Single- and double-acting cylinders

• 1. Introduction

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  Single- and double-acting cylinders sit at the heart of how reciprocating engines convert pressure into motion. The difference between them sounds simple: in one design, the working fluid pushes the piston from only one side; in the other, it pushes from both. But that single mechanical choice has shaped the design of steam locomotives, marine diesels, submarine engines, and every car engine on the road today.

  Why did internal combustion engines almost universally adopt one approach while steam engines settled on another? What happens when an engineer tries to force the wrong design into a confined space, as the US Navy discovered aboard a submarine in the 1930s? And how did a Scottish inventor named James Watt and an English engineer named Richard Trevithick set the course for the modern engine by arguing, in iron and steam, for the double-acting cylinder? Those are the questions this documentary will answer.

• 2. How Each Design Works

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  A single-acting cylinder lets the working fluid act on only one face of the piston. The return stroke depends on something else entirely: the load pressing back, a spring, another cylinder firing in sequence, or the spinning inertia of a flywheel.

  A double-acting cylinder removes that dependency. The working fluid is routed alternately to both sides of the piston, pushing it out and then pulling it back under pressure. To make that possible, one end of the cylinder must have a hole for the piston rod to pass through. That hole is sealed with what engineers call a gland or stuffing box, a tight packing that keeps the working fluid from escaping around the moving rod.

  Hydraulic double-acting cylinders illustrate the design clearly. Each end of the cylinder has its own port, and hydraulic fluid is supplied to one port for extension and to the other for retraction. This makes the cylinder capable of generating force in both directions of travel without relying on any external load to push the piston back. That capability became essential wherever gravity or spring return could not be guaranteed.

• 3. Steam and the Push for Rotation

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  Early steam engines, including atmospheric engines and certain beam engines, were single-acting. Because these machines typically pumped water from mine shafts, pulling the load in one direction was enough. The force was transmitted through chains attached to an arch head on the beam, a configuration that only needed tension in one direction.

  James Watt changed the calculation when he set out to build a rotative beam engine capable of driving machinery through a rotating output shaft. A single-acting cylinder delivers power in only one half of each stroke, which produces an uneven, lurching rotation. A double-acting cylinder fires on both the downstroke and the upstroke, smoothing the power delivery significantly. Watt adopted the double-acting design for exactly this reason.

  Richard Trevithick then took the double-acting piston further when he developed the high-pressure engine. His design became the template that most subsequent steam engines followed. The one group that later reversed course was the high-speed steam engine. These machines moved the crosshead into the piston itself, eliminating the piston rod. Removing the rod and its seals opened up the crankcase to the kind of oil lubrication that could not coexist with a stuffing box. Small steam models and toys followed the same logic, adding reduced manufacturing cost as another reason to stay single-acting.

• 4. Why Petrol Engines Went Single-Acting

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  Lenoir's original gas engines, built around 1860, were double-acting, following the steam engine tradition directly. Internal combustion engines moved away from that design quickly, and for reasons that had nothing to do with tradition.

  The first problem was mechanical force. The explosions inside a petrol engine place enormous loads on the piston and connecting rod. In a single-acting arrangement, that force always pushes in the same compressive direction along the rod, which allows engineers to design tighter bearing clearances. A double-acting setup would reverse the loading direction on alternate strokes, demanding much looser tolerances to survive.

  The second problem was space. A petrol engine needs a large valve area to flow enough gas in and out, but it also needs a small combustion chamber volume to achieve good compression. These two requirements compete for the same real estate in the cylinder head. The steam engine cylinder layout, with its wide passages suited to slower-moving steam, could not accommodate poppet valves efficiently. So engineers designed a new cylinder around the trunk piston, where the gudgeon pin connecting the piston to the connecting rod sits inside the piston itself. This removes the crosshead and piston rod entirely. It also opens the underside of the piston to a bath of lubricating oil, which simultaneously lubricates and cools the piston crown and rings, preventing the local overheating that would otherwise destroy them.

  Small two-stroke petrol engines, such as those used in motorcycles, technically use both faces of the piston: the underside acts as a compressor for the incoming charge. Engineers still classify these pistons as single-acting, because only one face produces actual power.

• 5. Exceptions and Outliers

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  Some internal combustion applications did persist with double-acting cylinders long after the mainstream had abandoned them. Extremely large gas engines built as blowing engines for blast furnaces, with one or two enormous cylinders running on furnace gas, used double-acting designs. The manufacturer Körting was particularly associated with these machines. Gas engines of this type require little or no compression of the incoming charge, unlike petrol or diesel engines, so the convoluted internal passageways of a double-acting cylinder did not hurt their performance the way they would in a high-compression engine.

  For marine propulsion, the Danish firm Burmeister and Wain built two-stroke cycle double-acting diesels before 1930. The first of these engines, rated at 7,000 horsepower, was installed in the British motor vessel Amerika, operated by the United Baltic Company, in 1929. By 1937, a later pair of Burmeister and Wain engines each produced 24,000 horsepower. These figures show that the double-acting principle could be scaled to genuinely impressive outputs when the engineering was matched to the application.

• 6. The USS Pompano Disaster

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  In 1935 the US Navy ordered the submarine USS Pompano as part of the Perch class. Six boats were built in total, and the Navy equipped them with three different diesel engine designs from different manufacturers, treating the class partly as a live test of competing technologies.

  Pompano received engines from H.O.R., which stood for Hooven-Owens-Rentschler. These were 8-cylinder double-acting diesels, built under licence from the MAN auxiliary engines fitted to the German cruiser Leipzig. The attraction of double-acting engines for submarine use was compactness: fitting more power into the confined hull space was a persistent design challenge, and double-acting and opposed-piston configurations both offered power-to-volume advantages over conventional single-acting engines.

  The H.O.R. engines failed catastrophically. They were wrecked during trials before Pompano had even left the Mare Island Navy Yard. The submarine sat idle for eight months until new engines could be fitted in 1938. Even those replacement engines were considered unsatisfactory, and in 1942 the boat was re-engined again, this time with Fairbanks-Morse engines.

  While Pompano was still under construction, the Salmon-class submarines were ordered. Three of these were built by Electric Boat with a 9-cylinder development of the same H.O.R. design. The Salmon-class engines were less catastrophic but still troublesome, and those boats were later rebuilt with the single-acting General Motors 16-248 V16 engines used by their sister vessels. Several Electric Boat submarines of the Sargo, Seadragon, and early Gato classes were also fitted with the 9-cylinder H.O.R. engines and later re-engined. The episode stands as one of the clearest demonstrations in naval history of what happens when a theoretically promising design encounters the unforgiving demands of operational service.