The radial engine is a reciprocating type internal combustion engine configuration in which the cylinders "radiate" outward from a central crankcase like the spokes of a wheel. It resembles a stylized star when viewed from the front, and is called a "star engine" (German Sternmotor, French moteur en étoile) in some languages. The radial configuration was very commonly used for aircraft engines before turbine engines became predominant.
Since the axes of the cylinders are coplanar, the connecting rods cannot all be directly attached to the crankshaft unless mechanically complex forked connecting rods are used, none of which have been successful. Instead, the pistons are connected to the crankshaft with a master-and-articulating-rod assembly. One piston, the uppermost one in the animation, has a master rod with a direct attachment to the crankshaft. The remaining pistons pin their connecting rods' attachments to rings around the edge of the master rod. Extra "rows" of radial cylinders can be added in order to increase the capacity of the engine without adding to its diameter.
Four-stroke radials have an odd number of cylinders per row, so that a consistent every-other-piston firing order can be maintained, providing smooth operation. For example, on a five-cylinder engine the firing order is 1, 3, 5, 2, 4 and back to cylinder 1. Moreover, this always leaves a one-piston gap between the piston on its combustion stroke and the piston on compression. The active stroke directly helps compressing the next cylinder to fire, so making the motion more uniform. If an even number of cylinders was used, the equally timed firing cycle would not be feasible. The prototype radial Zoche aero-diesels (below) have an even number of cylinders, either four or eight; but this is not problematic, because they are two-stroke engines, with twice the number of power strokes as a four-stroke engine.
The radial engine normally uses fewer cams compared to other types. As with most four-strokes, the crankshaft takes two revolutions to complete the four strokes of each piston (intake, compression, combustion, exhaust). The camshaft ring is geared to spin slower and in the opposite direction to the crankshaft. The cam lobes are placed in two rows for the intake and exhaust. For the example, four cams serve all five cylinders, whereas 10 would be required for a typical inline engine with the same number of cylinders and valves.
Most radial engines use overhead poppet valves driven by pushrods and lifters on a cam plate which is concentric with the crankshaft, with a few smaller radials, like the Kinner B-5 and Russian Shvetsov M-11, using individual camshafts within the crankcase for each cylinder. A few engines utilize sleeve valves such as the 14-cylinder Bristol Hercules and the 18-cylinder Bristol Centaurus, which are quieter and smoother running but require much tighter manufacturing tolerances.
C. M. Manly constructed a water-cooled five-cylinder radial engine in 1901, a conversion of one of Stephen Balzer's rotary engines, for Langley's Aerodrome aircraft. Manly's engine produced 52 hp (39 kW) at 950 rpm.
In 1903–1904 Jacob Ellehammer used his experience constructing motorcycles to build the world's first air-cooled radial engine, a three-cylinder engine which he used as the basis for a more powerful five-cylinder model in 1907. This was installed in his triplane and made a number of short free-flight hops.
Another early radial engine was the three-cylinder Anzani, originally built as a W3 "fan" configuration, one of which powered Louis Blériot's Blériot XI across the English Channel. Before 1914, Alessandro Anzani had developed radial engines ranging from 3 cylinders (spaced 120° apart) — early enough to have been used on a few French-built examples of the famous Blériot XI from the original Blériot factory — to a massive 20-cylinder engine of 200 hp (150 kW), with its cylinders arranged in four rows of five cylinders apiece.
Radial engines are regarded as being air-cooled almost by definition — so that it is interesting that one of the most successful of the early radial engines was the Salmson 9Z series of nine-cylinder water-cooled radial engines that were produced in large numbers during the First World War. Georges Canton and Pierre Unné patented the original engine design in 1909, offering it to the Salmson company — and the engine was often known as the Canton-Unné.
From 1909 to 1919 the radial engine was overshadowed by its close relative, the rotary engine — which differed from the so-called "stationary" radial in that the crankcase and cylinders revolved with the propeller. Mechanically it was identical in concept to the later radial except that the propeller was bolted to the engine, and the crankshaft to the airframe. The problem of the cooling of the cylinders, a major factor with the early "stationary" radials, was solved by the engine generating its own cooling airflow.
Little development of the radial engine was undertaken in Germany during World War I, where most aircraft used water-cooled inline 6-cylinder engines. The German Oberursel firm made licensed copies of the Gnome and Le Rhône rotary powerplants while Siemens-Halske built a number of their own designs including the Siemens-Halske Sh.III eleven-cylinder rotary engine, which was unusual for the period in being geared down, so that the engine's "total" rotational velocity could spin at a higher speed than the propeller — which was still bolted solidly to the crankcase and cylinders, as with most other rotary radial engines — with the crankshaft and associated internal parts spinning in the opposite direction through bevel gears in the rear of the crankcase.
By the end of the war the rotary engine had reached the limits of the design, particularly in regard to the amount of fuel and air that could be drawn into the cylinders through the hollow crankshaft, while advances in both metallurgy and cylinder cooling finally allowed stationary radial engines to supersede rotary engines. In the early 1920s Le Rhône converted a number of their rotary engines into stationary radial engines although most of the early radial engines were new.
By 1918, the potential advantages of air-cooled radials over the water-cooled inline engine and air-cooled rotary engine that had powered World War I aircraft were appreciated but remained unrealized. While British designers had produced the ABC Dragonfly radial in 1917, they were unable to resolve the cooling problems, and it was not until the 1920s that the Bristol Aeroplane Company and Armstrong Siddeley produced reliable air-cooled radials such as the Bristol Jupiter and the Armstrong Siddeley Jaguar.
In the United States, the National Advisory Committee for Aeronautics (NACA) noted in 1920 that air-cooled radials could offer an increase in the power-to-weight ratio and reliability, and by 1921 the U.S. Navy had announced it would only order aircraft fitted with air-cooled radials while other naval air arms followed suit. Charles Lawrance's J-1 engine was developed in 1922 with Navy funding, and using aluminium cylinders with steel liners ran for an unprecedented 300 hours, at a time when 50 hours endurance was normal. At the urging of the Army and Navy the Wright Aeronautical Corporation bought Lawrance's company, and subsequent engines were built under the Wright name. The radial engines gave confidence to Navy pilots performing long-range overwater flights.
Wright's 225 hp (168 kW) J-5 Whirlwind radial engine of 1925 was widely acknowledged as "the first truly reliable aircraft engine". Wright employed Giuseppe Mario Bellanca to design an aircraft to showcase it, and the result was the Wright-Bellanca 1, or WB-1, which was first flown in the latter part of that year. The J-5 was used on many advanced aircraft of the day, including Charles Lindbergh's Spirit of St. Louis with which he made the first solo trans-Atlantic flight.
In 1925, the American rival firm to Wright's radial engine production efforts, Pratt & Whitney, was founded. The P & W firm's initial offering, the Pratt & Whitney R-1340 Wasp, test run later that year, began the evolution of the many models of Pratt & Whitney radial engines that were to appear during the second quarter of the 20th century, among them the 14-cylinder, twin-row Pratt & Whitney R-1830 Twin Wasp, the most-produced aviation engine of any single design, with a total production quantity of nearly 175,000 engines.
In the United Kingdom the Bristol Aeroplane Company was concentrating on developing radials such as the Jupiter, Mercury and sleeve valve Hercules radials. France, Germany, Russia and Japan largely built licenced or locally improved versions of the Armstrong Siddeley, Bristol, Wright, or Pratt & Whitney radials.
Radial versus inline debate
- Weight: Liquid-cooled inline engines often weigh more than equivalent air-cooled radial engines.
- Damage tolerance: Liquid cooling systems are generally more vulnerable to battle damage. Minor shrapnel damage easily results in a loss of coolant and consequent engine seizure, while an air-cooled radial might be largely unaffected by small damage.
- Simplicity: Radials have shorter and stiffer crankshafts, a single bank radial needing only two crankshaft bearings as opposed to the seven required for a liquid-cooled six-cylinder inline engine of similar stiffness.
- Reliability:The shorter crankshaft also produces less vibration and hence higher reliability through reduced wear and fatigue.
- Smooth running: It is typically easier to achieve smooth running with a radial engine
- Cooling: While a single bank radial permits all cylinders to be cooled equally, the same is not true for multi-row engines where the rear cylinders can be affected by the heat coming off the front row, and air flow being masked.
- Drag: Having all the cylinders exposed to the airflow increases drag considerably, adding turbulence that can destroy the laminar airflow over the fuselage and adjacent wings.
- Power: Because each cylinder on a radial engine has its own head, it is impractical to use a multivalve valvetrain on a radial engine. Therefore, almost all radial engines use a two valve pushrod-type valvetrain which may result in less power for a given displacement than multi-valve inline engines.
- Visibility: Pilot visibility is often poorer due to the bulk of the engine
- Installation: The designer can be limited in engine placement, having to ensure adequate cooling air, which can be a challenge in a buried engine installation or pusher configurations.
The limitations of the poppet valve were largely overcome by the development of the Sleeve-valve, but at the cost of increased complexity, reduced reliability and increased maintenance costs.
The answer to some of the drag issues was the addition of specially designed cowlings with baffles to force the air over the cylinders. The first effective drag reducing cowling that didn't impair engine cooling was the British Townend ring or "drag ring" which formed a narrow band around the engine covering the cylinder heads, not only reducing drag, but adding a small amount of thrust. The National Advisory Committee for Aeronautics then studied the problem, developing the NACA cowling which further reduced drag, increased thrust and improved cooling. Nearly all aircraft radial engine installations since have used NACA type cowlings. The thrust generated by both the Townend ring and the NACA cowling was due to the Meredith Effect (discovered by British researchers), whereby the heat added during the cooling process was used to expand the exhausting cooling air through a nozzle producing thrust. The same effect was also put to use in the radiators of several aircraft that used liquid-cooled engines such as the Spitfire and Mustang.
Tight fitting cowlings also tended to reduce the bulk of engine installations and improve visibility, particularly for single engined fighter aircraft.
The German BMW firm came up with an innovation for their World War II-produced BMW 801 twin-row, 14-cylinder radial engine by integrating the 801's oil cooler within the forward cowl's structure — BMW designed the cowls used for every example of the 801 ever built, all with the integral oil cooler present in the forwardmost section of the cowl, which made it an appropriate choice for a powerplant to be "unitized" in the effort to produce a rapidly interchangeable aviation engine for maintenance purposes in front-line combat conditions, within the so-called German Kraftei (power-egg) unitized engine installation design philosophy. The BMW 801 ended up being the Third Riech's most-produced radial engine of any type, with some 28,000 examples built.
While inline liquid-cooled engines continued to be common in new designs until late in World War II, radial engines dominated afterwards until overtaken by jet engines, with the late-war Hawker Sea Fury and Grumman Bearcat, two of the fastest production piston-engined aircraft ever built, using radial engines. Factors influencing the choice of radial over inline were reliability and simplicity in maintenance.
Originally radial engines had one row of cylinders, but as engine sizes increased it became necessary to add extra rows. The first known radial-configuration engine to ever use a twin-row design was the 160 hp Gnôme "Double Lambda" rotary engine of 1912, designed as a 14-cylinder twin-row version of the firm's 80 hp Lambda single-row seven-cylinder rotary, with only the German Oberursel U.III clone of the Double Lambda reproducing the Gnome Double Lambda's twin-row design before the end of World War I. Most stationary radial engines did not exceed two rows, but the largest displacement mass-produced aircraft radial engine, the R-4360, with cylinders in corncob configuration, was a 28-cylinder 4-row radial engine used in a number of large American aircraft in the post-World War II period. Even though BMW had their best success with the BMW 801 14-cylinder twin-row radial during the war years, their later twin-row BMW 802 air-cooled eighteen-cylinder engine, and their complex, Wasp Major displacement-class water-cooled BMW 803 four-row radial never left the test bench. The Soviet Union built a limited number of 'Zvezda' engines with 56 cylinders but aircraft engines of this size, power and complexity were made obsolete by large turboprop engines which easily exceeded them in power without the weight or complexity. Large radials continued to be built for other uses though, as 112-cylinder diesel boat engines with 16 rows of 7 cylinders each displacing 383 liters (23,931 in3) and producing 10,000 hp (7,500 kW) were used on fast attack craft, such as Osa class missile boats.
A number of companies continue to build radials today. Vedeneyev produces the M-14P radial of 360–450 hp (270–340 kW) as used on Yakovlev and Sukhoi aerobatic aircraft. The M-14P is also used by builders of homebuilt aircraft, such as the Culp Special, and Culp Sopwith Pup, Pitts S12 "Monster" and the Murphy "Moose". 110 hp (82 kW) 7-cylinder and 150 hp (110 kW) 9-cylinder engines are available from Australia's Rotec Engineering. HCI Aviation offers the R180 5-cylinder (75 hp (56 kW)) and R220 7-cylinder (110 hp (82 kW)), available "ready to fly" and as a build-it-yourself kit. Verner Motor of the Czech Republic now builds several radial engines ranging in power from 25 to 150 hp (19 to 112 kW). Miniature radial engines for model airplanes are available from O. S. Engines, Saito Seisakusho of Japan and Shijiazhuang of China, and Evolution (designed by Wolfgang Seidel of Germany, and made in India) and Technopower in the USA.
While most radial engines have been produced for gasoline, there have been diesel radial engines. Two major advantages favour diesel engines — lower fuel consumption and reduced fire risk.
Packard designed and built a 9-cylinder 980 cubic inch displacement diesel radial aircraft engine, the 225 horsepower (168 kW) DR-980, in 1928. On 28 May 1931, a DR-980 powered Bellanca CH-300, with 481 gallons of fuel, piloted by Walter Edwin Lees and Frederick Brossy set a record for staying aloft for 84 hours and 32 minutes without being refueled. This record stood for 55 years until broken by the Rutan Voyager.
In 1932 the French company Clerget developed the 14D, a 14-cylinder two-stroke diesel radial engine. After a series of improvements, in 1938 the 14F2 model produced 520 hp (390 kW) at 1910 rpm cruise power, with a power-to-weight ratio near that of contemporary gasoline engines and a specific fuel consumption of roughly 80% that for an equivalent gasoline engine. During WWII the research continued, but no mass-production occurred because of the Nazi occupation. By 1943 the engine had grown to produce over 1,000 hp (750 kW) with a turbocharger. After the war, the Clerget company was integrated in the SNECMA company and had plans for a 32-cylinder diesel engine of 4,000 hp (3,000 kW), but in 1947 the company abandoned piston engine development in favour of the emerging turbine engines.
The Nordberg Manufacturing Company of the United States developed and produced a series of large two-stroke radial diesel engines from the late 1940s for electrical production, primarily at aluminium smelters and for pumping water. They differed from most radials in that they had an even number of cylinders in a single bank (or row) and an unusual double master connecting rod. Variants were built that could be run on either diesel oil or gasoline or mixtures of both. A number of powerhouse installations utilising large numbers of these engines were made in the U.S.
Compressed air radial engines
A number of radial motors operating on compressed air have been designed, mostly for use in model airplanes and in gas compressors.
Use in tanks
In the years leading up to World War II, as the need for armored vehicles was realized, designers were faced with the problem of how to power the vehicles, and turned to using aircraft engines, among them radial types. The radial aircraft engines provided greater power-to-weight ratios and were more reliable than conventional inline vehicle engines available at the time. This reliance had a downside though: if the engines were mounted vertically, as in the M3 Lee and M4 Sherman, their comparatively large diameter gave the tank a higher silhouette than designs using inline engines.
The Continental R-670, a 7-cylinder radial aero engine which first flew in 1931, became a widely used tank powerplant, being installed in the M1 Combat Car, M2 Light Tank, M3 Stuart, M3 Lee, LVT-2 Water Buffalo.
The Guiberson T-1020, a 9-cylinder radial diesel aero engine, was used in the M1A1E1, while the Continental R975 saw service in the M4 Sherman, M7 Priest, M18 Hellcat tank destroyer, and the M44 Self Propelled Howitzer.
Model radial engines
A number of multi-cylinder 4-stroke model engines have been commercially available in a radial configuration, beginning with the Japanese O.S. Max firm's FR5-300 five-cylinder, 3.0 cu.in. (50 cm3) displacement "Sirius" radial in 1986. The American "Technopower" firm had made smaller-displacement five- and seven-cylinder model radial engines as early as 1976, but the OS firm's engine was the first mass-produced radial engine design in aeromodeling history. The rival Saito Seisakusho firm in Japan has since produced a similarly sized five-cylinder radial four-stroke model engine of their own as a direct rival to the OS design, with Saito also creating a trio of three-cylinder radial engines ranging from 0.90 cu.in. (15 cm3) to 4.50 cu.in. (75 cm3) in displacement, also all now available in spark-ignition format up to 84 cm3 displacement for use with gasoline. The German Seidel firm formerly made both seven- and nine-cylinder "large" (starting at 35 cm3 displacement) radio control model radial engines, mostly for glow plug ignition, with an experimental fourteen-cylinder twin-row radial being tried out - the American Evolution firm now sells the Seidel-designed radials, with their manufacturing being done in India.
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