Some Engine Charts

[joeblogs]

It was suggested that one reason for the higher output to weight ratios is that water cooled engines could be run at a higher BMEP (brake mean effective pressure).  In otherwords, water cooled engines could take more supercharging.  Figure 5 shows that more engines with a higher BMEP were liquid cooled rather than air cooled.

[joeblogs]

Figure 6 plots maximum horsepower to weight against BMEP as a crude way to control for differences in supercharging.  Again there is a linear relationship and again the liquid cooled engines tend to dominate when dry weight is used in the calculation.  

The evolution of engine models is also evident in this chart.  Later versions of liquid cooled and air cooled engines (labeled in red) tend to have both higher BMEP and higher ratios of output to dry weight.  Most of the early versions of engines, which are not labeled, are clustered in the lower left hand corner of the diagram.  Even early Merlins have unremarkable power to weight and BMEP.

Notice that some of the really good German engines do not have a particularly high BMEP, but they do tend to have a good ratio of output to weight.  Low BMEP may reflect the lower octane gas used in German engines.  But high output to weight clearly reflects good design.

[joeblogs]

Figure 7 shows the distribution of maximum RPMS (typically at take-off) for air and liquid cooled engines.  Liquid cooled engines can generally be run at higher RPMs than can air cooled engines at this time.  That is usually explained on the basis of cooling and differences in valve gear (pushrod vs cams).

[joeblogs]

I was thinking of Clerget's 14F (a radial) and 16H (a 16 cyclinder V).

-Blogs

Originally posted by HoHun
Hi Joe,


I wasn't aware of the French engines! Do you have some examples?

The Jumo 205 was used not only in the Dornier Do 26 flying boat, but also in the Ju 86 reconnaissance aircraft. (Its high-altitude capability was the main reason for employing it in the latter - it was directly responsible for the development of the HF Spitfire marks.)

Regards,

Henning (HoHun)

[Nashwan]

The Napier Sabre was liquid cooled.

[joeblogs]

Yep yer right, It was miscoded.  

I have corrected the charts and text in the preceeding posts.  

Thanks for pointing this out.

-blogs

Originally posted by Nashwan
The Napier Sabre was liquid cooled.

[Captain Virgil Hilts]

The reason for increased fuel efficiency in the engines using higher octane is that engines requiring higher octane have higher static compression ratios and/or higher boost supercharging systems.

The higher cylinder pressure in engines with higher static compression ratios and/or higher boost makes more efficient use of fuel. It also makes more horsepower and torque (actually horsepower is just the rate at which an engine makes torque, which is the rotational force that actually does the work). This of course assumes all else being equal.

Good examples can be found in the history of the V1710 Allison engines. Later engines had more static compression (the early models were all under 6.5:1, as low as just under 6:1, while later engines were over 6.5:1, some near 7:1) and were run at higher boost levels. The later engines had more horsepower and better fuel efficiency.

In many cases, you find that engines with low static compression ratios run at higher boost levels make more PEAK horsepower. However, they are very slow to accelerate when they are not running at high boost, and they are much less efficient. Engines with lower compression are also more prone to foul spark plugs. Higher boost levels are more prone to cause headgasket and cylinder head failures.

Other things to notice are that turbocharged engines tend to be more efficient than engines with crank driven superchargers. The reason being that the faster you spin a crankdriven supercharger the more boost increases, but also more power is absorbed. At a certain point, the amount of power absorbed will begin to increase much faster than the amount of power gained by the increase in boost.

The turbocharger is more efficient because it uses the heat and expansion of exhaust gases to compress air in the intake tract. Using these gasses does not absorb power from the crankshaft. However, at some point, depending on the size of the turbocharger in relation to the size of the engine, the backpressure in the exhaust will increase to the point where no more power is gained by the increased boost level.

One reason for the increased power per cubic inch of displacement of the German engines was the fact that German engine manufacturers were the world leaders in FUEL INJECTION. True fuel injection eliminates problems like flooding, starvation, and poor mixture distribution.

[dtango]

Great posts joeblogs!

HoHun Said:
It may actually be that the engine control techniques and technology play a big role in getting a better specific fuel consumption. From what I've heard from civilian pilots, improved instrumentation enabled the pilots of C-54/Constellation type transports to hit the point of best economy with much greater precision than the cruder instruments typical for WW2 military aircraft.

I think that sums up some of the things that could be explored regarding the relationship between SFC's and fuel octane.

Here's an insightful article from AvWeb on the topic of fuel mixture, SFC, and engine readings for reference.
AvWeb Fuel Mixture Article

Tango, XO
412th FS Braunco Mustang

[joeblogs]

Originally posted by Captain Virgil Hilts


>The reason for increased fuel efficiency in the engines using >higher octane is that engines requiring higher octane have higher static compression ratios and/or higher boost >supercharging systems.

I think the logic goes the other way.  Higher octane/PN fuels permit higher compression and supercharging because, for such fuels, detonation occurs at a higher temperature.  

>The higher cylinder pressure in engines with higher static >compression ratios and/or higher boost makes more efficient >use of fuel. It also makes more horsepower and torque (actually >horsepower is just the rate at which an engine makes torque, >which is the rotational force that actually does the work). This of >course assumes all else being equal.

A higher detonation temperature implies less need for fuel cooling, that is adding extra fuel that is not fully burned just to cool the cylinder via evaporation of the fuel.

Horsepower and torque are clearly related, but the mapping from one to the other depends on the reduction gearing (if any) that is used.

>Good examples can be found in the history of the V1710 Allison >engines. Later engines had more static compression (the early >models were all under 6.5:1, as low as just under 6:1, while >later engines were over 6.5:1, some near 7:1) and were run at >higher boost levels. The later engines had more horsepower >and better fuel efficiency.

Noticed that pattern in the data.  Bill Gunston points out that it is only recently that you see an aircraft piston engine with a compression ratio of 10.  The Wright's orginal engine had a compression ratio over 4, but they had to go back to just 4 because of overheating (actualy detonation).

I'll have to look into the relationship between compression ratios and fuel economy.  If efficiency means specific fuel consumption, the adantage seems to lie with air cooled radials at somewhat lower average manifold pressures (BMEP) rather than highly supercharged water cooled engines that tend to run at higher RPMS.  

>Other things to notice are that turbocharged engines tend to be >more efficient than engines with crank driven superchargers. >The reason being that the faster you spin a crankdriven >supercharger the more boost increases, but also more power is >absorbed. At a certain point, the amount of power absorbed will >begin to increase much faster than the amount of power gained >by the increase in boost.

>The turbocharger is more efficient because it uses the heat and >expansion of exhaust gases to compress air in the intake tract. >Using these gasses does not absorb power from the crankshaft. >However, at some point, depending on the size of the >turbocharger in relation to the size of the engine, the >backpressure in the exhaust will increase to the point where no >more power is gained by the increased boost level.

Roughly 50 percent of the enegry in avgas is dissipated through hot exhaust.  Overall piston engine efficiency is on the order of about 25 percent (i.e. about one-quarter of the energy in avgas gets to the propeller), so any gains from harnessing energy in the exhaust is imoportant.  The first big gain was the turbo supercharer folowed by turbos that connected to the crankshaft via a quil drive (turbo compounds).

Superchargers were notoriously inefficient in the 1920s and 1930s.  After hiring an engieer with a background in fluid mechanics, Rolls Royce developed some of the best gear driven examples during the mid 1940s.  

Both Wright and Pratt and Whitney began to design their own gear driven superchargers in the early 1940s when they recognized significant gains could be made there if some R&D was spent.  Wright went on to develop its own turbo superchargers, replacing the ubiquitous GE models after the war.

The Germans did not have the most efficient gear driven superchargers during the war.  They would not have had even that had the British not sold them superchargers designed for the Rolls Royce Kestrel in the late 1930s.

One thing the Germans did have, in addition to good fuel injection systems, was a reliable variable speed coupling for their superchargers.  That allowed them to get more out of a single stage supercharger than the Americans (for example) could get out single stage, two speed superchargers.  At the very end of the war, some American engines (including the Allison) were offered with variable speed superchargers.

By the late 1940s, turbo superchargers could capture roughly 20 percent of the energy in the exhaust gases.  In other words, turbos became nearly as efficient as a piston engine.  As soon as that ocurred, it was only natural that the turbo itself would replace the internal combustion engine for many applications.

>One reason for the increased power per cubic inch of >displacement of the German engines was the fact that German >engine manufacturers were the world leaders in FUEL >INJECTION. True fuel injection eliminates problems like flooding, >starvation, and poor mixture distribution.

This is a good point.  Much of that technology ws mastered in the process of developing good diesel engines.

-Blogs

[straffo]

in your 1st chart what is this Arsenal 24H ?

[joeblogs]

It is a postwar French design, a liquid cooled engine with 24 cylinders arranged in an "H" configuration.  

France re-orgainzed its aircraft industry before and after the war.  So all these different names appear even though the actual designs tend to follow from previous vintages of the established firms like Gnome-Rhone or Hispano-Suiza.

-Blogs  

Originally posted by straffo
in your 1st chart what is this Arsenal 24H ?

[straffo]

thanks !

[MiloMorai]

Originally posted by straffo
in your 1st chart what is this Arsenal 24H ?

It was composed of 4 12H cylinder blocks in vertical opposed pairs driving 2 crankshafts geared to a common propeller shaft and displacing 140L.

The Arsenal 12H engine was based on the Jumo213.

H-S also produced a H-24 engine, the 24Z, from 2 of their 12Z type engines.

[straffo]

a monster so

I was not aware that Arsenal made anything else than the VG33 for exemple

[MiloMorai]

Originally posted by straffo
a monster so

I was not aware that Arsenal made anything else than the VG33 for exemple

Slight error, it was the 24H Tandem that was 140L.

The 24H weighed 1900kg and was rated at 4000hp.