science

Single- and double-acting cylinders

Single- and double-acting cylinders is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Single- and double-acting cylinders rather than just read about it. In short: 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 reciprocating engine is a cylinder in which the working fluid acts on one side of the piston only.

Key takeaways

Single- and double-acting cylinders belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Single- and double-acting cylinders to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Single- and double-acting cylinders from memory before moving on to harder problems.

Reference excerpt

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 reciprocating engine is a cylinder in which the working fluid acts on one side of the piston only. A single-acting cylinder relies on the load, springs, other cylinders, or the momentum of a flywheel, to push the piston back in the other direction. Single-acting cylinders are found in most kinds of reciprocating engine. They are almost universal in internal combustion engines (e.g. petrol and diesel engines) and are also used in many external combustion engines such as Stirling engines and some steam engines. They are also found in pumps and hydraulic rams.

Double-acting

A double-acting cylinder is a cylinder in which the working fluid acts alternately on both sides of the piston. In order to connect the piston in a double-acting cylinder to an external mechanism, such as a crank shaft, a hole must be provided in one end of the cylinder for the piston rod, and this is fitted with a gland or "stuffing box" to prevent escape of the working fluid. Double-acting cylinders are common in steam engines but unusual in other engine types. Many hydraulic and pneumatic cylinders use them where it is needed to produce a force in both directions. A double-acting hydraulic cylinder has a port at each end, supplied with hydraulic fluid for both the retraction and extension of the piston. A double-acting cylinder is used where an external force is not available to retract the piston or it can be used where high force is required in both directions of travel.

Steam engines

Steam engines normally use double-acting cylinders. However, early steam engines, such as atmospheric engines and some beam engines, were single-acting. These often transmitted their force through the beam by means of chains and an "arch head", as only a tension in one direction was needed. Where these were used for pumping mine shafts and only had to act against a load in one direction, single-acting designs remained in use for many years. The main impetus towards double-acting cylinders came when James Watt was trying to develop a rotative beam engine, that could be used to drive machinery via an output shaft. Compared to a single-cylinder engine, a double-acting cylinder gave a smoother power output. The high-pressure engine, as developed by Richard Trevithick, used double-acting pistons and became the model for most steam engines afterwards. Some of the later steam engines, the high-speed steam engines, used single-acting pistons of a new design. The crosshead became part of the piston, and there was no longer any piston rod. This was for similar reasons to the internal combustion engine, as avoiding the piston rod and its seals allowed a more effective crankcase lubrication system. Small models and toys often use single-acting cylinders for the above reason but also to reduce manufacturing costs.

Internal combustion engines

In contrast to steam engines, nearly all internal combustion engines have used single-acting cylinders. Their pistons are usually trunk pistons, where the gudgeon pin joint of the connecting rod is within the piston itself. This avoids the crosshead, piston rod and its sealing gland, but it also makes a single-acting piston almost essential. This, in turn, has the advantage of allowing easy access to the bottom of the piston for lubricating oil, which also has an important cooling function. This avoids local overheating of the piston and rings.

Crankcase compression two-stroke engines Small petrol two-stroke engines, such as for motorcycles, use crankcase compression rather than a separate supercharger or scavenge blower. This uses both sides of the piston as working faces, the lower side of the piston acting as a piston compressor to compress the inlet charge ready for the next stroke. The piston is still considered as single-acting, as only one of these faces produces power.

Double-acting internal combustion engines

Some early gas engines, such as Lenoir's original engines, from around 1860, were double-acting and followed steam engines in their design. Internal combustion engines soon switched to single-acting cylinders. This was for two reasons: as for the high-speed steam engine, the high force on each piston and its connecting rod was so great that it placed large demands upon the bearings. A single-acting piston, where the direction of the forces was consistently compressive along the connecting rod, allowed for tighter bearing clearances. Secondly the need for large valve areas to provide good gas flow, whilst requiring a small volume for the combustion chamber so as to provide good compression, monopolised the space available in the cylinder head. Lenoir's steam engine-derived cylinder was inadequate for the petrol engine and so a new design, based around poppet valves and a single-acting trunk piston appeared instead.

Extremely large gas engines were also built as blowing engines for blast furnaces, with one or two extremely large cylinders and powered by the burning of furnace gas. These, particularly those built by Körting, used double-acting cylinders. Gas engines require little or no compression of their charge, in comparison to petrol or compression-ignition engines, and so the double-acting cylinder designs were still adequate, despite their narrow, convoluted passageways. Double-acting cylinders have been infrequently used for internal combustion engines since, although Burmeister & Wain made 2-stroke cycle double-acting (2-SCDA) diesels for marine propulsion before 1930. The first, of 7,000 hp, was fitted in the British MV Amerika (United Baltic Co.) in 1929. The two B&W SCDA engines fitted to the MV Stirling Castle in 1937 produced 24,000 hp each.

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Illustrations

Worked examples

Example 1 — a first encounter with Single- and double-acting cylinders

Start with the simplest possible case. Write down what Single- and double-acting cylinders claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Single- and double-acting cylinders before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Single- and double-acting cylinders ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Single- and double-acting cylinders

In research: Single- and double-acting cylinders appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Single- and double-acting cylinders in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Single- and double-acting cylinders is common in secondary-school and first-year university syllabi. It links to neighbouring topics Hydraulics, Piston engines, Steam engines, so understanding it makes those chapters shorter.
In everyday life: Look for Single- and double-acting cylinders outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Single- and double-acting cylinders in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Single- and double-acting cylinders means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Single- and double-acting cylinders out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Single- and double-acting cylinders in simple terms?

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 reciprocating engine is a cylinder in which the working fluid acts…

Why does Single- and double-acting cylinders matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Single- and double-acting cylinders?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Single- and double-acting cylinders.