Hardest plane to take down in WW2?

[HoHun]

Hi Koolkitty,

>Everything I've read or heard has said that, despite offering excelent power and throttle performance, it offered poor fuel efficience and very high emmissions (even up through the 1970s) due to unburnt fuel resulting from injection every cycle and inherantly running rich (in the exaust).

Hm, from von Gersdorff at al. as well as from Müller (Junkers-Triebwerke), the German designers actually got both better power and better efficiency from direct fuel injection because the process of mixture generation could be controlled much more accurately.

Very rich running at high power is actually typical for the carburetted engines of the era.

Another example: According to the B-29 manual, the R-3350 of the B-29 used 1060 L/h for 2200 HP at take-off power. That's 346 g/HP/h specific fuel consumption in a turbo-supercharged engine which runs more efficiently than a mechanically supercharged one (if you look at shaft power as we do here).

The DB605A according to a Daimler-Benz datasheet had a minimum of 205 g/HP/h at 890 HP at 5.7 km. Without evaluating the B-29 power chart completely, it seems that the R-3350 had a minimum of about 195 g/HP/h at reduced power settings, which probably indicates the same efficiency of combustion as the R-3350 has a efficiency advantage not related to combustion as it does not have to spin a mechanically-driven supercharger like the DB605.

So the DB605A goes from 205 g/HP/h minimum over 215 h/HP/h at maximum continuous to 235 g/HP/h at take-off/emergeny power.

The R-3350 goes from 195 g/HP/h minimum over 311 g/HP/h at maximum continuous to 346 g/HP/h at take-off/emergency power.

The Jumo 213E, another fuel-injected, mechanically-supercharged engine, features values of 200 g/HP/h minimum, 210 g/HP/h at maximum continuous and 258 g/HP/h at take-off/emergency power.

I think the relatively flat incline of the specific fuel consumption curve is due to the use of fuel injection of the DB605A and the Jumo 213E. The minimum specific fuel consumption on the other hand is probably very close both for carburetted and fuel-injected engines. However, when you leave this optimum working point, the R-3350 at least begins to become noticably less efficient, and I think it's fairly typical for the engines of the time.

Regards,

Henning (HoHun)

[renrich]

Elvis, As I stated before I am not an engineer but please give me credit for being a "normal" American boy in the 50s. My first car was a 1950 Buick Special with around 60000 miles on it. I paid 495 dollars for it and drove all around Texas working on it, trying to keep it running. Many times with at least one cylinder not working. I can't remember whether it was a straight six or eight. It did not have much power when everything was working right but it was really a dog when one cylinder wasn't firing. The question I posed was how could an AC engine continue to run with a "jug" or cylinder was shot off like I have heard reported by several writers.

[Elvis]

Renrich,

I was giving you as much credit as I could, but your last post showed the difference between what you know and what I know.
To you, there's the engine in your old Buick and then there's an aircraft engine.
To me, an engine is an engine is an engine.
Whether you've got a dead cylinder in your old Buick or whether you've just had one shot off your P-47, to me, its all the same.
You still have an engine with an inoperative cylinder and the result is going to be the same.
Sorry if my post to you seemed demeaning in some way. It was not meant to. I was only trying to answer the questions I saw asked.





Elvis

[Elvis]

...btw, since it was a '50 Special, it should've had a 6.
The 8 was for the Roadmaster.

...of course, once the V-8 hit the scene, that all pretty much went by the wayside.



Elvis

[FLYBOYJ]

Originally Posted by HoHun

Very rich running at high power is actually typical for the carburetted engines of the era.

Hi Henning;

Actually that is somewhat correct - those fuel consumption charts are based on "normal" mixture settings and also using a richer mixture for engine cooling, something earlier mentioned. At altitude you can continue to lean for air density based on CHT and EGT. And remember, for the most part - maximum power setting (take off and WEP) are always assumed at "full rich."

[Elvis]

KK,

Fuel injection nomenclature - Direct vs. Indirect.

Direct Fuel Injection is when the fuel is shot directly into the cylinder.
Indirect Fuel Injection is when the fuel is shot into a secondary chamber that has a passage that leads to the combustion chamber.
This secondary chamber is known as a "Pre-Combustion Chamber".
I have only seen these terms used with diesel engines.
I once owned a 1980 VW Rabbit diesel and it used indirect fuel injection because the thinking at the time was that it offered better emissions.
With a gas engine, the fuel is usually shot into somewhere into the intake manifold, so, in a sense, it uses a direct fuel injection system...just indirectly.

Anyway, my point is, we're talking about gas engines here, so please refrain from the "direct..." term.
Its confusing and isn't totally correct.

The "inefficiency" of the fuel injection system used on the DB601 could stem from the fact (and I'd have to look this up to be certain) that I believesome of the early f.i. systems (and anything of WWII vintage would be "early") lacked a key component that is common on every f.i. system made today - a "fuel return line".
This is part of the reason why f.i. systems in cars are more efficient than carb's (and why no new auto has featured a carb since something like the 1991 model year) is that they don't use all of the fuel fed to them.
A carb does that. All of the fuel it sees, it dumps into the engine.
With an f.i. system, your injectors feed off a "log" or "manifold" (in this case, its just a pipe) and they only use the amount of fuel that they need. The rest of the fuel just heads back to the gas tank.
The lack of such a feature could be what the writer is getting at in the passage you quoted about the DB601 being "inefficient".

As for turbo's, they didn't really come into vogue until the mid-late 1970's, because they always had bearing problems.
Remember, an engine driven supercharger is running at about 10-15 times that of engine speed (i.e., engine turning 2000 rpm = supercharger impeller speed of 20,000-30,000 rpm). With an exhaust driven turbocharger, the impeller speed is easily 10 times that of the supercharger.
The turbo works by linking two impellers via a common axle. One sees exhaust gas under pressure, the other sees outside air.
By using the force of the exhaust gas, as its driven out of the exhaust manifold, outside air can be forced in under great pressure.
In theory, it tends to be a more effieicient system, compared to an engine driven supercharger, because the engine does not directly drive it. The force of the exhaust does, so no power is robbed from the engine.
The problem is in those super high speed that those impellers turn. Moreso, that super high speed that the axle that connects them turns, combined with the heat from the exhaust side transferring across that axle.
Keeping that axle, and its corresponding bearing, in one piece was quite the daunting task for many years and why the slower running supercharger was favoured.
It wasn't until the advent of the ceramic bearing, in the 1970's, that the "turbocharger dilemma" was quelled.
Since then, turbochargers have almost completely replaced superchargers, in the marketplace.
The only thing a supercharger has going for it, these days, is quicker reaction to the engine. Even with today's advanced technology, we still suffer from a little bit of that "turbo lag".



Elvis

[HoHun]

Hi Flyboyj,

>Actually that is somewhat correct - those fuel consumption charts are based on "normal" mixture settings and also using a richer mixture for engine cooling, something earlier mentioned. At altitude you can continue to lean for air density based on CHT and EGT.

The chart you couldn't see shows a detailed map of fuel consumptions for the different power settings and for altitudes up to 30000 ft. Obviously, one was expected to (have to) lean so that one ended up with these fuel consumptions (or close by, depending on environmental parameters).

I don't think the R-3350 could get a better fuel consumption than listed from additional leaning at altitude. The chart already takes altitude effect into account.

>And remember, for the most part - maximum power setting (take off and WEP) are always assumed at "full rich."

For the B-29, it is assumed at "Auto Rich", not at "Full Rich". For the late-model P-51D for which the V-1650-7 data listed above is valid, it's assumed at "Run", with "AR Emergency F. Rich" only to be used in a case of malfunction of the automatic regulator. For the early-model P-51D, it was "Auto Rich" like for the B-29.

However, that's just the label on the mixture quadrant. Just how rich the mixture is might vary according to the actual settings of the automatic or semi-automatic mixture regulator. 346 g/HP/h is the actual figure for the R-3350, so it certainly ran richer than the V-1650-7 which consumed just 277 g/HP/h.

Regards,

Henning (HoHun)

[FLYBOYJ]

Originally Posted by HoHun

Hi Flyboyj,

However, that's just the label on the mixture quadrant. Just how rich the mixture is might vary according to the actual settings of the automatic or semi-automatic mixture regulator. 346 g/HP/h is the actual figure for the R-3350, so it certainly ran richer than the V-1650-7 which consumed just 277 g/HP/h.

Regards,

Henning (HoHun)

That's where I was going with this. It was learned that under certain conditions when using a pressure carburetor with an automatic mixture regulator, further leaning is possible based on MP/ CHT/ and EGT at a specific RPM setting. That was the purpose of South Pacific Lindbergh's tour during WW2. A pilot at altitude could sit there and let the automatic fuel control do the work or he could monitor MP, CHT and EGT and lean more.

I've flown modern GA aircraft with FI and pressure carbs and would always lean at altitude - I would usually see anywhere between 5 and 10% fuel consumption savings.

[Crumpp]

Actually that is somewhat correct - those fuel consumption charts are based on "normal" mixture settings and also using a richer mixture for engine cooling, something earlier mentioned. At altitude you can continue to lean for air density based on CHT and EGT. And remember, for the most part - maximum power setting (take off and WEP) are always assumed at "full rich."

Engines curves over altitude always assume proper leaning is utilized. This will cause an accident and can lead to running out of fuel if a pilot does not properly lean as all the data assumes proper mixture control.

I think that is what you are saying as well FlyboyJ.

The pilot does have considerable control over how much or little he leans.

All the best,

Crumpp

[FLYBOYJ]

Originally Posted by Crumpp

Engines curves over altitude always assume proper leaning is utilized. This will cause an accident and can lead to running out of fuel if a pilot does not properly lean as all the data assumes proper mixture control.

I think that is what you are saying as well FlyboyJ.

Exactly!

Originally Posted by Crumpp

The pilot does have considerable control over how much or little he leans.

All the best,

Crumpp

My local airport is at 5,230' MSL. We take leaning a little serious up here!

[HoHun]

Hi Flyboyj,

>I've flown modern GA aircraft with FI and pressure carbs and would always lean at altitude - I would usually see anywhere between 5 and 10% fuel consumption savings.

Since the R-3350 runs about 50% richer than the DB605A at WEP settings to begin with, 5 to 10% do not make not much of a difference for the overall picture.

I at least don't see much evidence for direct fuel injection being less efficient than carburetion in WW2 - quite to the contrary.

Regards,

Henning (HoHun)

[FLYBOYJ]

Originally Posted by HoHun

Hi Flyboyj,

>I've flown modern GA aircraft with FI and pressure carbs and would always lean at altitude - I would usually see anywhere between 5 and 10% fuel consumption savings.

Since the R-3350 runs about 50% richer than the DB605A at WEP settings to begin with, 5 to 10% do not make not much of a difference for the overall picture.

It does if you're traveleing 2000 miles over an open ocean.

Originally Posted by HoHun

I at least don't see much evidence for direct fuel injection being less efficient than carburetion in WW2 - quite to the contrary.

I agree....

[HoHun]

Hi Flyboyj,

>It does if you're traveleing 2000 miles over an open ocean.

I don't picture me traveling 2000 miles over an open ocean at take-off/emergency power anyway.

Regards,

Henning (HoHun)

[FLYBOYJ]

Originally Posted by HoHun

Hi Flyboyj,

>It does if you're traveleing 2000 miles over an open ocean.

I don't picture me traveling 2000 miles over an open ocean at take-off/emergency power anyway.

Regards,

Henning (HoHun)

You won't unless you have to rapidly climb or decent - put it in B-29 operations, this was a common occurrence just in traveling through the jet stream let alone dealing with the enemy.

My point was based mainly on cruise. WEP settings are just for that - emergencies or some other event that will get you in trouble right away!

[HoHun]

Hi Flyboyj,

>>I don't picture me traveling 2000 miles over an open ocean at take-off/emergency power anyway.

>You won't unless you have to rapidly climb or decent - put it in B-29 operations, this was a common occurrence just in traveling through the jet stream let alone dealing with the enemy.

You are losing me - I would not travel 2000 miles while rapidly climbing or descending either, and I'd certainly not keep take-off/emergency power applied during a rapid descent as that would take me way past Vne.

>My point was based mainly on cruise. WEP settings are just for that - emergencies.

Oh well - my point was that as soon as you move away from the optimum working point of the carburetted engine, efficiency drops markedly. Even at maximum continuous, which could be used for longer spells, specific fuel consumption had risen by 60%.

It also seems that using automatic mixture control was standard operation procedure even in the B-29 though it had a flight engineer on board whose primary task was to operate the engines. The USAAF doesn't seem to have thought there was something to gain from manual leaning.

In fact, I seriously doubt that one could reduce the minimum 195 g/HP/h by any meaningful amount, but should you want to prove your point, here you can download the B-29 Flight Engineer Manual you might find helpful:

http://www.ww2aircraft.net/forum/oth...tml#post260968 (B-29 Engineering Flight book)

It lists a minimum of 201 g/HP/h at 15000 ft. Apparently, the automatic control knew how to lean with altitude, too. This is actually a higher figure than the minimum from the cruise control chart and reinforces my opinion that the abovementioned 195 g/HP/h are already the optimum. Daimler-Benz list their minimum with a tolerance of +10 g/HP/h - only towards higher figures. That's how I tend to read the US figures, too: no way you're going to shave off another 10% from the 195 g/HP/h figure.

Regards,

Henning (HoHun)