Super Charging Allison

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Supercharging the Allison

A Presentation to the

Aircraft Engine Historical Society

July 17, 2009

Indianapolis, Indiana

By

Daniel D. Whitney, P.E.


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Background

There are several aspects to the topic of Supercharging the Allisonthat involve more thanjust the technology of the supercharging methods applied to the Allison V-1710. In

particular, the engine was a key player in the technical and political evolution of large

aircraft engines during the period between the First World War and through the SecondWorld War. This paper reflects on these developments, and motivations, as well as the

technology of the V-1710.

Any engine used to power an aircraft is fundamentally a mechanical device interposed

into the process of burning a fuel, with the goal of extracting meaningful and usefulpower for the purpose of sustaining flight. Machines that do this are known to engineers

as Heat Power Engines, and for some 200+ years, people have been devising

interesting, sometimes unique, and hopefully effective, ways of doing this.

With the necessity to burn a fuel, thereby releasing its contained thermal energy, thefundamental requirements are to bring the fuel and oxidizer together at the same time and

place, then burn the fuel in a way that the heat doesnt damage the mechanicalcomponents of the engine, while deriving mechanical power that can be used to drive anairscrew, or provide a jet of gasses able to produce propulsive thrust. It quickly became

apparent to airplane designers that the more power available the more successful their

craft was likely to be. Unfortunately, early efforts at providing more power usually

meant more weight, which is the antithesis of a flying machine.

The first aviation engines were naturally aspirated, that is, they drew in ambient air in

which to burn the fuel, by the rapid movement of a piston down a cylinder. Soon

engineers began to describe the advantages of their particular machines by their

Volumetric Efficiency, that is, how well the cylinders were being filled during theintake stroke. This process could never be 100% efficient, and particularly so in an

airplane whose whole purpose was to fly high in the sky, where the air density issignificantly reduced as altitude increases. Clearly, there was a point where the available

power from the engine would decrease to the point that the airplane could go no higher.

This has been named the Critical Altitude.

Another type of efficiency (inefficiency) that works on any engine is the fact that it cannever fully extract, or convert, all the available fuel energy into power, for there are a

number of competing physical processes that conspire to waste the available energy. The

most demanding of these is the amount of energy required to heat the air in the cylinderas a consequence of burning the fuel. Peak temperatures during the combustion process

exceed 3,000oF, which is good for thermodynamics because it gives us a lot of thermal

energy to work with, but it also can be very hard on the machinery extracting mechanicalwork. Obviously, with this much heat in a confined space the machine is going to get

hot, so a cooling system must be included to keep the parts from being damaged. Next,

there is motion, for the oscillating piston in the combustion chamber is connected to acrankshaft, which has supporting bearings, all of which involves metal components

rubbing together, creating friction and wear. This can be overcome by incorporating a

lubrication system to keep the parts moving smoothly, through the lubricating oil is


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Figure 1 This is an example of how the heat from burning fuel inan engine is distributed. In this case only 29.6% of the fuelenergy is delivered to the propeller, and over half exits with thehot exhaust.

heated by this friction and the hot metal parts, so the airplane designer must devise a way

of cooling the oil. This again adds weight and drag to a fine flying machine.

Figure 1 is a schematicshowing how a typical

aircraft engine utilizes

the energy availablefrom the fuel. Notice

that only about 25% of

the fuel energy isdelivered to the

airscrew, another 25%

is wasted via the

cooling systems(cylinder/head and

lubricating oil), and

unfortunately, about

50% of the energyleaves via the hot

engine exhaust gas. Itstemperature having

been reduced to about

1500oF by the work done on the propeller and by the cooling systems, but this is still a

lot of energy. If only there was some way to recover it.

Early airplane designers quickly found that using larger, and therefore heavier, engines

was of dubious value. What was needed was for every pound of engine installed in the

airplane to do more work, the question was, how?

It is believed that GottliebDaimler was the first to propose supercharging, but it was nottried until used on a two-cycle engine in1878. With the start of WWI in 1914 FrenchmanAuguste Rateau came up with a way to get more power from an aircraft engine bysupercharging. His device forced more air into the engine, that is, to supercharge it,

and increasing volumetric efficiency. This not only produces more power, but has the

advantage that the power can be produced to a higher altitude, i.e., increasing the criticalaltitude. Rateau also advanced the concept by attempting to utilize some of the heat

being wasted in the exhaust to drive the supercharger (actually an air-compressor) by

powering it with a turbine. The device became known as the turbo-supercharger.

The TurbosuperchargerThe late 1800s and early 1900s were a very dynamic time for the development of heatengines. The early gasoline spark ignition 4-stroke, Otto Cycle, engines were being

invented and developed around the world. The Diesel engine cycle was also being

pursued, while at the same time stationary and marine steam turbines were being used inever larger scale. In about 1903 Sanford Moss, while working on a PhD in Engineering

at Cornell University, was able to obtain a small steam turbine for his research. With

modifications to this device he became the first to operate a turbine powered by hot


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combustion gas. This experience got him a position at General Electric, a company then

big in steam turbine driven electric generators and air compressors.

When the US was drawn into WWI in 1917 the aviation arm of the US Army Signal

Corps was interested in applying the Rateau turbosupercharger to the new Liberty-12

aircraft engine and Mosss earlier work was remembered. The GE Company was thenselected as one of several US entities given the charge to produce an indigenous

turbosupercharger. Only two machines came out of this work, one by Scherboney, and

the other by GE/Moss. Both were comparatively crude devices, with only the GE unitconsidered for development.

Moss and his team at GE had considerable resources available to them, specifically theyhad people who understood to some extent the dynamics of hot steam and gasses, the

elements of turbine design, and significantly, they understood air and had a lot ofexperience compressing it, albeit, for industrial applications.

By the end of WWI the GE/Moss team had built several operating turbosuperchargers

suitable for use with the 400 hp Liberty-12, and had successfully operated one at an

elevation of some 14,000 feet on Pikes Peak, Colorado.

Figure 2Although shown on an air-cooled engine, the Type B Turbo was used for the V-1710,particularly the P-38. Important features are the intercooler to reduce the temperature of the

compressed air going into the carburetor and the control of the wastegate, which determines thespeed of the turbine/compressor, and thereby the discharge air pressure.


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Following the war the turbosupercharger was one of the few programs to receive

continued funding and support by the military. The war had clearly shown the need andtactical advantage of operating combat airplanes at high altitudes.

Moss went through a number of significant evolutions of his turbosupercharger. The first

was to find a viable configuration for all of the machinery, for while a turbo is a fairly

simple device, having a single shaft, with a centrifugal supercharger on one end and anaxial turbine on the other, relatively large ducting needed to connect it to an intercooler,

the engines carburetor, and to the hot exhaust outlets.

The Use of Superchargers

The need and interest in fitting aircraft engines with superchargers was fairly widespread

in the early 1920s, however actually getting a supercharger to work on an engine was nota simple undertaking. While the centrifugal type of supercharger is preferred for

aircraft installations, (because of simplicity and light weight, having only one moving

part, the impeller), it must spin at very high speeds, inducing both very highmechanical stresses, while generating considerable heat. This heat (temperature rise) is a

physical consequence of the compression of the air and the inefficiency of the

supercharger as a machine. For these reasons superchargers remained in theexperimental laboratory, and in a few instances, applied to early racing airplanes. See

Figure 2 for details of a turbosupercharger installation.

With the exception of the efforts to develop the turbosupercharger, the earliest

production use of what could be considered a supercharger were the rotary inductionsystems fitted on some early radial engines built by Wright Aeronautical and Pratt &

Whitney. Early radials were plagued by uneven distribution of the fuel and air mixture to

the radially disposed cylinders. While their carburetors mixed the fuel and air fairly well,

the intake tubes from the carburetor to the cylinders resulted in uneven delivery, andoften times collected condensed or unevaporated fuel. The result was rough running andpoor performance. A solution was to interpose a mixer between the carburetor and

cylinders to insure a uniform distribution of the mixture to the various cylinders. This

took the form of what would now be called a mechanically driven supercharger in that an

impeller, driven at a speed higher than the crankshaft was used to fully mix the fuel withthe air and add enough heat to insure the fuel was fully evaporated prior to delivery to the

cylinders. This worked quite well on the low-power engines to which it was fitted.

With the interest in racing it was only natural that the step-up gear ratio to the impeller be

increased, with the result that the pressure of the mixture going to the cylinders wasgreater than atmospheric, alas Supercharging. However, the resulting improvement in

performance was not without consequences. First, the increase in power challenged thefundamental design of the engine components, requiring many to be strengthened.However the real constraint was the quality of fuel being used. In the US the typical

aviation grade fuel1had an octane rating of only about 40 60, while the best, known as

Fighting Grade, was only marginally better, in the 65 87 range. The higher pressures

1Innovation and the Development of Flight, By Roger D. Launius, page 105


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developed in the cylinders, the result of the greater density and temperature of the

mixture coming from the supercharger, caused these fuels to detonate, with disastrousconsequences for the pistons and engines. In racing situations special cocktails of fuel

could be prepared which allowed limited but safe operation, however this was not

practical for widespread application.

Spurred by the development of the supercharger and the need for more powerful engines,the development of improved aviation fuels, specifically the introduction of TEL (Tetra

Ethyl Lead) compound, allowed performance ratings of over 100. Engines could now

safely develop much more power. With improved fuels, and superchargers, mostmanufacturers of large aircraft engines worked to fit the device onto their existing

engines. However in most instances the marriage was not very successful.

An example of such an engine is the Curtiss Conqueror, a water-cooled V-12 having

1560 cubic inches of displacement. At 600 horsepower this was one of the mostpowerful engines of the era, with various models incorporating the many evolving

features coming from the rapid development of technology occurring during the 1920s.

These included propeller reduction gears, improved bearings, chemical-cooling

(ethylene glycol), mechanical supercharging, and turbo-supercharging. Only thereduction gears and improved bearings enjoyed more than limited production. By 1929

major changes to the engine were required if more power, efficiency, and reliability were

to be achieved.

The 1930s Developments

Wright Aeronautical acquired Curtiss in 1929, and largely abandoned the in-line V-12 in

favor of a new line of supercharged, air-cooled, radials to compete with the new Pratt &

Whitney air-cooled radials that were preferred by the US Navy. Pratt & Whitney had

introduced their air-cooled, R-1340 in 1925. It featured a rotary induction system.Concurrently, Packard had announced that they were no longer interested in providingthe US Army Air Corps with its in-line V-12s and ended for more than a decade its

involvement with aviation engines.

Back in the early 1920s the Curtiss D-12, a water-cooled V-12, was the most advanced

engine available. It was imitated around the world with racing versions setting newspeed records, particularly in the Schneider Cup competition. With the rapid dominance

of the type in racing, and with the objective of having a high-powered, streamlined, V-12

of their own, the US Army Air Corps issued a request for proposals in 1929 for the

industry to provide them with such an engine. Unfortunately the stock-market crash inOctober 1929 changed the ability and interests of the likely firms able to respond, with

the result that the Allison Engineering Company was the only firm to provide aresponsive offer.

The Air Corps did not want to have Allison join the industry as an aircraft enginemanufacturer given the number of firms already in the field. Allison had been a key

provider of unique, experimental, and limited production and modified aircraft engines,

reduction gears, and superchargers to the aviation industry beginning in WWI.Furthermore, they had developed the steel-backed plain bearing, and had fitted them into


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thousands of WWI era Liberty-12 engines, significantly improving the reliability of the

Liberty. It was not a question of Allisons ability; rather it was the dilution of support forthe available engine manufacturers and the lack of government funding that concerned

the Army.

The engine proposed by

Allison was the liquid-cooled V-1710. Its

primary designer was

Harold Caminez, one ofthe two or three most

experienced and capable

engine designers of the

day. He designed anengine able to operate

with high-temperature

ethylene glycol coolant,

with an integral propellerreduction gear, able to

develop 1000 horsepower,have an integral

supercharger, and be able

to operate with a turbosupercharger. The engine also incorporated advanced features

such as a roller cam operating four valves per cylinder, the new Allison steel-backedbearings, and was

adaptable to fuel injection.

In all, a very advancedengine, see Figure 3,

however the Army wasunable to fund the project

and it was not encouraged.

Allison did find a sponsor

for the engine, albeit in anunsupercharged, naturally

aspirated, form, as the US

Navy needed such anengine to power its new

airships. The Navy

sponsored the newly

designed mechanicallysupercharged V-1710-A

as a way to expedite

development, of theengine they wanted.

Meanwhile, Allison

developed the unique V-1710-B for airship use,

Figure 3 This is the V-1710-A engine designed by HaroldCaminez, showing many of the features that would identify itthrough its long life, including the four valve cylinders andpower section, separately removable cylinder banks, reductiongear and accessory sections.

Figure 4 The naturally aspirated V-1710-B for airshipsincluded several very unique features, including the ability toreverse directions from full power in seven seconds. Without asupercharger the carburetors were mounted in the veebetween the banks.


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Figure 4. Both of these developments were successful, however the airship program was

cancelled following the loss the USS Akron and USS Macon airships, just as the V-1710-Bs were ready for delivery. In the meantime the Army developed a requirement for a

large bomber and desired to power it with streamlined engines fitted into the wings,

rather than the high drag, low-powered air-cooled radials installations typical of the day.

With this requirement in mind Allison designed the V-1710-C, an engine with a longepicyclic reduction gear for easy streamlining into the airframe, integral mechanical

supercharger, and intended to operate with a turbosupercharger in a high altitude bomber.

This engine incorporated the many lessons learned in developing the earlier models, yetwas to endure a slow development because of the depressed economic times and minimal

funding. In 1936 this engine became the first Allison to fly, in the test bed Bell A-11A.

Because of the development delays the Boeing XB-15 bomber project had been built with

conventional air-cooled radials. As a way to expedite development of both the V-1710and turbosupercharger the Army then issued a new requirement for an in-line powered,

turbosupercharged Pursuit, the Curtiss XP-37. This airplane preceded a contract for 13

YP-37s, intended to introduce the high-speed, high-altitude fighter type aircraft to the US

military, along with its integrated engine and turbo installation. As a group these aircraftwere successful, although the primary lessons learned were that combining the

turbosupercharger with a high performance engine/airframe required a much moresophisticated control system than was available at the time. This work was immediately

undertaken and resulted in turbosupercharger installations that were very successful only

a few years later, during WWII, in the Lockheed P-38, Republic P-47, Boeing B-17,

Consolidated B-24, and Boeing B-29.

Figure 5Bell XP-39, powered by a V-1710-E2 extension shaft engine. In this original form,with intercooled turbosupercharger, Bell promised to exceed 400 mph at altitude. Whenflown it was unable to meet this requirement due to excessive drag, much of which was dueto the way the turbo was installed.


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Figure 9 Accessory and supercharger drive configuration on theV-1710-C15 engine showing the long quill shaft used to dampentorsional vibrations via the stabilizing inertia of the high speedsupercharger impeller.

Figure 8Often referred to as the Long-Nose Allison, the V-1710-Cwas intended to maximize the ability of airplane designers tostreamline the engine installation. It resulted in some very attractiveairplanes, such as the early P-40, XP-38, and P-37s.

requirement, as shown in Figure 7.

Concurrently, the follow-

on Pursuit competitionwas also faltering when

Curtiss decided to try the

V-1710-C Allison, Figure8, in their air-cooled P-36

which had a radial

engine. This was a verysuccessful installation,

although still not able to

reach altitudes over

20,000 feet. At the lastmoment in the

competition Allison

substituted a higher speed

supercharger drive gearand a larger carburetor on

the engine resulting inthe V-1710-19, see

Figure 9, which in May

1939 won the competition and was designated the P-40 Tomahawk. The seriousness of

the situation and times can be appreciated when it is realized that when Germany invadedPoland on September 1, 1939, beginning WWII, the only modern pursuit in the US

military was the single XP-40, AAF S/N 38-10, as the small batches of YP-38Lightnings

and YP-39 Airacobraswere not yet flying.

With the start of WWIIBritain and France came

to the US and purchased

large numbers of

aircraft, and the enginesto power them. These

purchases funded

construction of theneeded factories and the

rapid developments that

followed.

Allison had hand built atotal of fewer than 40 V-

1710 engines during the

preceding ten years, butnow had contracts for

many thousands. These

engines were to be


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Figure 10 The accessory drives in the V-1710-E/F/G are muchrevised compared to those in the V-1710-C. Item 31 is a hydraulicdamper that removes torsional vibration from the drive system,stabilized by the supercharger inertia. The starter shaft is rigidlygeared to the crankshaft. When it is used to drive the AuxiliaryStage Supercharger the Aux Stage hydraulic clutch and Impellerinertia provide the stabilizing inertia.

progressively increased in power and reliability, and to be built in a number of very

different configurations. The engineering and manufacturing challenges were immense.A key to meeting the required performance growth was to improve the supercharging of

the engine.

The Allison Integral Engine-Stage Supercharger

The basic centrifugal

supercharger containsonly two parts, one

rotating, and the other

stationary. These arethe high speed

impeller, which

comprises a number ofvanes radiating from a

hub, and a stationarydiffuser. The diffusertakes the very high

speed air coming from

the tips of the rotating

vanes, and converts theairs kinetic energy into

pressure energy.

This process heats theair because of the work

done compressing it,

while the pressureincreases its density.

This density increase isthe supercharging

effect, which is what

allows more air to beforced into the cylinder

than is possible in a naturally aspirated engine.

In the V-1710-A Allison used a very simple 8.25 inch diameter impeller design without a

backing plate, straight vanes and a vane-less diffuser. While it worked, it functionedmore like a rotary induction system and was not very efficient. Fortunately very little

supercharging, or boost, was required in this engine so supercharger efficiency was notan issue. When the V-1710-C was designed not only was the rated boost pressure higher,

it was also necessary to minimize the amount of heat in the mixture since the installationwas intended to be turbosupercharged, a process that adds heat as well. This meant that

the efficiency needed to be improved and the new supercharger shows that considerable

work had been done to improve its efficiency and performance. Still the technology wasin its infancy. See Figure 8 for the early engine supercharger drive train.


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Figure 11 The venturi type intake manifolds solvedthe Allison Time Bomb fears that had becomeepidemic for the P-38J by late-1943.

In addition to the thousands of V-1710-Cs built for the P-40 Tomahawk, early production

was also focused on the V-1710-E and V-1710-F engines for the P-39 and P-38respectively. These engines shared many parts and were a complete redesign of the V-

1710-C. They were intended to be easier to manufacture and have the capability for

increased power ratings. The supercharger in these engines featured an entirely new

drive design that offered a number of different drive ratios, allowing the designer to tailorthe engine/supercharger combination to the requirements of the aircraft type and model.

These two aircraft being excellent examples of this flexibility, with the altitude rated P-

39 having 8.8:1 gears while the turbosupercharged sea-level rated engines in the first P-38s needed only 6.44:1. See Figure 10 for the late type supercharger drive train. Both

engines were rated for 1150 horsepower. The supercharger impeller in the V-1710-C/E/F

engines was essentially the same, being 9.5 inches in diameter and having 15 vanes,made of forged and machined aluminum, with an integral backing plate to give the vanes

stability and eliminate harmful high frequency harmonic vibrations that would otherwise

cause them to fatigue and break. The efficiency of the supercharger, while good for theday, was in the 55 - 60 percent range.

With the initial models in production and military service, it was necessary to providemore power for the ever changing

combat conditions the aircraft andengines were engaged in. This

resulted in a new supercharger inlet

that featured steel guide vanes,

aligned with the aluminum vanes onthe impeller. This simple addition

improved the airflow into the

impeller, improved the efficiency ofthe supercharger, and allowed more

air to be delivered to the engine.This resulted in an increase in the

engine rated or critical altitude

because of the improvedsupercharger, and a slightly higher

horsepower delivered to the

propeller. Concurrent with thisimprovement Allison received

approval from the Army to

eliminate the backfire screens that

had been fitted in the intake

manifolds. A part of this changewas a new gas pipe between the

supercharger and streamlined intakemanifolds that incorporated a

venturi that would pickup and

reatomize any fuel that condensedin the gas pipe, a condition that was

occurring during extended low-


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Figure 12 Hundreds of thousands Type B Turbosuperchargers were built during WWII.

Each P-38 had two, one for each V-1710. The turbo was powered by hot exhaust gassescollected from each cylinder bank and directed to the turbine section. Maximum exhaust gastemperature was 1750

oF, though 1450

oF was more typical. Back pressure on the engine

was typically near sea-level, about 32 inHgA. Compressor discharge was controlled to meetengine requirements, up to about 32 inHgA into the carburetor. An intercooler was used tokeep this temperature below 100

oF

power cruise of the new P-38J with improved intercoolers, Figure 11. The new

manifolds allowed a further 2 3,000 foot increase in critical altitude.

Late in the war Allison introduced improved steel inlet guide vanes on the impellors ofsome limited production engines. These improved the efficiency of the engine stage

supercharger to nearly 80 percent, which is very good, particularly for a centrifugal

compressor of this date.

After the war a number of pending engine and supercharger improvements, which hadbeen delayed because they would require reconfiguration of production lines, were

incorporated on the two-stage V-1710-G6 engines used on the North American P-82E/F

Twin Mustang. This included an enlarged supercharger impeller of 10.25 inches diametercapable of efficiently passing more air. At the same time the carburetor was eliminated,

significantly reducing restrictions to airflow in the intake system. It was replaced by a

speed density fuel metering control, essentially an analog computer that calculates themass of air going into the cylinders and meters the appropriate quantity of fuel.


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Figure 13 This is the mechanically supercharger two-stage V-1710-93(E11) for the P-63A.A three-barrel carburetor was used to maximize altitude capability and is mounted on theinlet to the Auxiliary Stage Supercharger. This view shows the step-up drive for the AuxStage mounted on the starter shaft. Note that the starter is not mounted in this image.

Two-Stage Supercharging

The necessity of two-stage supercharging was well appreciated in the early days of

powered flight. During the 1920s the US Army strategic thinking was that in future warshigh-altitude, precision bombing, would be the deciding tactic. This was so widespread

that development of fighters (Pursuits) was not considered important to winning the

strategic battle. This strategy can be excused when it is understood that bombers of theday were usually faster and higher flying than contemporary Pursuits.

To reach these high altitudes the GE/Moss turbosupercharger, Figure 12, was to be

employed. Its design philosophy was that it would deliver air at sea-level conditions to

the engine, at up to the critical altitude of the supercharger, while imposing only sea-levelback pressure on the engine. In this way the engine would always operate as if at sea-

level, so the strains and power losses of operating at high altitude would not be an issue.

Of course, the turbosupercharger created significant temperature rise while compressingthe very cold air in the stratosphere, so an intercooler was fitted between the turbo and

the carburetor to cool the air to a temperature similar to that at sea-level. Since the turbo

was powered by the hot exhaust gasses coming from the engine the power required to

drive it could be obtained with little, if any, reduction in the amount of power delivered tothe propeller.

Alternatively, a mechanically driven second-stage of supercharging could also be done,

but this introduced additional mechanical complexity in the engine, as well as robbing

from the engine a considerable amount of power that would otherwise be available to the


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Figure 14 Two-Stage V-1710-119(F32R), with Merlin style Aftercooler. This configurationallows sustained operation at high manifold pressure because it cools the mixture,preventing detonation, without the necessity of carrying ADI fluid on the aircraft.Performance of this engine was comparable to the Merlin in every regard. It flew in anumber of experimental aircraft and would have been incorporated in the post-war V-1710-G10 had the jet engine not displaced that program.

propeller. In the 1930s Allison had proposed to the Army that they develop a

mechanically driven two-stage supercharger for the V-1710. The motivation had been inresponse to requests for such an engine from some of the airplane designers, and to

overcome the then apparent shortcomings of the turbosupercharger as then being

implemented. With the very tight budgets of the Depression era 1930s, and the

considerable long-term investment they had made in the turbosupercharger, the Armywould not support such a development, and did not want Allison to dilute their limited

engineering capacity by undertaking such a project.

With the onset of WWII, and the considerable allocation of new money and manpower tothe Allison engine program, development was begun on a mechanically supercharged

two-stage V-1710, Figure 13. This effort followed the same modular design philosophy

that had been applied to the earlier Allison V-3420 and V-1710-E/F engines; a

philosophy taught by the General Motors Technical Institute, to maximizeinterchangeability and adaptability. As a Division of General Motors, all of their top

engineers were products of this post-graduate school, so a standard of excellence and

communication existed throughout the corporation. An example of this philosophy is

that the V-1710-F3R had 7,161 parts by actual count, but only about 700 "piece" ordifferent parts. The competing Rolls-Royce/Packard Merlin V-1650 had about 11,000

parts, with some 4,500 "piece" parts.


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Figure 15 The family of Light-weight Mustangs developed by North American near the endof WWII included the XP-51J, which was powered by the two-stage, aftercooled, V-1710-119(F32R). This engine was the equal of the two-stage Merlin used in the other light-weightMustangs.

The Allison designed Auxiliary Stage Supercharger is a compact device that mounts on

the rear of the engine accessory section, connected by several struts, a air duct, and drivenby a step-up gear case that mounts onto the robust starter drive shaft. As a way to

improve the efficiency of the installation the Aux Stage operates at the optimum speed

for the conditions using a variable speed control of an integral hydraulic clutch. When

attached to the basic V-1710 the engine length is extended about 18 inches longer,requiring adaption of the airframe, see Figure 14. The primary wartime user of the two-

stage V-1710 was the Bell P-63King Cobra, an enlarged version of the P-39 fitted with a

laminar flow wing. In its two-stage form the V-1710 is about nine inches longer than thetwo-stage Rolls-Royce Merlin. This was the primary reason that the engine was not

fitted into the P-51 Mustangduring the war. To do so would have required moving the

firewall back, a major change that would have interruptedMustangproduction. Both thetwo-stage V-1710 and two-stageMerlinwere developed and available at the same time,

mid-1943, and delivered similar power at comparable altitudes. When the later light

weightMustangswere designed the P-51J and P-82E/F were fitted with the latest two-stage Allisons to give them outstanding performance, see Figure 15.

Horsepower Demands of Two-Stage Supercharging

The power needed to compress air has two components, first the power to increase the

density and temperature in an adiabatic (reversible) process, and second the losses (which

show up as even more temperature rise) due to inefficiency in the shapes of the impellerand diffuser. In the first process the power required is simply a function of the pressure

rise and the quantity of air being pumped through the supercharger, Figure 16. When

using two stages of compression the power required is the same as if a single stage was


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Figure 16 For the V-1710 Takeoff power at 3000 rpm is developed in the range of 9 10,000 pounds of air per hour. War Emergency at about 12,500 #/hr. It is clear that at highpower the amount of power taken from the crankshaft is considerable. The associatedtemperature rise increases in a similar manner, and unless an intercooler, or ADI/Waterinjection is used to cool the mixture, damaging detonation will occur at about 100

oC.

doing all of the work, however there really is a difference because the second stage

impeller is working with air already heated by the first stage compression andinefficiency. The result is a calculation that is somewhat empirical based upon two

stages with different efficiencies and pressure rise capability. In the case of the V-1710

the engine-stage supercharger, which operates at a speed which is a fixed multiple of the

crankshaft rpm, usually consumes more horsepower than the Auxiliary Stage. This isbecause of the laws of supercharging, that for a V-1710 with 8.1:1 engine-stage step-up

gearing develops a pressure rise ratio of about 2.0 at maximum rated engine speed of

3000 rpm. In a two-stage setup this will deliver up to about 56 inHgA2without any boost

provided by the Auxiliary Stage, or Turbosupercharger.

The Auxiliary Stage, or Turbosupercharger, is used to maintain sea-level manifold

pressure when operating at altitude, or alternatively, develop greater power at altitudesless than that where the first stage is developing its maximum pressure ratio, Figure 17.

The pressure ratios of the two stages, in series, multiply. This is how War EmergencyRatings of up to 100 inHgA, at 3200 rpm, can be achieved. At these very high ratings the

total amount of power required by the two superchargers can exceed 1200 horsepower.

2At sea-level, with ambient pressure of 30 inHgA the expected boost from the supercharger at take-off

should be 60 inHgA, however the losses through the carburetor and butterflys are about 2 inHgA, so the

net observed manifold pressure is 56 inHgA.


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Figure 17 The horsepower consumed by the Auxiliary Stage, when running with minimumhydraulic clutch skip can become very large. This only occurs when at maximum altitude, orWar Emergency Ratings.

Evolution of the Allison Two-Stage Supercharger

In its initial form the Allison two-stage fitted the Aux Stage directly behind the engine

accessory section, with the carburetor sitting on the inlet to the Auxiliary Stage. In this

form thousands were built for the P-63, the engine models being the V-1710-93(E11) andV-1710-117(E21), Figure 19. A consequence of having two-stages of compression is

heating of the air. Allison considered with their V-1710-119(F32R), Figure 14 and then

deferred as a near term production item, the Rolls-Royce approach of incorporating anaftercooler, and instead developed the first water injection system. This approach

required that the aircraft carry a small tank of water-alcohol and, when needed, that it be

metered, along with the fuel, and injected into the eye of the engine-stage supercharger.

This approach worked quite well, and is now a feature of almost all high-powered pistonaircraft engines.

There is a drawback to having the carburetor on the inlet to the supercharger, particularly

when the primary goal is to increase the operating altitude of the engine/aircraft. This isthe considerable reduction in air pressure ahead of the supercharger, and when combined

with the already low air density available at altitude, severely reduces critical altitude.


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Figure 18 Auxiliary Stage Supercharger is driven off the starter shaft and through theinfinitely variable speed hydraulic coupling.

Allison addressed this issue with the V-1710-109(E22) engine, Figure 20; here the outlet

of the Aux Stage was re-directed so that the compressed air would enter the standardengine-stage carburetor, now operating as an inter-stage carburetor. Modern engine

nomenclature would call such a configuration as a blow-through carburetor. While

even this arrangement resulted in pressure losses in the ducting and carburetor, these

losses were after the Aux Stage, and could be easily overcome by slightly increasing itsoperating speed. This arrangement is the same as that used when the engine was fed by a

turbosupercharger.


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Figure 19The V-1710-117(E21) was an improvement in details of the E11 two-stage engine. Here the airflow through the superchargers is shown schematically.

Figure 20The V-1710-109(E22) uses the interstage carburetor location, on theinlet to the engine-stage supercharger as done on turbosupercharged two-stageengines. While the airflow makes more turns, eliminating the restriction of thecarburetor at the inlet to the Aux Stage gave a sizeable improvement in altitudeperformance.


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Figure 21 The V-1710-143(G6R) was the last production model of the Allison V-12. Themajor improvements were adaptation of the Speed Density Fuel Control, eliminating therestrictions caused by a carburetor, an increased diameter engine-stage supercharger, andother detail changes that allowed rating the engine at 3200 rpm and 100 inHgA, resulting in aWar Emergency Rating of 2250 bhp.

All of the Allison Aux Stage Superchargers use a 12-3/16 inch diameter impeller. With

the various models the step-up drive gear ratio did change, with the trend being toincrease it for greater power and higher altitude capability. In all cases the hydraulic

driving clutch was able to provide the exactly optimum speed once the carburetor throttle

butterflys were fully open. Speed was controlled by the simple expediency of a dip

stick that determined the quantity of oil in the clutch, thereby controlling the degree ofslip between the input and output shafts. As with any hydraulic clutch there is some

horsepower lost due to heating of the oil in the clutch, although this is relatively minor,

being less than five percent of the power being transmitted. See Figure 18.

In the later production Aux Stage units the impeller inlet guide vanes were reshaped to an

elliptical curved section which improved the efficiency and quantity of air that could be

delivered to the engine.

When the V-1710 was to be used in a two-stage installation, turbosupercharged or withthe Aux Stage, Allison standardized on 8.1:1 step-up gears for the 9.5 inch diameter

engine-stage supercharger. Early models of the V-1710 for the P-38 had used 6.44 and

7.48:1 ratios, primarily because those models of the engine were not rated for the higherpower of the later models, which benefited from improved structure and internal system

details. These included use of silver plated bearings, larger oil pumps, improved air

intake manifolds, and sodium-cooled intake valves. When the post-war V-1710-G6engines, Figure 21, with 10.25 inch diameter engine-stage impellers were developed, the


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Figure 22The TurboCompound V-1710-127(E27) was based on the V-1710-109, with a newAux Stage air inlet cover, and an extension shaft to connect the GE Type C power turbine,which was geared directly to the crankshaft. The Aux Stage hydraulic clutch controlled the

Aux Stage as on other two-stage engines. These features resulted in 3000 bhp to thepropeller at altitude when running at 3200 rpm and 100 inHgA; not bad for an engineoriginally designed to produce 1000 bhp!

drive used the 7.48:1 gears, which in combination with the larger impeller gave

approximately the same impeller tip speed as the smaller impeller and higher gears in theearlier engines.

A feature of the V-1710 is that it can be assembled to turn the propeller either right or

left handed. This is accomplished by swapping the crankshaft end for end, fitting an

idler gear in the accessories drive (needed because the cams, distributors, magneto, andsupercharger always rotate the same direction), fitting ignition harnesses with the revised

firing order, and installing an opposite handed starter. For the Auxiliary Stage

Supercharger, which must also rotate in the original direction, a reversing idler gear isalso required. The result is that a left-handed Auxiliary Stage will have a slightly lower

overall step-up drive ratio, for example in the V-1710-G6L it is 1:8.03 versus 1:8.087 for

the G6R. This difference is so small that it is negligible in service.

V-1710 Turbo-Compound

Driving a supercharger takes horsepower from the propeller, and the discharged hotexhaust gasses waste energy. Both of these problems could be reduced, or eliminated, if

the wasted exhaust heat energy could be made to provide the power to the supercharger.

This is exactly what the turbosupercharger does, but the amount of power it can provideis limited to the amount of power needed to drive what is essentially an auxiliary stage

supercharger. Its power requirements are high at high altitudes, but otherwise the Aux


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Figure 23 Schematic of how the hardware was arranged to connect the turbine so that itspower was delivered to the crankshaft. While the turbine drives through the Aux Stage, itdoes not directly drive it. Power for the superchargers was obtained in the normal way. The

Aftercooler was not included in the V-1710-127(E27).

Stage does not need that much power to boost manifold pressure and make more engine

power. The concept of the turbo-compound is that the turbine wheel be mechanicallyconnected to the engine crankshaft so that it can have a place to use the considerable

power available in the hot exhaust. In this way it provides the power needed for

supercharging, and any excess is available through the crankshaft to the propeller.

Table 1: Turbocompound V-1710-127(E27) RatingsRating BHP/RPM/Altitude, ft/MAP Comments

War Emergency 3090/3200/28,000/100 inHgA Wet

Military 2320/3200/28,000/ 85 inHgA Dry

Normal 1740/3000/33,000/ 52.5inHgA Dry

Maximum Cruise 1340/2700/26,000/ Dry, BSFC 0.365 #/hp-hr

Allison was the first to develop such an engine, with development beginning in 1943. As

the V-1710-127(E27) program, Figure 22, it was intended to power the Bell P-63H

airplane, Figure 25. Both the engine and airplane were built, but never flown due to theending of hostilities and the introduction of jets.

The turbine selected for the project was from the General Electric Type C

Turbosupercharger, designated the CT-1. This turbine is similar to the Type B turbos for

the P-38, only larger, as it was being used on the Republic P-47 with 2,000 bhp P&W R-2800 engines. To take full advantage of the power available the developed engine was a

two-stage V-1710 with the turbine driving through the Aux Stage to a gear connected

directly to the crankshaft. Both supercharging stages being powered in their usual ways,though the hydraulic coupling and off of the starter shaft. See Figure 23. The turbine


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Figure 24V-1710-127(E27) performance curves show the resultsof the development program. Quite an engine!

extended the length of the engine, but resulted in a very efficient overall configuration,

although the exhaust pipes were exposed on the P-63H due to limited internal space inthe airframe.

Table 2V-1710-G10 Turbo-Compound Performance

Horsepower rpm Altitude, ft CommentsTakeoff: 2000 3200 Sea Level Grade 115/145, Dry

Military: 2250 3200 28,500 Critical Altitude, Dry

War Emergency

2400 3200 Sea Level Wet

2550 3200 20,000 Critical Altitude, Wet

Normal Cruise

1400 2700 Sea Level

1700 2700 30,000

One challenge for the projectwas that the Type C turbine

wheel rotated the wrong

direction relative to the AuxStage Supercharger, requiringthat a special wheel andnozzle assembly be provided

by GE. In this way the wheel

could be directly connected

to the Aux Stage. As can beseen in the photo of the

engine this was done. The

ratings for the engine areimpressive and listed in Table

1, with a War Emergencyrating of 3090 bhp beingavailable at 3200 rpm and

28,000 feet, with a manifold

pressure of 100 inHgA. See

Figure 24. The difference intake-off power, with and

without the feedback turbine,

was 500 bhp. This increasedto over 800 bhp additional at

altitude.

By comparison, and undersimilar operating conditions,late model P-38s with

turbosupercharged V-1710s

developed 1600 bhp, thoughat 3000 rpm,V-1710-G6 two-

stage engines in the P-82E/F

delivered 2250 bhp at 3200


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rpm, and at the same 100 inHgA manifold pressure. But the real advantage of the turbo-

compound is the significant improvement in cruise specific fuel consumption, achievedby recovering some of the otherwise wasted exhaust heat energy. With this power now

available to the propeller the efficiency of the engine is very much improved, to 0.365

pounds per brake horsepower-hour. This even improves on the efficiency of the non-

supercharged engines in the 1986 record setting Voyagerun-refueled around the worldairplane, which operated at 0.393 #/bhp-hr.

Some of this benefit is realized when the engine is turbosupercharged, and both

approaches are dramatically better than when a two-stage engine is mechanicallysupercharged. The V-1650-7 Merlin, used in the major production P-51D Mustang, in

cruise at 16,500 feet delivered 520 bhp at 2000 rpm, while burning 48 gallons of fuel per

hour, for a BSFC of 0.60 pounds per horsepower per hour. By comparison, the

contemporary (two-stage via turbosupercharging) Allison V-1710-111/113 used in the P-38L, cruising at 15,000 feet with 525 bhp from 1600 rpm, consumed 0.45 pounds of fuel

per horsepower per hour.

Because of the perceived improvement in performance of the two-stage Merlinover the

single-stage V-1710 in the P-51 Mustang, it has often been suggested that a similarengine swap should have been done to the P-38. This was extensively studied by

Lockheed during the war with the conclusion that in fact, the two engines provided very

similar performance in the airplane; however the range of the airplane would have beensignificantly curtailed with the Merlin. As shown above, the Merlin is not an efficient

engine.

The real ride for the turbo-compound V-1710 was to be in a follow-on model of the P-

82 Twin Mustang. This was to have been an evolutionary development; in fact the USAFactually had contracted for engines to replace the V-1710-143/145(G6R/L) that was in

the P-82E/F with similar engines having port fuel injection. These would have been the

V-1710-147/149(G9R/L), rated for 2450 bhp at 3200 rpm and 110 inHgA-wet, howeverAllison preferred to make an even bigger step and go directly to their port fuel injected,

aftercooled, and the turbo-compounded V-1710-G10. In a long-range escort fighter, withtwo pilots and two engines, the improved specific fuel consumption would have allowed

the airplane to fly unbelievably long missions. Table 2 provides key performance values

for the proposed V-1710-G10 engine.

Allison also proposed a new V-1710-H version of the engine that would have eliminatedthe engine-stage supercharger entirely, been fitted with an aftercooler, and used a two-

stage turbine to provide all of the supercharging, a way to further improve performance

and specific fuel consumption.

Conclusion

The key to making power in an engine is maximizing the quantity of air passing throughthe engine, while efficiently using the air for combustion in the cylinders. As can be

seen, the Allison V-1710 air induction system covered a wide range of methods, ranging

from naturally aspirated, in the V-1710-B, with rotary induction in the V-1710-A,fully supercharged in the V-1710-C, it operated with two-stages of supercharging in the


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Figure 25Bell P-63H was built, complete with the Allison TurboCompound V-1710-127(E27)engine, but not flown due the end of WWII. Note the large ventral exhaust pipe to dischargethe hot gasses leaving the turbine, which at War Emergency power added about 150 poundsof thrust.

V-1710-E/F, with both turbosupercharging and mechanical/Auxilary Stage

supercharging, and then being the first piston engine to be turbo-Compounded. Overthe 15 years of development of the basic engine, horsepower ratings rose from 700 bhp to

3000 bhp. When ratings reached 1350 bhp in 1942 the V-1710 became the first engine to

exceed one horsepower per pound of weight. The mechanical strength of the engine,

along with its building block design, allowed the many very different engineconfigurations to evolve and serve the diverse needs of aircraft designers. Over 70,000

V-1710s were built, mostly during the WWII war years. Normal time between overhaul,

by the end of the war was 500 hours, which for a high powered piston engine is verygood, a testament to the engines robust nature and reliability.

The Allison V-1710 was the product of the ingenuity, dedication, and hard work of many

people. It is indeed fortunate that a few individuals with vision persevered with its design

and development during the Depression of the 1930s, with the result that when it wasneeded it was available.

Super Charging Allison

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