Some Engine Charts

[joeblogs]

NOTE! IF YOU DOWNLOADED THE TEXT AND CHARTS PRIOR TO THE AFTERNOON OF FEB 23RD, THERE WAS AN ERROR IN THE DATA.  THE TEXT AND CHARTS HAVE BEEN CORRECTED.


I have been assembling a data set of information on about 80 separate engines of at least 800 CU displacement produced by about 35 companies in seven nations (France, Germany, Italy, Japan, UK, USA, and USSR). There are many different versions of certain engines (Cyclones, Merlins, Double Wasps, etc), and I have at least some information on about 400 different models.

The source data is a series of books published by Paul Wilkinson (Aircraft Engines of the World) published throughout the 1940s.  This source is a little thin on Japanese and Russian engines, so I am looking to augment this with information from other sources (Help!)

I've just begun to analyze this data, but I'll provide some tidbits in several posts that follow this one.

[joeblogs]

Figure 1 presents data on maximum horsepower vs. engine displacement.  There's a pretty obvious linear relationship between the two.  Most engine models with the same displacement generate about the same horsepower.  

I've identified most of the important engine families on the figure.  A vertical sequence of dots typically represents the development of a particular engine (e.g. Merlin or Double Wasp).  Some of these sequences are rather large, indicating how far some good designs could be stretched with better fuels, better supercharging, and internal strengthening.

But there are clearly some outliers.  These include Continental's X-1430 experimental engine (based on Wright field's "hyper cylinder"), Rolls Royce's Eagle and a few very late model Griffon and Merlin engines.  There's also the Napier Sabre VII.  All of those are liquid cooled engines.  Among the air cooled engines, the outliers include an E model of the Double Wasp, and Wright's turbo compound Cyclone 18.

Speaking of differences between the performance of air and liquid cooled engines, there appears to some systematic differences that vary over engine size:

For displacements under 2000 cu, it appears that liquid cooled engines enjoy a slight advantage over their air cooled brethren.

From 2000-2500 cu it looks like a toss-up.

From 2500-3500 cu, the air cooled engines have the advantage, but there are only a few liquid cooled engines that large.  

Among the monster engines--all post war models-- there are two liquid cooled French designs (Arsenal's 24H and Hispano Suiza's 24Z) and two air cooled designs (Pratt and Whiteny's Wasp Major and Bristol's coupled Centaurus).  To my knowledge, the only one of these engines produced in quantity was the Wasp Major.

[joeblogs]

Figure 2 shows the relationship between engine horsepower and weight.  Again there is pretty linear relationship between these two variables for most engines.  Liquid cooled engines have slight advantage here, and for a few engines a substantial one.  They should, as I am using dry weight here (not including the weight of a radiator, coolant, or oil).  Including a radiator and coolant should increase the the gross weight of a liquid cooled power plant by 20-40 percent.  

There are 33 liquid cooled engines with an HP/dry weight ratio of 1 or higher.  These include late war models of the Merlin, Griffon, Allison V-1710, the DB 601E, and the Jumo 213A.  The extreme outliers among the liquid cooled engines include a late model Griffon, Napier's Sabre VII, the post war Hispano 12Z, Continental's hyper engine, and the Russian M107.

The 11 air cooled engines with an HP/dry weight ratio of 1 or higher include late war models of the Double Wasp, Cyclone 9, and the post war models of Bristol's Hercules and Centaurus.

More on this later.  But I'll leave the debate over coolant and form drag to someone else.

[joeblogs]

Figure 3 presents the distribution of Specific Fuel Consumption (lbs of fuel per horsepower per hour) at cruise settings for about 200 engines.

The average SFC is about 0.45.  Quite a few have a worse fuel economy.  Somewhat surprisingly, these consist of many models of the Rolls Royce Merlin and Griffon engines.  I had thought that liquid cooled engines would be somewhat more fuel efficient than air cooled engines, but this is often not the case (I think because the best of these are run at higher RPMs).  

A number of engines have a better SFC, around 0.42.  These consist almost completely of the later versions of Pratt and Whitney's Twin Wasp (1830 cu) and Double Wasp (2800 cu) engines, Bristol's Hercules (2360 cu) and Centaurus (3270 cu) engines, and a few models of Allison's V-1710 liquid cooled engine.  

The most fuel efficient engines, with an SFC under 0.40, are the Turbo Compound version of the Wright 3350 and Pratt and Whitney's Wasp Major (4360 cu).  Both of these engines were not produced in quantity until after the war.

[joeblogs]

Figure 4 shows that that SFC seems to improve with the octane/performance rating of the fuel (the bars for engines rated on 87 octane avgas tend to be further to the right than the bars for engines rated on 100 octane, and so on).  I'm not yet sure whether it's the gas or the vintage of engine that explains the better performance (older engine models are typically rated on lower octane fuels).

[HoHun]

Hi Joe,

Great charts! :-)

>I'm not yet sure whether it's the gas or the vintage of engine that explains the better performance (older engine models are typically rated on lower octane fuels).

It's my impression that it's the fuel. Higher octance fuel contains more energy. If you'd map not specific fuel consumption, but energetical effectiveness eta (the ratio of energy output to energy input), the older engines probably would be a bit closer to the newer ones.

By the way, while eta is a dimensionless value, SFC is not (though it often seems to be listed as such). It's lbs/HPh (pounds of fuel per horsepower per hour). German values are often given as g/PSh (grams per horsepower per hour), so this information helps with conversions :-)

Regards,

Henning (HoHun)

[HoHun]

Hi Joeblogs,

>The average SFC is about 0.45.  Quite a few have a worse fuel economy.  Somewhat surprisingly, these consist of many models of the Rolls Royce Merlin and Griffon engines.  

It's important to remember that the specific fuel consumption is calculated based on shaft horsepower. Some engines like the Jumo 213 produced considerable amounts of exhaust thrust in addition to the shaft horsepower (for the Jumo 213E, this could be equivalent to another 500 HP shaft power at high altitude, high speed and special emergency power). I believe the Rolls-Royce engines were pretty good with regard to thrust, too, though of course at cruise settings it made less of an impact.

(German calculations showed that a turbo-supercharger was better for range than exhaust thrust, but that exhaust thrust gave better high-speed power. The conclusion was that turbo-superchargers were to be used for bombers, while fighters should have ejector exhausts. Since the German turbo-superchargers never really got into series production, this was mostly academical though, and both Focke-Wulf and BMW actually tried to create turbocharged fighters anyway.)

>The most fuel efficient engines, with an SFC under 0.40, are the Turbo Compound version of the Wright 3350 and Pratt and Whitney's Wasp Major (4360 cu).  Both of these engines were not produced in quantity until after the war.

Another very fuel-efficient engine was the Junkers Jumo 205, which achieved 170 g/HPh (0.37 lbs/HPh). Of course, it was a Diesel engine, so it had to be good! :-)

If you're not familiar with the Jumo 205: It was an upright six-cylinder inline engine with no cylinder heads, but opposing pistons pushing into the cylinder from both ends symmetrically. Accordingly, it had a crankshaft on top of the engine as well as at the bottom of the engine, and fresh air and exhaust were routed in and out of the cylinder through slits in the cylinder walls. The fuel was injected from the walls as well, and as a Diesel, the engine relied on self ignition. This was an important feature as it meant no high-tension ignition electrics were required, as in the 1930s the problem of insulating the ignition system for high-altitude operations hadn't been fully solved yet.

Late in WW2, the Jumo 207 came out which even had a turbo superharger, but it seems this only served to increase the high-altitude capability of the engine, not to improve its specific fuel consumption.

Regards,

Henning (HoHun)

[joeblogs]

HoHun -

Can you point me to an equation for deriving eta from sfc and fuel grades?  If you are referring to overall efficiency of the engine I guess we can compare the energy in a pound of fuel to horsepower implied by the SFC number.

I was thinking of doing something different, which is to map all octane numbers into performance numbers using some engineering charts developed in the war.  More on that later.

In the data I have, the only way exhaust thrust enters into the calculation is via a tubo-supercharger, and it is evident that well developed examples have a lower SFC.  

One thing I have noticed is that there is not a really big improvement in SFC until one gets to engines rated at 115 PN avgas.  These are end of war and post war engines.  

A lot of engines rated on 91-92 octane actually have better SFC than engines rated 100 octane.  The reason, I think, is that the former were mature engines developed for commercial markets (Cyclone 9 and Twin Wasp) where SFC really mattered.  The latter were new engines (Cyclone 18 and Double Wasp) used by the military through the war.

As for diesels,I have some data on a number of models, but I did not enter them in my data set.

The best diesels should have a better SFC than the gas engines and that is why both France and Germany developed some excellent examples for flying boats.  

-blogs

Originally posted by HoHun
Hi Joe,

Great charts! :-)

>I'm not yet sure whether it's the gas or the vintage of engine that explains the better performance (older engine models are typically rated on lower octane fuels).

It's my impression that it's the fuel. Higher octance fuel contains more energy. If you'd map not specific fuel consumption, but energetical effectiveness eta (the ratio of energy output to energy input), the older engines probably would be a bit closer to the newer ones.

By the way, while eta is a dimensionless value, SFC is not (though it often seems to be listed as such). It's lbs/HPh (pounds of fuel per horsepower per hour). German values are often given as g/PSh (grams per horsepower per hour), so this information helps with conversions :-)

Regards,

Henning (HoHun)

[Zigrat]

neat stuff!

[Zigrat]

do you have the info stil in excel to? If so I'd love to have it.. the first graph you have there displays linear behavior but there's alot of scatter.. I do alot of statistics work for my research and i'd like to try a multiple linear regression to see if I can add in some other factors like maximum boost to get a better fit.

[Zigrat]

hohun your explanation for why higher octane equals lower sfc doesn't make sense. it really does not contain more energy. the reason for lower sfc corresponding to higher octane is that higher compression ratios could be obtained with higher octane since you prevent detonation, and if you look at the otto cycle you'll see higher compression ratio = higher thermodynamic efficiency.

[HoHun]

Hi Zigrat,

>hohun your explanation for why higher octane equals lower sfc doesn't make sense. it really does not contain more energy

OK, I admit I was just guessing there :-) Thanks for the correction!

Regards,

Henning (HoHun)

[HoHun]

Hi Zigrat,

>the first graph you have there displays linear behavior but there's alot of scatter.. I do alot of statistics work for my research and i'd like to try a multiple linear regression to see if I can add in some other factors like maximum boost to get a better fit.

The scatter is indeed remarkable, especially as the second graph shows much less of it.

I'd say that shows that the old saying "There's no replacement for displacement but more displacement" is actually wrong - boost pressure and engine speed can easily make up for it :-)

However, that the scatter in the second graph is so much lower seems to indicate that using the same technology, you always get close results for the same engine weight regardless whether you concentrate on displacement, boost or rpm.

Regards,

Henning (HoHun)

[HoHun]

Hi Joe,

>One thing I have noticed is that there is not a really big improvement in SFC until one gets to engines rated at 115 PN avgas.  These are end of war and post war engines.  

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.

An important role seems to have been played by the BMEP display (brake mean effective pressure), which gave the pilot (or rather the flight engineer) a very accurate reading of the actual power output of the engines.

>The best diesels should have a better SFC than the gas engines and that is why both France and Germany developed some excellent examples for flying boats.  

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)

[joeblogs]

Yes I have a book that mentions the importance of improved instruments.


I've got some data on BMEP, but it'll have to wait until I get these taxes done....

-Blogs

Originally posted by HoHun
Hi Joe,

>One thing I have noticed is that there is not a really big improvement in SFC until one gets to engines rated at 115 PN avgas.  These are end of war and post war engines.  

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.

An important role seems to have been played by the BMEP display (brake mean effective pressure), which gave the pilot (or rather the flight engineer) a very accurate reading of the actual power output of the engines.

>The best diesels should have a better SFC than the gas engines and that is why both France and Germany developed some excellent examples for flying boats.  

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)