The Bentley AR1/BR1 Rotary engine was a British 9 cylinder rotary aircraft engine of WW1. The crankshaft remained stationary, the entire crankcase and the attached cylinders rotated around it as one unit. Its main application was in World War 1 aviation. The BR. 1 was built in large numbers, being one of the main powerplants of the Sopwith Camel.
The Bentley BR1 & BR2 Rotary Aero Engines of World War 1
This paper is a brief history of the Bentley BR1 & BR2 Rotary Aero Engines of World War 1. These particular engines, designed by WO Bentley whilst he was seconded to the Humber Company at Coventry, were by common consent the best of all the rotary engines used in that conflict. Also included in this paper are other rotary engines that preceded the Bentley.
All the rotary engines mentioned in this paper operated on the four-stroke Otto cycle, viz, Induction, Compression, Ignition and Exhaust. However, the manner in which they precisely achieved the cycle of events varied considerably from engine to engine, particularly with regard to the induction stroke. The fact that the whole engine rotated had no relevance on the fundamental cycle of operation. The vast majority of the French and British rotaries were manufactured under wartime conditions, but despite this they were all extremely well made using first rate materials.
The rotary aero engine, initially in the form of the French manufactured Gnome of the early twentieth century, was a phenomenon which broke upon the embryonic aviation scene in 1908. Coming seemingly from nowhere, the rotary engine held uncontested sway for just about a decade and was then relegated, almost as quickly, to relative obscurity and obsolescence.
The idea of an internal combustion engine rotating about its own fixed crankshaft axis was not new in 1908. At the Paris Universal Exposition of 1889, Felix-Theodore Millet (1844-1929) exhibited a patented cylinder rotary engine built into a five cycle wheel . Millet's rotary powered wheel was taken up commercially by the Darracq Company in 1900, and they enjoyed a limited amount of success manufacturing and marketing this invention. Although there are a few other possible claims, it does appear that Felix Millet may genuinely have originated the rotary engine concept.
Early water cooled aero engines were developed from their automobile equivalents but they tended to be heavy and their power to weight ratio was relatively poor. Although light in weight, air cooled engines were unreliable and prone to seizing due to unequal thermal expansion in their cylinders. Into this arena came the 7 cylinder Gnome 50 h.p. Omega rotary engine, which was first exhibited at the Paris Automobile Show in 1908.
Societe Des Moteurs Gnome of 49 Rue Laffitte, Paris, was formed by Louis Seguin in 1906 to initially manufacture stationary industrial engines. When Laurent, the younger brother of Louis, joined the company they set out to design and manufacture a lightweight rotary aero engine. Undoubtedly the Seguin brothers had seen or heard about Millet's rotary engine and perhaps believing that cylinder cooling would be significantly improved by having the entire engine rotate, they embarked upon the design of the Gnome.
At about this time nickel-chrome alloy steels were becoming available, with tensile strengths of 50/60 tons/sq.in (UTS). These are roughly equivalent to today's high tensile steels. The Seguin brothers took the opportunity to incorporate these alloy steels into their new engine. The cylinder wall thickness of all rotaries was very thin, in the region of only 1.5 mm (0.060") in the gnome, and nickel-chrome steels provided more than sufficient tensile strength to cope with the combined loads imposed by combustion pressures and centrifugal forces acting upon the rotating cylinders. The Gnome engine did not use shrunk-in cast iron cylinder liners, although most subsequent rotaries did, thus ending them with improved working surfaces for their pistons.
Cylinders of the Gnome and subsequent rotary engines were machined from solid billets of nickel-chrome or equivalent steels (Holtzer ND). In the case of the Gnome each cylinder started off as a 67lb billet; this being reduced to about 5lbs after 3 hours of machining time on a suitably tooled Turret Lathe. The cooling fins were also machined at the same set up using special form tools. A period photograph of this particular cylinder machining operation looks suspiciously like it is being performed on a Herbert Turret Lathe.
All rotaries employed a hollow crankshaft, again made from a nickel-chrome alloy steel forging, through which the fuel/air mixture was admitted into the crankcase. A rudimentary “carburettor positioned at the rear of the crankshaft supplied the mixture. From the crankcase the fuel/air mixture was directed to the cylinders. In the case of the Gnome a weakly spring loaded automatic poppet valve centrally located in each piston own permitted the fuel/air mixture to pass from the crankcase into the cylinder combustion chambers on the induction stroke. The poppet valve automatically closed as the individual pistons ascended on their compression stroke. The poppet valves demanded a high degree of attention, very careful setting and were not without their problems! There was of course always an explosive mixture below the pistons! A later version of the Gnome rotary introduced in 1913, called the Monosoupape (single valve) or “Mono”, utilised a series of drilled holes in the lower extremity of each cylinder together with corresponding transfer passages in the crankcase to allow the fuel/air /mixture, controlled by the piston itself, to enter the combustion chamber. This was very similar in operation to the gas flow in a two-stroke engine. All other rotary engines, viz. Le Rhone, Clerget, Siemens-Halske and the Bentley BR1 & BR2 utilised external transfer pipes, one per cylinder, to allow the fuel/air mixture to pass from the crankcase to the respective combustion chambers of each cylinder. Pushrod operated inlet valves, suitably timed off a cam ring, then admitted the fuel/air mixture into the respective cylinders.
Two other Paris based companies, already briefly mentioned, manufactured rotary aero engines and thus competed actively against the Gnome on the aviation scene. They were respectively Clerget-Blin & Cie established in 1911 at 37 Rue Cave, Levallois-Perret, a suburb to the north west of Paris noted for its industrial workshops, and the Le Rhone company founded in 1912 at 3 Rue La Boetie. In 1914 Le Rhone was taken over by the Gnome company and thus became Societe Des Moteurs Gnome et Rhone, although each part of the organization retained its own identity and range of engines.
Clerget rotaries were fairly orthodox in their valve operation, with separate pushrods and rockers for inlet and exhaust valves. This permitted valve timing overlap. In keeping with the Gnome and Bentley engines the Clerget crankshaft and big end assembly employed conventional master and articulated connecting rods. In contrast the Le Rhone used only a single push-pull rod to operate both the inlet and exhaust valves off a single rocker, and whilst this was a clever arrangement it did not permit any valve overlap. Additionally, the Le Rhone big end and connecting rod assemblies were very unusual indeed. The connecting rods had slipper ends which were captively held in a bronze thrust block comprising three concentric circular tracks - three equispaced rods in the outer track, three equispaced rods in the middle track and the remaining three in the inner track. It did of course mean that the nine connecting rods were made in three different lengths. This arrangement was claimed to give a smoother running engine as it obviated unbalance from unequal piston acceleration between cylinders.
All these engines, Gnome, Gnome “Mono”, Le Rhone and Clerget were extensively used in numerous types of allied First World War aircraft. The Sopwith Pup, for example, used the 80 h.p. Le Rhone C engine and the Sopwith Camel a number of different versions of the Clerget. Gnomes were generally reserved for training or non front line aircraft, although the Gnome “Mono” was used in many French and British combat machines. Each type had their adherents. Royal Flying Corps engine fitters rather liked the Clerget and recorded the fact in their logbooks. Some pilots preferred the smooth running of the Le Rhone and said so. “Mine’s a peach. It’s a genuine Le Rhone Le Rhone”. Others commented more generally. “The French are damn good at making engines……”.
It would be wrong to infer from the above that French designed rotaries were exclusively manufactured in France. Licensed manufacture of all three major rotary engines was undertaken in Great Britain during the First World War. Peter Hooker of Walthamstow held a licence for the manufacture of the Gnome, W.H. Allen of Bedford for the Le Rhone and Gwynnes of Hammersmith for Clerget. Additionally, a good deal of sub-contracting must also have taken place. Documents in the Warwickshire County Record Office confirm that Willans and Robinson at Rugby manufactured large quantities of Le Rhone components for W.H. Allen of Bedford. It would appear that the Daimler Company manufactured the Gnome “Mono”, Le Rhone and subsequently the Bentley BR1 and BR2 engines. Whether they were “officially licensed” to manufacture French engines is debatable as they seemingly “reverse engineered” the “Mono” from a sample engine supplied – not from drawings. Hardly a normal situation for licensed manufacture!
It has been suggested by FR (Rod) Banks in his autobiography, I Kept No Diary, that Peter Hooker Ltd., also known as The British Gnome and Le Rhone Engine Co., made the best and most reliable examples of the French designed rotary engines. The company was renowned for a number of precision made products, most notably the supply of special colour printing presses for the printing trade. They applied their engineering prowess to the manufacture of rotary aero engines. Their licence built Gnome engines achieved 80 hours between overhauls against 15-25 hours for the French built equivalents. This was in no small part due to the special tools they designed and manufactured for the rolling of bronze “L” section piston obturator rings – fitted to all French designed rotaries – of which more later in this paper.
Peter Hooker Ltd. also developed a process for forging aluminium alloy (Y Alloy) pistons and cylinder heads for aero engines. Alloy pistons of this material were used in the Gnome Monosoupape engine and replaced those made of cast iron, its superior thermal conductivity made it ideal for this application. “Y” Alloy had been developed by the National Physical Laboratory and its composition was; 4% Copper, 2 % Nickel, 1.5% Magnesium and the balance aluminium. Rod Banks relates in his autobiography that strongly against his advice a one- off cylinder for a Gnome engine was manufactured in “Y”alloy. Prior to it being trialled in an actual engine, Banks warned that the test should be conducted in the open on a Gnome “gun carriage” type test stand, rather than in the normal enclosed “escargot” type test units, and everyone was to keep clear! The engine had barley completed a revolution and when the cylinder made of “Y “alloy actually fired it was propelled 70 feet into the air over the test house!
Walter Owen (“WO”) Bentley (1888 – 1971) was born at Hampstead, London, on 16th September 1888, into a comfortably well off family. He was the youngest of nine children. His father Alfred was a retired business man and his mother Emily (nee – Waterhouse), was of Australian birth. In due course he had the good fortune to be educated at Clifton College, Bristol. At the age of 16 “WO” commenced a 5 year premium apprenticeship with the Great Northern Railway Company (GNR) at Doncaster, where he was inculcated into the world of railway engineering. The GNR at this time was probably engaged in the construction of the second batch of its Atlantic express locomotives.
On the completion of his apprenticeship with the GNR in1910, “WO” decided that he did not want to continue his career in the railway industry and opted to study Theoretical Engineering at King’s College, London. At this time he also raced various makes of motor cycles and competed unsuccessfully in the Isle of Man Tourist Trophy races of 1909 and 1910.
In 1912 “WO” joined his brother Horace Millner Bentley as a business partner in the firm of Bentley and Bentley to sell the French made DFP (Doriot – Flandrin – Paran) car from premises in London. “WO” was disappointed with the performance of the DFP and thought their engines could be improved with the incorporation of aluminium alloy pistons, then still a comparative novelty; the onal material being cast iron. This was done in 1913.
With the commencement of the First World War in 1914, “WO” volunteered for military service and was commissioned into the Royal Navy as a Technical Consultant. He was directed to Gywnnes of Hammersmith to oversee the manufacture of the Clerget 130 h.p. rotary aero engine being made under license by the firm. The Clerget was extensively used on the Western Front in many aircraft, including those used by the Royal Naval Air Service (RNAS). In addition to his work at Gywnnes “WO” was officially authorised to acquaint other firms, such as Rolls-Royce, with his own findings on aluminium pistons in DFP cars and the advantages that could be expected from their use. This was invaluable information which, if suitably disseminated, could aid the war effort considerably.
Unfortunately, although basically a fine piece of engineering, the Clerget suffered from a number of defects, including unequal cooling of its cylinders; steel not being a particularly good thermal conductor. Therefore, the cylinders were cool on their front leading edges, but very hot at the rear. This resulted in considerable asymmetric distortions in the cylinders, a problem not unique to the Clerget but experienced by all rotaries. In an attempt to mitigate these problems the Clerget used what was called a obturator ring, fitted at the top edge of each cast iron piston. The bronze obturator rings were of “L” section, very fragile with an operational life of only about 15 hours. To change the obturator rings necessitated a major engine strip down. Failure of an obturator ring in service usually resulted in piston seizure, followed by the total destruction of the engine. Bronze obturator rings were also used on the Gnome, Gnome “Mono” and Le Rhone engines, causing similar problems. On the Western Front there was consternation over the Clerget. “WO” visited France, perhaps on several occasions, to see the problems with the Clerget at the front.
Armed with this information “WO” attempted to implement a number of changes to the Clerget. These changes primarily focused on replacing the cast iron pistons with ones made from aluminium alloy. Pistons of this material had been previously tried in some Clerget engines with a noted improvement. However, Gwynnes were reluctant to implement this modification. Gwynnes position was perhaps understandable as they were not the design authority just licensees and Clerget in France retained close control of the design. “WO”and his navy superiors eventually became frustrated by the intransigence of Gwynnes and Clerget, and a decision was made to move “WO” to the Humber Company in Coventry with a view to designing a totally new engine.
This was new territory for “WO” who had never previously designed an engine, let alone an aero engine. With a small team at Humber he proceeded to conceive a new rotary, taking the best features of the Clerget and other engines, but incorporating new ideas that hopefully would obviate the problems of the past. Bentley retained the Clerget valve gear and cam ring assembly as he considered it was satisfactory, but completely altered the cylinder arrangement by substituting aluminium alloy L.8 cylinder barrels with shrunk in cast iron K.5 liners for the solid steel cylinders. An opportunity was also taken to incorporate detachable S.11 steel finned cylinder heads. Each cylinder barrel with its detachable head was secured to the crankcase by four long studs and castellated nuts and split pins; a method of construction that became the norm for many subsequent aero engines. The aluminium alloy L.8 pistons had slightly concave heads, each being fitted with five square section K.5 iron piston rings. Obturator rings were no longer used.
The Bentley AR1/BR1 Rotary engine was a British 9 cylinder rotary aircraft engine of WW1. The crankshaft remained stationary, the entire crankcase and the attached cylinders rotated around it as one unit. Its main application was in World War 1 aviation. The BR. 1 was built in large numbers, being one of the main powerplants of the Sopwith Camel.
The Bentley BR1 & BR2 Rotary Aero Engines of World War 1
This paper is a brief history of the Bentley BR1 & BR2 Rotary Aero Engines of World War 1. These particular engines, designed by WO Bentley whilst he was seconded to the Humber Company at Coventry, were by common consent the best of all the rotary engines used in that conflict. Also included in this paper are other rotary engines that preceded the Bentley.
All the rotary engines mentioned in this paper operated on the four-stroke Otto cycle, viz, Induction, Compression, Ignition and Exhaust. However, the manner in which they precisely achieved the cycle of events varied considerably from engine to engine, particularly with regard to the induction stroke. The fact that the whole engine rotated had no relevance on the fundamental cycle of operation. The vast majority of the French and British rotaries were manufactured under wartime conditions, but despite this they were all extremely well made using first rate materials.
The rotary aero engine, initially in the form of the French manufactured Gnome of the early twentieth century, was a phenomenon which broke upon the embryonic aviation scene in 1908. Coming seemingly from nowhere, the rotary engine held uncontested sway for just about a decade and was then relegated, almost as quickly, to relative obscurity and obsolescence.
The idea of an internal combustion engine rotating about its own fixed crankshaft axis was not new in 1908. At the Paris Universal Exposition of 1889, Felix-Theodore Millet (1844-1929) exhibited a patented cylinder rotary engine built into a five cycle wheel . Millet's rotary powered wheel was taken up commercially by the Darracq Company in 1900, and they enjoyed a limited amount of success manufacturing and marketing this invention. Although there are a few other possible claims, it does appear that Felix Millet may genuinely have originated the rotary engine concept.
Early water cooled aero engines were developed from their automobile equivalents but they tended to be heavy and their power to weight ratio was relatively poor. Although light in weight, air cooled engines were unreliable and prone to seizing due to unequal thermal expansion in their cylinders. Into this arena came the 7 cylinder Gnome 50 h.p. Omega rotary engine, which was first exhibited at the Paris Automobile Show in 1908.
Societe Des Moteurs Gnome of 49 Rue Laffitte, Paris, was formed by Louis Seguin in 1906 to initially manufacture stationary industrial engines. When Laurent, the younger brother of Louis, joined the company they set out to design and manufacture a lightweight rotary aero engine. Undoubtedly the Seguin brothers had seen or heard about Millet's rotary engine and perhaps believing that cylinder cooling would be significantly improved by having the entire engine rotate, they embarked upon the design of the Gnome.
At about this time nickel-chrome alloy steels were becoming available, with tensile strengths of 50/60 tons/sq.in (UTS). These are roughly equivalent to today's high tensile steels. The Seguin brothers took the opportunity to incorporate these alloy steels into their new engine. The cylinder wall thickness of all rotaries was very thin, in the region of only 1.5 mm (0.060") in the gnome, and nickel-chrome steels provided more than sufficient tensile strength to cope with the combined loads imposed by combustion pressures and centrifugal forces acting upon the rotating cylinders. The Gnome engine did not use shrunk-in cast iron cylinder liners, although most subsequent rotaries did, thus ending them with improved working surfaces for their pistons.
Cylinders of the Gnome and subsequent rotary engines were machined from solid billets of nickel-chrome or equivalent steels (Holtzer ND). In the case of the Gnome each cylinder started off as a 67lb billet; this being reduced to about 5lbs after 3 hours of machining time on a suitably tooled Turret Lathe. The cooling fins were also machined at the same set up using special form tools. A period photograph of this particular cylinder machining operation looks suspiciously like it is being performed on a Herbert Turret Lathe.
All rotaries employed a hollow crankshaft, again made from a nickel-chrome alloy steel forging, through which the fuel/air mixture was admitted into the crankcase. A rudimentary “carburettor positioned at the rear of the crankshaft supplied the mixture. From the crankcase the fuel/air mixture was directed to the cylinders. In the case of the Gnome a weakly spring loaded automatic poppet valve centrally located in each piston own permitted the fuel/air mixture to pass from the crankcase into the cylinder combustion chambers on the induction stroke. The poppet valve automatically closed as the individual pistons ascended on their compression stroke. The poppet valves demanded a high degree of attention, very careful setting and were not without their problems! There was of course always an explosive mixture below the pistons! A later version of the Gnome rotary introduced in 1913, called the Monosoupape (single valve) or “Mono”, utilised a series of drilled holes in the lower extremity of each cylinder together with corresponding transfer passages in the crankcase to allow the fuel/air /mixture, controlled by the piston itself, to enter the combustion chamber. This was very similar in operation to the gas flow in a two-stroke engine. All other rotary engines, viz. Le Rhone, Clerget, Siemens-Halske and the Bentley BR1 & BR2 utilised external transfer pipes, one per cylinder, to allow the fuel/air mixture to pass from the crankcase to the respective combustion chambers of each cylinder. Pushrod operated inlet valves, suitably timed off a cam ring, then admitted the fuel/air mixture into the respective cylinders.
Two other Paris based companies, already briefly mentioned, manufactured rotary aero engines and thus competed actively against the Gnome on the aviation scene. They were respectively Clerget-Blin & Cie established in 1911 at 37 Rue Cave, Levallois-Perret, a suburb to the north west of Paris noted for its industrial workshops, and the Le Rhone company founded in 1912 at 3 Rue La Boetie. In 1914 Le Rhone was taken over by the Gnome company and thus became Societe Des Moteurs Gnome et Rhone, although each part of the organization retained its own identity and range of engines.
Clerget rotaries were fairly orthodox in their valve operation, with separate pushrods and rockers for inlet and exhaust valves. This permitted valve timing overlap. In keeping with the Gnome and Bentley engines the Clerget crankshaft and big end assembly employed conventional master and articulated connecting rods. In contrast the Le Rhone used only a single push-pull rod to operate both the inlet and exhaust valves off a single rocker, and whilst this was a clever arrangement it did not permit any valve overlap. Additionally, the Le Rhone big end and connecting rod assemblies were very unusual indeed. The connecting rods had slipper ends which were captively held in a bronze thrust block comprising three concentric circular tracks - three equispaced rods in the outer track, three equispaced rods in the middle track and the remaining three in the inner track. It did of course mean that the nine connecting rods were made in three different lengths. This arrangement was claimed to give a smoother running engine as it obviated unbalance from unequal piston acceleration between cylinders.
All these engines, Gnome, Gnome “Mono”, Le Rhone and Clerget were extensively used in numerous types of allied First World War aircraft. The Sopwith Pup, for example, used the 80 h.p. Le Rhone C engine and the Sopwith Camel a number of different versions of the Clerget. Gnomes were generally reserved for training or non front line aircraft, although the Gnome “Mono” was used in many French and British combat machines. Each type had their adherents. Royal Flying Corps engine fitters rather liked the Clerget and recorded the fact in their logbooks. Some pilots preferred the smooth running of the Le Rhone and said so. “Mine’s a peach. It’s a genuine Le Rhone Le Rhone”. Others commented more generally. “The French are damn good at making engines……”.
It would be wrong to infer from the above that French designed rotaries were exclusively manufactured in France. Licensed manufacture of all three major rotary engines was undertaken in Great Britain during the First World War. Peter Hooker of Walthamstow held a licence for the manufacture of the Gnome, W.H. Allen of Bedford for the Le Rhone and Gwynnes of Hammersmith for Clerget. Additionally, a good deal of sub-contracting must also have taken place. Documents in the Warwickshire County Record Office confirm that Willans and Robinson at Rugby manufactured large quantities of Le Rhone components for W.H. Allen of Bedford. It would appear that the Daimler Company manufactured the Gnome “Mono”, Le Rhone and subsequently the Bentley BR1 and BR2 engines. Whether they were “officially licensed” to manufacture French engines is debatable as they seemingly “reverse engineered” the “Mono” from a sample engine supplied – not from drawings. Hardly a normal situation for licensed manufacture!
It has been suggested by FR (Rod) Banks in his autobiography, I Kept No Diary, that Peter Hooker Ltd., also known as The British Gnome and Le Rhone Engine Co., made the best and most reliable examples of the French designed rotary engines. The company was renowned for a number of precision made products, most notably the supply of special colour printing presses for the printing trade. They applied their engineering prowess to the manufacture of rotary aero engines. Their licence built Gnome engines achieved 80 hours between overhauls against 15-25 hours for the French built equivalents. This was in no small part due to the special tools they designed and manufactured for the rolling of bronze “L” section piston obturator rings – fitted to all French designed rotaries – of which more later in this paper.
Peter Hooker Ltd. also developed a process for forging aluminium alloy (Y Alloy) pistons and cylinder heads for aero engines. Alloy pistons of this material were used in the Gnome Monosoupape engine and replaced those made of cast iron, its superior thermal conductivity made it ideal for this application. “Y” Alloy had been developed by the National Physical Laboratory and its composition was; 4% Copper, 2 % Nickel, 1.5% Magnesium and the balance aluminium. Rod Banks relates in his autobiography that strongly against his advice a one- off cylinder for a Gnome engine was manufactured in “Y”alloy. Prior to it being trialled in an actual engine, Banks warned that the test should be conducted in the open on a Gnome “gun carriage” type test stand, rather than in the normal enclosed “escargot” type test units, and everyone was to keep clear! The engine had barley completed a revolution and when the cylinder made of “Y “alloy actually fired it was propelled 70 feet into the air over the test house!
Walter Owen (“WO”) Bentley (1888 – 1971) was born at Hampstead, London, on 16th September 1888, into a comfortably well off family. He was the youngest of nine children. His father Alfred was a retired business man and his mother Emily (nee – Waterhouse), was of Australian birth. In due course he had the good fortune to be educated at Clifton College, Bristol. At the age of 16 “WO” commenced a 5 year premium apprenticeship with the Great Northern Railway Company (GNR) at Doncaster, where he was inculcated into the world of railway engineering. The GNR at this time was probably engaged in the construction of the second batch of its Atlantic express locomotives.
On the completion of his apprenticeship with the GNR in1910, “WO” decided that he did not want to continue his career in the railway industry and opted to study Theoretical Engineering at King’s College, London. At this time he also raced various makes of motor cycles and competed unsuccessfully in the Isle of Man Tourist Trophy races of 1909 and 1910.
In 1912 “WO” joined his brother Horace Millner Bentley as a business partner in the firm of Bentley and Bentley to sell the French made DFP (Doriot – Flandrin – Paran) car from premises in London. “WO” was disappointed with the performance of the DFP and thought their engines could be improved with the incorporation of aluminium alloy pistons, then still a comparative novelty; the onal material being cast iron. This was done in 1913.
With the commencement of the First World War in 1914, “WO” volunteered for military service and was commissioned into the Royal Navy as a Technical Consultant. He was directed to Gywnnes of Hammersmith to oversee the manufacture of the Clerget 130 h.p. rotary aero engine being made under license by the firm. The Clerget was extensively used on the Western Front in many aircraft, including those used by the Royal Naval Air Service (RNAS). In addition to his work at Gywnnes “WO” was officially authorised to acquaint other firms, such as Rolls-Royce, with his own findings on aluminium pistons in DFP cars and the advantages that could be expected from their use. This was invaluable information which, if suitably disseminated, could aid the war effort considerably.
Unfortunately, although basically a fine piece of engineering, the Clerget suffered from a number of defects, including unequal cooling of its cylinders; steel not being a particularly good thermal conductor. Therefore, the cylinders were cool on their front leading edges, but very hot at the rear. This resulted in considerable asymmetric distortions in the cylinders, a problem not unique to the Clerget but experienced by all rotaries. In an attempt to mitigate these problems the Clerget used what was called a obturator ring, fitted at the top edge of each cast iron piston. The bronze obturator rings were of “L” section, very fragile with an operational life of only about 15 hours. To change the obturator rings necessitated a major engine strip down. Failure of an obturator ring in service usually resulted in piston seizure, followed by the total destruction of the engine. Bronze obturator rings were also used on the Gnome, Gnome “Mono” and Le Rhone engines, causing similar problems. On the Western Front there was consternation over the Clerget. “WO” visited France, perhaps on several occasions, to see the problems with the Clerget at the front.
Armed with this information “WO” attempted to implement a number of changes to the Clerget. These changes primarily focused on replacing the cast iron pistons with ones made from aluminium alloy. Pistons of this material had been previously tried in some Clerget engines with a noted improvement. However, Gwynnes were reluctant to implement this modification. Gwynnes position was perhaps understandable as they were not the design authority just licensees and Clerget in France retained close control of the design. “WO”and his navy superiors eventually became frustrated by the intransigence of Gwynnes and Clerget, and a decision was made to move “WO” to the Humber Company in Coventry with a view to designing a totally new engine.
This was new territory for “WO” who had never previously designed an engine, let alone an aero engine. With a small team at Humber he proceeded to conceive a new rotary, taking the best features of the Clerget and other engines, but incorporating new ideas that hopefully would obviate the problems of the past. Bentley retained the Clerget valve gear and cam ring assembly as he considered it was satisfactory, but completely altered the cylinder arrangement by substituting aluminium alloy L.8 cylinder barrels with shrunk in cast iron K.5 liners for the solid steel cylinders. An opportunity was also taken to incorporate detachable S.11 steel finned cylinder heads. Each cylinder barrel with its detachable head was secured to the crankcase by four long studs and castellated nuts and split pins; a method of construction that became the norm for many subsequent aero engines. The aluminium alloy L.8 pistons had slightly concave heads, each being fitted with five square section K.5 iron piston rings. Obturator rings were no longer used.
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