Supercharger vs Turbocharger

[Juha]

It's not high pressure effectively zero pressure, its cryogenic. N2O is weakly cryogenic.

Yes, I corrected that in my message #77

... however there is a reference to the N2O system being made safe. It's a non inflamable liquid that could be protected by some kind of a self sealing system.

Yes but it is as the data sheet of it says ""Oxidant. Strongly supports combustion. May react violently with combustible materials." The last part made self-sealing difficult maybe impossible, at least with the materials Germans used to self-seal their fuel tanks and according to Fernández-Sommerau it wasn't self-sealed, at least not in the 3rd system used in 109G-5 and I really doubt that the 2 earlier system were self-sealing.

Juha

 

[wmaxt]

Nitrous itself is not flammable.

Nitrous does three things,

First - it cools the Air/Fuel charge allowing more (denser charge) to be stuffed into the combustion chamber and

Second - the cooling reduces detonation

Third - it adds oxygen - EVERY Nitrous system adds extra fuel in proportion to the oxygen in the Nitrous charge to get maximum power.

 

[Shortround6]

I would disagree with a fiar bit of what you've state about the DB600, Improvements in DB605 performance (aside from tuned scavenging and spark plugs) came about as a result of improvements in piston compression ratio and compressor fluid dynmamics, latter on still the supercharger was increased in size in the D and AS series engines but still remained a single stage. 1.98 ata boost, the maximum used by the 2000hp DB605DCM at CR of 8.5 is only 14 psig or 59 inches of mercury. To achieve somewhat lower power levels the Merlin needed 25psig (about 2.8 ata boost) for the same wieght and worse fuel consumption. Two stage compressors are not more efficient than single stage, they may draw less shaft power but this comes at the expense of intercooling which is basically throwing away energy into the intercooler for disposal by a bulky radiator of some kind.

Improvements to the 605 over the 601 came from a number of things, 1. a 5.3% increase in displacement. 2. a 12-15% increase in rpm (2400-2500rpm to 2800rpm). 3. an 9% increase in boost (1.3 Ata to 1.42). Since some of these improvements compound it means that the increased valve overlap, increased compression ratio and improved compressor fluid dynamics are responsible for under 1/2 the improvement.

With the later versions it was mostly the increased boost as can be seen by comparing the percentage increase in boost to the percentage increase in power. It is not exactly the same but it is close. The fact that some times (certain models) of the 605 used "over boost" to reach their power levels is also evident by looking at some of the charts on Kurfurst's site. By over boost I mean using more boost than can be sustained to the critical height of the previous model.

Two stage compressors ARE more efficient than single stage compressors for a given level of boost. This was not new even in the late 30s, Mercedes used a two stage supercharger on their 1939 Grand Prix cars for that reason. comparing one supercharger to another at different levels of boost isn't really comparing the superchargers.

This line is a real hoot (joke) "at the expense of intercooling which is basically throwing away energy into the intercooler for disposal by a bulky radiator of some kind"

While true in a very theoretical sense regarding the whole system as a "heat engine" for ultimate fuel efficiency from a practical, real world sense the intercooler is a necessity to get high power output. High fuel efficiency and high power from the size size/weight engine not really being possible at a given level of technology. Charge cooling does at least three things for an engine. Since the power limitation comes from oxygen that can be moved through an engine per unit of time (mechanical strength being ignored) the more air means the more power. We mean the mass or weight of air. At a given pressure say 1.8 ata (12 lbs boost) the actual amount of air is also governed by the temperature of the air. The lower the temperature the more dense the air is even at the same pressure and the more weight (mass) of air there is. Charge cooling means higher destiny air at the same pressure for more power. The second thing is the fact that at times the engine is material limited. Certain parts of the engine can only operate at certain high temperatures limits without quickly failing. Intake air temperature affects the temperature of the charge flowing through the engine at all points. Increase the intake charge temperature by 200 degrees and peak temperature in the cylinder will be 200 degrees higher and the exhaust temperature will be 200 degrees higher. using charge cooling can affect things like exhaust valve life. The third thing is that there is a limit to the amount of boost, cylinder compression and charge temperature that a particular engine (they vary a bit due to spark plug placement, exhaust valves, combustion chamber shape and cooling) will tolerated before getting to detonation levels with a particular grade of fuel. The lower the intake charge temperature the more boost and/or cylinder compression can be used.
MW/50 is a form of charge cooling. The extra fuel sprayed into the supercharger intake of the BMW 801D did some charge cooling in addition to providing the extra fuel. GM-1 did some charge cooling in addition to providing the extra oxygen. Some late model Jumo 211s used an intercooler (or more properly an after cooler) on a single stage supercharger (perhaps the only single stage supercharger to use an inter/after cooler in WW II ?). For the Jumo 211 it was worth about 100 extra horsepower or several thousand ft of altitude. Of course it fit much better on bomber type aircraft than on small fighters.

There are figures around for the Merlin XX, it's supercharger in high gear heated the intake charge about 148 degrees C over the incoming air. when providing 9lb of boost. It had a pressure ratio of around 3.5 to 1. Higher pressure ratio superchargers are going to heat the intake charge much more (the Merlin 61 could compress the air 5.1 times at 23,500ft) and such high supercharger ratios at high altitude would have been unusable due to detonation problems without some form of charge cooling. Please note that the WER ratings on the P&W R-2800 were achieved using BOTH intercoolers and water/alcohol (MW/50) injection.


When assessing the supercharger set ups and what they achieved it is also good to remember in what order things happened and why.

Note I didn't say the DB605 with a single stage supercharger was better than engines with a two stage unit, I said it was far dependant on high supercharger compression ratios and provided sufficient performance; it clearly still faded away slightly earlier but not at all by much especially the latter englarged supercharger versions. The Merlin used its supercharger not to compensate for altitude but to overboost its engine while the DB605 did less so and it seems gained considerable fuel economy.

When assessing the supercharger set ups and what they achieved it is also good to remember in what order things happened and why. When the Merlin 61 was developed and introduced it's purpose was to improve altitude performance. Initial Boost was limited to 15lbs which was achievable with the single stage supercharger at low altitudes. Getting 15lbs of boost ( 60in manifold pressure) at 23,500 ft was NOT achievable with a single stage supercharger at that time. The Merlin needed a strengthened supercharger drive to get to 18lbs of boost and beyond and better fuel or water injection in order to use the higher than 18lbs boost at any altitude. The water injection was used in non flying experimental engines. It took several years to get to the 21 and 25lb boost levels even at low altitudes after the 2 stage supercharger was introduced. The P&W two stage engines were always about altitude performance as were the the American turbo installations, all two stage installations.

 

[tomo pauk]

Another great post

 

[wuzak]

Nice post SR.

 

[Siegfried]

Improvements to the 605 over the 601 came from a number of things, 1. a 5.3% increase in displacement. 2. a 12-15% increase in rpm (2400-2500rpm to 2800rpm). 3. an 9% increase in boost (1.3 Ata to 1.42). Since some of these improvements compound it means that the increased valve overlap, increased compression ratio and improved compressor fluid dynamics are responsible for under 1/2 the improvement.

Multiply the displacement increase by RPM increase by boost increase one barely gets a 36% increase.

An increase of compression ratio from 6.9 (DB601) to 8.5 (DB605DC) however figures prominently.

Valve overlap and multipoint fuel injection can get 10% to 15% more power.

The DB605A and AS/ASM had a CR of 7.5 the latter DB605DB, DC 8.5.

With the later versions it was mostly the increased boost as can be seen by comparing the percentage increase in boost to the percentage increase in power. It is not exactly the same but it is close. The fact that some times (certain models) of the 605 used "over boost" to reach their power levels is also evident by looking at some of the charts on Kurfurst's site. By over boost I mean using more boost than can be sustained to the critical height of the previous model.

The DB605 series has a quirky proportionality of boost levels at sea level and power increase however at full power height it was about 150hp more.

Two stage compressors ARE more efficient than single stage compressors for a given level of boost. This was not new even in the late 30s, Mercedes used a two stage supercharger on their 1939 Grand Prix cars for that reason. comparing one supercharger to another at different levels of boost isn't really comparing the superchargers.

Two stage compressors are used when technology is inadaquet to produce the required compression in a single stage.

This line is a real hoot (joke) "at the expense of intercooling which is basically throwing away energy into the intercooler for disposal by a bulky radiator of some kind"

It is true. Modern gas turbines are renowned for reducing the number of stages as part of efficiency improvements.

Of course it all gets down to detail design, but its clear the DDs engineering avoided dependance on high supercharger pressures and intercooling; so touting the Merlins intercooler supercharger setup rather ignores the point that it was in a way a fix to the limitations of carburation and throttle body injection.

While true in a very theoretical sense regarding the whole system as a "heat engine" for ultimate fuel efficiency from a practical, real world sense the intercooler is a necessity to get high power output.

From a practical point of view the DB series managed to operate high piston compression ratios and avoid the need for high pressure ratio superchargers and inter-coolers.

High compression ratios allow greater expansion of the hot gases and therefore more power and efficiency. There is more danger of pre ignition, however no energy is lost as in intercooling and while intercooling can clearly demonstate advantages it comes at a price as well.

High fuel efficiency and high power from the size size/weight engine not really being possible at a given level of technology.

The DB605DC managed 1850hp on C3 a 96/130 fuel without MW50 it weighed 745 kg.
The Merlin (66) seemed to need 100/150 fuel (really 110/150) to do the same it weighed 744kg.

The Daimler Benz engine seems to have been more fuel efficient, suggesting a smaller radiator as well.

Higher power levels (eg Merlin 100) resorted to 100/150 plus ADI.

Charge cooling does at least three things for an engine. Since the power limitation comes from oxygen that can be moved through an engine per unit of time (mechanical strength being ignored) the more air means the more power.

SNIP

There are figures around for the Merlin XX, it's supercharger in high gear heated the intake charge about 148 degrees C over the incoming air. when providing 9lb of boost. It had a pressure ratio of around 3.5 to 1. Higher pressure ratio superchargers are going to heat the intake charge much more (the Merlin 61 could compress the air 5.1 times at 23,500ft) and such high supercharger ratios at high altitude would have been unusable due to detonation problems without some form of charge cooling. Please note that the WER ratings on the P&W R-2800 were achieved using BOTH intercoolers and water/alcohol (MW/50) injection.


When assessing the supercharger set ups and what they achieved it is also good to remember in what order things happened and why.

Again the boost level of 9 psi is equal to 1 ata plus 9/14 = 1.65, a level not introuced (1.7 ata) early 1944 on the DB engine.

The technology the DB605 used simply didn't need high boost levels nor did it need inter cooling.
The supercharger was simply less importation a factor to this engine.

The DB605L for the almost in service Me 109K-14 (and DB603LA) had two stage superchargers but did not need to use inter cooling

When assessing the supercharger set ups and what they achieved it is also good to remember in what order things happened and why. When the Merlin 61 was developed and introduced it's purpose was to improve altitude performance.

For the high altitude Wellington, supposedly.

Initial Boost was limited to 15lbs which was achievable with the single stage supercharger at low altitudes. Getting 15lbs of boost ( 60in manifold pressure) at 23,500 ft was NOT achievable with a single stage supercharger at that time.

1.8 ata boost 11 psig
1.98 boost 14 psig.
The DB605 could get away with a single stage supercharger.

 

[Shortround6]

Instead of discussing the advantages and disadvantages of the different systems you seem to want a Rolls Royce vs Daimler Benz argument.

Multiply the displacement increase by RPM increase by boost increase one barely gets a 36% increase.

That "barely gets a 36% increase" would have been good for for 1598hp at sea level for a DB 605A compared to a DB601Aa instead of the 1475. at 3700meters it should have been good for 1496hp (ok I am using HP instead of PS for convenience but all I am comparing here are DB engines) instead of about the 1400 hp/PS the 605A got at that altitude. Since the 605A did NOT display even a 36% total increase in power over the 601Aa it is rather hard to see how those 3 improvements didn't contribute significantly to the increase that was achieved.

An increase of compression ratio from 6.9 (DB601) to 8.5 (DB605DC) however figures prominently.

You do, of course, have a source to back that up?
There is no question that the DB series did increase form the 6.9 to 8.5 compression. However a chart from "Aircraft Power Plants" by Jones, Insley, Caldwell and Kohr (1926) shows that there is a 10% increase in power going from 5.4:1 to 7.0:1 compression ratio and another 5% increase going from 7.0:1 to 8.5:1. Similar charts from other books, like "Aircraft Engine Design" by Liston (1942) show similar even if not identical increases. There is usually a diminishing return in increasing compression, there is less of an increase going from 9.0 to 10.0 than in going from 6.0 to 7.0. They also show about a 40% increase in peak cylinder pressure for the same 7.0:1 to 8.5:1 change, they do show about a 5-6% improvement in fuel efficiency though.

Valve overlap and multipoint fuel injection can get 10% to 15% more power.

They can, the question is did they? maybe they did and maybe they didn't, I don't know, but we do know that the point fuel injection didn't provide the charge cooling that the evaporation of fuel in the supercharger of some carburetor or single point (throttle body?) injection systems did. Without a detailed analysis or report comparing both on ONE engine we are just guessing as to which was better. R-R figured that the Fuel evaporation was good for about 20-25 degrees C reduction in intake charge temperature in the Early Merlins.

The DB605 series has a quirky proportionality of boost levels at sea level and power increase however at full power height it was about 150hp more.

No more than some other engines that were throttle back at take-off or sea level compared to what they could do a several thousand meters. But the 150hp claim may be pushing things just a bit. From the chart for the DB605A on the Kurfurst site it seems it topped out at about 1525-1550 HP or so at 2100meters compared to the 1475 for take-off?

Looking a the 605AM compared to the 605A we get 1.42ata of boost at sea level for 1475hp and 1.8 ata (using C-3 fuel plus MW-50?) for 1800hp at sea level. Note how the power and boost track fairly closely? But with the same supercharger this high level of boost can only be maintained to 4000meters and by 5600meters or so and without MW-50 the power has fallen to the same level as a 605A. The use of MW-50 (charge cooling) is still worth about 100hp if I am reading the chart right. Please note that the C-3 fuel has allowed for over boosting at low altitudes but has done little or nothing for high altitude performance.

With the bigger supercharger used on the 605AS DB gained several thousand meters of altitude. Or to put it another way it made around 200hp more at 8000meters than a 605A. But the bigger supercharger cost about 40hp at sea level at 1.42Ata. Granted with the bigger supercharger 'over boosting' at low altitude was quite possible subject to engine strength or temperature limits and the use of C-3 fuel and/or MW-50 (charge cooling).

Two stage compressors are used when technology is inadaquet to produce the required compression in a single stage.

While that is true and it is true that nobody was going to design a two stage Supercharger to get a pressure ratio of under 3 to 1 which could easily be done with a single stage supercharger (after 1940/41) it is also true that NOBODY had a single stage centrifugal compressor that could reach a 5 to 1 pressure ratio in WW II, either supercharger or jet engine compressor, in fact you would be hard put to find very many centrifugal compressors today that exceed 5 to 1.

Of course it all gets down to detail design, but its clear the DDs engineering avoided dependance on high supercharger pressures and intercooling; so touting the Merlins intercooler supercharger setup rather ignores the point that it was in a way a fix to the limitations of carburation and throttle body injection.

I think you are mixing cause and effect again. DBs engineering avoided dependence on high supercharger pressures by using a large displacement engine. An extra 25.5-32% of displacement can certainly allow for some different choices. In the 1930s when design and development of both engines started and they went into initial production the DB used 39 in of manifold pressure to the Merlin's 42in and the BMEP of the two engines were with a couple of percent of each other. The Merlin hung on as long as it did partially because of the better fuels. Had these fuels not been available perhaps more work would have been done on the Vulture or the Griffon been pushed along faster. Given the displacement difference the ONLY way for the Merlin to compete was to use higher boost pressure. Using point fuel injection would have helped mixture distribution problems but would have done little to solve the the displacement difference. A 5-10% increase doesn't cover the 32% displacement difference.

From a practical point of view the DB series managed to operate high piston compression ratios and avoid the need for high pressure ratio superchargers and inter-coolers.

Actually the high piston compression ratios prohibited the use of of high pressure ratio superchargers and without the high pressure ratio superchargers there was a lot less need for the inter coolers. Figure it out for yourself, Say you boost the piston compression ratio by 20%, you do not get 20% more power, you may get 5-10% more power but if you had kept the same piston compression ratio and increased the boost pressure by 20% you could burn 20% more fuel per power stroke. Even counting the extra power to drive the supercharger you come out ahead.

High compression ratios allow greater expansion of the hot gases and therefore more power and efficiency. There is more danger of pre ignition, however no energy is lost as in intercooling and while intercooling can clearly demonstate advantages it comes at a price as well.

the energy LOST in the intercooler is pretty much theoretical as this energy is pretty much destructive to a high power engine. If you want to design a fuel economy special go ahead but the increased power from the denser charge has been proven over and over again (some German engines could pick up 4-5% in power just from using MW-50 WITH NO INCREASE IN BOOST). Speaking which doesn't MW-50 cause a loss of power as it reduces the heat (energy) in the intake charge? The Price of an inter cooler is weight, bulk and drag. But then lugging around 100-200liters if water/alcohol brings a few penalties too doesn't it?

The DB605DC managed 1850hp on C3 a 96/130 fuel without MW50 it weighed 745 kg.
The Merlin (66) seemed to need 100/150 fuel (really 110/150) to do the same it weighed 744kg.

How about we go back to the Merlin 61 and the DB605AS? Merlin 61 does 1390hp at 7170meters. DB605AS does 1200PS at 8000meters
Or the Merlin 71 vs the DB605DC? Merlin has 1475hp at 6742 meters compared to the DB605DC's 1550ps at 6000meters while using MW-50?
And the Merlins don't need anything more than 100/130 fuel.

For the high altitude Wellington, supposedly.

You have proof of another explanation?

1.8 ata boost 11 psig
1.98 boost 14 psig.
The DB605 could get away with a single stage supercharger.

Yes it could, for low altitude work. And it only needed better fuel of it's own (late war C-3) and/or charge cooling (MW-50) to do it.

there is nothing wrong with MW-50 (water/alcohol) Plenty of American aircraft used it. Just don't claim how superior one countrie's or companie's "technology" is for not using intercoolers whenthey were using an alternative form of charge cooling.

 

[wuzak]

It is true. Modern gas turbines are renowned for reducing the number of stages as part of efficiency improvements.

Is this so?

Have you any numbers to back that up?

FYI:
RR RB211 - 1 fan stage, 7 intermediate compressor stages, 6 high pressure compressor stages.
RR Trent - 1 fan stage, 8 intermediate compressor stages, 6 high pressure compressor stages.


Turbine manufacturers continue to work to improve the efficiency of the compressor stages, which means les work is required to drive the compressor stage, which means less power needs to be extracted in the turbine and therefore more thrust (or shaft hp).

If they have reached an overall pressure ratio with which they are satisfied it wouldn't surprise me if try to do it with fewer stages. Fewer stages mean higher pressure ratios per stage, and more heat generated per stage, unless there is a gain in efficiency of the compressor design.

 

[Edgar Brooks]

--

 

[Shortround6]

ADI is anti-detonation injection. Just a different term for water (water/alcohol) injection /MW-50.

 

[Juha]

What's ADI?

Anti-Detonant Injection, OK SR beats me

 

[Juha]

The DB605DC managed 1850hp on C3 a 96/130 fuel without MW50 it weighed 745 kg.
The Merlin (66) seemed to need 100/150 fuel (really 110/150) to do the same it weighed 744kg.

In fact Merlin 66 produced appr 1980hp at sea level with 100/150 fuel and 25lb boost.

Juha

 

[wuzak]

In fact Merlin 66 produced appr 1980hp at sea level with 100/150 fuel and 25lb boost.

Juha

According to Lumsden the Merlin 66 with 100/150 fuel and +25psi boost made 2000hp @ 3000rpm at 5250ft and 1860hp @ 3000rpm @ 11,000ft.

On 100/130 fuel it was 1750hp @ 5250ft and +18psi, and 1625 @ 12,500ft, +18psi.

 

[Edgar Brooks]

--

 

[wuzak]

Higher power levels (eg Merlin 100) resorted to 100/150 plus ADI.

I don't think any Rolls-Royce built Merlins ever entered service with ADI. Some Packard Merlins did.

Anyone know a definitive answer?

Rolls-Royce did play with ADI on the test bench, at least.

RM.17SM made 2620hp (corrected from 2640hp) @ 3150rpm with +36psi boost, special fuel (extra TEL) and ADI.

A development Merlin 66 ran at 2380hp @ 3300rpm and +30psi boost for a 15 minute "sprint" run in 1943.

 

[wuzak]

Thank you (both) since it means I've been able to confirm my suspicion that the Merlin 100-series didn't have it, they just had "standard" fuel injection, which did sufficient cooling on its own.

Yes, the fuel injection was in the throttle body. Or, if not, in the supercharger impeller eye.

 

[Siegfried]

Thank you (both) since it means I've been able to confirm my suspicion that the Merlin 100-series didn't have it, they just had "standard" fuel injection, which did sufficient cooling on its own.

Packard V-1650-9 (Merlin 100) of the P-511H used it.

North American P-51H Mustang
Specs of the P-51H-5-NA:

One Packard Merlin V-1650-9 twelve-cylinder Vee liquid cooled engine rated at 1380 hp for takeoff and a a war emergency power of 2218 hp at 10,200 feet and 1900 hp at 20,000 feet with water injection.

 

[wuzak]

Yes, but only Packard Merlins.

 

[Edgar Brooks]

--

 

[Tante Ju]

the energy LOST in the intercooler is pretty much theoretical as this energy is pretty much destructive to a high power engine. If you want to design a fuel economy special go ahead but the increased power from the denser charge has been proven over and over again (some German engines could pick up 4-5% in power just from using MW-50 WITH NO INCREASE IN BOOST). Speaking which doesn't MW-50 cause a loss of power as it reduces the heat (energy) in the intake charge? The Price of an inter cooler is weight, bulk and drag. But then lugging around 100-200liters if water/alcohol brings a few penalties too doesn't it?

I dont think "100-200 liter" MW was carried. More like 70-80 liter.. also one big plus for MW that it also massive cools engine, as it evaporates in combustion chamber, so you do not only no intercoolar radiator needed, but also smaller coolant radiator will do. Without the added drag and bulk of using larger radiator to compete with increased engine temperatures during high power. So you can actually size radiators to be smaller and to cope just with engine temps at military power, and MW takes care of rest at high powers, high power I mean 5-700 HP more than normal.. This is significant benefit, that you do not have to size radiators for power of say in case of DB, 2000 HP but 1300 HP. It means less bulk added, but more important, much less drag.

So essence - MW is like charge cooling and evaporate cooling. Otherwise very learnable post, it was good to read!