Analysis of the BMW IIIa and the function of Höhengas

[ZachariasX]

This report tries to explain the design principles of the BMW IIIa engine functions and discusses its power envelope in quality as well as in quantity. I try to go more in-depth about the engine than other contributions about the engine. The aim of this essay is to give an understanding of how the engine works and what can be expected from it in terms of performance in various conditions. It is based on all the info I could gather, both in this community and over at TheAerodrome. I am also most grateful for @Holtzauge and his support without which I could not have made this analysis.

 

 

TL;DR:

 

·         The BMW IIIa engine is essentially 260 hp engine that by design is limited to a maximum power output of ~210 hp.

·         Any power in excess of that is not available to the pilot and he has no means of making use of it, not even for a short time.

·         Using alternative fuels, the engine can produce a theoretical maximum of ~235 hp at sea level.

·         The BMW IIIa has fixed fuel jets in the carburetor and it is designed to run lean of peak at lower altitudes. It has a considerably better mileage than traditionally regulated engines like the Hisso or rotaries.

·         The Fokker D.VIIF in FC performs almost the way it should, but the engine does behave like a Hisso8 rather than an “altitude engine” and it’s behavior during over compression might warrant another look.

 

 

I am aware that most of the readers are very versed in engine knowledge. But this is the internet, so for the following discussion I assume no prior knowledge.

To understand the engine, we must first have some basic understanding of the principles of an internal combustion engine as well as an understanding of the fuels used. An engine is only as good as the fuel.

 

 

1.  The principle of an internal combustion engine

 

An internal combustion engine functions essentially as an air pump. The absolute amount of air pumped per timeframe sets the theoretical maximum power output and represents the limiting factor for the power output of any internal combustion engine. Whatever you do, you cannot produce more power than the amount of oxidizer (air) at hand the engine provides. On the other hand, it is trivial to add any practical amount of fuel to the burn.

Any inefficiencies will deduct from this theoretical max power envelope. An ideal engine follows that power envelope.

For the following analysis, the above will be the base to model engine power.

 

 

2.     Fuel

 

The following fuels were used in WW1:

Tab.1

 

Allied----Old Export Grade------------------ ~0.734 SG & 62.0 octane
Allied----New Export Grade----------------- ~0.724 SG & 65.9 octane
Allied----Fighter Grade--------------------- ~0.716 SG & 68.0 octane
Allied----RAF or French Fighter Grade------- ~0.712 SG & 71.0 octane

German-Heavy Benzine--------------------- ~0.748 SG & 67.5 octane
German-Medium Benzine-------------------- ~0.733 SG & 70.5 octane
German-Light Benzine----------------------- ~0.710 SG & 71.2 octane

 

This is data from very detailed report posted by Kacey over at TheAerodrome. It shows that the engineers back then were aware what was in their fuel blends. What they came up with was the suitable balance of what was practical to source, what could provide sufficient power according to engine design and what was practical in handling as well.

Those fuels used are fast burning fuels (unlike the more expensive fuels at your local gas station) that allow only limited compression in the cylinder. To get an idea of octane grade requirements for a given compression ratio, we can refer to:

Tab.2

 

Automotive Gasolines, Recommended Practice, SAE J312 Jan93, SAE Handbook, volume 1:

For a typical carbureted engine, without engine management:

   Compression       Octane Number    Brake Thermal Efficiency       

     Ratio                 Requirement         (Full Throttle)

      5:1                 72                      -
      6:1                 81                       25 %
      7:1                 87                       28 %
      8:1                 92                       30 %
      9:1                 96                       32 %
     10:1                100                     33 %
     11:1                104                     34 %
     12:1                108                     35 %

 

 

Using this table, we can see that above 5:1 compression, things will get difficult for most WW1 fuels. Thus, engines compressed beyond said safe limit were called “over-compressed” in some instances. As stated by the table above, thermal efficiency increases considerably when upping compression.

 

 

1.     BMW IIIa engine design principle

 

 

The BMW IIIa engine, as well as other German engines of the time, have no mixture regulation at all. It is not automatic, as some might put it. It is a fixed jet with a fixed capacity, being only regulated by the Venturi of the carburetor. Hence, the air-fuel mixture will vary according to the surrounding atmosphere. As a gross simplification, at half air density (~5000 m altitude), the engine will have twice the fuel flow for the same amount of air pumped through the engine, making the mixture twice as rich. This is actually somewhat reflected in this game (FC), where you have to retard the mixture lever some 50% from best power setting at sea level.

 

Using that rule of the thumb, then if I have an air-fuel ratio of 12.5 that gives me most power at sea level, this makes for an air-fuel ratio of 6.25 at 5000 m. This means the engine is running incredibly rich (and inefficient) and the pilot has no means to rectify that.

The solution that is incorporated in the Höhen-engines is that those engines run far lean of peak. The BMW IIIa is set for a sea level air-fuel ratio of 22 (!) as seen later in Fig.3 later on. This means, for ideal mixture, the aircraft has to climb to about 3500 m. Lower than that, the engine will run very lean and has this a far better economy especially than the rotaries, that generally run very rich.

 

In consequence, this particular design feature gives the “altitude engines” an incredible fuel efficiency compared to other engines of the time.

 

 

4.     BMW IIIa power curves

 

After the war, the British and the Americans (like in this popular post war NACA report No.135 where the graph is from) made extensive tests on these engines. The power vs. altitude (ambient air density) chart of the BMW IIIa looks like this:

Fig.1

 

 

For the most part, the engines power output scales linear with rpm as well as air density, consistent with our initial assumptions (1). Deviations occur toward the extremes, the left side of the chart as well as the top graph representing 1600 rpm power output falling short. The latter point can be explained by the engine having reached its practical rpm limit. (This is also reflected in the BMEP graphs in that report but that are off topic for this discussion)

For the sake of simplicity, I will now only consider the 1400 rpm graph, as 1400 rpm is the practical limit of the engine in terms of rpm.

The left part of all graphs reflect two distinct design features of the engine:

 

1.       It is “over compressed” and excessive chamber pressure will cause a collapse of the power output caused by pre-detonation.

2.       The engine is designed to run lean of peak and does not regulate the fuel jets in the carburetors at all. The non-linear power drop-off at air densities higher than 0.055 lbs per cu. ft. reflect the significant leaning of the mixture under these conditions.

 

 

The first finding proves that when compression of about 5.7 is reached, pre-detonation sets in to a degree that makes the power output collapse. I would speculate that slightly before that compression is reached, you will start to hear the knock and when you advance the throttle further, pre-detonations sets in with said results.

 

This first finding also tells me that at 1400 rpm, I cannot pass beyond 210 hp power output. Should I advance the throttle more, then I do not get just more “knock” and more power, but much more “knock” and much less power. Actually, I get some 195 hp while actively destroying my engine in short time.

 

 

The second design feature is more subtle, but has equally important consequences. It means that, in principle, if I have half the atmospheric pressure, I will still get a roughly similar fuel flow than at sea level. Thus, the air-fuel ratio is only about half at said altitude!

Fig.2

 

 

Looking at the standard textbook (from the interweb) curve of Power vs. mixture, we can see that a combustion engine prefers a certain mix ratio of air and fuel. In the depicted case of automotive gasoline, best power is obtained with a slight excess of fuel in the burn. At the stoichiometric ideal mixture, the burn temperature is the hottest, dropping both toward lean or rich. Also, the speed of the burn and the burn temperature drops, especially when running the engine lean.

The BMW IIIa, as detailed in (3), is designed to run at a very lean mixture. Throttle and Höhengas all open produce an air-fuel ratio (AFR) of 22, Höhengas closed will produce an AFR of 20, as shown here in Fig.3:

Fig.3

 

 

The engine hence runs most of the time with a mixture lean of peak and according to Fig.1. Doing so, it runs at a mixture that does not produce 100% power efficiency.

 

If a mixture is set to an AFR of 22, then at ~5000 m with half ambient pressure, the carb will produce an AFR of ~11, that is slightly rich of peak (that is probably AFR=12.6, depending on the fuel type used). Hence, the engine starts out at ground level with a relatively inefficient, far lean of peak mixture (on the far right-hand side of the red power graph of Fig.2), but as the aircraft climbs, the mixture will shift left on the curve of Fig.2, constantly improving its burn efficiency until it reaches best mixture around 4000 m.

 

A note to this: It becomes evident that the altitude where the engine runs most efficient is dependent on local weather! It also is dependent on the fuel type used. If we were to run the engine ethanol (with a corresponding octane grade of ~120!), then I had to climb higher until AFR 9:1 is reached, or with methanol even higher to reach AFR 6.74. This is about 9000 m. Fuel used is absolutely critical for the engine to work as intended. To set the altitudes again, I had to replace the fuel jets in the carb with higher capacity ones. The only way to adjust this is to exchange the fuel jets installed in the carburetor.

 

 

4.     Consequences of running an engine lean of peak with fixed fuel jets.

 

Looking at Fig.1, we can see the power curve dropping off in the left third of the graph. This is the result of running rich of peak at lower air density. Knowing about this intrinsic property, I can project the theoretical power output of the engine as if it would be not limited in any practical way by mixture or compression:

Fig.4

 

 

 

 

If I extrapolate the power graph from the (more or less) linear part on the right-hand side onwards and upwards, then it intersects roughly at the 260 hp power mark for corresponding sea level performance. I can also extrapolate the impaired, actual power output curve past the pre-detonation limit and get the maximum power output the engine would deliver, as if there was no pre-detonation happening.

 

Hence, it shows that the BMW IIIa is in fact a 260 hp engine that is limited to 235 hp max output at sea level due to the carburetor concept used, if there was no pre-detonation limiting the practical maximum power output to some 210 hp. This is more than just semantics, as a 260 hp engine has a different power curve than a 235 hp engine. This will become apparent as we discuss altitude performance later on.

This understanding is about the only way to give the sometimes stated 235 hp as “maximum power” some meaning. The engine can never reach that power output, unless it was fed with at least 85 octane fuel. No changes on the engine itself (that can be assumed as possible back then and that still make this engine a BMW IIIa) can make the engine reach that power.

 

There is a Hermann Hermann Göring quote, him saying that they would lace their gasoline such that they could fly “full power”. So let’s say Indeed that was the case and the Jasta was ready to put up with the trouble that benzene brings to fuel handling. Let’s say these 19 year olds were willing to run an engine rated for 185 hp at 235 hp and accept a worsening of an already terrible service rate (engines back then do not compare with what we are used today). That is what he would have gotten: Some 10% added power where it is of little use operationally at the cost of reduced service rates. I doubt that Hermanns bragging was widespread use, as it not only goes against regulations, but also against operational use.

 

 

Let’s go back to the chart. We can transpose this power drop (loss in efficiency) from 260 hp to 235 hp (Fig.4) to the curve from Fig.2 :

Fig.5

 

 

The green triangle is the power penalty imposed on the engine by letting it run lean of peak. We start out at AFR 22 and we know that the actual penalty is ~10% power, because the engine theoretically only reaches 235 hp, and not 260 hp. We don’t have to be all to exact here, but for the purpose of an illustration of the issue it shall suffice.

 

That green triangle is visible as well superimposed on the power chart of Fig.1:

Fig.6

 

 

It becomes apparent that the BMW IIIa in no way really suffers from running at a very lean mixture. It even makes the Göring claim slightly pointless. The green triangle is largely outside the 210 hp power and 80% ambient air density envelope. This means, the real downsides only occur at ratings that are not and should not be available to the pilot.

 

 

When I trace the 1400 rpm power graph and transpose the values into (what I think is) a more illustrative chart by matching power with altitude (of a standard atmosphere), we get this:

Fig.7

 

 

This is the normal power [hp] vs. altitude [m] chart. (Somewhat exact.) It is of note that the NACA chart shows power readings up to what corresponds some ~7200 m altitude. This in the real world very much depends on local weather! For the rest I just linearly extrapolated the NACA chart.

I can compare that now to the maximum performance that is theoretically possible, by just having the seal level performance as a function of atmospheric decrease:

Fig.8

 

 

We see that the power curve nears the maximum theoretical power envelope near where the engine reaches the most efficient AFR. The relative efficiency between the actual power curve from the reference chart and the maximum theoretical power gives me an efficiency curve analogous to Fig.2:

Fig.9

 

 

This is the mixture efficiency curve of the engine with both throttle and Höhengas wide open. We do not reach 100% efficiency, showing off my tracing abilities. But it comes close enough for our purposes. A tracing error of ~2.5% is not really relevant for this discussion.

 

Having this curve, I can plot the simulated power curve to check my readouts:

Fig.10

 

 

With this solid orange graph, I have functionally simulated the power output of the engine with throttle and Höhengas fully open, now also capped at 210 hp, as the engine cannot deliver more at the hands of the pilot.

 

A comment to this curve. The increasing drop from theoretical power to actual power at altitude results from two main factors: the efficiency drop of the engine from running at a mixture rich of peak and the other is the effect low ambient pressure has on the engine efficiency as a whole. The first part is specific to engines like the BMW IIIa and the later applies to all engines; also the ones that feature a mixture lever. How pronounced this latter effect is depends on the engine design, reflecting, compression, rpm, etc.

 

If we were to separate the effects in the chart, it would look something like this:

Fig.11

 

 

This is just an arbitrary illustration, but it is supposed to show where what kind of power loss can occur. The same projects on the mixture curve as well:

Fig.12

 

 

The orange mixture curve hence is a composite from several factors and cannot be directly used for simulating other German engines that function after the same principle (like the Mercedes IIIa). The black line would reflect what a mixture curve could look like without low air pressure effects. Compression ratio has a big impact on retaining performance at altitude, hence only engines that compare in specs like compression and rpm would be suitable for using the same efficiency curves.

 

The same applies for the right hand side of the graph, but there, the altitude effects are very small and it is almost exclusively a drop due to lean mixture related efficiency drop. This is why I did not draw the black line all the way to the right.

 

Now, why going through the pains of simulating what I have on the chart as original data anyway? This is because the engine has two levers, the throttle and the Höhengas. Knowing how the engine preforms allows me to simulate the engine power curve with Höhengas closed. This is the one that I have to produce myself, as there is no such power chart.

 

As a first step, we know the mixture settings that the throttle lever alone can give us. It is shown in Fig.3 above. What I do now is I trace the mixture at respective altitudes on the curve of Fig.9, extending that curve.

Fig.13

 

 

The mixture range of the throttle alone is shifted slightly to the rich side (left). At sea level, we start at the right-hand side of the curve, moving left as we climb.

 

Now, the Höhengas is a very particular arrangement. It adds two further carb barrels to the three that are controlled by the throttle lever. Having Höhengas closed restricts manifold pressure to by 20%, as for instance detailed in the NACA report:

Fig.14

 

 

If I assume 20% power loss for the reduction of airflow to 24 inches MAP, I can simulate the whole power curve vs. altitude that is obtained on the throttle alone by normalizing that to the corresponding AFM efficiency:

Fig.15

 

 

The solid blue line is the simulated power output; the dashed blue line is the theoretical maximum power according to the initial assumption (1). Hence, I get 192 hp in this simulation for actual takeoff power. If the engine ran at best mixture, I would theoretically have 208 hp.

 

This chart shows that the power differential between full throttle and full throttle plus full Höhengas is not directly an assumed 20% power reduction due to the throttling to 24 inches MAP, but it’s actually a tad less, since the engine has less penalty due to running less lean. Conversely, at altitude, just flying on the throttle creates a larger power penalty than having Höhengas open.

 

It is of note that this simulation does not take into account for self-reinforcing power loss due to altitude effects beyond 7000 m, hence the nominal values are very much on the generous side and tend to converge at a similar point for 0 hp power output. In reality, this cannot be the case, as a certain minimum of power is required to keep the engine running. The right-hand side of the chart is definitely on the optimistic side of things.

 

Taken together:

The throttle arrangement separate from the Höhengas does two things:

 

•         It acts like a choke providing a richer mixture for ground and low altitude operations, making them both more reliable and more efficient.
•         It ensures the engine not really exceeding rated power.

 

On takeoff with just throttle, I have 192 hp. After takeoff, I can then gradually open the Höhengas, pushing the power output a tad above 200 hp. Should I be less nervous, I can just wait until I am above 2000 m and then just open that lever fully. How much you want to push such an engine above its rated 185 hp is up to the pilot. But 210 hp is a hard limit.

 

As an operational consequence, in flight reducing the throttle or Höhengas has an influence on the mixture in the engine. If I fly with both levers forward and retard the Höhengas, I get a richer mixture and hence more power from the same amount of fuel. If I retard the Throttle instead of the Höhengas, I get a leaner mixture.

 

 

5.     Comparison to other engines

 

Siemens-Halske Sh.III

 

Another fancy German engine is of course the Siemens-Halske Sh.III. That one is rated at 160 hp. As we shall see, it is also a very surprising engine. Although rated at 160 hp, it also features a power graph trajectory (in a way I made it for the BMW IIIa) that makes it a 240 hp engine. Here an illustration from “Der Motorwagen”, Oct. 1919:

Fig.16

 

 

British post war tests confirmed that chart on actual engine runs and I can extrapolate these figures analogous to Fig.6. This yields exactly the chart of Fig 16 above.

 

The Sh.III does not have a “Höhengas” arrangement. It has a fine adjust mixture lever (as other rotaries), plus the provision of a pin that can be inserted to restrict the throttle range to some 80%, capping power to some 200 hp on takeoff. For high altitude flying, the pilot can remove the pin (on the ground), but then has to take care not going past 1400 rpm, or the engine will not only suffer due to excessive power output but the performance will also cap out above ~200 hp and drop from there.

 

Assuming perfect mixture and efficiency and giving the engine 190 hp power cap that the pilot should not exceed (or he might walk home), the power chart looks like that then:

Fig.17

 

 

I capped the engine at 190 hp because the chart looks still healty at that power. From then on, the BMW will retain a slight edge, as it follows a 260 hp power curve vs the 240 hp power curve of the Sh.III. Takeoff power on the BMW with only the throttle will give about the takeoff power available to the pilot with a Sh.III.

 

That the green curve is eventually surpassing the orange (BMW) line is an artifact of the simulation due to the BMW engine graph reflecting adverse altitude effects (as discussed in Figs.11 and 12) to a higher degree. We can safely assume the BMW engine always be superior to the Sh.III in terms of net power output.

 

 

Hispano 8B

 

The Hisso8 had usually some 220 hp output and it featured a mixture lever. I shall simulate it with maximum efficiency:

Fig.18

 

 

The red dashed line represents the generic 220 hp Hisso8, the green line the Sh.III and the orange line is the BMW IIIa. Only below 2000 m can the Hisso surpass the competing engines, quiet marginally so from 500 m on. At 3000 m, the Hisso8 only has 152 hp (ideally!) while the BMW has 175 hp. If anyone needed a reason why the Entente wanted all Fokker D.VII in the garbage compressor, then a 15% power margin at operational altitude will do. The chart also shows how well performing the Siemens-Schuckert SSW D.III was. That aircraft itself is aerodynamically highly competitive. Together with that engine, it is certainly more than a match for the common 220 hp SPAD.

 

 

6.     The BMW IIIa in FC

 

On the forum spec sheet of the Fokker D.VIIF, it says:

 

“[…]The Fokker D.VII was generally equipped with the Mercedes D.IIIa engine, but a new BMW engine type with the D.VIIF designation was also fitted. This new high compression BMW engine with high altitude control gave the D.VII much better performance. Its climb rate was almost twice that as the Mercedes-equipped version, and when flown at maximum throttle the engine was capable of generating almost 250 horsepower at ground level for a short time. With a nominal rating of 230 horsepower and a newly designed carburettor, this engine very much improved the aircraft's high altitude performance and pushed the aircraft’s performance to a new level. However, there was a persistent shortage of BMW engines, and as a result, only every third or fourth aircraft had this type of engine installed.

 

This BMW IIIa engine had a special control lever - "Höhengashebel" - which was used to gain more power at higher altitudes. Engaging it at lower altitudes could lead to engine malfunction (detonation). […]”

Emphasis mine.

 

It is impressive again how diligent research done by the dev team is. Without losing many words on the peculiarities, they got it right that the engine in principle certainly is performing to higher specs than 230 hp. Whether the engine theoretically does 250 hp or 260 hp is largely academic, as the differences imposed on the simulation will be near tracing accuracy of the original chart. Also, the engine is most likely based on the NACA report reflected in Fig.1.

 

However, some subtleties cannot be reconciled with the original data shown above. 230 hp as “rated power” I have not seen yet on any chart. Much rather the 185 hp as stated in Fig.1.  Fig.1 also shows that 250 hp on ground level not likely. Opening Höhengas at sea level does not just cause engine malfunction, it causes sever power drop and engine malfunction, as pre-detonation acts against engine timings in the worst way possible.

 

To go beyond 210 hp, one would require higher-grade fuel than available at the time (see Tabs.1 and 2). No matter how you strengthen the engine to withstand 250 hp power, the fuel at hand will cause pre-detonation and limit power to the same 210 hp unless you use non-conventional fuel.

 

Taken together, the D.VIF in FC is certainly simulated well enough for out gaming purposes, but it does not behave like a BMW IIIa. What we should see is actually the power output ratio between Höhengas on/off vary between altitudes, as the engine runs on different mixture settings and hence at different efficiencies.

 

In the simulation, using nominal 20% power penalty for Höhengas off, then I have only 9% power drop at SL, and 24% at 2000 m. The game does not reflect that. When I go the FC and take the D.VIIF to seal level (autumn map), then I get 193 km/h on the throttle and 204 km/h with Höhengas on. (It fails quickly after reaching max. speed). As a linear speed increase relates to a power increase to the cube, then this results in an (204/193)^3 -> 18% shaft power increase at sea level. This fits well if the engine is not simulated as running “lean of peak” and hence does behave like a Hisso8 rather than a German altitude engine.

 

Now how about a possible max. power output for the BMW IIIa in FC? The Fokker D.VII reaches a speed of 189 km/h at sea level. The Fokker D.VIIF reaching 204 km/h on both throttle and Höhengas fully open. This makes the D.VIIF produce 180*(204/189)^3 = 226 hp at sea level with throttle and Höhengas open, engine knocking. This is again very much a conservative projected power output analogous to Fig.4 that is also somewhat in line with the ultimate power you can draw from the engine if there was no pre-detonation.

 

Hence, by using the correct charts, the Fokker D.VIIF performs how it should even though the engine behavior is not really consistent with the actual engine. I also take the text in the spec sheet as a misunderstanding of the design principle of the engine. This is of little consequence for the game of course, but not helpful for the ones that have more interest.

 

 

The only thing that is really particular about the game’s engine simulation, is that pre-detonation does not cause power loss, but just triggers the timer. I figure if power loss would be simulated, players would be far less greedy on the throttle. (Did someone say P-40?) Maybe if @AnPetrovich gets up one morning and happens to be incredibly bored, he might want to implement that behaviour?

 

 

Edited November 28, 2021 by ZachariasX

[US103_Baer]

Interesting, thanks.

Note. In the comparison with Hispano-Suiza, you've used the nominal output, not max for the Type 35 S.

The factory figures as given in elsewhere in the forums are max 238hp at 2240rpm ( https://forum.il2sturmovik.com/topic/74661-data-showing-incorrect-engine-used-in-the-spad-xiii-its-impact-on-performance-proposed-corrections/ ).

NACA has a test giving 234hp.  Will dig it up if you want. Of course these will drop with altitude but starting with 238hp is a lot better than starting with the nominal output of 220hp.

Edited November 29, 2021 by US28_Baer

[Zooropa_Fly]

Holy sh*t !

[1PL-Husar-1Esk]

Nice research you did, Greg also futured those engines on his yt channel ,  yours get to me better.  I also wish we had more advanced combustion engine modeling in the game featuring  engine  knock and its sound, vibration and exhaust. More feedback than just timers with technochat and addintinal engine breaking sounds after damage done on top of ideal engine work sound. 

[BlitzPig_EL]

Fascinating read sir.  Thank you.

So, in essence, our D.VIIF is not too far off the mark as far as it's performance goes, even if some of the granular operational details are not quite correct.  Is that a fair assessment?  

 

Also, thanks for mentioning the P40, you know it's my baby.

 

Interesting also is the octane levels of Great War aviation fuels.  As some of you may know, I work on classic cars for a living, so I have some knowledge of automotive fuels over the history of the car.  In the US, we didn't see octane levels approaching those numbers till the late 1930s, when 66 octane became available as "high test" as it used to be called.  It wasn't till after WW2, thanks to advances in the production of aviation fuel, that we saw octane levels rise above 70 for motorcar fuel.

[J99_Sizzlorr]

Two more graphs on the BMW 185hp engine one with Benzol and the other with Benzin....

 

And some graphs for the Mercedes D.IIIaü for comparison also with consumption for Benzol and Benzin

 

Edited December 1, 2021 by J99_Sizzlorr

[ZachariasX]

1 hour ago, BlitzPig_EL said:

So, in essence, our D.VIIF is not too far off the mark as far as it's performance goes, even if some of the granular operational details are not quite correct.  Is that a fair assessment?  

That is my impression as well.

 

1 hour ago, BlitzPig_EL said:

In the US, we didn't see octane levels approaching those numbers till the late 1930s, when 66 octane became available as "high test" as it used to be called.  It wasn't till after WW2, thanks to advances in the production of aviation fuel, that we saw octane levels rise above 70 for motorcar fuel.

Actually, I was surprised about that as well, coming from a similar background with classic cars.

[No.23_Starling]

This is amazing! Could you do a comparison between the overcompressed AU Merc engine and the 1917 version? Am keen to understand the difference it would make to the Albi Dva and DVII, and the difference in performance by varying alts (does the over compression make lower alt performance worse?).

 

Likewise, I’d love you to follow up on Baer’s post on the 220hp HS engine (235hp+ output). I’ve read somewhere that the late SPAD XIII should have a performance edge on the DVIIF under 2km (maybe from Greg’s video?)

[ZachariasX]

10 hours ago, US93_Rummell said:

This is amazing! Could you do a comparison between the overcompressed AU Merc engine and the 1917 version? Am keen to understand the difference it would make to the Albi Dva and DVII, and the difference in performance by varying alts (does the over compression make lower alt performance worse?).

I can try that, but in essence it would duplicate (in it's own way) the chart that @J99_Sizzlorr posted above (thanks a lot for those charts!!), as the full power enveloppe I always traced from those charts. It is the power chart for "throttle only" that is really the original part of my above write-up.

 

I think Greg is certainly not far from any thruth is his videos. Hence, nominally I'd agree with his assesments. I do have more problems sometimes with him contextualizing his findings that sometimes ask IMHO for drawing wrong analogies. A through understanding of something should enable you to draw the right analogies to gauge other related issues.

 

In the case of the BMW engine, for instance by ignoring the fact how the carburettor is designed I can make up the claim that the idea of putting in benzene as fuel a great, because you "could take off with 260+ hp power". And it was supposedly just that what Göring was bragging about. But in reality it would be stupid to tune the engine like that, because at altitudes where your purpose of flying in the first place is, you'd have less power then with the original configuration. Only if you make it a totally different engine by giving it a different carb with mixture regulator, you'd have a 260 hp engine the way the Hisso8 is a 200/220/235 hp engine. And then you have to further ignore how smart it is to have an inherently unreliable engine (according to our standards today) going 40% above rated design power.

 

Greg seems to be pretty agnoistic about such, hence for instance he takes the P-40 overboost issue at face value and makes power estimates that I find a tad optimistic. MAP is not a direct funtion of engine power, it merely correlates. Your engine will be reacting to the current density altitude, something he does briefly mentions in his new video, but not in the context of what it means when you operate an engine at the edge of its capacities in either very cold or very hot climates. Also, if certain power curves were never produced, that tells you something about what works on the bench with the engine hooked up on the dynamometer and what just doesn't. Because it is an easy thing to do, unless you are convinced of a certain outcome. Pilots may have done what they said they did. But there are sometimes a lot of "if's" to substantiate certain deductions from that. While they may have pulled a duck ton of MAP, I still put a question mark on the deduction that they hit that supposed projected power output. Any engine chart (as does the BMW IIIa chart!) will show you how the real engine does not scale power linear anymore with MAP and rpm at the edges of the design enveloppe.

 

I have gotten wary of and classic engine hitting any supposed power figure. I'm happy if they just run.

 

I know that is the kind of doubt that doesn't sell well on YouTube, but all my experience with old engines tell me that hooking them up to a dynamometer will produce two things: It will hurt the engine (because you apply max. torque at very low rpm already and then you have to repair it again) and the power reading will be a disapointment.

 

The great charm about any form classic locomotion is that it is a lot of fun even when doing it in slow and careful manner.

[Chill31]

German 6 Cylinder Engines

 

Here are all of the documents I have on German 6 Cylinders

On 11/28/2021 at 9:10 AM, ZachariasX said:

On the forum spec sheet of the Fokker D.VIIF, it says:

 

“[…]The Fokker D.VII was generally equipped with the Mercedes D.IIIa engine, but a new BMW engine type with the D.VIIF designation was also fitted. This new high compression BMW engine with high altitude control gave the D.VII much better performance. Its climb rate was almost twice that as the Mercedes-equipped version, and when flown at maximum throttle the engine was capable of generating almost 250 horsepower at ground level for a short time. With a nominal rating of 230 horsepower and a newly designed carburettor, this engine very much improved the aircraft's high altitude performance and pushed the aircraft’s performance to a new level. However, there was a persistent shortage of BMW engines, and as a result, only every third or fourth aircraft had this type of engine installed.

 

It is impressive again how diligent research done by the dev team is. Without losing many words on the peculiarities, they got it right that the engine in principle certainly is performing to higher specs than 230 hp. Whether the engine theoretically does 250 hp or 260 hp is largely academic, as the differences imposed on the simulation will be near tracing accuracy of the original chart. Also, the engine is most likely based on the NACA report reflected in Fig.1.

 

However, some subtleties cannot be reconciled with the original data shown above. 230 hp as “rated power” I have not seen yet on any chart. Much rather the 185 hp as stated in Fig.1.  Fig.1 also shows that 250 hp on ground level not likely. Opening Höhengas at sea level does not just cause engine malfunction, it causes sever power drop and engine malfunction, as pre-detonation acts against engine timings in the worst way possible.

See the attached chart.  There is definitely a flavor of BMW fitted to Fokker D7 aircraft that were producing 230 hp.  My previous post has links to all of the 6 Cylinder engine docs I have for German engines.

 

Edited December 2, 2021 by Chill31

[Chill31]

On 11/28/2021 at 9:10 AM, ZachariasX said:

In the simulation, using nominal 20% power penalty for Höhengas off, then I have only 9% power drop at SL, and 24% at 2000 m. The game does not reflect that. When I go the FC and take the D.VIIF to seal level (autumn map), then I get 193 km/h on the throttle and 204 km/h with Höhengas on. (It fails quickly after reaching max. speed). As a linear speed increase relates to a power increase to the cube, then this results in an (204/193)^3 -> 18% shaft power increase at sea level. This fits well if the engine is not simulated as running “lean of peak” and hence does behave like a Hisso8 rather than a German altitude engine.

Can you get more power out of an engine than the fuel mixture provided to the engine?  I don't know the answer here...but the power of an engine is derived from extracting energy from the fuel by burning it, right? So if I look at the chart below, the X-axis is the weight of the fuel/air mixture.  Higher weight = more power available.

 

Assuming the plots correlate to throttle position (big assumption), the difference between the full-throttle position with no altitude control and the full throttle position with full altitude control is 27.3-24.3=3 lbs.   3lbs/24.3 = .123.  Based upon that 12.3% increase, I would be very surprised to discover a power increase of 20% using by using full throttle + altitude control.  Thoughts?

[ZachariasX]

 

5 hours ago, Chill31 said:

Thoughts?

You are quiet right, hence I wouldn't discount 20% power from the net power output, but from the 260 hp power trajectory and take this as a baseline. And this is why I get almost 190 hp from the throttle alone instead of ~182 hp. The differences in fuel flow I accounted for by normalizing the efficiency curve, and you can see the blue graph in Fig.13 starts to the left of the orange graph. It is the net efficiency accounting for fuel flow as well, as it is devised directly from the actual power chart.

 

In my model I used the airflow as baseline for power and deducted observed efficiency loss at given mixture ratio. This is far easier than if I stared with just the fuel flow, where it is difficult to gauge the efficiency. I tried that as well (and again went the way of the coyote).

 

All numbers of course come with the caveat that I indeed could reach 235 hp on benzene at sea level (or the corresponding density altitute, of course). That number I find on the optimistic side of things but it is often quoted, hence I used it as I could make it fit. And it produced all results to be in a plausible range. In my model, the less power you give that setting, the more you lift what you get on throttle alone, to the point where thottle alone will get you more power than both lever open. Hence, your idea is very much in line with my simulation, I just started on a different premise.

 

The main rationale of my work was to seek an answer to the question of how much Höhengas can contribute to the power output down low. And both ways to look at it, yours and mine, say that it is less than one might think at the price of actively hurting the engine. And this is why I think Göring was, if they were doing as he recounted, bragging, and doing as he said is actually less cool than he makes it appear. It's not likely the lever that saved you from the nasty SPAD on the deck, but it was probably advisable to have something else up your sleeve as well.

[Chill31]

3 hours ago, ZachariasX said:

All numbers of course come with the caveat that I indeed could reach 235 hp on benzene at sea level (or the corresponding density altitute, of course). That number I find on the optimistic side of things but it is often quoted, hence I used it as I could make it fit. 

In the BMW document I linked above, the British report says they ran the engine at 90% throttle for an hour!  After 10 hours of start and stop running, a cylinder blew off.

 

According to the chart I posted above, they were getting 234 hp at 1400 rpm.

 

The full report is interesting to read regarding the operation of the  carburetor as well as the testing of the motor.

[ZachariasX]

49 minutes ago, Chill31 said:

The full report is interesting to read regarding the operation of the  carburetor as well as the testing of the motor.

Most of those docs I didn't have so far, thanks a lot!!

Edited December 2, 2021 by ZachariasX

[JGr2/J5_Klugermann]

Thank God I am only interested in flying the planes.

[Holtzauge]

About the 234 hp at SL: The British post war test did go up to those levels but for those tests they were spiking the fuel with 20% Benzol and for the 90% power torture run they used 50%!

 

Question is how far can you go on the best German aviation fuel (without additives) they had at the time which was “Leichtbenzin” with an octane rating of 70?

 

In the tests NACA did after the war they did not run the engine with Benzol added and IIRC then they did not take out more than slightly over 200 hp above an atmospheric pressure in the test chamber that corresponded to around 2000 m altitude. So when I simulate the Fokker D.VIIF I’m assuming a constant slightly over 200 hp up to the FTH of a bit over 2000 m.

 

I don’t know much about engines but I would assume that if a German pilot in WW1 running on standard Leichtbenzin opened up the Höhengas throttle he would not only hurt the engine through the detonation, he would not be gaining all the 234 hp either. Question is how much power do you lose when you get detonation? I would assume it’s a sliding scale with a rather minor loss as it’s starting and then quite a substantial one when you get the really bad knocking just before the engine gives up the ghost.

 

My most recent experience with detonation was in the early 90’s when someone I was riding with in a SAAB refused to downshift from 3rd to 2nd after taking a sharp corner and instead insisted on torturing the engine by going full throttle and ever so slowly managing to accelerate the car!

 

Since we have MW50 and 150 octane fuel options for the WW2 birds then maybe a Benzol option for the DVIIF? Grabs hat starts running………

 

Edited December 2, 2021 by Holtzauge

[Chill31]

39 minutes ago, Holtzauge said:

About the 234 hp at SL: The British post war test did go up to those levels but for those tests they were spiking the fuel with 20% Benzol and for the 90% power torture run they used 50%!

 

In the tests NACA did after the war they did not run the engine with Benzol added and IIRC then they did not take out more than slightly over 200 hp above an atmospheric pressure in the test chamber that corresponded to around 2000 m altitude. So when I simulate the Fokker D.VIIF I’m assuming a constant slightly over 200 hp up to the FTH of a bit over 2000 m.

I think the Germans may have been using better fuel that leichtbenzin? Sizzlors post above looks like German testing with Benzol...

Edited December 2, 2021 by Chill31

[Holtzauge]

12 minutes ago, Chill31 said:

I think the Germans may have been using better fuel that leichtbenzin? Sizzlors post above looks like German testing with Benzol...

For sure, testing could have been done with the better stuff but question is what did they have in the cans out at the Jastas? This becomes a bit like the infamous 150 octane/1.98 ata wars that raged over in the WW2 forums: What fuel were they running the engines on on a regular basis? Did some of the Jastas have Benzol? OTOH the Höhengas is meant for just this purpose: The intention is not to use it until they got up to altitude. If the idea was to use Benzol on a regular basis, then why bother with the complexity of the Höhengas?

[ZachariasX]

5 minutes ago, Chill31 said:

I thinknthe Germans may have been using better fuel that leichtbenzin.  Sizzlors post above looks like German testing with Benzol...

Benzol was known to improve knock rating. The problem with it is more of a practical nature. You cannot store it easily (it tends to form raisin) and it eats your rubber sealings in your piping and then you get less mileage from the fuel (also shown in thoise graphs). Naphta is a popular and potent solvent. None of which are a problem on the bench, but operationally it is up to you if you want power where you don't need it at the price of such hassle. It is only below 1200 m or so where the fuel really makes a difference. And all you can get is some 10% power at the maximum. Knowing that you are still get power above rated power conventionally it comes down to the question "do I want to deal with all that crap to go from 10% excess power to 20% excess power?" How quickly the engine blew on the British test bench shows what "reducing TBO" can mean.

 

They could also run the engine on pure alcohol and get all the power they want (they would have to put different fuel jets in the carb) at the cost of a lot of mileage and handling of the fuel would be far, far easier. (Well, if the gang wouldn't drink it when nobody is looking.)

[Holtzauge]

27 minutes ago, ZachariasX said:

Benzol was known to improve knock rating. The problem with it is more of a practical nature. You cannot store it easily (it tends to form raisin) and it eats your rubber sealings in your piping and then you get less mileage from the fuel (also shown in thoise graphs). Naphta is a popular and potent solvent. None of which are a problem on the bench, but operationally it is up to you if you want power where you don't need it at the price of such hassle. It is only below 1200 m or so where the fuel really makes a difference. And all you can get is some 10% power at the maximum. Knowing that you are still get power above rated power conventionally it comes down to the question "do I want to deal with all that crap to go from 10% excess power to 20% excess power?" How quickly the engine blew on the British test bench shows what "reducing TBO" can mean.

 

They could also run the engine on pure alcohol and get all the power they want (they would have to put different fuel jets in the carb) at the cost of a lot of mileage and handling of the fuel would be far, far easier. (Well, if the gang wouldn't drink it when nobody is looking.)

 

Yes, I think this you said could be a key "It is only below 1200 m or so where the fuel really makes a difference": Maybe the Luftstreitkräfte tacticians late in the war had concluded that combat took place at higher latitudes so they did not prioritize or think low alt performance was all that important? In addition: those issues you mentioned with Benzol sounds like something you'll want to avoid if you can. They could of course also have reasoned that if the "Tommies" start coming in low we can always spike up the fuel with some Benzol!