Category Archives: Aircraft Engines

Pratt Whitney R-2060 Yellow Jacket

Pratt & Whitney R-2060 ‘Yellow Jacket’ 20-Cylinder Engine

By William Pearce

Around 1930, the United States Army Air Corps (AAC) was interested in a 1,000 hp (746 kW), liquid-cooled aircraft engine. Somehow, the AAC persuaded Pratt & Whitney (P&W) to develop an experimental engine at its own expense to meet this goal. The engine was the R-2060 Yellow Jacket, and it carried the P&W experimental engine designation X-31. The “Yellow Jacket” name followed the “Wasp” and “Hornet” engine lines from P&W.

Pratt Whitney R-2060 Yellow Jacket

The Pratt & Whitney R-2060 Yellow Jacket was an experimental liquid-cooled engine. Note the annular coolant manifold around the front of the engine that delivered water to the water pumps.

While the R-2060 would be P&W’s first liquid-cooled engine, the company had experimented with liquid-cooled cylinders as early as 1928. In addition, many of P&W’s engineers had experience with liquid-cooled engines while working for other organizations—in particular, those workers who had helped develop liquid-cooled engines at Wright Aeronautical.

The R-2060 had a one-piece, cast aluminum, barrel-type crankcase. Attached radially around the crankcase at 72-degree intervals were five cylinder banks. The lowest (No. 3) cylinder bank was inverted and hung straight down from the crankcase. Each cylinder bank consisted of four individual cylinders arranged in a line. This configuration created a 20-cylinder inline-radial engine. Attached to the front of the crankcase was a propeller gear housing that contained a planetary bevel reduction gear. Mounted to the rear of the crankcase was the supercharger and accessory section.

The crankshaft had four throws and was supported by five main bearings. Mounted to each crankpin was a master connecting rod with four articulated connecting rods—a typical arrangement found in radial engines. Each individual cylinder was surrounded by a steel water jacket. Mounted atop each bank of cylinders was a housing that concealed a single overhead camshaft. The camshaft actuated the one intake valve and one exhaust valve in each cylinder. Each camshaft was driven from the front of the engine by a vertical shaft and bevel gears. Some of the camshafts drove magnetos at their rear. The magnetos fired the two spark plugs in each cylinder. The spark plugs were installed horizontally into the combustion chamber and placed on each exposed side of the cylinder. The camshaft housing on the lower cylinder bank was deeper and served as an oil sump.

Pratt Whitney R-2060 Yellow Jacket right

The 20-cylinder R-2060 was a fairly compact and light engine. Note the camshaft housings atop each cylinder bank and that the housing of the lower bank was deeper to serve as an oil sump. (Tom Fey image via the Aircraft Engine Historical Society)

Air was drawn into the downdraft carburetor mounted at the rear of the engine. Fuel was added, and the mixture then passed into the supercharger, which was primarily used to mix the air and fuel rather than provide boost. The air and fuel flowed from the supercharger through five outlets—one between each cylinder bank. The outlets were cast integral with the crankcase. Attached to each outlet was an intake manifold that branched into two sections, with each section branching further into two additional sections. The four pipes were then connected to the four cylinders of the cylinder bank. The exhaust ports were on the opposite side of the cylinder bank.

Cooling water flowed from the radiator into two inlets on an annular manifold mounted around the rear of the engine. The manifold had five outlets, one for each cylinder bank. Water flowed from the annular manifold into a pipe that ran along each cylinder bank. Branching off from the pipe were connections for each cylinder, with the mounting point near the exhaust port. The water passed by the exhaust port and through the water jacket, exiting near the intake port. The water from each cylinder was collected in another pipe that led to a smaller annular manifold mounted around the front of the engine. Two water pumps driven at the front of the engine took water from the front manifold and returned it to the radiator.

Pratt Whitney R-2060 Yellow Jacket left close

For each cylinder bank, the inlet for the intake manifold was cast into the crankcase. Unfortunately, the intake manifold did not provide equal distribution of the air and fuel mixture to the cylinders and caused the engine to run rough. The electric starter can be seen mounted on the left. (Tom Fey image via the Aircraft Engine Historical Society)

The Pratt & Whitney R-2060 Yellow Jacket had a 5.25 in (133 mm) bore and a 4.75 in (121 mm) stroke. Creating an oversquare (bore larger than the stroke) engine was not typical for P&W and was repeated only with the R-2000, which was derived from the R-1830 with minimal changes. However, the comparatively short stroke helped decrease the engine’s diameter. The R-2060 displaced 2,057 cu in (33.7 L) and was projected to produce 1,500 hp (1,119 kW) at 3,300 rpm. The Yellow Jacket was 70 in (1.78 m) long and 42 in (1.07 m) in diameter. The engine weighed 1,400 lb (635 kg).

Design work on the R-2060 was started in May 1931, and single-cylinder testing began in August of the same year. The engine was first run in 1932, and issues were soon encountered with rough running. The intake manifolds were of unequal lengths and caused inconsistent air and fuel distribution to the cylinders. Efforts to smooth out the engine’s operation by altering the firing order were tried but not successful. On its last test, the R-2060 achieved 1,116 hp (820 kW) at 2,500 rpm, but reaching 1,500 hp (1,119 kW) at 3,300 rpm was beyond what the engine could handle. The Yellow Jacket project was cancelled in late 1932 after accumulating just 35 hours of test running. Only one R-2060 engine was built.

Cancellation of the R-2060 allowed P&W to focus on the development of the air-cooled, two-row, 14-cylinder R-1830 Twin Wasp radial engine. The R-1830 became the most produced aircraft engine of all time, with 173,618 examples built. The sole R-2060 Yellow Jacket was preserved and is part of Pratt & Whitney’s Hangar Museum in East Hartford, Connecticut.

Pratt Whitney R-2060 Yellow Jacket rear

Rear view of the R-2060 illustrates the engine’s carburetor and supercharger housing. The annular manifold around the rear of the engine supplied cooling water to the five cylinder banks. (Kimble D. McCutcheon image via the Aircraft Engine Historical Society)

– The Liquid-Cooled Engines of Pratt & Whitney by Kimble D. McCutcheon (presentation at the 2006 Aircraft Engine Historical Society Convention)
Development of Aircraft Engines and Fuels by Robert Schlaifer and S. D. Heron (1950)
The Engines of Pratt & Whitney: A Technical History by Jack Connors (2009)

Farman 18T engine

Farman 18T 18-Cylinder Aircraft Engine

By William Pearce

The rules of the Schneider Trophy Contest stated that any country that won the contest three consecutive times would retain permanent possession of the trophy. By 1930, Britain had two consecutive victories and were favored to win the next contest scheduled for September 1931. Frenchman Jacques P. Schneider had started the contest, and France won the first competition held in 1913. The possibility of losing the contest forever spurred France to action, and the STIAé (service technique et industriel de l’aéronautique, or the Technical and Industrial Service of Aeronautics) ordered at least five aircraft types and three different engines for the 1931 contest. One of the engines ordered was the Farman 18T.

Farman 18T engine

The Farman 18T was specifically designed for installation in the Bernard flying boat. The unusual 18-cylinder engine had no other known applications.

Avions Farman (Farman) was founded in 1908 by brothers Richard, Henri, and Maurice. In October 1917, the company moved to produce engines built under license to support the war effort. The first of these engines was built in mid-1918, and production stopped after World War I. In 1922, Farman started to design their own line of engines under the direction of Charles-Raymond Waseige.

The Farman 18T was designed by Waseige and had an unusual layout. The water-cooled engine had three cylinder banks, each with six cylinders. The left and right cylinder banks were horizontally opposed, with a 180-degree flat angle across the engine’s top side. The lower cylinder extended below the crankcase and was perpendicular to the other cylinder banks. This configuration gave the 18-cylinder engine a T shape.

The engine used a two-piece cast aluminum crankcase that was split vertically. Steel cylinder liners were installed in the cast aluminum, monobloc cylinder banks that were bolted to the crankcase. The four valves of each cylinder were actuated via pairs of rockers by a single overhead camshaft. Each camshaft was driven by a vertical shaft at the rear of the engine.

The 18T used aluminum pistons and had a compression ratio of 6.0 to 1, although some sources say 8.5 to 1. The connecting rods consisted of a master rod for the lower cylinder bank and two articulated rods for the left and right cylinder banks. Each cylinder had two spark plugs, one installed in each side of the cylinder bank. The spark plugs were fired by magnetos driven from the rear of the engine. A nose case at the front of the engine contained the Farman-style bevel propeller reduction gear that turned the propeller at .384 crankshaft speed.

Farman 18T Paris Air Show 1932

The 18T (lower left) was proudly displayed as part of the Farman exhibit at the Salon de l’Aéronautique in November 1932. The other Farman engines are a 350 hp (261 kW) 12G (middle) and a 420 hp (313 kW) 12B (right).

For induction, air passed through carburetors at the rear of the engine and into a centrifugal supercharger that provided approximately 4.4 lb (.3 bar) of boost. The air/fuel mixture flowed from the supercharger into an intake manifold for each cylinder bank. The intake manifolds ran along the bottom of the cylinder bank for the left and right banks and along the right side (when viewed from the non-propeller end) of the lower cylinder bank. The exhaust ports were on the opposite side of the cylinder head from the intake.

The 18T had a 4.72 in (120 mm) bore and stroke. The engine displaced 1,491 cu in (24.4 L) and produced a maximum of 1,480 hp (1,104 kW) at 3,700 rpm. The 18T was rated at 1,200 hp (895 kW) at 3,400 rpm for continuous output. The engine was 65.98 in (1.68 m) long, 44.65 in (1.13 m) wide, 32.56 (.83 m) tall, and weighed 1,069 lb (485 kg).

Two Farman 18T engines were ordered under Contract (Marché) 289/0 (some sources state Marché 269/0) issued in 1930 and valued at 3,583,000 Ғ. The two engines were to power a flying boat built by the Société des avions Bernard (Bernard Aircraft Company). An official designation for the flying boat has not been found, and it was not among the known aircraft ordered for the 1931 Schneider Contest. There is some speculation that a lack of funds prevented the aircraft from being ordered for the 1931 race, but it would be ordered in time for the 1933 race.

Farman 18T Paris Air Show 1932 display

The display at the air show in Paris announced the 18T’s 1,200 hp (895 kW) continuous rating. Note that the supercharger housing extended above the crankcase, which was otherwise the engine’s highest point.

The design of the Bernard flying boat was led by Roger Robert and developed in coordination with the 18T engine. The all-metal aircraft had a low, two-step hull with sponsons protruding from the sides, just behind the cockpit. A long pylon above the cockpit extended along the aircraft’s spine, and the pylon supported the engine nacelle and wings. The engines were installed back-to-back in the middle of the nacelle. The engines’ lower cylinder banks extended into the pylon, and the left and right cylinder banks extended into the cantilever wings, which were mounted to the sides of the nacelle. Surface radiators for engine cooling covered the sides of the pylon, and extension shafts connected the propellers to the engines. The aircraft had a 36 ft 1 in (11.0 m) wingspan and was 35 ft 5 in (10.8 m) long. The engine nacelle was 17 ft 1 in (5.21 m) long. A 12.5 to 1 scale model of the flying boat was tested at the Laboratoire Aérodynamique Eiffel (Eiffel Aerodynamics Laboratory) in Auteuil (near Paris), France.

The 18T engines were bench tested in 1931, but the most power achieved was only 1,350 hp (1,007 kW). While further development was possible, at the time, the chance of France fielding a contestant in the 1931 Schneider Contest was virtually non-existent. The chances of the Bernard flying-boat being built were even worse. Although the aircraft had an estimated top speed of over 435 mph (700 km/h), and a detailed study was submitted to the Service Technique (Technical Service), the flying boat was seen as too radical and was never ordered. The limited funds were needed for the more conventional racers.

The Supermarine S.6B went on to win the 1931 Schneider Contest, giving the British permanent possession of the trophy. The 18T was marketed in 1932 and displayed at the Paris Salon de l’Aéronautique (Air Show) in November. However, there was little commercial interest in the 18T, and the project was brought to a close without the engine ever being flown; most likely, full testing was never completed.

Bernard - Farman 18T Schneider 3-view

Powered by two 18T engines, the Bernard flying boat racer had an estimated top speed of over 435 mph (700 km/h). This speed was substantially faster than the Supermarine S.6B that won the 1931 Schneider race at 340.08 mph (547.31 km/h) and went on to set an absolute speed record at 407.5 mph (655.8 km/h). However, the estimated specifications of unconventional aircraft often fall short of what is actually achieved.

Aerosphere 1939 by Glenn D. Angle (1940)
Les Moteurs a Pistons Aeronautiques Francais Tome 1 by Alfred Bodemer and Robert Laugier (1987)
Schneider Trophy Seaplanes and Flying Boats by Ralph Pegram (2012)
Les Avions Bernard by Jean Liron (1990)
Les Avions Farman by Jean Liron (1984)

Napier Nomad II rear

Napier Nomad Compound Aircraft Engine

By William Pearce

D. Napier & Son (Napier) was a British engineering firm that designed and manufactured aircraft engines since World War I. In 1931, Napier began experimental design work on a sleeve-valve, 24-cylinder, diesel (compression ignition) engine. Designated E101, the engine had a 5.0 in (127 mm) bore, a 4.75 in (121 mm) stroke, and a displacement of 2,238 cu in (36.7 L). While a two-cylinder test engine was built, and possibly a full bank of six cylinders, it is not clear if a complete H-24 E101 was constructed. However, the E101 served as the foundation for the E107, which was converted to spark ignition and became the first of the Sabre engine line. In 1933, Napier acquired licenses to produce the Junkers Jumo 204 and 205 aircraft engines as the Culverin (E102) and Cutlass (E103). Although not commercially successful, the experience with the Junkers engines provided Napier with detailed knowledge of two-stroke, high-powered diesel engines.

Napier Nomad I front

The Napier Nomad I was perhaps the most complex aircraft engine ever built. Of the contra-rotating propellers, the front set was driven by the turbine, and the rear set was driven by the 12-cyinder diesel engine. (Napier/NPHT/IMechE image)

In late 1944, the British Ministry of Aircraft Production (later, Ministry of Supply, MoS) issued a specification for an economical 6,000 hp (4,474 kW) aircraft engine to be used in large, long-range aircraft. Harry Ricardo, a prominent engine designer and researcher, suggested that combining a two-stroke diesel with a gas turbine would be the best way to create a powerful, compact, and economical aircraft engine.

Napier took Ricardo’s suggestion and combined it with their diesel engine experience. For the 6,000 hp (4,474 kW) engine, Napier proposed the E124: an H-24 diesel with a displacement of approximately 4,575 cu in (75 L) that incorporated an axial flow recovery turbine. Both of the upper and lower cylinder banks formed an included angle of 150 degrees, while the left and right banks formed an angle of 30 degrees. This spacing was done to accommodate exhaust manifolds in the 30-degree left and right Vees. Single- and twin-cylinder tests had begun, as well as tests on the axial-flow compressor, but Napier felt that such an engine would have a very limited market. The project was halted in 1946.

While the E124 was not built, it laid the foundation for a new engine capable of 3,000 hp (2,237 kW) and designed to achieve the lowest fuel consumption under any operating conditions. The new engine was the E125 Nomad I, and Napier began preliminary design work in 1945, with the MoS giving its support by 1946. In a way, the Nomad I was half of the H-24 engine with a reworked recovery turbine. The Nomad I was a liquid-cooled, horizontally-opposed, 12-cylinder, two-stroke, valveless, diesel engine that incorporated a gear-driven, two-speed supercharger and an exhaust-driven turbine that drove a compressor integral with the bottom of the engine. Alone, the compressor could not create the high-level of boost that was desired, so the supercharger was included to reach the design goal.

Napier Nomad I org exhaust rear

Rear view of the Nomad I with its original exhaust manifold illustrates the complexity of the system with its many pipes and flexible joints. The round housing for the supercharger impeller can be seen in front of the turbine. (Napier/NPHT/IMechE image)

The engine’s two-piece magnesium-zirconium alloy crankcase was split vertically and held together by 28 through bolts. A cast aluminum, six-cylinder, monobloc cylinder bank was attached to each side of the crankcase via studs. Wet cylinder liners were installed in the cylinder banks and covered with individual cylinder heads made from aluminum. A magnesium-alloy propeller gear reduction housing was secured via studs to the front of the crankcase. The housing also incorporated air intake on each of its lower sides. The intakes led to the compressor, which had an upper housing cast integral with the bottom of the crankcase, and a lower housing that was bolted on to the crankcase. Behind the compressor was a bifurcated air outlet, an oil sump, and the lower supercharger housing—all bolted to the crankcase.

Air entered the inlets on each side of the Nomad I and flowed into the 10-stage (some sources say 11-stage) axial flow compressor, which was the first stage of supercharging. The compressor had a maximum pressure ratio of 5.62 to 1. The air then exited the compressor via the bifurcated duct, which split the air along both sides of the engine and led back to the supercharger. An air to water intercooler (never installed) was positioned on both sides of the engine, between the compressor and the supercharger. After passing through the engine-driven centrifugal supercharger, the air was ducted into two passageways—one each for the left and right cylinder banks. Pressurized at 95.5 psi (6.58 bar) absolute, the air passed through a compartment in each cylinder bank that interfaced with the intake ports for each cylinder.

Air entered the loop-scavenged cylinder via a series of intake ports around the cylinder liner wall that were uncovered by the piston. The cylinder’s compression ratio was 8 to 1. As the piston moved toward the combustion chamber, fuel was injected via an injector located in the center of the cylinder head. The injected fuel was ignited by the heat of compression as the piston moved toward the cylinder head. On its power stroke, the piston uncovered exhaust ports which were situated slightly higher in the cylinder wall than the intake ports. The high level of supercharging ensured that an ample amount of air passed through the cylinder, which also helped cool the piston crown, cylinder wall, and cylinder head.

Napier Nomad I side

The Nomad I’s original (upper) and revised (lower) exhaust system and turbine can be compared in these images. In the lower image, the compressor’s intake can be seen near the front of the engine. The polished duct between the compressor and supercharger is where the intercooler would have been installed. (Napier/NPHT/IMechE images)

The exhaust gases and scavenging air flowed from the uncovered exhaust ports in the cylinder liner into manifolds positioned above and below the cylinder bank. The two exhaust manifolds for each cylinder bank merged together at the rear of the engine. Here, fuel could be injected, mixed with the surplus air, and ignited to increase the flow of exhaust gas energy to the turbine to create more engine power (for takeoff). The hot gases then flowed to a primary axial flow turbine at the extreme rear of the engine. The gases powered the primary turbine and then flowed out the exhaust nozzle at the end of the engine, generating some thrust. If more power was being harnessed by injecting fuel into the exhaust, a valve allowed the gases to flow into a secondary axial flow turbine positioned between the engine and the primary turbine. After powering the secondary turbine, the gases flowed into the primary turbine and then out the exhaust nozzle. The turbines were mounted in a tubular frame attached to the rear of the engine.

It should be noted that the description above applies to the second version of the exhaust system that was used by 1951. An earlier, original exhaust system had two manifolds above and below each cylinder bank, with each manifold collecting exhaust from three cylinders. The four manifolds from each cylinder bank joined into pairs at the rear of the engine and then merged into a single pipe. Immediately before the exhaust pipes connected to the primary (rear) turbine, an upper and a lower pipe branched off. The upper pipes of the left and right manifolds and the lower pipes of the left and right manifolds joined together at their respective spots as they fed into the secondary (front) turbine. At this point, extra fuel could be injected and ignited for additional power, as in the previous exhaust system described above. The original exhaust system incorporated around 28 flexible joints and was far more complex than the later system. Undoubtedly, issues with the original system were encountered that led to its replacement.

The exhaust turbines were mounted coaxially to the same shaft. This turbine shaft extended forward to power the compressor and led into the propeller gear reduction housing. The turbine shaft was geared to the front (outer) propeller of a contra-rotating set. The front propeller rotated counterclockwise. The rear (inner) propeller rotated clockwise and was geared to the crankshaft. There was nothing that linked the two propeller sets together, but they could not be run independently of each other. In other words, the piston engine section was needed to power the rear propeller, and the engine’s exhaust gases powered the turbine that was needed to run the front propeller. The turbine could not power itself, and the engine’s exhaust gases could not bypass the turbine.

Napier Nomad I Avro Lincoln install

The Nomad I installed in the nose of the Avro Lincoln test bed. The installation required significant modifications to the aircraft. Note the engine’s intake duct and the reversable-pitch propeller. (Napier/NPHT/IMechE image)

The Nomad I’s compressor and turbine were based on those developed for the 1,590 ehp (1,186 kW) Napier Naiad turboprop engine. The six-throw crankshaft of the Nomad I was supported between the left and right crankcase sections by seven main journals. The front of the crankshaft was geared to the propeller and a flexible shaft that extended to the rear of the engine to drive the supercharger impeller. The connecting rods were of the fork-and-blade type. The two-piece pistons had an austenitic stainless steel crown attached to a Y-alloy (aluminum alloy) body. The steel crown was used because of the high temperatures in the cylinder, and the piston was further cooled with oil flowing between the piston body and crown. The center of the crown could reach 1,300° F (700° C) when the engine was running at full power. A camshaft just below each cylinder bank drove three fuel injection dual pumps, and each pump provided the fuel to two cylinders via a single injector in each cylinder. The front of each camshaft also drove a coolant pump. A spark plug positioned just below the injector in each cylinder was used to start the engine. The spark plugs were fired by a magneto driven from the rear of the engine.

Despite its complexity, the Nomad I was designed to be operated by a single lever in the cockpit. The Napier Nomad I had a 6.0 in (152 mm) bore and a 7.375 in (187 mm) stroke. The engine displaced 2,502 cu in (41.0 L) and was rated at 3,080 ehp (2,297 kW) at 2,050 rpm, which was 3,000 shp (2,237 kW) combined with 320 lbf (1.42 kN) of thrust from the turbine. The 3,000 shp (2,237 kW) was combined from 1,450 shp (1,081 kW) from the diesel engine and 1,550 shp (1,156 kW) from the turbine, spinning at 15,600 rpm. For estimated cruising power at 30,250 ft (9,220 m), the diesel engine produced 725 shp (541 kW) at 1,650 rpm and the turbine produced 750 shp (559 kW) at 17,000 rpm, for a combined 1,475 shp (1,100 kW). The Nomad I had a specific fuel consumption (sfc) of 0.36 lb/ehp/hr (219 g/kW/h). The engine was 126.5 in (3.21 m) long, 58.25 in (1.48 m) wide, 49.25 in (1.25 m) tall, and weighed 4,200 lb (1,905 kg).

The design of the Nomad I was laid out by a team led by Ernest Chatterton, Chief Engineer of the Piston Engine Division at Napier. The compressor and turbine sections were tested in 1948. The prototype engine was completed in 1949 and first run in October. After running for a total of 860 hours on the test stand, contra-rotating propellers were installed, and the engine underwent a further 270 hours of tests. In 1950, an Avro Lincoln bomber (serial SX973) that had been loaned to Napier’s Flight Test Department at Luton, England was modified to install the Nomad I in the aircraft’s nose. This conversion entailed a fair amount of work, with everything forward of the cockpit needing to be fabricated. SX973 made its first flight with the Nomad I in 1950. While the aircraft’s four Rolls-Royce Merlin engines were retained, they could be shut down in flight and the Lincoln held aloft solely by the Nomad I. The Nomad-Lincoln made its only public appearance at the Society of British Aircraft Constructors flying display at Farnborough in September 1951. Another Nomad I engine was also on display at the show. The Nomad I accumulated 120 hours of flight time in the Lincoln.

Napier Nomad I Avro Lincoln feathered

The Napier Nomad I had enough power to keep the Avro Lincoln aloft with the four Rolls-Royce Merlin engines shut down and feathered. (Napier/NPHT/IMechE image)

After a total of approximately 1,250 hours of operation, the Nomad I program was brought to a close in September 1952. The complex engine had proven to be temperamental, although it did exhibit very good fuel economy when it was running correctly. While Nomad I engine tests were underway, an updated and simplified version of the engine had been designed and designated E145 Nomad II. The design of the Nomad II took advantage of lessons learned from the Nomad I and the latest developments of axial compressors.

The Nomad II was designed in 1951, and the program was supervised by Chatterton and A. J. Penn, Napier’s gas turbine chief engineer. Although similar in configuration and possibly sharing some components with the Napier I, the Napier II was a new design. The Napier II retained the horizontally-opposed 12-cylinder layout incorporating a turbine and compressor, but the contra-rotating propellers and mechanically-driven centrifugal supercharger were discarded. The wet cylinder liners of the Nomad I were replaced by dry liners, which were made of chromium-copper alloy with chrome-plated bores. The crankcase was again cast of magnesium-zirconium (RZ-5) alloy.

Napier Nomad I and II geartrain

A simplified comparison of the Nomad I (top) and Nomad II (bottom) power systems. Not shown on the Nomad I was the two-speed supercharger drive. Not shown on the Nomad II was the second quill shaft to the variable-speed coupling. Neither drawing shows the engines’ accessory camshafts.

The improved axial flow compressor had a diameter of 10.88 in (276 mm) and was hung below the engine via four flexible mounts. The compressor had 12 stages, a maximum pressure ratio of 8.25 to 1, and a maximum mass air flow of 13 lb/sec (5.9 kg/sec). Its inlet faced forward to take full advantage of ram air. The pitch of the compressor’s inlet guide vanes automatically adjusted to improve airflow at lower speeds. The first five stages of the compressor used cobalt-steel blades, and the remaining seven stages used aluminum-bronze blades.

The Nomad II’s loop-scavenged system was improved over that of the Nomad I. Air from the compressor was routed forward in a manifold mounted below each cylinder bank. The pressurized air entered the revised cylinder banks and passed through guide vanes to flow into each cylinder via eight intake ports. Two pairs of four ports were positioned in the upper sides (top side of the engine) of the cylinder wall. The specially-designed intake ports directed the flow of air toward the hemispherical combustion chamber, where it circulated back toward the piston and the uncovered exhaust ports. The six exhaust ports consisted of three large ports, each with a smaller port below (toward the piston). The exhaust ports were positioned on the bottom side of the cylinder (lower side of the engine) and closer to the combustion chamber than the intake ports.

Napier Nomad II front

The Napier Nomad II was a simpler engine and was improved in every way compared to the Nomad I. Note the single rotation propeller shaft and simplified exhaust system. The compressor can be seen under the engine. (Napier/NPHT/IMechE image)

The exhaust gases were collected in an exhaust manifold mounted below each cylinder bank. The exhaust gases flowed back to a three-stage axial flow turbine mounted at the rear of the engine. The turbine and the compressor were mounted on separate shafts that were coaxially coupled. The turbine shaft was also connected to the crankshaft via an infinitely variable-speed fluid coupling (Beier gear). At low power (under 1,500 rpm), the turbine did not create the power needed to drive the compressor. This resulted in the variable-speed coupling delivering power from the crankshaft to drive the compressor. At high power (above 1,500 rpm), the turbine created more power than what was needed to drive the compressor. The variable-speed coupling fed the extra power back to the engine’s crankshaft. The fluid coupling drive set was mounted to the upper-rear of the engine.

While the cylinders’ compression ratio was 8 to 1, air was fed into the cylinders at 89 psi (6.14 bar) absolute for takeoff, creating an effective compression ratio of 27 to 1. A set of six fuel injection pumps were located above each cylinder bank. The pumps were driven by a camshaft from the front of the engine. The fuel injector in the center of the cylinder head had six orifices: one sprayed toward the piston, and the other five were equally spaced radially around the nozzle and sprayed toward the combustion chamber walls. The fuel was injected into the cylinder at 3,675 psi (253 bar).

Napier Nomad II cutaway

The cutaway view of the Nomad II reveals that the engine was still very complex compared to a conventional piston engine. Note the gearset at the front of the engine that powered the propeller shaft, fuel injection cams (upper), and quill shafts (lower) to the variable-speed coupling. (Napier/NPHT/IMechE image)

When the engine was viewed from the rear, the propeller turned counterclockwise. In the reduction gear housing at the front of the engine, the crankshaft drove the propeller shaft via four pinions. Although the exact gear reduction used in the test engines has not been found, a variety of reduction speeds were available: .526, .555, .569, .614, or .660 times crankshaft speed. Each of the lower two pinions were mounted to separate quill shafts that extended back to the rear of the engine and drove (or were driven by) the variable-speed gearset coupled to the turbine shaft. The crankshaft was supported by eight main bearings, with two I-beam connecting rods attached to each crankpin. The connecting rods used slipper-type bearings with two fairly-light straps securing the pair to the crankshaft. Since the engine was a two stroke, there was no downward pull on the connecting rod that required a more robust cap. The small end of the connecting rod that attached to the piston had a slipper-type eccentric bearing. As the connecting rod articulated from top dead center to bottom dead center, the bearing would rock slightly on the piston, opening a small gap for lubrication. This provided the proper oil flow that otherwise would not have occurred with the unidirectional loads of the two-stroke engine.

For starting, two ignition coils and two distributors driven from the front of the engine fired a spark plug in each cylinder. However, some photos appear to show two spark plugs in each cylinder. For installation, the engine was hung by two supports above the front cylinders and two supports above the rear casing.

The Napier Nomad II had the same 6.0 in (152 mm) bore, 7.375 in (187 mm) stroke, and 2,502 cu in (41.0 L) displacement as the Nomad I. The engine initially had a takeoff rating of 3,135 ehp (2,338 kW) at 2,050 rpm, which was 3,046 shp (2,271 kW) combined with 250 lbf (1.11 kN) of thrust from the turbine. As development continued, water injection was added that increased the Nomad II’s takeoff rating to 3,570 ehp (2,662 kW) at 2,050 rpm. This power was a combination of 3,476 shp (2,592 kW) and 230 lbf (1.02 kN) of thrust. At full power, the turbine shaft turned at 18,200 rpm, 8.88 times crankshaft speed. The engine’s maximum continuous rating was 2,488 ehp (1,855 kW) at 1,900 rpm, which was 2,392 shp and 145 lbf (1,855 kW and .64 kN). The Nomad II had a sfc of 0.345 lb/ehp/hr (210 g/kW/h). The engine was 119.25 in (3.03 m) long, 56.25 in (1.43 m) wide, 40 in (1.02 m) tall, and weighed 3,580 lb (1,624 kg).

Napier Nomad II parts

Various components of the Nomad II. Clockwise from the upper left: compressor and compressor housing, parts of the turbine, the Beier variable-speed fluid coupling, two connecting rods, and a piston with its stainless steel crown. (Napier/NPHT/IMechE images)

The Nomad II was first run in December 1952 and had accumulated 350 hours by mid-1954. The engine underwent various bench tests and tests with a 13 ft (3.96 m) diameter, constant-speed, reversable-pitch propeller. It was found that running the engine on diesel, kerosene, or jet fuel (wide-cut gasoline) resulted in little difference in power. Some tests indicated that a sfc as low as 0.326 lb/ehp/hr (198 g/kW/h) could be achieved, this being realized at 22,250 ft (6,782 m) with the engine producing 2,027 ehp (1,511 kW) at 1,750 rpm. The Nomad II maintained takeoff power up to 7,750 ft (2,362 m), and a constant boost, power, and sfc could be maintained up to 25,000 ft (7,620 m). At sea level, the turbine developed 2,250 hp (1,678 kW), but 1,840 hp (1,372 kW) was used to power the compressor. The Nomad experienced a two percent drop in power for every 20° F (11° C) increase in air temperature. Since the engine only burned 70 percent of the air passing through the cylinders, the ability to inject and ignite fuel into the exhaust manifold was experimented with, resulting in 4,095 ehp (3,054 kW) for a sfc of .374 lb/hp/hr (227 g/kW/h).

For flight tests, Napier proposed installing Nomad II engines in place of the outer two Rolls-Royce Griffons on an Avro Shackleton maritime patrol aircraft. In October 1952, the MoS loaned the second prototype Shackleton (VW131) to Napier for conversion and subsequent Nomad II flight testing. The aircraft arrived at Napier’s center at Luton on 16 January 1953. Dummy engines were first installed, and vibration tests were conducted in April 1954. The Nomad II installation and cowlings were clean and refined, but flight-cleared engines were slow to arrive. Eventually, two Nomad II engines were installed and some ground runs were made, but the Nomad program was cancelled in April 1955, before the aircraft had flown. While the Nomad II had unparalleled fuel economy for the time and was simpler, lighter, smaller, and more powerful than the Nomad I, there was little demand for the engine. Napier kept all Nomad data for a time, believing that interest in the engine might be rekindled and spark further development, but that was not the case.

Napier Nomad II rear

The 12-stage turbine was mounted in a tube frame behind the engine. The housing above the turbine contained the variable-speed coupling that linked the crankshaft to the turbine shaft. Note the single spark plug (used for starting) in each cylinder. (Napier/NPHT/IMechE image)

Before the project was cancelled in 1955, the E173 Nomad III was designed as a continuation of the engine’s development. The Nomad III incorporated fuel injection into the exhaust manifold and an air-to-water aftercooler between the compressor and the cylinders. With these changes, the engine had a wet takeoff rating of 4,500 ehp (3,356 kW) at 2,050 rpm, which was 4,412 shp (3,290 kW) combined with 230 lbf (1.021 kN) of thrust from the turbine. The Nomad III weighed 3,750 lb (1,701 kg), 170 lb (77 kg) more than the Nomad II, but a complete engine was never built.

While the Nomad demonstrated excellent economy and impressive power for its weight, the engine was overshadowed by development of turboprops and turbojets. Money for development was tight, and the Nomad program had cost £5.1 million. In cases like the Avro Shackleton, it was less expensive to use Griffon engines than continue development of the Nomad. For other projects, the turboprop offered greater potential in the long run. While the Nomad engine was designed to cruise around 345 mph (556 km/h), the turbojet offered significantly higher cruise speeds compared to any other type of aircraft engine.

The exact number of Nomad I engines constructed has not been found, but it was at least two. A nicely restored Nomad I engine is preserved and on display at the National Museum of Flight at East Fortune Airfield in Scotland. The Nomad I underwent a restoration in 1999, and it was discovered that there were no propeller gears, pistons, or a crankshaft in the engine. This engine may be the Nomad I that was displayed at Farnborough in 1951. Of the six Nomad II engines built, two are preserved and on display—one at the Steven F. Udvar-Hazy Center in Chantilly, Virginia and the other at the Science Museum at Wroughton, England.

Napier Nomad II prop test

The Nomad II setup for tests with a 13 ft (3.96 m) propeller. Note that two spark plugs appear to be installed in each cylinder. Although not finalized, the top-mounting system made it fairly easy to install or remove the engine. (Napier/NPHT/IMechE image)

– “Napier Nomad Aircraft Diesel Engine” by Herbert Sammons and Ernest Chatterton, SAE Transactions Vol 63 (1955)
– “Napier Nomad” by Bill Gunston, Flight (30 April 1954)
– “Napier’s Nomad Engine” The Aeroplane (30 April 1954)
– “Compound Diesel Engine Design Analyzed” Aviation Week (17 May 1954)
Aircraft Engines of the World 1952 by Paul H. Wilkinson (1952)
Aircraft Engines of the World 1956 by Paul H. Wilkinson (1956)
By Precision Into Power by Alan Vessey (2007)
Turbojet: History and Development 1930–1960 Volume 1 by Antony L. Kay (2007)
Men and Machines by Charles Wilson and William Reader (1958)
Napier Powered by Alan Vessey (1997)

IAM M-44 sectional view

IAM M-44 V-12 Aircraft Engine

By William Pearce

In 1925, the Soviet Air Force (Voyenno-Vozdushnye Sily or VVS) approached the TsAGI (Tsentral’nyy Aerogidrodinamicheskiy Institut, the Central Aerohydrodynamic Institute) and requested proposals for a large, heavy bomber. Under the direction of Andrei Nikolayevich Tupolev, the Tupolev OKB (Opytno-Konstruktorskoye Byuro, the Experimental Design Bureau) started design work on the aircraft in 1926, and the government finalized the aircraft’s operational requirements in 1929. The aircraft created from this program was the Tupolev ANT-6, which was given the military designation TB-3.

Tupolev TB-6 6M-44 top

Model of the Tupolev TB-6 6M-44 with its six M-44 engines. Gunner stations are seen outside of the outer engines and in the wing’s trailing edge.

The large, four-engine TB-3 lifted its 137 ft 2 in (41.80 m) wingspan from earth for the first time on 22 December 1930, but plans for even larger and more ambitious aircraft were underway. In October 1929, the Scientific and Technical Committee of the Air Force (Nauchno-tekhnicheskiy komitet upravleniya Voyenno-Vozdushnye Sily or NTK UVVS) instructed Tupolev to design bombers capable of carrying a 10-tonne (22,046 lb) and a 25-tonne (55,116 lb) payload. With a 177 ft 2 in (54 m) wingspan, the 10-tonne bomber became the ANT-16, which was given the military designation TB-4. The 25-tonne bomber had a 311 ft 8 in (95 m) wingspan and became the ANT-26, which was given the military designation TB-6. However, this line of developing very large aircraft, the TB-6 in particular, quickly illustrated that there was a lack of powerful engines and that numerous smaller engines were required for the aircraft. The TB-4 required six 800 hp (597 kW) engines, and the TB-6 required twelve 830 hp (619 kW) engines. If an engine with a 2,000 hp (1,491 kW) output could be built, not only could it power these large aircraft, but it would also simplify their construction, maintenance, and control.

Back in 1928, the TsAGI had realized the need for more powerful engines and initiated work on a single-cylinder test engine to precede the design of a large, high-power bomber engine. This test engine was designated M-170; “170” was the anticipated horsepower (127 kW) output of the cylinder. The results were encouraging, and in 1930, the Institute of Aviation Motors (Institut aviatsionnogo motorostroyeniya or IAM) was tasked with the construction of a V-12 engine based on the M-170 cylinder. The 12-cylinder engine was designated M-44, and the single-cylinder test engine was renamed M-170/44.

The design of the M-44 was initiated in February 1931 under the supervision of N. P. Serdyukov. The design progressed rapidly and was completed in May. The M-44 was a four-stroke, water-cooled, 60-degree V-12. Based on a sectional drawing, the crankcase was split horizontally with main bearing caps for the crankshaft machined integral into the lower half of the case. The main bearings were secured by long bolts that passed through the lower crankcase half and screwed into the upper half. The crankshaft accommodated side-by-side connecting rods with flat-top aluminum pistons.

IAM M-44 sectional view

Sectional drawing of the IAM M-44 reveals some of the engine’s inner workings. The design was fairly conventional, just extremely large. Unfortunately, no images or other drawings of the engine have been found.

The individual steel cylinders were secured to the crankcase via hold down studs. A steel water jacket surrounded the cylinder barrel. The cylinder had a flat-roof combustion chamber, and four spark plugs were positioned horizontally at its top, just below the valves. Two spark plugs were on the outer side of the cylinder and the other two on the Vee side. Each cylinder bank was capped by a monobloc cylinder head with dual overhead camshafts. One camshaft operated the two intake valves for each cylinder, and the other camshaft operated the two exhaust valves for each cylinder. An intake manifold was attached to the Vee side of the cylinder head, and individual exhaust stacks were attached to the outer side of the cylinder head.

The normally aspirated M-44 had a compression ratio of 6 to 1 (some sources state 5 to 1). A propeller gear reduction (most likely using spur gears) was incorporated onto the front of the engine. The IAM M-44 had an 8.74 in (222 mm) bore and a 11.26 in (286 mm) stroke. Each cylinder displaced 675.6 cu in (11.07 L), and the engine’s total displacement was 8,107 cu in (132.9 L). The M-44 was the largest V-12 aircraft engine ever built. The engine produced 2,000 hp (1,491 kW) for takeoff and 1,700 hp (1,268 kW) for continuous operation. Some sources indicate that 2,400 hp (1,790 kW) was expected out of the engine after it was fully developed. The M-44 was approximately 118 in (3.00 m) long, 46 in (1.16 m) wide, and 65 in (1.66 m) tall. The engine weighed around 3,858 lb (1,750 kg).

With development of the 2,000 hp (1,491 kW) M-44 engine underway, studies were started to incorporate the engine into the ANT-16 (TB-4) and ANT-26 (TB-6) aircraft designs. Proposals to re-engine the ANT-16 with four M-44s were quickly abandoned so that work could focus on using six M-44 engines to power the ANT-26. This version of the aircraft is often cited as TB-6 6M-44. The ANT-26 design was ordered in July 1932, with construction starting soon after. Delivery of the ANT-26 prototype was expected in December 1935. Some sources state that an even larger, 30-tonne (66,139 lb) bomber with a 656 ft (200 m) wingspan and powered by eight M-44 engines was conceived, but it appears this aircraft never progressed beyond the rough design phase.

The Tupolev TB-6 6M-44 had two engines installed in each wing and two engines positioned back-to-back and mounted above the aircraft’s fuselage. The aircraft had a 311 ft 8 in (95 m) wingspan and was 127 ft 11 in (39 m) long. The TB-6 6M-44’s top speed was 155 mph (250 km/h), and it had a ceiling of 22,966 ft (7,000 m). The aircraft had a maximum bomb load of 48,502 lb (22,000 kg) and could carry a 33,069 lb (15,000 kg) bomb load 2,051 miles (3,300 km). Its maximum range was 2,983 miles (4,800 km).

Tupolev TB-6 6M-44 side

This rear view of the TB-6 6M-44 illustrates the tandem engines mounted above the fuselage.

The construction of three M-44 prototypes was planned, but the first engine was delayed by continued trials of the M-170/44 test engine, which was given a higher priority. The manufacture of the first M-44 engine began in early 1933, and the engine was first run later that year. The second engine was built and run in 1934. Plans to build the third M-44 engine were suspended on account of issues with the first two engines. The M-44 test engines had trouble producing the desired power and suffered from reliability issues. It became clear that the engine was not going to be successful, and the program was cancelled in 1934.

A supercharged version of the engine, known as the M-44H, had undergone preliminary design work in 1932. However, performance specifications for this engine have not been found, and it is doubtful that detailed design work was completed. In 1935, a decision was made to build the third M-44 engine, modified for marine use. This engine was designated GM-44 and incorporated a reversing gearbox. The GM-44 produced 1,870 hp (1,394 kW), but it was no more reliable than the M-44 aircraft engine. The GM-44 engine was cancelled in 1936.

With the M-44 engine program dead, the ANT-26 design reverted back to using 12 engines (1,200 hp / 895 kW Mikulin M-34FRN). However, studies concluded that the multitude of engines created additional drag that impacted the aircraft’s performance, and the engines added so much complexity that the ANT-26 would be difficult to fly and very difficult to maintain. Simply put, the giant aircraft was impractical, and it was subsequently cancelled in July 1934. A transport/commercial version of the aircraft, designated ANT-28, was also cancelled. The ANT-26’s airframe was 75 percent complete at the time of cancellation.

Tupolev TB-6 12M-34FRN

With the M-44 cancelled, the 12-engine TB-6 12M-34FRN was designed to preserve the aircraft’s capabilities with reliable engines. However, one would question the practicality of such an aircraft. Note the set of tandem engines that was placed above each wing.

Russian Piston Aero Engines by Vladimir Kotelnikov (2005)
Самолеты- гиганты СССР by Vladimir Kotelnikov (2009)
Unflown Wings by Yefim Gordon and Sergey Komissarov (2013)
OKB Tupolev by Yefim Gordon and Vladimir Rigmant (2005)

Isotta Fraschini Asso 750 front

Isotta Fraschini W-18 Aircraft and Marine Engines

By William Pearce

In late 1924, the Italian firm Isotta Fraschini responded to a Ministero dell’Aeronautica (Italian Air Ministry) request for a 500 hp (373 kW) aircraft engine by designing the liquid-cooled, V-12 Asso 500. Designed by Giustino Cattaneo, the Asso 500 proved successful and was used by Cattaneo as the basis for a line of Asso (Ace) engines developed in 1927. Ranging from a 250 hp (186 kW) inline-six to a 750 hp (559 kW) W-18, the initial Asso engines shared common designs and common parts wherever possible.

Isotta Fraschini Asso 750 front

The direct drive Isotta Fraschini Asso 750 was the first in a series of 18-cylinder engines that would ultimately be switched to marine use and stay in some form of production for over 90 years.

The Isotta Fraschini Asso 750 W-18 engine consisted of three six-cylinder banks mounted to a two-piece crankcase. The center cylinder bank was in the vertical position, and the two other cylinder banks were spaced at 40 degrees from the center bank. The cylinder bank spacing reduced the 18-cylinder engine’s frontal area to just slightly more than a V-12.

The Asso 750’s crankcase was split horizontally at the crankshaft and was cast from Elektron, a magnesium alloy. A shallow pan covered the bottom of the crankcase. The six-throw crankshaft was supported by eight main bearings. On each crankshaft throw was a master rod that serviced the center cylinder bank. Articulating rods for the other two cylinder banks were mounted on each side of the master rod. A double row ball bearing acted as a thrust bearing on the propeller shaft and enabled the engine to be installed as either a pusher or tractor.

The individual cylinders were forged from carbon steel and had a steel water jacket that was welded on. The cylinders had a closed top with openings for the valves. The monobloc cylinder head was mounted to the top of the cylinders, with one cylinder head serving each bank of cylinders. The cylinder compression ratio was 5.7 to 1. The cylinder head was made from cast aluminum and held the two intake and two exhaust valves for each cylinder. The valves were actuated by dual overhead camshafts, with one camshaft controlling the intake valves and the other camshaft controlling the exhaust valves (except for the center bank). A single lobe on the camshaft acted on a rocker and opened the two corresponding valves for that cylinder. The camshafts for each cylinder bank were driven at the rear of the cylinder head. One camshaft of the cylinder bank was driven via beveled gears by a vertical drive shaft, and the second camshaft was geared to the other driven camshaft. The valve cover casting was made from Elektron.

Isotta Fraschini Asso 750 RC35 crankcase

The cylinder row, upper crankcase, and cylinder head (inverted) of an Asso 750 RC35 with gear reduction. The direct drive Asso 750 was similar except for the shape of the front (right side) of the crankcase. Note the closed top cylinders. The small holes between the studs in the cylinder top were water passageways that communicated with ports on the cylinder head.

Three carburetors were mounted to the outer side of each outer cylinder bank. The intake and exhaust ports of the outer cylinder banks were on the same side. The intake and exhaust ports of the center cylinder bank were rather unusual. When viewed from the rear, the exhaust ports for the rear three cylinders of the center bank were on the right, and the intake ports were on the left. The front three cylinders were the opposite, with their exhaust ports on the left and their intake ports on the right. This configuration gave the cylinders for the center bank crossflow heads, but it also meant that each camshaft controlled half of the intake valves and half of the exhaust valves. A manifold attached to the inner side of the left cylinder bank collected the air/fuel mixture that had flowed through passageways in the left cylinder head and delivered the charge to the rear three cylinders of the center bank. The right cylinder bank had the same provisions but delivered the mixture to the front three cylinders of the center bank. Presumably, the 40-degree cylinder bank angle did not allow enough room to accommodate carburetors for the middle cylinder bank.

The two spark plugs in each cylinder were fired by two magnetos positioned at the rear of the engine and driven by the camshaft drive. From the rear of the engine, the firing order was 1 Left, 6 Center, 1 Right, 5L, 2C, 5R, 3L, 4C, 3R, 6L, 1C, 6R, 2L, 5C, 2R, 4L, 3C, and 4R. A water pump positioned below the magnetos circulated water into a manifold along the base of each cylinder bank. The manifold distributed water into the water jacket for each individual cylinder. The water flowed up through the water jacket and into the cylinder head. Another manifold took the water from each cylinder head to the radiator for cooling. Starting the Asso 750 was achieved with an air starter.

Motore Isotta Fraschini Asso 750

Two views of the direct drive Asso 750 displayed at the Museo nazionale della scienza e della tecnologia Leonardo da Vinci in Milan. Note the three exhaust stacks visible on the center cylinder bank. The front image of the engine illustrates the lack of space between the cylinder banks, which were set at 40 degrees. (Alessandro Nassiri images via Wikimedia Commons)

The Isotta Fraschini Asso 750 had a bore of 5.51 in (140 mm), a stroke of 6.69 in (170 mm), and a total displacement of 2,875 cu in (47.1 L). The original, direct drive Asso 750 produced 750 hp (599 kW) at 1,600 rpm, and weighed 1,279 lb (580 kg). An improved version of the Asso 750 was soon built that produced 830 hp (619 kW) at 1,700 rpm and 900 hp (671 kW) at 1,900 rpm. This engine weighed 1,389 lb (630 kg). The direct drive Asso 750 was 81 in (2.06 m) long, 40 in (1.02 m) wide, and 42 in (1.07 m) tall.

A version of the Asso 750 with a spur gear reduction for the propeller was developed and was sometimes referred to as the Asso 850 R. Available gear reductions were .667 and .581, and the gear reduction resulted in the crankshaft having only seven main bearings. The Asso 850 R produced 850 hp (634 kW) at 1,950 rpm, and weighed 1,455 lb (660 kg). This engine was also further refined and given the more permanent designation of Asso 750 R. The 750 R had a .658 gear reduction. The engine produced 850 hp (634 kW) at 1,800 rpm and 930 hp (694 kW) at 1,900 rpm. The Asso 750 R was 83 in (2.12 m) long and weighed 1,603 lb (727 kg).

Isotta Fraschini Asso 750 rc35 front

Front view of the Asso 750 RC35. The gear reduction required new upper and lower crankcase halves and a new crankshaft, but the other components were interchangeable with the direct drive engine.

Around 1933 the Asso 750 R engine was updated to incorporate a supercharger. The new engine was designated Asso 750 RC35. The “R” in the engine’s designation meant that it had gear reduction (Riduttore de giri); the “C” meant that it was supercharged (Compressore); and the “35” stood for the engine’s critical altitude in hectometers (as in 3,500 meters). The engine’s water pump was moved to a new mount that extended below the oil pan. The supercharger was mounted between the water pump and the magnetos, which were moved to a slightly higher location. The supercharger was meant to maintain sea level power up to a higher altitude, and it provided .29 psi (.02 bar) of boost up to 11,483 ft (3,500 m). The Asso 750 RC35 produced 870 hp (649 kW) at 1,850 rpm at 11,483 ft (3,500 m). The engine was 87 in (2.20 m) long, 41 in (1.03 m) wide, 48 in (1.21 m) tall, and weighed 1,724 lb (782 kg).

In 1928, Isotta Fraschini designed a larger, more powerful engine that had both its bore and stroke increased by .39 in (10 mm) over that of the Asso 750. The larger engine was developed especially for the Macchi M.67 Schneider Trophy racer. The M.67’s engine was initially designated Asso 750 M (for Macchi) but was also commonly referred to as the Asso 2-800. The “2” designation was most likely applied because the engine was a “second generation” and differed greatly from the original Asso 750 design.

Isotta Fraschini Asso 750 rc35 rear

The single-speed supercharger on the Asso 750 RC35 is illustrated in this rear view. Note the relocated and new mounting point for the water pump. The supercharger forced-fed air to the engine’s six carburetors.

The Asso 2-800 had a bore of 5.91 in (150 mm), a stroke of 7.09 in (180 mm), and a total displacement of 3,434 cu in (57.3 L). The engine used new crossflow cylinder heads and a new crankcase. The cylinder heads had intake ports on one side and exhaust ports on the other. Air intakes for the engine were positioned behind the M.67’s spinner, with one intake on the left side for the left cylinder bank and two intakes on the right side for the center and right cylinder banks. Ducts delivered the air to special carburetors positioned between the cylinder banks. The modified engine also had a higher compression ratio and used special fuels. Under perfect conditions, the special Asso 2-800 engine produced up to 1,800 hp (1,342 kW), but it was rarely able to achieve that output. An output of 1,400 hp (1,044 kW) was more typical and still impressive. At speed, the Asso 2-800 in the M.67 reportedly made a roar like no other engine.

Isotta Fraschini made a commercial version of the larger engine, designated Asso 1000. With the same bore, stroke, and displacement as the Asso 2-800, the Asso 1000 is often cited as the engine powering the M.67. However, the Asso 1000 retained the same configuration and architecture as the Asso 750, except the Asso 1000 had a compression ratio of 5.3 to 1. Development of the Asso 1000 trailed slightly behind that of the Asso 750.

The direct drive Isotta Fraschini Asso 1000 produced 1,000 hp (746 kW) at 1,600 rpm and 1,100 hp (820 kW) at 1,800 rpm. The engine was 86 in (2.19 m) long, 42 in (1.06 m) wide, and 44 in (1.12 m) tall. The Asso 1000 weighed 1,764 lb (800 kg). Like with the original Asso 750, a gear reduction version was designed. This engine was sometimes designated as the Asso 1200 R. The gear reduction speeds available were .667 and .581. The Asso 1200 R produced 1,200 hp (895 kW) at 1,950 rpm and weighed 2,116 lb (960 kg).

Isotta Fraschini Asso 1000

The Isotta Fraschini Asso 1000 was very similar to the Asso 750. Note the intake manifolds between the cylinder banks, each taking the air/fuel mixture from one of the outer banks and feeding half of the center bank.

The Asso 750 and Asso 1000 engines were used in a variety of aircraft, but most of the aircraft were either prototypes or had a low production count. For the Asso 750, its most famous applications were the single engine Caproni Ca.111 reconnaissance aircraft (over 150 built) and the twin engine Savoia-Marchetti S.55 double-hulled flying boat. Over 200 S.55s were built, but only the S.55X variant was powered by the Asso 750. Twenty-five S.55X aircraft were built, and in 1933, 24 S.55X aircraft made a historic formation flight from Orbetello, Italy to Chicago, Illinois. The Asso 750 powered many aircraft to numerous payload and distance records. Six direct-drive Asso 1000 engines were used to power the Caproni Ca.90 bomber, which was the world’s largest landplane when it first flew in October 1929. The Ca.90 set six payload records on 22 February 1930.

Although not a complete success in aircraft, the Asso 1000 found its way into marine use as the Isotta Fraschini ASM 180, 181, 183 and 184 engines. ASM was originally written as “As M” and stood for Asso Marini (Ace Marine). The marine engines had water-cooled exhaust pipes and a reversing gearbox coupled to the propeller shaft. The Isotta Fraschini marine engines were used in torpedo boats before, during, and after World War II by Italy, Sweden, and Britain.

Isotta Fraschini ASM 184

The Isotta Fraschini ASM 184 engine with its large, water-cooled exhaust manifolds and drive gearbox. Note that the center bank only has its rear (left) cylinders feeding into the visible exhaust manifold. One of the two centrifugal superchargers can be seen at the rear of the engine. The engine is on display at the Museo Nicolis in Villafranca di Verona. (Stefano Pasini image)

The ASM 180 and 181 were developed around 1933, and produced 900 hp (671 kW) at 1,800 rpm. Refinement of the ASM 181 led to the ASM 183, which produced 1,150 hp (858 kW) at 2,000 rpm. Development of the ASM 184 started around 1940; it was a version of the ASM 183 that featured twin centrifugal superchargers mounted to the rear of the engine. The ASM 184 engine produced 1,500 hp (1,119 kW) at 2,000 rpm. Around 1950, production of the ASM 184 was continued by Costruzione Revisione Motori (CRM) as the CRM 184. In the mid-1950s, the engine was modified with fuel injection into the supercharger compressors and became the CRM 185. The CRM 185 produced 1,800 hp (1,342 kW) at 2,200 rpm.

CRM continued development of the W-18 platform and created a diesel version of the engine. Designated 18 D, the engine retained the same bore, stroke, and basic configuration as the Asso 1000 and earlier ASM engines. However, the 18 D was made of cast iron, had revised cylinder heads, and had a compression ratio of 14 to 1. The revised cylinder head was much taller and incorporated extra space between the valve springs and the valve heads. The valve stems were elongated, and a pre-combustion chamber was positioned between the valve stems and occupied the extra space in the head. Some versions of the engine have a fuel injection pump consisting of three six-cylinder distributors driven from the rear of the engine, while other versions have a common rail fuel system.

CRM 18 D engines

Four CRM 18 D engines, which can trace their heritage back to the Asso 1000. The three engines on the left use mechanical fuel injection with three distribution pumps. The engine on the right has a common fuel rail. Note the three turbochargers at the front of each engine. (CRM Motori image)

The exhaust gases for each bank were collected and fed through a turbocharger at the front of the engine (some models had just two turbochargers). Pressurized air from the turbochargers passed through an aftercooler and was then fed into two induction manifolds. Each of the manifolds had three outlets. The front and rear outlets were connected to the outer cylinder bank, and the middle outlet was connected to the center bank. For the center bank, induction air for the rear three cylinders was provided by the left manifold, and the front three cylinder received their air from the right manifold.

Various versions of the 18 D were designed, the most powerful being the 18 D BR3-B. The BR3-B had a maximum output of 2,367 hp (1,765 kW) at 2,300 rpm and a continuous output of 2,052 hp (1,530 kW) at 2,180 rpm. The engine had a specific fuel consumption of .365 lb/hp/hr (222 g/kW/h). The BR3-B was 96 in (2.45 m) long, 54 in (1.37 m) wide, 57 in (1.44 m) tall, and weighed 4,740 lb (2,150 kg) without the drive gearbox. CRM, now known as CRM Motori Marini, continues to market 18 D engines.

Isotta Fraschini Asso L180

Other than having a W-18 layout, the Isotta Fraschini L.180 did not share much in common with the Asso 750 or 1000. However, the two-outlet supercharger suggests a similar induction system to the earlier engines. Note the gear reduction’s hollow propeller shaft and the mounts for a cannon atop the engine.

In the late 1930s, Isotta Fraschini revived the W-18 layout with an entirely new aircraft engine known as the Asso L.180 (or military designation L.180 IRCC45). The Asso L.180 was an inverted W-18 (sometimes referred to as an M-18) that featured supercharging and a propeller gear reduction. The engine’s layout and construction were similar to that of the earlier W-18 engines. One source states the cylinder banks were spaced at 45 degrees. With nine power pulses for each crankshaft revolution, this is off from the ideal of having cylinders fire at 40-degree intervals (like the earlier W-18 engines) and may be a misprint. The crankshaft was supported by seven main bearings in a one-piece aluminum crankcase. The spur gear reduction turned at .66 crankshaft speed and had a hollow propeller shaft to allow an engine-mounted cannon to fire through the propeller hub. The single-speed supercharger turned at 10 times crankshaft speed.

The Isotta Fraschini L.180 had a 5.75 in (146 mm) bore and a 6.30 in (160 mm) stroke. The engine displaced 2,942 cu in (48.2 L) and had a compression ratio of 6.4 to 1. The L.180 had a takeoff rating of 1,500 hp (1,119 kW) at 2,360 rpm, a maximum output of 1,690 hp (1,260 kW) at 2,475 rpm at 14,764 ft (4,500 m), and a cruising output of 1,000 hp (746 kW) at 1,900 rpm at 14,764 ft (4,500 m). It is doubtful that the L.180 proceeded much beyond the mockup phase.

A number of Isotta Fraschini aircraft and marine engines are preserved in various museums and private collections. Some marine engines are still in operation, and the German tractor pulling group Team Twister uses a modified Isotta Fraschini W-18 engine in its Dabelju tractor.

Dabelju IF W-18 57L

The modified Isotta Fraschini W-18 in Team Twister’s Dabelju. The engine’s heads have been modified to have individual intake and exhaust ports. These crossflow heads are similar in concept to the heads used on the Macchi M.67’s engine. (screenshot of Johannes Meuleners Youtube video)

Isotta Fraschini Aviation (undated catalog, circa 1930)
Isotta Fraschini Aviation (1929)
Isotta Fraschini Aviazione (undated catalog, circa 1931)
Istruzioni per l’uso del motore Isotta-Fraschini Tipo Asso 750 (1931)
Istruzioni per l’uso del motore Isotta-Fraschini Tipo Asso 750 R (1934)
Istruzioni per l’uso del motore Isotta-Fraschini Tipo Asso 750 RC 35 (1936)
Istruzioni per l’uso del motore Isotta-Fraschini Tipo Asso 1000 (1929)
Aeronuatica Militare Museo Storico Catalogo Motori by Oscar Marchi (1980)
Aircraft Engines of the World 1941 by Paul H. Wilkinson (1941)
Jane’s All the World’s Aircraft 1931 by C. G. Grey (1931)