Aero-Engine basics

[Badboy]

Hi,

We have seen a number of threads, and one rather long thread in particular (Near the end of this one http://www.hitechcreations.com/forums/showthread.php?threadid=115995 ) where the discussion has revolved around terms such as the “full throttle height”,  the “critical altitude”, and also the “rated altitude” for an aero engine typical of those used in WWII. For those who would like to be able to follow those threads, but who are lost in the technicalities and terminology, and would like to understand the principles underlying the ebb and flow of such discussions, I would like to provide the following explanation… Seeker, sit up and pay attention.  

But firstly a disclaimer! In trying to make any explanation as easy to understand as possible, it is necessary to cut out as much of the technical jargon as possible, to make the explanation as simple as possible, and if it helps folk to “get it” even a few sacrifices to precision and absolute correctness are necessary, to avoid becoming bogged down with the need to qualify every statement, so please, for those of you who are already knowledgeable, no nit picking please If I wanted to write a text book it would probably be even easier, a lot shorter, a lot more precise, but fewer of the people trying to get to grips with this stuff would get it first time, if at all… Also, I’m going to refer to English or Imperial units because those were the units used in England through the war years.  

Let’s start at the very beginning, and forgive me if it sounds as though I’m stating the obvious in some cases, but I think it is important, if you want to understand the finer points, to get right back to basics. So please be patient and don’t skip too much, because leaving holes or gaps in your knowledge is probably the biggest cause for misunderstanding something later, when we really get down to the nitty gritty. Lastly, it is rather long winded, and it’s easy to get lost along the route, to lose sight of where we are going, sorry about that, all I can say is, all good things are worth taking your time over, so try to bear with me.

So here goes… A petrol engine derives its power from the burning of petrol and air. If you own a motor car (or any kind of surface vehicle) you will have some idea of the mechanical workings of the engine, but like most motor car owners you may be inclined to regard the burning of petrol to be the more important partner in the process of combustion. This is natural, since the air is free and you have to pay for petrol. But from the point of view of the engine designer the air is even more important than the petrol, because it is harder to get the right amount of air into the cylinders than the right amount of petrol. Petrol is compact and easy to handle, whereas air is just the opposite.

In order to burn 1lb of petrol, from 12 to 15 lb of air are needed. For every gallon of petrol used, about 1200 cubic feet of air must be made to enter the engine. The internal combustion engine must therefore be designed to handle very large quantities of air. In fact the engine is really an air pump, and the power that can be obtained from it depends on how much air it can draw in. The small amount of petrol can always be supplied. The conception of the engine as primarily a machine for handling air is fundamental to a good understanding of the principles of the internal combustion engine. This explains why an increase in the bore or stroke of the cylinders, or in their rotational speed, usually results in an increase of power.

This brings us to an important concept, that we are going to need later… Volumetric Efficiency.

It might be expected that the cylinder would draw in on each induction stroke a volume of air equal to the swept volume, and this is approximately true. The really important thing, however, is not the volume, but the weight of air that enters on each induction stroke.

Now because of the resistance offered by the intake manifold, and even more, by the intake port, there is a distinct pressure drop of the air on its way to the cylinder even when the throttle valve is wide open. This means that the pressure, and therefore the density of the air on the cylinder side of the intake port, during the induction stroke is always less than that of the atmosphere   surrounding the aeroplane. In this state, each cubic foot of air in the cylinder weighs less than a cubic foot of air outside.

Another factor tending to reduce the weight of air (or mixture) drawn into the cylinder is the temperature of the charge at the time the inlet valve closes. The inlet manifold maintains a temperature about equal to the boiling point of the petrol and the cylinder and piston are even hotter. As it passes from the carburettor to the engine the air is warmed, and warm air will always weigh less per cubic foot than cold air at the same pressure.

If it were possible to leave the intake valve open long enough for the pressure inside and outside to become equal and if, in addition, the parts of the engine were no warmer than the ordinary air outside, then, under these circumstances, a greater weight of air would fill the cylinder than actually does. The engine would be a more efficient air pump, and other factors being equal, it would deliver more power when the oxygen in the air was burnt. Unfortunately, this ideal state of affairs can only be obtained by turning over the engine infinitely slowly whilst it is cold, but although such an operation is of no practical use, it may be used as an imaginary standard, against which the actual usefulness of the engine as an air pump may be assessed.

This is the idea that lies behind the definition of volumetric efficiency. This quantity is defined as follows the weight of mixture entering cylinder per stroke divided by the weight of mixture to fill the swept volume at normal temperature and pressure multiplied by 100 to get a percentage.

Normal temperature and pressure are defined in order to give an exact value to the standard part of this expression and in England at the time of WWII these values were taken to be 15 degrees centigrade and 14.7 lb/sq inch.

The design of the engine has an important bearing on volumetric efficiency. The passage of the air through the inlet (or air scoop) and carburettor along the inlet manifold, through the inlet valves should be as unrestricted as possible. It is for this reason that the Merlin engines, for example, have twin inlet ports and valves in each cylinder.

The timing of the valves also has a considerable effect, so for example, the momentum of the inrushing charge is utilized by leaving the inlet valve open until some way past Bottom Dead Centre in order that the largest possible weight of air (and therefore mixture) may be drawn in.

So you can see that in order to get air into the engine, ideally, the temperature of the charge should be as low and the pressure outside the inlet as high as possible, consistent with other practical considerations. It is these two factors that we should deal with next and that leads us naturally to think about the effect of altitude on the temperature and pressure of the atmosphere.

Continued...

[Badboy]

We live at the bottom of a sea of air, the lower layers of which are pressed down by those above. Atmospheric pressure is a measure of the weight of the overlying air, and an observer who sets out with a barometer to measure this pressure at different levels would discover that his barometer readings diminished in a regular manner as his height above the earth’s surface increased. If the air were to have the physical properties of sea-water, this would be the end of the matter, but unlike water, all gases are easily compressible, and the lower layers of air, subject to a greater pressure than layers above, are compressed into a smaller volume. A cubic foot of air at sea level contains more air by weight than a cubic foot at any point above it. In scientific terms, its density decreases with increase in altitude. The same thing would be noted of a pile of porous rubber blocks, and anyone buying by the cubic foot from such a pile would choose from the lower layers if he wished to get the most for his money.

It is also well known that the farther the air is from the warmth of the earth’s surface, the colder it becomes, and on this account it might be expected, according to well established laws governing the behaviour of all gases, that the density of the air would increase as it became colder. However, even though the two factors, temperature and pressure, have opposite effects, the density of the atmosphere still decreases with increase in height, because the effect of pressure is predominant, the part the temperature plays is a very much smaller one.

An engine operating at any considerable height above the earth’s surface is surrounded by “thin” air, and although it may still pump in as many cubic feet as at sea level, it finds difficulty in getting a sufficient weight of it.

It is interesting to note, by way of analogy, that what is true of a petrol engine is also true of a human being. Human beings can also be though of as a kind of internal-combustion engine, we get our “fuel” from food. The food is “burnt” or oxidized by the air taken into the lungs. On the top of a high mountain the human engine finds difficulty in taking in a sufficient weight of air, and partly adapts itself by deeper and more rapid breathing. The human engine that attempts even greater heights by flying in an aircraft must have its oxygen supply augmented from a separate source.

The aircraft engine also finds difficulty in breathing at altitude. Now, remember how I explained how power depends on the amount of air by weight that passes into the cylinders? The term “Volumetric Efficiency” has been used to express this idea numerically. It is clear that there will be a progressive falling off in volumetric efficiency as the aircraft climbs. So unless something is done to offset decreasing density an engine would get the full weight of air at sea level, only 3/4 as much air at 10,000ft, only 1/2 as much air at 20,000ft and 1/3 as much air at 30,000ft and only 1/4 as much air at 40,000ft. The serious effects of decreased air density could be overcome if some means were found of deceiving the engine into believing that it was still operating at sea level. This was accomplished by supercharging. But before we talk about supercharging, we can just make a few observations about engines that are not supercharged. They are called “normally aspirated” engines, which means that they are provided with no assistance in breathing. They can be run at full throttle at sea level but their power begins to drop as soon as they gain altitude.  

Now, in order to counter that, the early supercharger was a pump or blower, the purpose of which was to maintain the pressure of the air in the intake manifold equal to its value at sea level. In this way, volumetric efficiency and the power output could be maintained up to an altitude called the rated altitude or the critical altitude. Beyond this, the supercharger could not maintain the normal pressure in the manifold, and engine power began to fall off. Of course in WWII, when victory usually went to the airplane that could fly highest and fastest, this was of the utmost importance.

The power to drive the supercharger is derived from the engine. On some engines the drive is taken directly through a train of gears, but an alternative is provided by the use of a turbine, driven by the exhaust gases from the engine. Some American superchargers employ this means of turning the impellor. The construction of a suitable turbine was fraught with difficulty, for the blades are called upon to endure the very high temperatures of the exhaust gases. Considerable research at the time led to the discovery of metals capable of standing this rough treatment, and the blades of the turbines used on such famous aircraft as the P-38 Lightning, Fortress, and Thunderbolt operated successfully at dull-red heat. This turbo-supercharger offers some advantages over the gear driven type, and I will talk about those advantages later.

However, back to the point, there are some problems associated with supercharging. The extra power that the supercharger enables the engine to deliver does not all reach the propeller shaft. A price must be paid, because some power is needed to drive the supercharger itself.

For example, the power of an unsupercharged, or normally aspirated engine, as it is called, will have fallen to about half its sea level value at 20,000 ft. Suppose that it is required to restore the full power of a 1000hp engine at this height. A net gain of 500hp will be needed. A pumping apparatus of sufficient capacity will require about 100hp to drive it, because it is a general rule of thumb that the supercharger absorbs about 10% of the rated power of the engine.

Now, we face another problem, the full delivery of such a supercharger cannot be used near sea level, let’s just think about that… At 20,000ft it is doubling the pressure of the atmosphere surrounding it, in order to reproduce sea level pressure in the manifold. If it were allowed to go on doing this when the aircraft was actually surrounded by air at sea level pressure, the pressure inside would be far in excess of that for which the engine was designed. Detonation and overheating would certainly result, adding to already excessive pressure and temperatures in the cylinders. The engine would be severely damaged. So some means must be found for maintaining a constant air pressure in the manifold as the aircraft descends, and this means for controlling boost pressure is naturally found in the throttle lever and its throttle valve. By gradually closing the throttle, maximum permissible boost may be kept constant from the fully open throttle altitude down to sea level.

Near the ground, therefore, the manifold pressure is no higher than in the same engine without a supercharger, and power is being consumed by the supercharger to no good purpose. Under these conditions the normally aspirated engine would actually deliver more power. In the case of the example quoted the difference would be 100hp.

If you consider two engines, one without a supercharger, the other with a supercharger designed to maintain sea level pressure up to 20,000 ft. The unsupercharged engine gives a better performance at ground level, but its power begins to fall off immediately as the aircraft climbs. The supercharged engine shows a slight increase in power as the aircraft climbs to its rated altitude, (or full throttle or critical altitude), at that height it actually delivers more power than at ground level. Two factors contribute to this. One is the decrease of temperature that takes place with increased altitude, at the same pressure (and these are the conditions operating inside the manifold) the colder air will be denser, and some gain in volumetric efficiency is to be expected on this account. The other factor concerned is the decreased back pressure at the exhaust. This gives more efficient scavenging and a lower induction (or “suction”) pressure. The throttle is gradually opened as the aircraft climbs until a height is reached at which the throttle is fully open. This is the height referred to as rated altitude or the full throttle altitude, or the critical altitude. Beyond this point there is nothing “in hand”, and the engine power shows a progressive decline similar to that of the unsupercharged engine. In reality the full throttle altitude varied, even for the same aircraft and on the DB601 engine being discussed in that other thread, for example, the supercharger hydraulic fluid coupling was subject to sludging, because it acted as a centrifuge and so the efficiency would fall off over a period of time and so would the full throttle altitude.
   
So, here is another idea, the pilot should regard his throttle lever as a boost control, in directly controlling engine power. The lever should not be advanced to give manifold pressure in excess of that for which the engine was designed, and, when high boost is necessary for extra power, it should not be maintained for longer than is specified in the handbook. A gauge indicates the actual pressure inside the intake manifold. The British practice was to call this a boost gauge and have it reading in pounds per square inch above or below atmospheric pressure. In America the indicator was called a manifold pressure gauge and was calibrated in inches of mercury absolute. Now there is a rough and ready way to convert between the two systems. If atmospheric pressure is taken as 30 inches of mercury and approximately 15lb per square inch, then a boost of plus 2 on one indicator would read as 34 inches of mercury on the other. The two different methods of calibration are not important, but it is nice to be aware of both. I’ll say more about boost control, and more importantly, automatic boost control later, but let’s continue...

[Badboy]

To return to the problem of the power wasted in driving a supercharger at ground level, the logical outcome was either some form of clutch that would allow the supercharger to remain idle when it was not wanted, or the use of a second low gear that would consume only a small part of the engine output. With an engine designed to withstand a moderate degree of ground boosting, and using high octane petrol to avoid detonation, a low-gear supercharger was found to pay for itself in increased power near to sea level. So, the typical gear driven supercharger was equipped in this way with two gear ratios. With this mechanism the supercharger would turn at moderate speed in low gear, giving good power at low altitudes by the avoidance of unnecessary pumping. At greater heights a second gear was arranged to drive the blower at higher speed and maintain the permissible boost at greater altitude. The change from low to high gear was made at an altitude where both gear ratios gave the same actual available engine power, but above which the low gear was less effective than the high. The gear change was usually operated by hydraulic means. A later development in gear-driven superchargers made use of two stages of compression. When the air is compressed it is heated and that represents an unavoidable loss of energy, but more serious is the drop in volumetric efficiency occasioned by feeding the engine with hot air. With a two stage supercharger the problem is acute, and it is usual to cool the gases by passing them through a radiator situated either between the two stages, or between the final stage and the induction manifold. Such a combination provides a gain in horse power over and above the single-stage blower, and gives the aircraft engine so equipped an even higher ceiling.

Now I promised to say some more about turbo chargers and the actual blower of a turbo driven supercharger is basically the same design as that of a supercharger, but the novelty lies in the method used to drive it. Instead of being allowed to pass directly to the outside air, the exhaust gases are made to pass first through the blades of a turbine, the rotor of which is on the same shaft as the impellor. In this way mechanical energy is derived from the hot exhaust gases as they leave, and very little power is absorbed from the engine. Apart from the obvious advantage of using the waste gases as a source of power, there is another important advantage.

Each of the two gear ratios of a two speed supercharger provides its maximum effectiveness at a given altitude, that means two critical or rated altitudes. Between these two rated altitudes there is a drop in power due to one gear being too low and the other too high. The ideal way out of this dilemma would be to use a supercharger driven through a gear of infinitely variable ratio. The turbine of the exhaust driven supercharger provides, in effect, just such an infinitely variable gear, for the speed of the blower may be regulated by allowing more or less of the exhaust gases to escape directly to the air through a variable “waste gate”. The turbine has, however, a variable speed characteristic of its own, which causes the change in speed to take place more or less automatically.

The speed of the turbine depends on the difference of pressure between the exhaust gases and the outside air. The greater the difference, the higher the speed of the turbine, and hence the greater the degree of compression by the supercharger. So long as sufficient mixture is supplied to the cylinders the pressure of the exhaust gases will remain the same, whereas the pressure of the outside air decreases steadily with increase in altitude. In consequence, as the aircraft rises higher and higher, the turbo-driven supercharger automatically picks up speed and provides the engine with a greater volume of air. The limit is only reached when the speed and temperature of the turbine rotor approach the safe limits for the materials used in its construction.

It was usual to employ two stages of compression, of which the second is gear driven. An intercooler is placed between the two. Such a combination can be constructed to supply the engine with an almost constant weight of air up to very great heights, which is why the P-38 is so good at high altitude, for example.

The turbo-supercharger was an outstanding innovation but it was not a “cure-all”. There are disadvantages to be weighed against the advantages, and, as usual in all matters relating to aircraft, the designer was faced with the problem of compromise. For example, the bulk and weight of the turbo-supercharger, with its exhaust collectors and complicated plumbing, increase the difficulties of installation and provide some limit to its usefulness.

The expansion of the exhaust gases as they pass through the turbine greatly reduces their speed. It must be remembered that normally this speed is very high, and can be increased by specially shaped outlets to the exhaust manifold. The use of such “ejectors” gives appreciable extra impetus to the aircraft in the form of exhaust thrust. Think of them like miniature jet-propelling engines. The use of a turbo-supercharger precludes any effective use of the exhaust in this way. Both are successful attempts to obtain more useful work from the exhaust gases before they escape, but the designer can’t have it both ways.

Lastly then, It has been explained why it is necessary to close the throttle of a supercharged engine gradually as the aircraft descends. If this were left entirely to the pilot, he would need to be constantly adjusting the throttle lever and watching the boost gauge. With one more thing to think about, it would be unusual if the engine did not occasionally suffer from “over-boosting”. So, there is an automatic boost control, designed to relieve the pilot of the necessity for constant adjustment of the throttle lever, and to prevent the possibility of over-boosting.

Essentially the boost pressure itself is made to operate the throttle valve. The pilot selects the manifold pressure he requires by moving the throttle lever and a butterfly valve linked to it. The automatic control does the rest. If the aircraft climbs to greater altitudes, the throttle valve is slightly opened, if it descends, the valve is closed. Manifold pressure at the selected value is maintained automatically at all altitudes up to the rated altitude. That makes life easier!

Well, that’s about it!

Hope you found that helpful

Badboy

[HoHun]

Hi Badboy,

Great article! In depth-coverage of the topic, and no nits to pick :-)

The only thing that occurred to me that two related topics didn't get mentioned that might be of interest, too:

- ram effect
- engine speed control

(The first affects boost, the second about everything and was specially mentioned by Seeker.)

Below some information on boost conversion that I just posted in another thread, but as I think your article is going to be referred to in future posts, it should be here for reference purposes, too :-)

Regards,

Henning (HoHun)

---cut------------------

Proper boost conversions of course have to be done via the international system.

The international unit of pressure is 1 Pa = 1 N / m^2.

1 ata = 98.0665 kPa
1 psi = 6.89476 kPa
1 mm Hg = 133.322 Pa
1 in Hg = 3.3863788 kPa

Since psi are often provided as pressure relative to the standard atmosphere, for a correct conversion you need to add/subtract the standard sea level atmospheric pressure:

P0 = 101.325 kPa

(Note the difference to the technical atmosphere ata is based on.)

When do I have to add the standard sea level pressure? It depends on the boost indicator.

If the airplane is parked with the engine off at sea level in a standard atmosphere and the boost indicator reads zero, you have to add sea level pressure to get true pressure. British instruments work like that. I think Japanese do, too.

American, Russian and German instruments usually provide absolute pressure (the second "a" in "ata") and indicate correct ambient pressure when sitting on the ground with engine off.

With standard sea level pressure being P0 and indicator reading Pi, you'd have the following conversion to SI units:

For psi:

P = Pi *C(psi) + P0

For Japanese instruments and mm Hg:

P = Pi * C(mm Hg) + P0

For Soviet instruments and mm Hg:

P = Pi * C(mm Hg)

It can be tricky at times to find out whether adding P0 is required or not if you don't have good documentation on the aircraft.

The German practice of using ata for absolute and atü for relative pressure provided good clarity there - one of the few cases in which I can say something good about non-SI units :-)

[Seeker]

Eminently well written; Badz, thanks a lot!

One curiousity remains; however. As you mention abouve; Turbo-supercharged engines often incorporate a waste gate or blow off valve to prevent over pressurisation of the inlet side.

Why is it that gear driven superchargers do not also incorporate such a pressure limiter on the inlet side? I would have thought that with an appplicable pressure threshold; one could install a supercharger with a single, high altitude rated gearing and rely on such a pressure reducing arrangement to enable full throttle running until the engine had reached an altitude whereby alll the inlet pressure was required.

[MiloMorai]

Seeker, in a way they do, at least on the R-R engines, the throttle lever could be full forward but the butterfly valve would only be open enough to allow the max boost. As the altitude increased the valve would open more until FTH was reached when it would be fully open.

White's book on Allied engines gives a full description.

Btw, nice post Badboy.

[Slobberdonkey]

Seeker I think you're getting confused on how it works.

Think of the supercharger simply as a fan blowing air into the engine.

With a fan that is gear driven, the speed of the fan is fixed to the speed of the engine (driveshaft).

With a fan that is turbo driven, the speed of the fan is independant of the mechanical aspect of the engine itself.  Instead, it's speed is regulated by the high pressure exhaust gasses that are blown through an impeller that in turn drives teh fan. Thus the speed of the fan is regulated through the amount of exhaust gas pressure relative to outside air applied to the impeller driving it.  Waste gates are used to tune this exhaust system pressure to the desired amount.

With a gear driven supercharger, exhaust gases aren't involved thus no pressure differential to capitalize off of an no possible use of waste gates since the desired intake air pressure is going to always be greater than the outside air feeding it, even counting the ram air effect.


However,  it may be possible to design a mechanical supercharger using some sort of CVT to achieve the desired effect of good high alt performance without having to sacrifice low alt power.

[joeblogs]

The only things I would add is a discussion of the use of rich fuel mixtures to cool the charge at high manifold pressures and possibly a discussion of the various forms of WEP - water, GM, etc.

Note that the other way to see the "tax" imposed by a supercharger is to examine specific fuel consumption curves of the engine at different supercharger settings. The higher the gear ratio, the higher the fuel consumption per unit of gross output.

-Blogs

[Pongo]

Great write up.

[Seeker]

Originally posted by joeblogs
The only things I would add is a discussion of the use of rich fuel mixtures to cool the charge at high manifold pressures and possibley a discussion of the various forms of WEP - water, GM, etc.
-Blogs

I'm quite familiar with how charge and ignition effects have an influence (along with compression ration) on an engine; as I've tuned many a screaming two stroke to melt down.

To this extent:

Brands Hatch has a long loop going through woodland on it's grand prix circuit. We'd have to upjet bikes when running the long circuit late in early summer; going through the unusually oxygenated air of the woodland loop plus the extra full throttle running it's nature provides was enough to melt the piston crown of a suzuki 750 triple set up for the short circuit (when one is fresh and extreme  )

I'm also familiar that with engines sharply tuned to provide maximum power at a specific engine speed; they can behave very weakly when "off the curve"; I appreciate that many engines can't tolerate full throttle under a certain engine speed; and that load can be applied to balance that engine speed to produce the desired torque (the perfect drag racing start.....)

I wasn't sure what all this talk of "Full throttle hieght" was (I'm used to thinking of full throttle RPM); and I'm still a bit hazy over what "critical alt" is.

It also strikes me that all our discussions concentrate on maximum acceleration and power. However; I've heard dogfighting referred to as a race to get to the fireing position; and in any race; brakes are what you want.

I wonder which engine worked best when slamming the anchors on for the curve , if I may use a racing analagy. I know Badboy is of the mind one should never brake; one should go up; and as a theoritician he may well be right  ; nonetheless; I see many a good spit pilot chopping throttle for a kill; Levi strikes to mind.

Engine/prop braking; an under appreciated phenomenon?  :-)

[Ecliptik]

"Full throttle height" and "critical" or "rated" altitude are all the same thing - the altitude at which the supercharger provides the maximum power boost.  Badboy mentioned this.

For example, if an engine has a single-stage supercharger with a critical altitute of 20,000 feet, then at altitudes below 20,000 feet, the throttle is automatically kept partially closed to prevent the supercharger from overboosting the engine, thus it is not providing its full potential additional power ouput.  Additionally, being coupled to the engine driveshaft, it is sapping a certain relatively constant amount of power from the engine.  As critical altitude is approached, this power deficiency is overcome by the additional power from the pressure boost, and overall power output rises as you climb to 20k.  Above 20,000 feet, the supercharger will be unable to maintain full sealevel manifold pressure despite blowing for all its worth with the throttle wide open, and power output will gradually decrease as you climb higher than 20k.  At exactly 20k, the supercharger is providing exactly the full sealevel manifold pressure, with the throttle open fully, and is thus operating at maximum efficiency.

[joeblogs]

Short of deploying flaps or skidding, the best anology to brakes for these A/C is the use of the vertical.

-blogs

Originally posted by Seeker

It also strikes me that all our discussions concentrate on maximum acceleration and power. However; I've heard dogfighting referred to as a race to get to the fireing position; and in any race; brakes are what you want. ...

 

[Dead Man Flying]

Originally posted by Seeker
I wonder which engine worked best when slamming the anchors on for the curve , if I may use a racing analagy. I know Badboy is of the mind one should never brake; one should go up; and as a theoritician he may well be right  ; nonetheless; I see many a good spit pilot chopping throttle for a kill; Levi strikes to mind.

This is where theory and virtual reality depart in Aces High.  In the main arena environment, I find that killing fast is the single most important element of surviving.  Even if I'm outnumbered, I rely on hitting my shots and making those shots count.  I just don't have the luxury of using the vertical rather than chopping the throttle or kicking myself into a skid in order to line up a shot.  On those occasions where I do enjoy the luxury of uninhibited fighting, I'll use the vertical as much as the next guy.

It's all about what works.  

-- Todd/Leviathn

[joeblogs]

Here's an example of what I mean, using a mature radial engine with a single stage two-speed supercharger:



SFC is measured in lbs. of fuel per horsepower per hour. As is typical for most American performance data, the chart is calculated on the basis of the standardized atmosphere.

Note how the curves rise rapidly once the engine is operating above 65 percent of rated power (1,000 HP in this case). It is at this point that fuel mixture switches from auto-lean to auto-rich.

Note also the fuel economy penalty imposed by higher supercharger gear ratio.

The specs for this engine are an exact match for the R1820-59 version of the cyclone. This is one example of the abundant G200 series, which featured forged cylinder heads and crankcases. This series went into production in March of 1939 and over 20,000 were built by the end of the war.

-Blogs

Originally posted by joeblogs
The only things I would add is a discussion of the use of rich fuel mixtures to cool the charge at high manifold pressures and possibly a discussion of the various forms of WEP - water, GM, etc.

Note that the other way to see the "tax" imposed by a supercharger is to examine specific fuel consumption curves of the engine at different supercharger settings. The higher the gear ratio, the higher the fuel consumption per unit of gross output.

-Blogs