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 exactly new in 1908. At the Paris Universal Exposition of 1889, Felix-Theodore Millet (1844-1929) exhibited a patented five cylinder rotary engine built into a 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 claimants, 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 of 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 present day 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 utilise shrunk-in cast iron cylinder liners, although most subsequent rotaries did, thus endowing 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 crown 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 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 organisation 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.
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 exactly new in 1908. At the Paris Universal Exposition of 1889, Felix-Theodore Millet (1844-1929) exhibited a patented five cylinder rotary engine built into a 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 claimants, 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 of 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 present day 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 utilise shrunk-in cast iron cylinder liners, although most subsequent rotaries did, thus endowing 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 crown 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 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 organisation 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.
No comments:
Post a Comment