Should aircraft engines be liquid-cooled or air-cooled? This “difference of opinion” is about a hundred years old and without a doubt the argument will continue as long as piston engines power the airplanes we fly. The manner in which the question is stated is misleading, however, since all waste heat that comes through the structure of an engine is eventually delivered to the air. In “liquid-cooled” engines the coolant can be water, ethylene glycol, a mixture of the two, or one of the many other liquids that have been tried and found wanting. Its primary purpose is to carry heat from the cylinder barrel and head to the radiator through which air, the actual cooling medium, flows. Proponents of liquid-cooling–now as in the past–can point to some benefits and operational advantages such as lessened hazard of shock cooling an engine, being able to direct dedicated coolant flow to critical areas in the cylinder head such as the exhaust valve seat and guide area, flexibility in radiator placement, greater structural rigidity in the engine, and having the option of designing airframes with a relatively small cross-sectional area that could still house a powerful engine. With every advantage, imagined or real, there is almost always a price to pay. Those who opted for liquid-cooled engines had to accept added weight, greater possibility of battle damage in military applications, and greater system complexity as the penalties. Such is life.

The general concept of “liquid-cooling” an engine has remained basically the same since before the Wright brothers made their historic flight, except for some significant mechanical, chemical, and thermal improvements. Those who chose to cool engines by the seemingly simpler direct transfer of waste heat from the cylinder to the air have had a much more tortuous and rocky path to follow, generally speaking. The developers of effective air-cooled engine installations had to, among other things, invent effective engine cowlings, conduct extensive studies of the aerodynamic behaviour of air inside a cowling and around cylinders, and deal with myriad metallurgical and other problems in the engine itself in order to extend the life of critical components. Many choices had to be made with regard to cylinder structure and arrangement, valve placement and actuation, the number of valves per cylinder, and the ratio of heat dissipation between the air and oil, to name but a few. As in most engineering activities where there is not an established body of information from which decision-making assistance can be drawn, wrong choices were made that doomed some promising engines and drastically extended the development process of others. Probing the “edge of the envelope” has never been for the faint of heart.

Inline and V-type engines. Conceptually, the air-cooled cylinder has always been associated with low weight and simplicity since no secondary means of heat transfer was necessary. Pioneer engine designers were well aware of this and one of the earliest successful air-cooled aviation engines was the V-8 that Glenn Curtiss used to power the June Bug in 1908. It reflected the technology of that era and the individual cylinders with integral heads were grey iron castings with relatively widely spaced fins. The choice of gray cast iron as a cylinder material was logical at that time. Its machining and wear characteristics were relatively well understood since it had been used extensively in manufacturing engines of all types. Curtiss no doubt understood that aluminium would provide much better heat transfer but it had been in commercial use for only about 25 years and suitable alloys for producing dense, strong, heat-treatable castings had not yet been developed. Also, an aluminium cylinder would have required a cast iron or steel sleeve, bronze or cast iron valve guides, and valve seat inserts, making the construction of the engine considerably more complex. While Curtiss no doubt also understood the value of deep, closely spaced fins on air-cooled cylinders regardless of the material used, foundry technology, particularly the making of baked sand moulds and cores necessary for such castings, had apparently not progressed to the point that cylinders of acceptable quality could be cast consistently.

European thinking tended to follow the same trends with regard to materials and engine layout. Renault in France introduced an air-cooled V-8 with individual cast iron cylinders with integral heads in 1909. Attempts to increase the power output of this engine brought on drastic cooling problems that were only partly alleviated by use of an engine-driven cooling fan. Larger versions of the Renault engine in V-8 and V-12 form were developed and built in France and also by the Royal Aircraft Factory in Britain during World War I, but regardless of size the engines were characterized by very short exhaust valve life and extremely high fuel consumption. According to one author, (L.J.K. Setright) these engines travelled a fine line between thermal and mechanical disaster.

With the excellent vision provided by hindsight one can see that the Renault and similar engines were, to a considerable extent, fuel-cooled as a means of extending the life of certain critical components, particularly exhaust valves. This was a common characteristic of practically all of the air-cooled in-line and V-type engines of the World War I era. Specific fuel consumption on the order of one pound. per horsepower per hour at full power was not unusual. Incidentally, fuel cooling is not a phenomenon limited to the distant past. Aircraft of the World War II era powered by large radial engines generally left a trail of black smoke when the engine was running at take-off power, a certain indication that some of the fuel was not completely burned. This generally served to keep cylinder head temperatures within the prescribed limits and to cool exhaust valves and other hot spots in the combustion chamber thereby preventing detonation and/or pre-ignition.

The unusual 3 cylinder Anzani engine that Louis Bleriot used in his flight across the English channel had cast iron cylinders with “atmospheric” intake valves and cam-operated exhaust valves. Note the priming cup on the centre intake tube.

In 1909, a year that has been called “the year of practical powered flying” by some aviation historians, the air-cooled three cylinder “fan type” Anzani engine powered Blériot's monoplane on its epic 37-minute flight across the English channel. This somewhat unusual engine had cast iron air-cooled cylinders with camshaft-operated exhaust valves and “atmospheric” intake valves that were kept closed by light springs and opened in reaction to the differential between atmospheric pressure and lowered pressure in the cylinder as the piston moved down on the intake stroke. It is surprising that this type of intake valve arrangement was used in early aircraft engines such as the Anzani when in practically all automotive engines of that era both the intake and exhaust valves were cam-operated. It did, however, eliminate one pushrod and rocker arm per cylinder, simplify the cam, and save weight.

Rotary radial engines. Direct air cooling was the natural choice for the designers of the rotary radial engines used extensively in World War I military aircraft. The machining capability necessary to produce the cylinders was readily available, and the major parts of the engine were machined from billets and forgings of alloy steel rather than from castings. The materials were very likely one of the low-to-medium carbon steels alloyed with nickel that were popular in that era. The first of the well-known French rotaries, the 50 horsepower Gnome, had been flown successfully in 1909. The power-to-weight ratio of the rotaries was generally better than that of other aircraft engines, a fact that made them attractive to aircraft designers. In response to military needs, larger rotary engines were manufactured in relatively large quantities in Germany as well as in France and Britain. Some rotary engines were manufactured in the U.S. under license agreements with the French. Near the end of World War I some twin-row fourteen and eighteen cylinder rotaries had been designed and tested but it is doubtful that any of these were used operationally.

 
The cylinders of the Le Rhone rotary engine of early World War I vintage were machined from steel billets and had relatively closely spaced fins. The single pushrod operated both the intake and exhaust valves by means of a semi-desmodromic cam ring.

Since the typical rotary engine used in World War I fighters turned at about 1,200 RPM at full power and was enclosed in a partial cowling, the relatively shallow fins machined as an integral part of the cylinder were adequate for heat dissipation. The cylinder walls were quite thin and the head was usually an integral part of the cylinder, resulting in a clean, simple, and light structure. There were no exhaust manifolds on rotary engines and when the exhaust valve on top of the head was open the exhaust gases, which generally contained liberal amounts of castor oil, vented directly to the atmosphere inside the partial cowling used on tractor installations such as the Nieuport and Sopwith aircraft. Since there was no way to incorporate an oil sump or any sort of an oil recovery system into the structure of the engine, the lubrication system inevitably was of the “total loss” type.

While the rotary radial engine was quite satisfactory for certain specialized military uses, its idiosyncrasies–and there were many–made it unsuitable for commercial applications. By the end of World War I it was considered obsolete. One of its major drawbacks was that in operation it produced gyroscopic forces that were a challenge to many pilots–and a death warrant to some–when controls were actuated to change the aircraft's direction of flight. Another basic disadvantage of the rotary engine was that the windage losses were quite high because of air resistance to the motion of the cylinders as they rotated. After World War I, surplus rotaries were readily available but efforts to convert them to static radial engines were generally unsuccessful since cylinder head and exhaust valve cooling were very inadequate unless the cylinder was moving rapidly through the air. Today, the only operators of rotary engines are dedicated restorers of World War I aircraft and builders of replicas who strive for maximum authenticity.

Static Radial Engines. By the middle years of World War I a number of engine designers in England had come to the conclusion that the static radial engine layout offered the best path to developing militarily and commercially viable engines. There was also support for the development of air-cooled engines from the British Navy since Admiralty planners were convinced that such engines would be lighter for a given power output, easier to maintain, and less subject to battle damage, a matter of more than passing interest to pilots flying single-engined aircraft over water. Incidentally, U.S. Navy planners and aviators came to essentially the same conclusions in the very early 1920s. While there was some interest the 1930s and early 1940s in liquid-cooled engines such as the experimental Lycoming XH-2470 and Pratt & Whitney XH-3730, a 24 cylinder sleeve valve engine, it was of short duration. In the U.S. Navy, the air-cooled radial engine would reign supreme throughout World War Il and beyond in piston-engined aircraft

The cylinder developed by Prof. A. H. Gibson and Sam Heron at the Royal Aircraft Establishment in 1918 had many modem features, including a mercury-cooled exhaust valve and an aluminium head with relatively deep fins. Note the unusual valve springs.

The closed-end poultice type cylinder barrel of the British ABC Dragonfly engine built in 1918 was made of steel and the head was held in place with cap screws and studs. The intake valve is shown.

The cast aluminium cylinder head on the Lawrance opposed twin engine used on the World War I “Penguin” trainers was attached to the cylinder with studs. Note the unusual hairpin valve springs and the adjustable length pushrods.

After the end of World War I, the Lawrance engineers worked with both the Army and the Navy in developing a nine-cylinder radial engine, the Model  J-1. It was the best American air-cooled engine at the time and passed its 50-hour test in 1922. A unique feature of the engine was the use of exhaust valves with hollow stems that were partly filled with mercury as a means of carrying heat from the head of the valve through the stem to the valve guide. Heron and Gibson had experimented with hollow stemmed valves for a number of years using water or mercury as heat transfer agents. The use of mercury was not really satisfactory since it would not wet the surface of the inside of the valve stem and therefore did not transfer heat well. Water had been found to be wholly unsuitable.

The U.S. Navy had decided to build aircraft carriers and since it badly needed light, reliable engines it gave a contract to Lawrance for the J-1 radial and ceased buying the liquid-cooled Hispano-Suiza engines manufactured by Wright under license as a means of pressuring Wright and other companies into developing radial engines. The Wright Aeronautical Corporation bought the Lawrance Company, largely at the urging of the Army and Navy, and the later engines were known as Wrights. These engines, known as the J series, were dominant in the 1920s and the J-5, which was the first American production engine to use salt-cooled exhaust valves, achieved everlasting fame as the powerplant that carried Charles Lindbergh across the Atlantic.

Samuel Heron, ever on the move, had emigrated to the U.S. in 1921 after a disagreement with his employer, J.D. Siddeley of the Siddeley Deasy Company, over Siddeley's efforts to alter one of Heron's cylinder designs. It has been said that Heron did not suffer those he considered fools gladly-or at all- and apparently he did not make exceptions for employers or company owners. After his arrival in the U.S. he went to work for the U.S. Army Air Service at McCook Field (now Wright-Patterson AFB) in Dayton, Ohio as a development engineer. In 1926, he joined the Wright Company, of which Lawrance was now vice-president, and his work in cylinder development was largely responsible for the success of the Wright J-5 engine. The Lawrance cylinder design had evolved from an all-aluminium cylinder with a steel liner that suffered from breakage of the aluminium mounting flange to the J-5 type that had a finned steel cylinder barrel with a screwed-on head and much more fin area, especially around the exhaust valve port. A major step had been taken in improving the radial air-cooled engine but much remained to be done.

Pratt & Whitney set a new standard for cylinder design with its first engine, the Wasp

Wright responded to this challenge with an even larger engine, the 1,790 cubic inch single row direct-drive nine cylinder Cyclone. Its displacement was soon increased by 30 cubic inches and as the R-1820, it powered a number of military and civilian aircraft. Its rated power output rose from about 500 horsepower in 1927 in its original direct-drive form to as much as 1,525 horsepower in the versions produced after World War II. The author clearly remembers an instance in the early 1980s during forest fire season when a heavily loaded (very probably overloaded) ex-Navy Grumman S2F “borate bomber” took off from the Ramona Airport in southern California on a fairly hot day. The rate of climb was minimal but the sound of the two very hard-working Wright R-1820s echo off the surrounding hills was memorable.

The unique Comet engine, manufactured in Wisconsin in 1929, had a semi-desmodromic valve actuation system that required only one push rod and one rocker arm. The cam ring follower was similar to that used on the LeRhone rotary engines. Note the fore-and-aft alignment of the valves.

By the early 1930s, cylinder design in the U.S. for large radial engines had become somewhat standardized, with bores of about six inches and two large valves per cylinder in a hemispherical combustion chamber. Valve actuation was by means of ring cams housed in the nose section or the crankcase, enclosed push rods, and rocker arms in individual housings cast integrally with the cylinder head. While all engines of any consequence had enclosed valve mechanisms, it wasn't until 1932 that automatic pressure lubrication with engine oil was introduced on the Pratt & Whitney Wasp. Much of the subsequent development work involved experimentation with materials, sodium-cooled valves, improvements in piston rings, better foundry practices, and many other minor improvements that would extend engine life and allow higher power outputs. Near the end of World War II the much more widespread use of forged cylinder heads with machined fins was a major factor in allowing engines to be operated at higher manifold pressures because of the greater mechanical strength and better heat dissipation capabilities of such heads.

The cast aluminium head on this late 1930s Kinner engine was attached to the steel barrel by a ring of studs and nuts. The valve mechanism was enclosed (rocker covers have been removed).

Materials and Processes, Small Air-Cooled Engines 

Over about a twenty-year period starting in 1921, the exhaust valves used in Wright engines were changed substantially to cope with higher power outputs and leaded fuels. The valve on the lower right has Stellite facing on the seating surface and the stem.

This sodium-cooled exhaust valve from a late World War II radial engine weighs 14 ounces and the head is 2.5 inches in diameter. At take-off power (2,700 RPM) the valve opens and closes 22 times per second.

This is a typical valve train for a radial engine. Note the use of rollers on the tappet and the end of the rocker arm that contacts the valve.

This cross section through a piston wall shows the end result of piston ring development. The top three rings are compression rings. At the centre is the oil control ring. The bottom ring is an oil scraper ring. On the right is an enlarged view of a keystone compression ring.

This air-cooled radial engine cylinder has a cast head. The cylinder barrel is machined from an alloy steel forging. The large number of studs used to attach the cylinder to the crankcase spreads the cyclic loads.

 materials and processes

Materials and how they were processed played a major role in the evolution of the air-cooled cylinder and it was fortunate that the science and practice of metallurgy had made rapid strides during the critical periods of cylinder development. Cyclic mechanical stresses and the process of waste heat dissipation are the two main enemies of the aircraft engine. Metallurgical and mechanical improvements were prime factors in extending cylinder life. The magnitude of the problem was well stated by an engine developer many years ago who, probably while viewing some failed parts, described the aircraft engine as a machine bent on destroying itself mechanically while busily trying to incinerate its exhaust valves. 

 exhaust valves

This was definitely the most trouble-prone and vexing part of the air-cooled aircraft engine. All engine designers made serious efforts to improve exhaust valve reliability and durability. Experiments with internally cooled valves were started as early as 1913. Initially, water and mercury, as previously stated, were tried as the heat transfer agents in the sealed, hollow valve stems. While the material in the stems was almost always called a coolant, in actuality it was a heat transfer agent used to carry heat from the head of the valve to the stem where it could be passed through the valve guide and into the fins on the cylinder head. As mentioned earlier, water proved to be impractical and mercury did not work well because it did not wet the interior of the valve stem and therefore did not transmit heat well. During his early years at McCook Field the ever-ingenious Sam Heron had observed the characteristics of various sodium compounds which are normally used in heat-treating operations. These materials are solid at room temperature and become liquid at engine operating temperatures. He observed that since these compounds wet the surface of steel alloys readily and transfer heat very well, their use should be effective in extending the life of exhaust valves. The ancestor of our present-day sodium-cooled valves had arrived, thanks to Mr. Heron, and almost ninety years later we are still enjoying the benefits of his ingenuity though even today such valves are not completely fault free.

 Cylinder barrels

Initially, cast iron was thought to be the only satisfactory cylinder barrel material but the builders of rotary radial engines proved that a variety of steel alloys could be used. Practically all air-cooled aircraft engines made in the last eighty years have used steel cylinders and engine developers have concentrated on selecting the most appropriate alloys and methods of heat treating and finishing the interior surface. Probably the most widely used steel was-and is-a chromium-molybdenum alloy generally known as SAE 4140 (now known as UNS G41400). This material was used by Pratt & Whitney in the late 1920s and still serves us well today. It can be used in through-hardened condition or nitrided depending on the application and severity of service conditions.

Piston ring shapes and materials and their compatibility with the cylinder were studied extensively, especially during World War II and resulted in development of the keystone ring as well as much more effective oil control rings. There was also widespread use of chrome plating of cylinder bores as a reconditioning and salvage procedure during the war, processes still widely used on air-cooled aviation engines of all sizes.. It is probably safe to say that the ultimate combination of piston ring and cylinder materials has not yet been found, and indeed there may not be one.

Metallurgy played an important part in valve and seat development and it was fortunate that a number of heat and corrosion resistant steel alloys became available in the 1920s. Materials such as high speed steel, a cutting tool material that contains as much as 20 percent tungsten along with molybdenum and several other materials, and various austenitic stainless steel alloys were used but none provided the desired degree of durability and resistance to burning in service. The advent of highly leaded fuels helped to increase power outputs but also caused serious valve and seat erosion problems. The solution can be credited in part, at least, to Mr. Heron and others who were facing the same problem in England. He used Stellite, a cast cutting tool material composed of 65 percent cobalt, 30 percent chromium, and 5 percent tungsten, as an overlay material which was gas-welded onto the seating area of the valve as well as on valve seat inserts and finished by grinding. This process greatly increased valve life and was used extensively during World War II. It is still considered one of the best procedures for producing reliable valves and seats for use in large engines that used leaded fuels.

 small air cooled engines

Well before World War II, it became evident that for small personal and business aircraft the radial engine, even in its smallest forms, was not really a practical source of power. Radials were inherently more expensive to produce, could not be cowled as easily as opposed engines, and were generally less economical to operate. In the late 1920s Harold Morehouse, in conjunction with the Wright Aeronautical Corporation introduced an opposed twin of rather archaic design and the Bristol Cherub of about 30 horsepower became available in England. Very few of the Wright-Morehouse twins were produced commercially while the Cherub enjoyed some popularity powering the light sport aircraft of that era.

By the mid 1930s, as the need for small engines to power inexpensive light aircraft in the U.S. increased, opposed twins such as the L-head Aeronca E-107 and the overhead valve E-113, the four cylinder Continental A-40 and its Lycoming counterpart, the 0-145, appeared and a new generation of opposed air-cooled engines was born. The names of the smaller radials familiar to those of us who are older-Kinner, Szekeley, Warner, LeBlond (later Ken-Royce), Lenape (remember the rare J-3 Cubs powered by the three-cylinder Papoose?), Velie, and others have faded into the mists of the past. 

conclusion