Axial engines (sometimes known as barrel or Z-crank engines) are a type of reciprocating engine with pistons arranged around an output shaft with their axes parallel to the shaft. Barrel refers to the cylindrical shape of the cylinder group (result of the pistons being spaced evenly around the central crankshaft and aligned parallel to the crankshaft axis) whilst the Z-crank alludes to the shape of the crankshaft.
The key advantage of the axial design is that the cylinders are arranged in parallel around the output/crank shaft rather than at 90 degrees as in crankshaft engines. As a result, it is a very compact, cylindrical engine, allowing variation in compression ratio of the engine while running. In a swashplate engine the piston rods stay parallel with the shaft, and piston side-forces that cause excessive wear can be eliminated almost completely. The small-end bearing of a traditional connecting rod, one of the most problematic bearings in a traditional engine, is eliminated. An alternate design, the Rand cam engine, replaces the plate with a sine-shaped cam.[clarification needed]
A wobble-plate is similar to a swashplate, in that the pistons press down on the plate in sequence, forcing it to nutate around its center. This motion can be simulated by placing a Compact Disc on a ball bearing at its centre and pressing down at progressive places around its circumference. The difference is that while a wobble plate nutates, a swash-plate rotates.
Axial engines are challenging to make practicable at typical engine operating speeds.
In 1911 the Macomber Rotary Engine Company of Los Angeles marketed one of the first axial internal-combustion engines, manufactured by the Avis Engine Company of Allston, Massachusetts. A four-stroke, air-cooled unit, it had seven cylinders and a variable compression ratio, altered by changing the wobble-plate angle and hence the length of piston stroke. It was called a "rotary engine" because the entire engine rotated apart from the end casings.
Ignition was supplied by a Bosch magneto directly driven from the cam gears. The high voltage current was then taken to a fixed electrode on the front bearing case, from which the sparks would jump to the spark plugs in the cylinder heads as they passed within 1/16 inch from it. According to Macomber's literature, it was "Guaranteed not to overheat".
The engine was claimed to be able to run at 150 to 1,500 rpm. At the normal speed of 1,000 rpm, it reportedly developed 50 hp. It weighed 230 pounds (100 kg) and it was 28 inches (710 mm) long by 19 inches (480 mm) in diameter.
In 1913 Statax-Motor of Zurich, Switzerland introduced a swashplate engine design. Only a single prototype was produced, which is currently held in the Science Museum, London. In 1914 the company moved to London to become the Statax Engine Company and planned on introducing a series of rotary engines; a 3-cylinder of 10 hp, a 5-cylinder of 40 hp, a 7-cylinder of 80 hp, and a 10-cylinder of 100 hp.
It appears only the 40 hp design was ever produced, which was installed in a Caudron G.II for the British 1914 Aerial Derby but was withdrawn before the flight. Hansen introduced an all-aluminum version of this design in 1922, but it is not clear if they produced it in any quantity. Much improved versions were introduced by Statax's German division in 1929, producing 42 hp in a new sleeve valve version known as the 29B. Greenwood and Raymond of San Francisco acquired the patent rights for the US, Canada, and Japan, and planned a 5-cylinder of 100 hp and a 9-cylinder of 350 hp.
In 1917 Anthony Michell obtained patents for his swashplate engine design. Its unique feature was the means of transferring the load from the pistons to the swashplate, achieved using tilting slipper pads sliding on a film of oil. Another innovation by Michell was his mathematical analysis of the mechanical design, including the mass and motion of the components, so that his engines were in perfect dynamic balance at all speeds.
In 1920 Michell established the Crankless Engines Company in Fitzroy (Australia), and produced working prototypes of pumps, compressors, car engines and aero engines, all based on the same basic design.
A number of companies obtained a manufacturing licence for Michell’s design. The most successful of these was the British company Waller and Son, who produced gas boosters.
The largest Michell crankless engine was the XB-4070, a diesel aircraft engine built for the US Navy. Consisting of 18 pistons, it was rated at 2000 horsepower and weighed 2150 pounds.
Experimental barrel engines for aircraft use were built and tested by Mr J.O. Almen of Seattle in the early 1920s, and by the mid-1920s the water-cooled Almen A-4 (18 cylinders, two groups of nine each horizontally opposed) had passed its United States Air Corps acceptance tests. It however never entered production, reportedly due to limited funds and the Air Corps' growing emphasis on air-cooled radial engines. The A-4 had much smaller frontal area than water-cooled engines of comparable power output, and thereby offered better streamlining possibilities. It was rated at 425 horsepower (317 kW), and weighed only 749 pounds (340 kg), thus giving a power/weight ratio of better than 1:2, a considerable design achievement at the time.
Heraclio Alfaro was a Spanish aviator who was knighted at the age of 18 by King Alfonso XIII of Spain for designing, building, and flying Spain's first airplane. He developed a barrel engine for aircraft use which was later produced by the Indian Motorcycle Company as the Alfaro. It was a perfect example of the "put in everything" design, as it included a sleeve valve system based on a rotating cylinder head, a design that never entered production on any engine. It was later developed further for use in the Doman helicopter by Stephen duPont, son of the president of the Indian Motorcycle Company, who had been one of Alfaro's students at MIT.
The Bristol Axial Engine of the mid-1930s was designed by Charles Benjamin Redrup for the Bristol Tramways and Carriage Company; it was a 7-litre, 9-cylinder, wobble-plate type engine. It was originally conceived as a power unit for buses, possibly because its compact format would allow it to be installed beneath the vehicle's floor. The engine had a single rotary valve to control induction and exhaust. Several variants were used in Bristol buses during the late 1930s, the engine going through several versions from RR1 to RR4, which had a power output of 145 hp at 2900 rpm. Development was halted in 1936 following a change of management at the Bristol company.
Perhaps the most refined of the designs was the British Wooler wobble-plate engine of 1947. This 6-cylinder engine was designed by John Wooler, better known as a motorcycle engine designer, for aircraft use. It was similar to the Bristol axial engine but had two wobble-plates, driven by 12 opposed pistons in 6 cylinders. The engine is often incorrectly referred to as a swashplate engine. A single example is preserved in the Aeroplane Gallery of The Science Museum, London.
The Dyna-Cam engine originally came from a design by the Blazer brothers, who worked for Studebaker in 1916. They sold the rights to Karl Herrmann, Studebaker's head of engineering, who developed the concept over many years, eventually taking out US patent 2237989 in 1941. It has 6 double-ended pistons working in 6 cylinders, and its 12 combustion chambers are fired every revolution of the drive shaft. The pistons drive a sine-shaped cam, as opposed to a swashplate or wobble-plate, hence its name.
In 1961, at the age of 80, Herrmann sold the rights to one of his employees, Edward Palmer, who set up the Dyna-Cam Engine Corp. along with son Dennis. Edward's son Dennis and daughter Pat then helped get the engine installed in a Piper Arrow. The engine was flown for about 700 hours in the Piper Arrow from 1987 through 1991. Their longest engine ran for nearly 4000 hours before overhaul. Dyna-Cam opened an R & D facility in around 1993 and won many various awards from NASA, US Navy, the US Marine Corps, California Energy Commission, Air Quality Management District, and Los Angeles Regional Technology Alliance for different variations of the same Dyna-Cam Engine. About 40 prototype engines were built by the Herrmann Group and another 25 built by the Dyna-Cam Group since they acquired the engine and opened their shop. A new patent was granted to Dennis Palmer and Edward Palmer first in 1985 and then several more around 2000 to Dennis Palmer. In 2003 the assets of the Dyna-Cam Engine Corp were acquired by first Aero-Marine Corp. who changed their name to Axial Vector Engine Corporation. Axial Vector then totally re-designed the cam engine. Axial Vector's new engine, like many of the others on this list, suffers from the "put in everything" problem, including piezoelectric valves and ignition, ceramic cylinder liners with no piston rings, and a variety of other advanced features. It has almost no similarity to the original Herrmann and Dyna-Cam Engine since the Dyna-Cam Engine used conventional valves, piston rings, accessories, had no unproven ceramic materials and actually flew in a Piper Arrow and also powered a 20-foot (6.1 m) Eliminator Ski Boat for over four years.
UK company FairDiesel Limited is designing two-stroke diesel opposed piston barrel engines which use non-sinusoidal cams, for industrial applications and aviation use. Their designs range from a 2-cylinder, 80 mm bore engine to a 32-cylinder, 160 mm bore one.
New Zealand company Duke Engines started in 1993 has created several different engines and installed one in a car in 1999. The engine runs a 5-cylinder, 3 litre, 4-stroke internal combustion engine platform with its unique axial arrangement which is in its 3rd generation. During development the Duke has been tested at Mahle Powertrain in the UK & in the USA; test results are available with it also having multi fuel capabilities.
Cylindrical Energy Module
The Cylindrical Energy Module is a sine-waved Swashplate engine that can also be used as a stand-alone pump powered by an alternate source. The rotating swash-plate rotor assembly is moved back and forth with the help of piston drive pins which follow a stationary Sinusoidal Cam Track that encircles the rotor assembly.
- The most well-known application is in torpedoes, where the cylindrical shape is desirable. The modern Mark 48 torpedo is powered by a 500 hp swashplate engine geared to a pump-jet propulsor. It is fueled by Otto fuel II, a monopropellant that requires no oxygen supply and can propel the torpedo at up to 65 knots (120 km/h) (74.56 mph).
- Other applications include pneumatic and hydraulic motors, hydrostatic transmissions such as Honda's Hondamatic CVT, and air conditioner pumps. Also, some Stirling engines use a swashplate arrangement, e.g. Stirling Thermal Motors' STM 4-120 engine.
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