Supercharger Lectures by Dr. Stanford A. Moss, Ph.D

[Boost Engineer]

Dr Stanford A. Moss, PhD, is considered by many engineers to be the actual "Father of the Turbocharger"  

1918 - General Electric company started a gas turbine division. Dr. Stanford A. Moss developed the GE turbosupercharger engine during W.W.I. It used hot exhaust gases from a reciprocating engine to drive a turbine wheel that in turn drove a centrifugal compressor used for supercharging.

http://inventors.about.com/library/inventors/blenginegasturbine.htm

I have decided to post some of his lectures for the board to read. The lectures were written in the 1940s time frame when 1710 cubic inch engines, making over a thousand HP, were powering some of the fastest aircraft in the world.

SUPERCHARGERS
BY SANFORD A. MOSS Ph.D.

Lecture #1
1)  The power from an Internal Combustion Engine depends directly on the WEIGHT OF THE CHARGE that is taken in by the cylinders, afterwards to be ignited.

2) The charge is initially Air, but presently is combined with fuel in different ways (depending on the particular type of engine).  

2) The charge enters each cylinder during that part of the engine cycle many years ago called called "Inlet" but today usually is called the "intake" stroke .

3) When the "inlet" portion of the cycle is completed, the volume which the piston has displaced is expected to be filled with a given charge.

4) The heat energy (liberated by the ignition of each cylinderful of charge) obviously depends directly on the weight of the charge, all of which presently goes through a chemical process called COMBUSTION..

5) For many years, all internal combustion engines accepted the charge that entered the cylinders, generated a given power output, and if more power was desired the cylinder was made bigger or the stroke of the engine was increased.

6) Then the SUPERCHARGER came along. Its function being to induce into the cylinder a greater amount of charge than would normally enter the cylinder using atmospheric pressure alone.

7) The details of this process will be described in the next series of posts, not from the point of view of an Engine Designer with a lot of mathematics,  but for the benefit of those who are interested in Superchargers in a general way and want to know the "how" and "why" and the historical background.

The posts will examine how the Supercharger Designers have applied, to the problem, previously existing engineering knowledge, as well as their own inventive skill.

a) The Supercharger is an Air Compressor attached to an internal combustion engine, and arranged in a way that allows filling the cylinder with charge at pressure and density greater than would otherwise have occurred in the normal inlet stroke.

b) The weight of the charge is increased nearly in proportion to its absolute pressure, and therefore the engine power will naturally go up by a similar amount. Add a larger amount of MASS FLOW to the engine by using a supercharger and see a large increase in engine power.

c) The alternative is is the much more expensive procedure of increasing the displaced volume of the engine by a new design, (a larger engine size), more weight, and higher cost for premium parts to handle the additional loads on the engine in proportion to the power increase desired.

It has taken many years for the Supercharger (Turbo) Enthusiast to get the recognition that they have today.

Note: Much of the above post was taken Verbatum from a series of articles by Dr Moss and published in a book called "Aeronautics" in 1940. The concepts and words are valid even today.

Tom Vaught, Senior Engineer, Boosted Engine Research.

ps I have changed some of the wording slightly to fit calendar year 2010 sentence usage.

  

 

[Boost Engineer]

Lecture #2

1)  The earliest extensive use of the supercharger was to obtain "sea level power" of an airplane when it flies at an appreciable altitude. The density of the atmosphere decreases rapidly with altitude, and the weight of a cylinderful of charge and hence the engine power, decrease proportionately. A Supercharger compresses the altitude atmosphere restoring "sea level density" with-in the cylinders and will therefore generate "sea level" power.

2) Such a use of superchargers to restore "sea-level" conditions at altitude was extended to also give an increase of power of an airplane at sea level. This increase of power at seal level was called "Ground-Boost". This increase in "sea-level" power was typically maintained as the airplane climbed in altitude until it reached the same "sea-level" power as a naturally aspirated engine at max altitude.  (Note: So Dr. Moss was the First Boost Engineer! TV)

3) Early in the progress of the "supercharger art", this possibility of an increase in power of an internal combustion engine was extended to Diesel engines operating at altitude in the mountains, and then to diesel engines operating at sea level. This use was extended to superchargers for Diesel engines for ships, for land plants, for locomotives, and for automotive purposes like trucks and buses.  

4) Another supercharger application has been with automotive gasoline engines. This started with racing automobiles, and has extended to some extent to passenger cars. (Note: By 2011, the Ford Motor Company will have at least one Boosted Engine on 90% of their product line-up so it took many years to get Boosting accepted by the auto manufacturers on a large scale basis. TV)  Superchargers are principally used on 4 cycle engines but can be applied to 2 cycle engines as well.

5) As a preface to a detailed discussion of the supercharger, a history of its development will next be given. The writer has been associated with this development since the beginning in the United States, and can tell of the history from personal knowledge. (The writer being Dr Moss).

6) This history is not only interesting in itself, but shows the theoretical basis of supercharger fundamentals. A careful distinction will be made between apparatus operated experimentally in the course of the developments, and that successful enough to be operated commercially.  

Tom Vaught, Senior Engineer, Boosted Engine Research.

[AlkyV6]

Interesting stuff. Thanks for posting it.
Looking forward to the next lecture.

[Boost Engineer]

Lecture #3 Gas Turbines

The Supercharger development was a by-product of an effort to develop a Gas Turbine, and the modern Turbocharger resembles this Gas Turbine. So a few high-lights of its history will assist in an understanding of the supercharger itself. (NOTE: A Turbocharger is an exhaust driven Supercharger so Dr Moss calling a Turbocharger a Supercharger is correct) TV

1) The Gas Turbine is an apparatus for obtaining power from products of combustion, which directly operate a Turbine Wheel.

This has been a dream of engineers since John Barber's British patent of 1791, which shows most of the modern elements. The mechanical resources of 1791 could never have produced an operative apparatus, and so the idea lay dormant for a hundred years. Then the Steam Turbine and the Internal Combustion Engine independently because successful as means of obtaining power.

2) So beginning about 1890, lots of people thought of combining the two (Steam Turbine and IC Engine) many of them supposing that they themselves invented a gas turbine.

The plan is as follows, mostly unchanged, from the 1791 Barber patent to a machine operating in Neuchatel, Switzerland, whose 1940 tests are described later.

Air and Fuel, usually oil, are enforced into a combustion chamber, and burned in it while at a pressure appreciably above atmosphere, about 50 to 150 lbs. per sq.in.  The combustion occurs at this pressure just as it would in the open atmosphere, and liberates the same amount of heat, so that the products of combustion leave the combustion chamber at appreciable pressure and at a temperature approaching that of the flame.

These products then pass through nozzles and serve to drive a wheel similar to that of a Steam Turbine. This turbine wheel then drives a compressor wheel which supplies compressed air to the combustion chamber.  The Turbine Wheel is expected also to give additional power, which would represent the net product of the fuel burned.

But time after time Enthusiast Inventors (of which the writer was one) got gas turbines running, but the turbine barely delivered enough power to drive the compressor, and there wasn't any net power.  

3) ONE OF THE EARLY GAS TURBINE DIFFICULTIES WAS THE DESIGN OF A TURBINE WHEEL WHICH WOULD ENDURE WHEN OPERATED BY THE NEAR-FLAME OF THE PRODUCTS OF COMBUSTION. Note (Today Turbo Manufacturers and Automotive Companies are driven to higher and higher exhaust outlet temperatures. These temperatures can approach and exceed 1000 degrees C. It is extremely difficult to make a turbine wheel and housing live at these temperatures.) TV

4) Finally during the World War, about 1917, an engineer in France, Auguste Rateau, commercially operated an Aviation TURBOSUPERCHARGER. This had a turbine wheel operated by the products of combustion from the exhaust of an aviation engine, so as to drive a centrifugal compressor which supercharged the engine at altitude.  A lecture on the Centrifugal Supercharger will be given at a later time.

More on Lecture #3 to follow tomorrow

Tom Vaught, Senior Engineer, Boosted Engine Research.

[rwon]

I'm following this on a daily basis...

[Boost Engineer]

Lecture #3 Continued

The first French Turbosupercharger was followed by a United States design with different details, the history of which will be given presently.  Here we need only note that the turbosupercharger uses products of combustion after they have been through an internal combustion engine, and so are at a lower temperature than that possible with the direct products of combustion of the gas turbine, unless the latter have some sort of cooling.  .

Nevertheless, turbosupercharger temperatures are much higher than have ever occurred in any previous sort of turbine. Hence the metallurgical development of the materials of the turbine wheel of the gas turbine and the turbosupercharger, have gone hand in hand.

From about 1890 to about 1925 various experimenters worked on the gas turbine in Europe, and the General Electric Co. conducted some early research in the United States. However, for some years the turbosupercharger was the only commercial example of the turbine wheel operated by products of combustion as the driver of a centrifugal Compressor, as an auxiliary of their Velox power plant system, and this application continues commercially.

Next came an oil-refinery process, the Houdry system, using a turbine driven by hot gases as the driver of the centrifugal compressor, all as part of a chemical cycle. However, although the turbosupercharger, the Velox application, and the Hondry process, all use turbine wheels driven by the gases of combustion to drive centrifugal compressors, none of them are really what are called PRIME MOVERS, which are primarily designed to produce mechanical power from heat.

The first gas turbine prime mover to emerge from the experimental stage is probably a stand-by electric plant in Neuchatel, Switzerland, built by Brown, Boveri, and Company, whose tests were widely reported in the techincal press in January and February, 1940. Similar plants for various commercial uses are being proposed, and the gas turbine may at last become a commercial reality, partly due to the help which the turbosupercharger development gave.

 

Lecture #4    Nozzles and Diffusers

To have a real understanding of supercharger fundamentals, we must make a detour so as to combine a little theory with the history of two of the elements.

The Nozzle in this theoretical discussion is exactly the same as the nozzle of the garden hose. It comprises a passage of properly changing cross-section wherein the fluid passes from a conduit where the velocity is low and the pressure is high, to a region at a lower pressure. Energy is available when a fluid thus passes from a high to a low pressure, and when this passage occurs in a nozzle, the energy appears as Kinetic Energy, or energy represented by fluid velocity.

In more simple language, the fluid spouts out of the nozzle.  As the fluid goes from the nozzle into a region of lower pressure, it get to the pressure of this region. Hence the full energy due to the passage from the high to the lowpressures produces the spouting velocity, but with deduction for friction losses.

The turbine wheel gives a means of applying the Kinetic Energy of Velocity so as to product power to drive a shaft. So a jet directed from a nozzle against the vanes, blades, or buckets of a turbine wheel, finally produces mechanical power as a result of the drop of pressure of the fluid from the value in the conduit preceding the nozzle, to the value in the region at the exit of the turbine buckets.

More on Nozzles Tomorrow (TV)

Tom Vaught, Senior Engineer, Boosted Engine Research.

[RyanMayo]

This stuff is getting good.  Thank you Tom!

[99TTGT]

thank you.!

[rwon]

Keep it coming, I love this stuff

[Boost Engineer]

Lecture #4    Nozzles and Diffusers (continued)

 Another way of obtaining power, when a fluid passes from a high pressure region to a region of lower pressure, is the use of a reciprocating piston, such as in a steam engine. The total available power is the same, whether from the reciprocating piston or the turbine, and is equal to the net shaft power plus the losses. 

These losses are quite a different kind in the two cases, but the fundamental principal remains, THAT THE AVAILABLE POWER IS THE SAME, WHETHER IN A TURBINE OR A RECIPROCATING ENGINE. In a similar way, THE POWER REQUIRED TO PRODUCE PRESSURE RISE OF A FLUID IS THE SAME WHETHER WITH A RECIPROCATING PISTON OR A CENTRIFUGAL COMPRESSOR.

In all of the above theory, the word FLUID has been used, as covering both of the cases of liquid and a gas, and the theory of an incompressible fluid such as water, to be extended to the compressible fluids such as air, steam, or gas, was only recently acquired.  The success of the supercharger depends upon this knowledge and the writer can give the history of its acquirement from personal experience.

So lets go back to the period about 1889, and see the theories and practices then existing. This data has particular interest for the writer, as he then became a Machinist Apprentice, and so the development to be mentioned has occurred in one engineering lifetime.

The theory of obtaining power from expansion of the compressible fluid, steam, in a reciprocating engine is well known. The theory of the nozzle in a turbine or water wheel using the incompressible fluid, water, is well known. As the pressure of the water gradually decreases during the passage through the nozzle, THE VELOCITY GRADUALLY INCREASES. To provide for this, THE NOZZLE AREA MUST GRADUALLY DECREASE, AND SO THE INCOMPRESSIBLE FLUID NOZZLE IS CONVERGENT.

When such an incompressible fluid flows through the nozzle, or even through a simple hole in the wall of a vessel, WITH A GIVEN PRESSURE WITHIN THE VESSEL, THE LOWER THE PRESSURE ON THE OUTSIDE OF THE VESSEL, THE GREATER THE FLOW.

But the English engineer, Napier, had shown not many years before 1889, that this was not the case with a compressible fluid such as steam or air. THEN, AS THE PRESSURE ON THE OUTSIDE OF THE VESSEL DECREASED, THE FLOW THRU THE ORIFICE WALL INCREASED UNTIL THE OUTSIDE PRESSURE BECAME ABOUT ONE HALF OF THE INSIDE PRESSURE. IF THE OUTSIDE PRESSURE STILL FURTHER DECREASED, THERE WAS NO FURTHER INCREASE OF FLOW.

This showed the difference in the behavior of incompressible and compressible fluids in a nozzle or orifice.

One of the consequences of this was thought to be that steam or other compressible fluid, could not be used as could an incompressible fluid, in a turbine with usual pressure drops, to give the same power as could be obtained with a reciprocating engine.

The centrifugal pump was well known at this time. This had an impeller which discharged fluid from its periphery, with some rise of pressure, and with an appreciable velocity. Beyond the impeller sometimes was a Diffuser, identical in theory as we shall see later, with the Diffuser of the supercharger.

This Diffuser had DIVERGENT PASSAGES, WITHIN WHICH THE VELOCITY OF THE FLUID DECREASED, SO THAT THE ENERGY OF THE MOTION WAS TRANSFORMED INTO ADDITIONAL PRESSURE RISE. In this way, CENTRIFUGAL PUMPS FOR WATER WERE USED TO OBTAIN VERY HIGH PRESSURES.

They were first called Turbine Pumps because the diffuser was the reverse of the nozzle of the Turbine Water Wheel.

Fan blowers also were well known for pumping air, but no appreciable pressure rise of air had ever been produced with a wheel or impeller of any kind. On the other hand, reciprocating air compressors were always used when air had to be raised to any pressure which gave such a change in volume that it really could be said to be "compressed".

In the case of the Fan Blower, the pressure rise was so slight that no real change of volume or compression occurred. There was no knowledge of use of the impeller wheel and the diffuser of the centrifugal pump to take the place of the reciprocating air compressor.

More on Compressors Tomorrow (TV)

Tom Vaught, Senior Engineer, Boosted Engine Research.

[Boost Engineer]

Lecture #4    Compressors (continued)

From this approximate date of 1889, until about 1909, there ensued a Dark Age in engineering development which is somewhat discreditable. The theories of mechanics and hydraulics relating to incompressible fluid turbines and pumps were well known, as well as the thermodynamic theories giving the pressure, volume, and the like, of compressible fluids; but the combination of the two theories was not made.

The lack was the complete application of the known laws to the design of passages for a compressible fluid, taking proper account of the changes of volume. While progress was made throughout this Dark Age, even near the end of it papers were published in learned transactions with fallacious statements "that a turbine using products of combustion could never be efficient".

The first advance was when Swedish engineer, Gustave DeLaval, showed that a nozzle for a steam turbine with appreciable pressure drop, required a passage having first a convergent and then a divergent part.  (I have attached a link to this information):  http://en.wikipedia.org/wiki/De_Laval_nozzle   (TV)                            

The Nozzle in the link above would have exhaust flow entering the nozzle from the left and driving the Turbine wheel with the flow exiting from the right of the nozzle and impacting on the Turbine blades.

The English scientist, Osborne Reynolds, first applied the known fundamentals to the complete theory of such a convergent-divergent nozzle for any compressible fluid.

Next the exact reverse of a Turbine was proposed as an air compressor, with a reversed convergent-divergent nozzle as a Diffuser, regardless of the fact that most circumstances did not require such a shape.  

The writer showed in a patent application of 1904, the cases when a diffuser for a compressible fluid should be first convergent and then divergent, and when it should be wholly divergent. This patent was not issued until 1913, but Centrifugal Compressors based on it were in use in 1906. In the meantime, the engineers Parsons, Samuelson, Reteau, Zolley, Curtiss, Emmet, Hodgkinson, and Jungren developed steam turbines of several sorts. Soon it was realized that the turbine could operate with products of combustion as well as with steam. So now the now commonplace complete theory of design of turbines and compressors for compressible fluids was gradually built up.

This enables an engineer skilled in the art to make the areas of the passages of a turbine or compressor to suit the velocities and the volumes of a compressible fluid, as they occur from point to point.

FOR VELOCITIES LESS THAN A CRITICAL VALUE WHICH TURNS OUT TO BE THE VELOCITY OF SOUND, THE VOLUME CHANGES OF A COMPRESSIBLE FLUID ARE SECONDARY AND THE VELOCITY CHANGES GOVERN, JUST AS IN THE CASE OF AN INCOMPRESSIBLE FLUID.

Then a nozzle for conversion of pressure into velocity is convergent, and a diffuser for conversion of velocity into pressure is divergent.

FOR VELOCITIES GREATER THAN THE VELOCITY OF SOUND, THE CHANGE OF VOLUME OF A FLUID IS A MAJOR ITEM. Then, at the low pressure end of the nozzle, the passage area MUST INCREASE as to give room for the volume increase due to the pressure decrease, so that the area near the nozzle is "expanding".

But with most diffusers for centrifugal compressors, the velocity of sound is not reached, so this divergent-convergent requirement does not occur, and the diffuser is wholly divergent as in a centrifugal pump.

Lecture 5 (starting tomorrow) will be on SUPERCHARGER SPEEDS   (TV)

Tom Vaught, Senior Engineer, Boosted Engine Research.

[Boost Engineer]

I took the day off, I will try and start the first post (Lecture) on Supercharger Speeds on Sunday Night.

Tom Vaught

[rwon]

 

[Boost Engineer]

Lecture # 5    Supercharger Speed

(This lecture is a very short lecture, therefore I will post only the key points of the lecture)  TV

1) The earliest commercial really high-speed machines were the steam-turbines of DeLavel, which operated about 20,000 to 28,000 rpm.

2) The Compelling Advantage of a High Speed Compressor is a great reduction in size of apparatus for any give horsepower produced. 

3) During the early steam turbine days pictures were frequently shown of a immense reciprocating steam engine at about 75 rpm, and down in the corner a little bit of a high speed steam turbine, drawn at the same scale, and developing the same power. So when supercharger development started, the pathway had been prepared for really high speed operation. However, a prejudice was encountered from the mistaken notion that high speed meant unsafe stresses. 

We will start on Lecture 6   Centrifugal Compressors  tomorrow  (TV)

Tom Vaught, Senior Engineer, Boosted Engine Research.

[rwon]

I'll be back for class tomorrow . lol

[Boost Engineer]

Lecture 6   Centrifugal Compressors

 Centrifugal Compressors are machines for increasing the pressure of compressible fluids by an appreciable amount, by means of a high speed wheel called an Impeller, succeeded by a diffuser, which has passages designed to convert the exit velocity of fluid leaving the impeller, into pressure.

 Centrifugal compressors differ from centrifugal pumps and fan blowers in that: a)  they produce pressure such that there are appreciable changes of volume of the fluid as it passes through the compressor: b) they have diffusers which must be designed with areas to suit the velocities and fluid volumes which exist from point to point, as was discussed in the diffuser history already given: c) they operate the fluid velocities, pumps and blowers. Therefore, passages, blade angles, and other items of design must be arranged with such care that the high speed flow is handled without loss or shock.

Centrifugal compressors were first constructed by the French engineer, Rateau, probably around 1900, but the details of Rateau's early work do not seem to be available and he may not have used a diffuser.  The first work in the United States was by the General Electrick Co., which began research in 1904 as part of its early research on the gas turbine.  The first machine was put into commercial service in the Lynn Works of the company in 1906 and it continues to operate at the present time.   

 An extensive comercial business was built up, and machines were made for pressures of from 1 to 30 lb. per square inch for foundry cupolas, blast furnaces for pig iron, pneumatic tube and conveyor systems, air supply for gas and oil furnaces, exhaustion and compression in fuel and illuminating gas plants, and similar purposes. Some machines had a single impeller and others had a number of stages in series.  These machines operate with impeller peripheral speeds of from 500 to 1,500 ft. per second, and at rotational speeds of from 3600 to 20,000 RPM.  The machine of Rateau and the General Electric Co. had impellers with blades at inlet and exit which were similar in a general way to those of other designers at the time.  These machines were soon paralleled by machines of many other manufacturers in Europe and America, sometimes with different details of construction. 

 Many centrifugal compressors are driven by turbines or electric motors, and operate at practically constant speed. A typical pressure-volume curve for such a case is given in this link (TV): http://www.pdhengineer.com/courses/m/M-2019.pdf   This shows that for a wide range of flow or volume change in the vicinity of the rated volume, the pressure produced by a centrifugal  compressor is practically constant.

 For instance if such a machine (operated at practically constant speed) supplied a number of oil or gas burners, then if some of the burners are turned off or on, the pressue given by a pressure-gage attached to the air main will show but little change.  However, for an overload somewhat beyond the rated flow, the pressure will begin to drop.  Also, if the amount of load, such as the number of oil or gas burners, is reduced to a small amount, or cut off altogether, the pressure takes a small sharp drop, and will begins to pulsate.  THIS PULSATION IS CAUSED BY THE FACT THAT AS THE LOAD IS DECREASED, THE FLUID LEAVING THE IMPELLER IS NO LONGER PROPERLY DIRECTED TO SUIT THE DEFUSER BLADES, AND THE DEFUSER FUNCTIONS INTERMITTENTLY. Thus, we have built up the theory and history of the elements of a supercharger.

This concludes Part I of the "Supercharger Lectures". We will start on Part II of the lectures by Sanford A. Moss, PhD on Thursday. (TV)

Tom Vaught, Senior Engineer, Boosted Engine Research.

[MCA]

Awesome information. 

Thanks for sharing.

[littlebluefoxbody]

good stuff!

[J. Fast]

Whoa, great read! Any pictures?

[Boost Engineer]

The lectures were presented in the years prior to 1940. I have no pictures to post.

I came across the hard copies of the lectures some years ago and wanted to pass on the info to others interested in the man's knowledge of Supercharging.

Hopefully I start on Part II Lectures in the next couple of days. Been busy with projects around the house.

Tom Vaught

[dug]

this is some good stuff

[Boost Engineer]

SUPERCHARGERS
BY SANFORD A. MOSS Ph.D.

Part II
Lecture #1

In the preceeding lectures we have shown the foundation on which the various elements of Superchargers are based. From these elements, designers during the period beginning about 1917, developed the supercharger so that it has become an essential part of nearly all aviation engines. (And modern automotive engines like the Corvette and the Ford Shelby 500 Mustang TV).

Next we will show just what the finally produced supercharger is, and how it is combined with the engine. This will give us the basis so that we can take a flash-back and consider the history and the theory of the supercharger development, with knowledge of what the parts look like.

The development has crystallized so that a particular sort of supercharger has a very extensive use. This consists of an impeller driven by a step-up gearing from the crankshaft, surrounded by a casing which is formed by parts of the engine structure, and succeeded by a difuser, also supported by the engine structure. Next is a collecting passage to which are attached the inlet pipes which lead the charge to the inlet valves.  

(Note: The typical modern Supercharger (Vortech or Ati) has a similar step-up gear arrangement in the case but also uses a second ratio multiplier with the drive belt pulleys attached on the supercharger and crankshaft. There have been some efforts to use gear drive mechanisms from the crankshaft to the impeller on high horsepower automotive racing engines recently. TV)

Preceeding the Impeller is the carburetor, with a suitable passage connecting the two. Preceeding the Carburetor is an inlet duct leading from an Inlet scoop which faces exactly in the direction in which the plane is flying so that the atmospheric air is received with ram. The parts of the supercharger itself will be illustrated by photographs of the parts used in commercial engines with the permission of the manufacturers. (Note, I do not have a way to post these photos from the lectures on the board TV).

Impeller Drive

The impeller is driven by a double set of gears from the end of the crankshaft of the engine, and the "train" as used in one model of the Pratt and Whitney engine is shown in Figure 1. A similar "train" in the engine of the Wright Aeronautical Corporation is shown in Figure 2.

Other parts of the engine, such as the starter, magnetos, and generator, also need gears, and are usually combined with the supercharger "train", as part of a single gear drive system.  

For reasons which will be explained presently, two speed geared Superchargers are needed for some altitudes, and a typical gear "train" is shown in figure 3. This involved clutching arrangements so that the impeller can be driven at either of the two speeds. (Note: I worked on a two speed centrifugal Supercharger concept some years ago with a transmission engineer named David Jansen who later filed a patent on that system.) The link is here: http://www.freepatentsonline.com/6609505.html). ( He left off my contributions to the system when he filed the patent, LOL  TV)  

A centrifugal supercharger is a supercharger operating at very high speed, and must have a mathematical design exactly suited to the proportions of the engine to which it is attached.

It so happened that the specialists of the General Electric Company (in high speed turbines and compressors), participated in the design and manufacturer of superchargers at an early date.

A few early geared superchargers were attached to the engine as separate accessories, as will be described later. But it was obvious that this involved extra weight and complication, and there was begun the almost universal practice of arranging the supercharger casing, (which surrounds the impeller, as well as the support  for the difuser ring) as part of the engine casing. Then the Supercharger is "Built-in" as part of the engine.

The geared superchargers already mentioned, both simple and two speed, have a single impeller. But geared superchargers also have been used with two impellers in series, called two stage superchargers. However these have not had sufficient use to make them desireable here to give reasons that have been advanced in their use nor the details of their construction. (Note, Many years later, the two stage supercharger is in production on the Ford 6.7L Diesel Engine and will be a large production volume engine.  TV)  A link to that engine is shown below.  

http://www.autoblog.com/2009/08/31/beware-the-scorpion-2011-ford-super-duty-gets-all-new-6-7-liter/

Quote:  "The Scorpion features a brand new turbocharger design from Honeywell that we will likely see in many more applications in the coming years. It combines the principle of the sequential turbo approach with the variable vane system. However, instead of two separate turbochargers, two smaller diameter ones are mounted in one housing on a common shaft. The smaller diameter reduces the rotational inertia of the system while still allowing for sufficient flow capacity at higher speeds. It also eliminates the extra plumbing complexity of a dual turbo setup. "

Note the dual impeller in one of the photos and be sure and read the article.

Tomorrow Part II, Lecture #2 (if I can make some time to type it up) TV

Tom Vaught, Senior Engineer, Boosted Engine Research.

[Boost Engineer]

Part II
Lecture # 2

The single speed or two speed superchargers described in the previous lecture, are used for moderate altitudes, but for really high altitude flying, such as needed in aviation warfare, the supercharger impeller instead of being driven by the engine crankshaft, is in the United States driven by a turbine wheel operated by the engine exhaust gases.

As in the preceding lecture, we will first discuss the general arrangement, so that we can afterward go into details of history and theory with a more comprehensive knowledge of what we are talking about.

Figure 7 is a diagrammatic representation of the complete turbocharger cycle. (as I cannot post that cycle, I will describe what was shown on the illustration. TV)

Photo shows a turbine housing, (as part of a turbocharger), a cylinder head, a cutaway of the piston, connecting rod, and crankshaft, an inter-cooler, and the intake and exhaust valves.

The wording is, "Red Hot Exhaust Gases from Engine to Turbine Housing" (entering inlet of Turbine).

"Air Compressed to Sea Level Density" (By Inter-cooler).

"Air Flow from Inter-cooler to Carburetor to Intake Valve"

The Engine exhausts into a closed exhaust manifold, as arranged that there is maintained within it practically sea-level pressure at all altitudes.

This exhaust manifold discharges into a nozzle box which is immediately behind the turbine wheel.  The nozzles typically cover a 360 degree arc. The nozzle box has nozzle openings exactly like those of a steam turbine, which direct the exhaust gases against the vanes or buckets of the turbine wheel.

This wheel is mounted on a shaft so as to drive an impeller similar to the typical centrifugal compressor. This wheel of the centrifugal Compressor supercharges the engine

The casing of the Centrifugal Compressor is an independent unit with provision for support for the nozzle box. This unit of Centrifugal Compressor and exhaust gas Turbine, is located on a convenient part of the airplane with connections with some flexibility, to the Exhaust and Inlet manifolds.

The Centrifugal Compressor delivers air to the carburetor, and then to the Inlet manifold. Usually there is a Geared Supercharger in addition.

As an airplane with a Turbocharger ascends, the pressure with-in the Exhaust Manifold and the Nozzle Box is maintained at the sea level value, by automatic regulation of the Waste Gate. The pressure outside the Nozzle Box is the low pressure at altitude. The difference between these two pressures furnishes a stream of Exhaust Gas at a High Velocity which drives the Turbine Wheel.

This drives the Centrifugal Compressor so as to compress the air entering the Engine from the low value (at altitude), to practically sea level pressure in the Intake Manifold.

So in spite of the fact that the engine may be at High Altitude which normally would GREATLY DECREASE the power, the engine exhausts at Sea Level Pressure, and receives its Intake Charge at Sea Level Pressure, and so continues to give Sea Level Power to the engine.

(Note: Having the ability to offset changes in Density Altitude by using a Boosting Device should make the race car more consistant as far as  horsepower generation and more repeatable as far as track times. TV)   

This ends Lecture 2.    Lecture 3  "Reasons for Supercharging an Internal Combustion Engine" will be posted tomorrow . TV

Tom Vaught, Senior Engineer, Boosted Engine Research.

[mccabeke]

excellent reading! thanks!

[Boost Engineer]

Lecture 3  "Reasons for Supercharging an Internal Combustion Engine"

The "Induction" or Inlet stroke

As the piston moves through the Intake Stroke, a void is created within the cylinder, and the pressure of the outside atmosphere pushes charge in to fill up the void.

It is often said that the charge is "sucked" into the cylinder, but there is really no sucking, and the only action is the pushing of the charge into the cylinder by the atmosphere. But this push is against the resistance of the inlet duct, carburetor, inlet manifold, and intake valve. 

With this resistance, and the lag due to the engine speed, the pressure inside the cylinder is appreciably below atmosphere during the Intake stroke. Furthermore, all of the engine parts are quite hot, particularly the intake valve and the cylinderwalls, so that the charge is heated perceptibly as it enters the cylinder.

The ratio of the weight of the charge actually within the cylinder displacement, to the weight if atmospheric pressure and temperature had existed within the cylinder, is called "Volumetric Efficiency" and is from 75% to 85%. That is, the weight of charge is only 80% of the value corresponding to the cylinder displacement full of charge at atmospheric conditions. 

All of this applies whether the engine is at near Sea-level or at Altitude. But at Altitude the charge is further decreased from the 80% possible at Sea-level. The weight of a Cubic Foot of Air, at average Sea-level conditions is .0734 lbs per cubic foot. But at an Altitude of 20,000 feet the weight is about .0403 lbs per cubic foot. which is about 55% of the Sea-level value. 

The engine power is decreased nearly in the same proportion, and so at 20,000 feet the engine power is .55 x .8 or 44% of the value which might have been obtained if the cylinder displacement were filled with charge at    Sea-level conditions.

Also, it has been found that the impeller of the centrifugal supercharger has an important effect in the mixing of the charge. This mixing is supposed to be performed by the carburetor, but the carburetor always introduces the fuel in droplets, and even with multiple jets and good spraying, the mixture is far from being homogeneous. 

This results in a rough running engine with poor fuel economy.  The requirement for good mixing is that the fluid shall be reduced to minute droplets, each of which shall be introduced into a new portion of the body of air, not previously saturated by evaporation of the other droplets.

A droplet thus surrounded by new air readily evaporates. This requirement seems to be met when the imperfect mixture of air and fuel passes through a centrifugal supercharger, and there has been a marked improvement in engine performance, apart from the mere increase of power due to the additional amount of charge. However, the intake manifold must be designed to give uniform distribution to all cylinders. 

It has been thought that improvement in mixture may be obtained merely by a lot of indiscriminate mixing, with eddies and whirls. However, there is a minimum of this sort of thing in a well-designed centrifugal supercharger, and the flow is made as smooth as possible, with a minimum of eddies.

Under such circumstances, an unevaporated drop moves at a different velocity than the surrounding air, due to its greater density, and so necessarily comes in contact with a new portion of the air body, which has been stated, is the condition for complete evaporation.  On the other hand an eddie or whirl will send the unevaporated drops to the outer diameter by centrifugal force, and throws them together, apart from the main body of air.

Note: This is one major reason why an Annular Booster is far superior for evaporation of the fuel (as well as for a higher signal strength to the main well and jets.)  TV

Tom Vaught, Senior Engineer, Boosted Engine Research.