compactness that a trunk engine would otherwise provide, as it has that already from its own design features). Each approach to expansive working compromises the advantage of simplicity but still retains the advantage of compactness.
Reversing
An oscillating cylinder engine cannot be reversed by means of the valve linkage (as in a normal fixed cylinder) because there is none. Reversing of the engine can be achieved by reversing the steam connections between inlet and exhaust or, in the case of small engines, by shifting the trunnion pivot point so that the port in the cylinder lines up with a different pair of ports in the port face. In the latter case, the fixed port face is usually provided with three ports, the central one being the steam feed and the outer two being exhausts, only one of which being in use at any time, depending on the required direction of running.
Marine
Examples
See also
*
Elbow engine
An elbow engine is a piston-based steam engine typically fed by steam or compressed air to drive a flywheel and/or mechanical load. It is based on a mechanism known as a Hobson's joint. Although not commonly used today for practical purposes, ...
- another cylinder-ported engine, not requiring additional valvegear
*
Carpet railway
Birmingham Dribbler or carpet railway describes a type of very early model railway. It is a bit of a misnomer, as the railway featured a model live steam railway locomotive, but no track – the locomotive was simply run across the floor. In som ...
– the 'Birmingham Dribbler' toy locomotives used this type of engine
References
{{Steam engine configurations
Steam engines