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.

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.

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.

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.

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.

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.

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.

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).

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).

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, reversible-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.

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.

Sources:
– “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)
– https://www.thegrowler.org.uk/avroshackleton/the-nomad-proposal.htm
– http://www.apss.org.uk/projects/completed_projects/nomad/index.htm
– http://www.apss.org.uk/projects/completed_projects/nomad/detail/index.htm