Altitude Compensating Carburettors, Pt.3: German (continued) A

Bletchley · 11 August 2009, 12:40 PM

Thanks for the feedback, KC and mik

KC: I am sure that you are right, and thank you for correcting me - the 1300 m altitude also struck me as an anomaly, but I did not notice the change in the compression ratio for the D.IVa on p.6, and the other version on p.193, assuming that they were the same - I conflated the two together, and will try to disentangle them now.

The lower compression (early) version, with CR of 4.8:1 is, I guess the same version captured by the British in 1917 (CR 4.9). I would think, now, that this version was 'leaned down' by approximately 4%-5% from an altitude of 1300 m, from the rated 260 Ps to 250 Ps at ground level (although why it wasn't therefore rated at 250 Ps is another anomaly).

The overcompressed version, with a higher compression ratio of 5.6:1, is probably the 'bis 1' version referred to in mossie's list, and the one listed on p.193 of Dechamps & Kutzbach. This, as you say, must have had an altitude compensating carburettor (probably very similar, even identical, to that used on the overcompressed D.IIIa), that would be engaged from 1300 m (i.e. the mixture would be 'leaned down' by the carb. to 95% power at 250 Ps, just as the early version was, and then it would be leaned down further, by another 13% on the throttle, from an altitude of 1300 m upwards). I guess it would have maintained that 250 Ps to around 1800 m or more (leaned down on the throttle from 286 Ps to 250 Ps), and it would then have declined, only slowly at first, but faster from around 3000 m as the compensator then continued to maintain the mixture balance but was no longer compensating fully for the 'normal' altitude effects.

mik: yes, I am sorry - that is the point I was trying to get across in the earlier thread - the NACA tables assume a 'perfect' mixture balance, and therefore do not take into account the power reduction/increase from variable leaning and enriching of the mixture at different altitudes. They are OK for most of the Allied engines from late 1916 onwards, as these had a 'vacuum' type compensator that was used to manually meter down the fuel as the aircraft gained altitude (keeping mixture more-or-less constant at all altitudes above 3000 ft), and rotaries had the 'fine adjustment' that did the same. The German engines are complicated - none of them had, with the exception of the rotaries and the BuS IVa, an altitude compensating carburettor of this type or any obvious form of mixture control. Up to 1918, it appears, they were 'adjusted' or 'leaned down' to a variety of altitudes (depending on the engine, the compression ratio, the fuel, and the amount of power it could reasonably 'sacrifice' at low altitude for a higher altitude gain). From the Spring of 1918 onwards there was a variety of 'overcompressed' or high-compression and over-dimensioned engines, with, it seems, as many compensating carburettors as there were different makes of engine (some of them very complicated, like the Maybach, and some, no doubt, highly experimental).

In addition, you have to take into account that these 'lean burn' German engines were operating at or near the limits of combustion at full throttle and low altitude, and were therefore much more sensitive to both the seasonal or weather-related changes in atmospheric temperature and pressure than one operating at around an AFR of 15:1. At this near-stoichiometric setting there is a kind of 'buffer' or 'dead zone', that limits the effect of these variations on the power output of the engine. This means, you cannot say "it could hold its power up to x altitude with y amount of 'leaning'", but can only talk in approximations that may vary from day to day or from winter to summer. We also still don' t have enough information on the fuels that they were using in the high compression engines - we know that they used benzol, but are still unsure of the proportions (or if it changed from winter to summer, or from one engine to another). To understand the performance of these engines fully, and their limits, we have to understand the fuel technology.

Bletchley

Additional note: looking again at the relative fuel consumption graph for the captured (lower compression) D.IVa, which is not 'dish' shaped but is, rather, shaped more like a hockey-stick with a bent toe, I think the D.IVa might have been 'leaned' on the final movement of the throttle (instead of a sub-optimum capacity of the main fuel jet, which would have leaned it across the full throttle range). If this is correct, the AFR would have been 'normal' at most points on the throttle curve (around 15:1, richer at low rpm and leaner in the middle range), and would then have been 'leaned' to around 95%-96% power to 250 Ps at 1400 rpm, holding this power to around 1300 m. This would make sense of the 260 Ps rating for the D.IVa, as there may have been an earlier version that was not 'leaned' at all, but was 'normal' across the full throttle range, and produced 260 Ps at ground level and up to approx. 0.9 m (3000 ft). The overcompressed version would then have simply extended this further, with the movement of the throttle lever into a 'high altitude' range (leaning it down to an AFR of around 20:1 at fully open throttle). This thought has been prompted by further examination of the Benz Bz IV series:

This engine came in at least two (possibly three) variants. The example captured by the British in 1917 had a compression ratio recorded around 4.91:1 (5.0, I think, according to Dechamps & Kutzbach). This is rated at 200-240 at 1400 rpm by D&K, and the British found that it produced around 230 hp, adjusted to sea level, at 1400 rpm with the throttle fully open. They record that the AFR was 'normal' across most of the throttle range, until the final movement of the throttle - when the movement of the throttle weakened the mixture. The relative fuel consumption graph shows that the mixture is rich at low rpm, declines until weaker at half-throttle, flattens out slightly, and then dips to an even weaker mixture at fully open throttle. I think this indicates that the mixture was 'leaned' probably by about 4%-5% at full throttle (from a 'nominal' 240 Ps down to 230 Ps) which would have held until around 1300 m. Turning to p.193 of D&K, the overcompressed Bz IV appears to have been 'leaned down' to 225 Ps at 1400 rpm, held to 1300 m, and then at 225 Ps by 13% from a 'nominal' 260 Ps at an altitude up from 1300 m (probably to around 1800 m) - all on the movement of the one throttle lever, as with the Daimler Mercedes engines. We know, from descriptions of the Benz Bz IV in the Shuttleworth Collection, that a 'button' had to be pushed in to move the throttle lever into the 'high altitude' range on the (overcompressed?) Bz IV. Yavor, in an earlier thread, posted some details of a Bz IV rated at 200 Ps and producing 220 Ps at 1400 rpm. I think this is possibly an earlier, lower powered, version - one that might not have been 'leaned' at all.

Kacey · 12 August 2009, 09:53 AM

Quote:

Originally Posted by Bletchley

Thanks for the feedback, KC and mik

This thought has been prompted by further examination of the Benz Bz IV series:

This engine came in at least two (possibly three) variants. The example captured by the British in 1917 had a compression ratio recorded around 4.91:1 (5.0, I think, according to Dechamps & Kutzbach). This is rated at 200-240 at 1400 rpm by D&K, and the British found that it produced around 230 hp, adjusted to sea level, at 1400 rpm with the throttle fully open. They record that the AFR was 'normal' across most of the throttle range, until the final movement of the throttle - when the movement of the throttle weakened the mixture. The relative fuel consumption graph shows that the mixture is rich at low rpm, declines until weaker at half-throttle, flattens out slightly, and then dips to an even weaker mixture at fully open throttle. I think this indicates that the mixture was 'leaned' probably by about 4%-5% at full throttle (from a 'nominal' 240 Ps down to 230 Ps) which would have held until around 1300 m. Turning to p.193 of D&K, the overcompressed Bz IV appears to have been 'leaned down' to 225 Ps at 1400 rpm, held to 1300 m, and then at 225 Ps by 13% from a 'nominal' 260 Ps at an altitude up from 1300 m (probably to around 1800 m) - all on the movement of the one throttle lever, as with the Daimler Mercedes engines. We know, from descriptions of the Benz Bz IV in the Shuttleworth Collection, that a 'button' had to be pushed in to move the throttle lever into the 'high altitude' range on the (overcompressed?) Bz IV. Yavor, in an earlier thread, posted some details of a Bz IV rated at 200 Ps and producing 220 Ps at 1400 rpm. I think this is possibly an earlier, lower powered, version - one that might not have been 'leaned' at all.

Hugh

Here's what I'm seeing. In D&K pg#6 you see a Benz Bz.IV listed as:

Bz.IV: 6 cyl inline, 145/190mm(b/s), rated 200ps@1400rpm@msl@5.0:1 CR
Bz.IV: 6 cyl inline, 145/190mm(b/s), rated 240ps@1400rpm@msl@5.8:1 CR

Now on pg#193 you see a Benz Bz.IVü (also referred to as Bz.IVsü) listed as follows:

Bz.IV: 6 cyl inline, 145/190mm(b/s), rated 225ps@1400rpm@msl@5.8:1 CR

So what's going on here? Well it's just what you are surmising.

Different carburetor systems. The low compression version most likely had a non AC-carb.

The frist high compression version on pg#6 most likely had this same non-AC carb system and could produce 240ps@msl.

The third high-altltude version on pg#193 had a leaned out sea-level carb system, so it could only produce 225ps@msl. However, it could maintain it up to 1300-1800m altitude.

The EXACT same engine w/o the leaned out AC-carb system could produce 240ps@msl but by 1000m would have been down close to 215ps and at 1800m would only produce around 192ps.

Therefore, one can see the ADVANTAGE of the leaned-out AC-carb system offerring 225ps versus the 192ps at 1.3-1.8km altltiude! Above these heights the engine performance would begin to follow normal degradation curves.

So we are saying the same things but arriving there from different directions.

Respectfully Submitted,

KC

ps (non hp type)

By the way you can readily see the two factors effecting MSL(sea-level) output of these type aeroengines with this available data.

The POTENTIAL power of this Bz.IVsü (5.8:1 "overcompressed") was cal'd by D&K (pg#193) as 260ps@1400rpm@msl (275ps@1500rpm@msl).

The "overcompressed" part (i.e. detonation) holds it back to 240ps@1400rpm@msl. That's the reported performanceat sea-level with a correct carb mixture.

So the 5.8:1 CR is holding back 20ps at MSL.

Now when you add a leaned-out carb system onto the same engine it can only provide 225ps@1400rpm@msl.

So the AC-carb system is holding it back another 15ps at sea-level.

In other words the CR is costing (1-240/260) 7.7 percent reduction.
The AC-carb ('overlean" @msl) is costing (1-225/240) another 6.25 percent power loss.

Total reduced effect of (1-225/260) 13.5 percent@msl. About 55% coming from "overcompressed" and 45% coming from "overleaned".

I've done this for all the German "overcompressed" HA aeroengines and the factors lead to a DETONATION reduction factor and a AC-carb factor TOGETHER that account for the reported MSL(sea-level) performance figures on these engines.

And they all hold together one to each other.

Again,

KC

PSS

Actually I rechecked my data analysis of these "overcompressed" inline German aeroengines and found I used three factors to explain the reported performance figures.

We have to consider that these engines were also being throttled back at sea-level (MSL) to avoid detonation. They SHOULD have been adjusting spark advance INSTEAD but I won't get into that right now!

Anyway, the correct figures for the Benz Bz.IVsü are:

The POTENTIAL power of this Bz.IVsü (5.8:1 "overcompressed") was cal'd by D&K (pg#193) as 260ps@1400rpm@msl (275ps@1500rpm@msl).

The "overcompressed" part (i.e. detonation) holds it back around -13ps to 247ps@1400rpm@msl. The reported performance of 240ps in D&K, must also have some ACC and throttling effects taking place.

So the 5.8:1 CR is holding back -13ps at MSL.

Now when you add in a leaned-out carb system onto the same engine it loses another -17ps so it can only provide 230ps@1400rpm@msl. Which is correctly in line with the 7-10% reduction expected from a "overleaned" AC carb system.(NACA Report#108,1921)

It appears the throttling effect on this particular engine is only adding another 2% decrease of about 5ps.

So you start with 260ps max deduct 13 for CR effect plus 17 for AC carb system effects and add anoher 5ps for throttling effects and you end up with 260-35 or 225ps at sea-leel and 1400rpm.

Hope I haven't confused anybody with this, it seems pretty simple to me.

The other OC'd aeroengines have differring degress of octane debt and throttle debt and the AC carb debit stays around 7-10%. And it fits all reported performance data for those engines.

KC

Bletchley · 16 August 2009, 12:06 AM

Thank you KC

It is good that, coming independently at this from another direction, you come to the same conclusion.

Although I have touched on the BMW in a previous thread, it might be useful if I include it here again and add more detail - I hope this will also stack up well against your own calculations:

The BMW IIIa was a high compression (6.4:1 - 6.5:1) and over-dimensioned engine - or as Greybeard elegantly put it in a previous thread, it was like a 'moon buggy' designed specifically to opperate at higher altitudes. I would add to this, however, that unlike a 'moon buggy' it also had to be used as a 'sand buggy' when brought back down to lower altitudes, i.e. it was, in effect, a rather difficult to achieve compromise between the two, arrived at by combination of very severe 'leaning' and great attention to cooling at low altitude (in an engine that would otherwise run very hot), with a strict regime of 'throttling back' that extended to a second throttle lever specifically for use at higher altitude.

The British capture report on an example taken from a Fokker D.VIIF notes that the pistons were made of aluminium, and aluminium also replaced steel or iron in other parts of the engine wherever possible, whilst "cooling of the crankcase and heating of the carburettor have evidently received very careful consideration". The carburettor was also unusual, as it had three chambers - the central one had the usual arrangement of fuel jets, with a small 'compensating' jet to supply a rich mixture at idle and low running, and a larger 196 cc capacity main jet to supply most of the fuel requirements from idle up to half-throttle. With the throttle lever moved just up to the half-throttle position the carburettor would be supplying a 'normal' mixture strength from the central chamber only, but as the throttle was moved up again towards the 'fully open' position the carburettor started to draw on a weaker mixture from the two side chambers - a combined mixture from all three chambers that became progressively weaker as this throttle lever was moved further from the half-throttle position towards full throttle. With this throttle fully open, at ground level and 1400 rpm, British engineers measured an Air Fuel Ratio of 20:1, commenting that, at this point, "the carburettor is adjusted to run at the weakest possible mixture on ground level". The reduction in power from 'leaning' the mixture to this extent would have been in the region of 20%, and this is confirmed from the British tests using a 'blower' to get 234 hp, adjusted to sea level, at fully open throttle (234 less 20% gives around 187 hp, adjusted to sea level, or about 185 hp at ground level). Interestingly, Gitter provides us with a German figure of 226 Ps, which indicates a rated 'leaning' of around 18% (226 Ps less 18% gives approx. 185 Ps) probably from a slightly lower altitude, a margin that allows for changing atmospheric temperature and pressure conditions.

This would maintain a constant power of 185 Ps to around 2300 - 2500 m, at which point the pilot would engage the 'over gas', using the second throttle lever. This would draw more air and fuel from the two side chambers of the carburettor, to give a progressively weaker mixture but maintain a constant 185 Ps (if it was used correctly, as it now depended on the pilot's judgement of how far this second throttle should be engaged to compensate for altitude effects) to a rated altitude of around 3200 m (at a constant 1400 rpm and a 'normal' AFR, depending on whether the pilot required max. power, full power or fuel economy at a 'cruise' setting). Although 3200 m was the rated height, the carburettor was capable of supplying even more fuel to the engine at this altitude (the two 'secondary' fuel jets were individually equal in their capacity, at 196 cc, to the main jet in the main chamber of the carburettor) to give a maximum sea-level equivalence of around 300 Ps that would then maintain a constant 185 Ps power up to at least 4000 m, and is the reason, I think, for the astonishing test flights recorded to altitudes of 30,000 ft and above. Engaging the secondary throttle at ground level or low altitude appears to have been possible, to a limited extent, to get around 10-20 hp extra 'emergency power' for a few minutes only, but was officially frowned upon, although I think it is highly unlikely that the engine would ever have been able to provide the theoretical maximum 300 hp at ground level - the mixture would have had an AFR of way above 20:1 at this point, and the engine would almost certainly have cut out long before. When the British tested it with a 'blower' they found that the maximum they could get out of it was 254 hp (close to the rated 260 Ps ground level equivalence that was 'leaned down' from around 3200 m), at which point the engine was starting to self-destruct and suffering from burnt valves and cracked cylinders. This gives a maximum usable 'emergency power' of probably less than 20 hp, at low altitude, although this could have been achieved only at great risk to the engine. A figure of 195 Ps is given in data from the Fighter Competion at Adlershof in 1918 (quoted in the Pfalz D.XII Profile), which indicates that an 'emergency power' of just 10 Ps was regarded as a 'safe' limit at low altitude. To this, however, I will add my usual 'caveat' that (a) we still do not fully understand the fuel technology, and (b) when 'leaned' to this extent even small changes in atmospheric temperature and pressure could have an effect on the amount of power proudeced by the engine. I therefore still maintain something of an open mind on the 'maximum power at low altitude debate', although I do not believe the evidence from

Hermann Goering who, I believe, was making exagerated claims to get priority in supply.

Bletchley

Note: for those who may be following this thread, and are wondering where the British test data comes from, it all comes from test reports compiled at the Royal Aircraft Establishment in Farnborough. These original printed reports, from around 1917/18 can be viewed either at the Farnborough Public Library (in the reference section of the Aviation Collection), or the National Archive at Kew (some are also held by the Imperial War Museum, London, and the RAF Museum at Hendon). They can also be purchased in a digitised format for £25.00 (high res. 300 dpi) in a 2 CD set from Hampshire County Libraries: Piston Aero Engines of the Great War

Bletchley · 21 August 2009, 11:38 PM

Turning to Argus engines, joegertler posted a useful set of factory data from a 1916 manual for the As.II, As.III and As.IV (in a post dated 4th Dcember 2007). This indicates that the As.III 6 cylinder 145x160 rated at 180 Ps at 1400 rpm had a 'normal' power of 175 Ps and a 'high' power (short duration) of 195 Ps.

We can compare these figures to an Adlershof test conducted between 8th August 1917 and 11th September 1917 (printed on pp.71-76 of Dechamps & Kutzbach), a series of runs culminating in a non-continuous duration run of 60 hours between 13th August and 21st August 1917.

On 13th August 1917 the engine (Nr.2785) was run up to full power, in a single run from 1:10 to 2:15 pm (atmospheric pressure 761 mm Hg, temp. between 17-18 degrees C), and readings were taken (adjusted to a standard temp. of 15 deg. C and msl 760 mm, as in previous British tests referred to in earlier posts). The engine produced 181.5 Ps at rpm of 1390, and a maximum 199 Ps at the top of the power curve at 1703 rpm (on the previous day the engine had been run 'warm', radiator temp. 75 deg.C as compared to 65-75, and the reading then was slightly less at 175.8 at 1394 rpm).

This compares well to the factory figures provided by joegertler, but it is interesting to note that the engine appears to have been 'leaned down' by 500 m (from a pressure height of 716.2), which resulted in approx. 5% less power at msl than would have otherwise been achieved - but would have meant that this engine could have maintained ful power at around 180 Ps to approximately 1300-1400 m (just like the Benz Bz.IV). There is, however, no indication that this was done on the throttle, like the Benz, as the relative fuel consumption curve is the standard 'dish' shape, so it was most likely 'leaned' with a less than optimum main fuel jet capacity (as with the Daimler Mecedes D.III and D.IIIa). We can see this clearly in the data from the final full-power 'duration' run of 60 hours between 13th August and 21st August 1917 (a series of 8 ndependent runs, not all continuous, of between 4 and 11 hours each) where the standard Argus carburettor appears to have been removed. An mean figure of 189 Ps at 1406 rpm was recorded (with power fluctuating very little, mostly around 189-190), with no indication this time that the engine was 'leaned' from 716.2 (the figures simply adjusted to msl), which can, I think, be taken as the 'nominal' power output at sea level if the carburettor was not 'leaning' the mixture.

The tests were conducted using a 'Benzin' fuel, although there is a reference to 'Benzol' within the test report, which suggests that some benzol may have been added, at least in small amounts, even though this was very clearly not an 'altitude' engine (but still with a relatively high compression ratio of about 5.06:1, rather like the Benz).

Bletchley

Bletchley · 4 September 2009, 10:37 PM

Here is further evidence, I think, from 1918 that German engineers were using a 'lean burn' approach to adjust their carburettors for better altitude performance on both the older 'low compression' engines and the later 'high compression' or 'overcompressed' engines: it is from a paper by K. Kutzbach, 'Adaptation of Aeronautical Engines to High Altitude Flying', in Technische Berichte vol.III no.4 1918, translated into English and published as NACA Technical Note no.142 in 1923 (the reference at the start to Bader is a reference to the paper that I quoted from at the start of this thread).

The Use of Poor Mixture

Bader has already pointed out (Technische Berichte vol.II no.1 p.95) that fuel consumption and climbing capacity can be improved by working, at ground level, not with the mixture which produces maximum power, but with a poorer mixture (smaller jet)...This proposal is however, practicable only to a certain extent, as an engine which works with a poor, slow burning mixture at ground level or at low altitude has a tendency to retarded ignition, detonation and undue heating. It adapts itself, however...to a low altitude engine, which, in this way, requires no additional regulation for altitude.

This can be carried further when the maximum charge...'H' position of the throttle lever... is only used for greater altitude, and another... the 'V' position... nearer the earth, in which the mixture is richer due to the smaller air ratio... in the carburetor... By gradually changing from the 'V' position to the economical 'H' position during ascent, the full advantage of fuel economy may be attained by the high altitude engine. The characteristic feature of this design lies in the decrease in the speed of the engine at the 'H' position as compared with the 'V' position, and this, at the same time, is an indication to the pilot not to open the carburetor too soon.

It is interesting, though, that there is no mention here of the use of benzol to overcome the problem of detonation at lower altitude, weak mixture and open throttle. I will add some details for altitude control used on Maybach engines in a day or so.

Bletchley

Bletchley · 14 September 2009, 10:58 PM

As promised, sorry for the delay

Turning to the Maybach engines, the Maybach Mb.IV (in two versions, a 'CX' of 1914 at 210 PS, 6 cylinder inline 150x180 CR 5.48:1, and a later 'HSL' of 1915 at 240 PS) and Mb.IVa (6 cylinder inline, 165x180, 245 PS, CR around 5.9:1 - 6.0:1) were probably the most commonly employed during the war, although there also appear to have been earlier and smaller versions of 170 PS or less (Mb.III ?). The Mb.IV was used from about 1915 onwards, I think, almost exclusively as the engine that powered the big German Zeppelin and Schutte-Lanze airships. Several were acquired by the British from airships destroyed over England, but in almost every case they appear to have suffered so severely in the subsequent crash and burn that the British engineers had to use bits from various different engines to get an idea of how they worked, and how much power they produced (although they would have been familiar with this type's general design features from the smaller pre-war Maybach-Wolseley airship engine). The most intact engine appears to have been one recovered from the wreckage of the Schutte-Lanze airship S.L.11, shot down by Lt. W.L. Robinson on 3 September 1916. Most of the information that we now have (at least in English) is probably derived from the AID (Aeronautical Inspection Department) report of an examination of the remains of this engine at Farnborough between September and November 1916. This AID report, in its original and uncorrected form, was then published in a condensed version by The Automobile Engineer as "The 200 hp Maybach" in December 1916. Rather coyly, the author of the piece states that "It should be understood at the outset that no attempt will be made to analyse design considerations, to criticise or to suggest in what direction the engines are peculiar, as such comment might conceivably, by careful deduction or inference, be of service to the enemy", although this actually hid the fact that the report was based on a severely damaged and incomplete engine that could not be run up to obtain actual performance figures. Importantly, two of the corrections to the original AID report, added in a covering note dated 11 November 1916, were apparently not passed on to The Automobile Engineer (or were passed on with the report, but were then missed by the author). These corrected the mistakes that the engine had a stroke of 190 mm (corrected to 180 mm), and a compression ratio of 9.4:1 (corrected to 5.48:1, but printed as 5.94:1 in The Automobile Engineer). So far as I am aware, this Automobile Engineer publication is the only published and printed original source of information in English about this engine (up until the release of the original AID report, now available in digital format on CD-ROM), and so it is possible that these same mistakes may still pop up in secondary sources.

The Automobile Engineer describes the 'unusual' features of the Maybach carburettor, indicating that it was fed by a constant head gravity feed reservoir instead of the normal float chamber, and that it allowed for some adjustment to changing altitude:

"The rotary barrel throttle opens on one side to the inlet pipe, on the other to a mixing chamber. In the lower portion of the barrel is a large extra air port registering with a port in the casing. The size of this port is further controlled by an air shutter. The main air regulator is of the sliding shutter or guillotine type. It opens or shuts as the throttle opens or shuts, being interconnected by bell crank levers. The jet has an eccentric hole covered by a cap, also with an eccentric hole, so that rotation of the cap regulates the area of the jet hole... The jet cap is also interconnected with the throttle and air slide, opening when they open, and vice versa. The constant head gravity petrol feed to the jet, in lieu of the usual and adjacent float feed, is provided by a small reservoir placed a little higher than the jet, that is fed from the float chamber, in turn fed from the pump and the separate air vessel or valve box... the quality of the mixture is, at any throttle opening, kept constant by mechanical means, i.e., the interconnection of the throttle air and jet orifices. Means of adjustment of the relative settings... are provided."

The main float chamber was kept well away from the carburettor, no doubt to protect the airship from the danger of fire from blow-backs, feeding the much smaller 'reservoir' that supplied the fuel jet by gravity and was therefore not affected by any difference in pressure between atmospheric pressure in the float chamber and the (reduced) pressure at a venturi. Instead, the adjustable fuel jet, adjustable throttle, and an adjustable auxilliary air valve were all mechanically controlled by the single movement of a throttle lever - adjusted on the ground, this could be calibrated to give either leaner or richer running across the full range of throttle lever movement. The British engineers estimated the power of the engine as 200 hp at 1200 rpm, but have little to say about how it was worked or how the altitude control was used (the controls clearly did not survive the crash). It is only when they got a more complete example of this engine type, the higher compression and overdimensioned (bore of 165 mm as against 150 mm) Mb.IVa high altitude engine from a Rumpler C.IV shot down in January 1918, that we get a report (in English) of the engine and altitude controls.

(Continued)

Bletchley

Bletchley · 14 September 2009, 10:59 PM

This "Report on the 300 hp Maybach aero engine" produced by the RAE (Royal Aircraft Establishment) in May 1918 was based on tests conducted on engine no.1261, taken from a Rumpler C.IV brought down in France on 18 January 1918 and, after some repairs, restored to running condition. This "new Maybach" was "credited with attaining an increase of 200 rpm at altitudes above 2000 feet" by the French, the first to test one, and it is described as "undoubtedly a great improvement in general design and efficiency". Alluminium alloy was used wherever possible in the construction of the engine, to cut down on weight, but the high compression ratio nevertheless "necessitates the use of very heavy [cast iron] pistons and connecting rods" although "in comparison with the Zeppelin-Maybach engines, the cylinders... are of a wonderfully clean design". The two carburettors "follow the distinctive principles of the well known Maybach design, but are slightly modified in many of their details as compared with the earlier engines". Unlike the earlier report on the "200 hp" Maybach Mb.IV, this "300 hp" Mb.IVa was captured with the controls intact, and a detailed description is provided:

"When running slowly, the throttle is slightly open, the supplementary air port closed, and the air shutter practically closed; in this position the small or pilot jet only is open. On opening the throttle, the supplementary air port commences to open in conjunction with the throttle opening. The main air shutter automatically opens in proportion, admitting more air, which passes directly across the top of the jets, and the jet area increases until the main jet orifice is fully open. Owing to the interconnections of the control levers, there comes a point when the supplementary air port area increases out of all proportion to the increase in the jet area, the latter reaching a limit with no further increase by reason of the arrangement of the levers. This point represents the opening for maximum power at ground-level. If the throttle is opened beyond this point, the air supply rapidly becomes out of proportion to the jet opening and petrol supply. By this arrangement a simple form of altitude control is provided. The various positions for 'slow running' [Leer, L], 'slow speed' [Langsam, LA], 'full speed' [Voll, V] and 'altitude' [Hohe, H] are marked on the body of the carburettor, and are indicated by a pointer attached to the throttle lever".

The report continues by stating that, because of the interconnections between the air and petrol controls, "any required throttle curve could be obtained" and, when adjusted by the British engineers to run at best power at ground level, the engine gave 258 hp at 1200 rpm, 279 hp at 1300 rpm, and 294.5 at 1400 rpm - with a maximum 304.5 at 1500 rpm. A one hour test at 1400 rpm produced an average of 290 hp, without any indication that the engine was overheating or was under any undue mechanical stress at this setting. This is perhaps surprising, as it is clear from German sources that in operational use the Mb.IVa engine was rated at 260 PS, and adjusted to run at 245 PS at 1400 rpm - leaned down and throttled back from a nominal 320 PS at 2500 m. Dechamps & Kutzbach indicate that at full throttle for ground running and at ground level this engine was 'leaned down' by about 18%, to an AFR of around 20:1, and it was throttled back by 6% to give a constant 245 PS up to 2500 m, with further 'leaning' and an increase in power as the throttle lever was advanced from the 'Voll' section of the throttle quadrant into the 'Hohe' section, to give an extension in the engine's full throttle height to around 3500 m at 1400 rpm. At lower altitudes the advance of the throttle lever into the altitude range could probably give an 'emergency power' of up to 20 PS or more, although advancing the throttle further would then only 'lean' the mixture further without the addition of any more petrol - thus maintaining a balanced mixture at higher altitudes, although the power would now fall from 245 PS as altitude was gained.

The robust construction of the engine clearly gave it a useful flexibility in operational use. It appears to have been capable of being adjusted to run either as a relatively high powered low-altitude engine running at around 300 hp up to about 1300 m, or as a 'high altitude' engine running at 245 PS up to an altitude of around 3500 m or more. The British engineers, however, were not that impressed with this, and regarded the engine as an "unnecessarily complicated instrument... which would need careful first adjustment and constant adjustment for wear", comparing the carburettor to that of the White and Poppe, and concluded their report with the observation that "if any dust is present the mechanism is very liable to stick or to render the control very hard to operate."

I don't have any information on the intermediate Mb.IV type, the 240 PS/hp 'Mb.HSL', but it seems likely that it was closer in design and construction to the 210 hp/PS 'Mb.CX' than the Mb.IVa, rated at 260 PS at 1400 rpm and leaned down on the throttle by 6% to 245 PS at ground level - to give a full throttle height of around 1300 m.

Bletchley

Sources

Wikipedia. 'Maybach Mb.IVa'. The designation of the MB.IV 210 PS engine as the 'Mb.CX', and the Mb.IV 240 PS as 'Mb.HSL' comes from this source, but I think that the source is incorrect in stating that the Mb.IVa engine had alluminium pistons (both the AID report on the Mb.IV and the RAE report on the Mb.IVa state very clearly that the pistons were cast iron), and that the Mb.IVa had "three different carburettor settings, each optimised for performance at different altitudes" (there were in fact four, three for engine speed/load, and the last for altitude).

Aeronautical Inspection Department. 'General particulars of Maybach engine captured from German airship S.L.11', September 1916 (with covering note containing amendments, dated 11 November 1916).

'The 200 hp Maybach', The Automobile Engineer, February 1916.

Royal Aircraft Establishment. 'Report on the 300 hp Maybach aero engine', May 1918.

(The above are all digitally reproduced in high-res. in 'Piston Aero-Engines of the Great War', ISBN: 9781859758014)

Dechamps, H., & Kutzbach, K. 'Prufung, Wertung und Weiterentwicklung von Flugmotoren', Richard Carl Schmidt, 1921.

Kutzbach, K. 'Results of experimental flights at high altitude with Daimler, Benz and Maybach engines to determine mixture formation and heat utilization of fuel', Technische Berichte, vol.III pt.1 1918 (translated into English in NACA Technical Report no.125, 1923).

YavorD · 15 September 2009, 12:47 AM

Thank you, Bletchley!
A small correction: The last reference is NACA Technical Note No.125. NACA Technical Report No.125 is about "Aeronautic Instruments".
Regards,
Yavor

Bletchley · 15 September 2009, 10:55 PM

Thank you for the correction, Yavor

I have been doing some digging into the fuel technology of both the Allied and German aviation fuels, as I think that is the key now to a better understanding of the altitude performance of these engines - and I will post something on this soon, either here or in a new thread (but probably in a new thread, as I think it may be be quite a long post). It is a subject that has been largely overlooked, I think.

There has been some discussion of this thread on the RoF Forum:

riseofflight.com • View topic - Fokker D.VII climb

Bletchley

Bletchley · 18 September 2009, 11:20 PM

There is a further report that I missed from the list of sources:

Sparrow, S.W. Performance of Maybach 300-horsepower airplane engine. NACA Report no.134, 1923. http://naca.central.cranfield.ac.uk/...report-134.pdf

Although there is not much information here on the construction aspects of the engine, or the way in which the throttle was worked, there are some interesting test resuts from tests made in a high altitude chamber, on an Mb.IVa no. 2026. The tests were done using 20 per cent benzol / 80 per cent American 'X' grade gasoline "for the purpose of preventing pre-ignition at air densities corresponding to ground level", which indicates that this engine was designed to run on a petrol/benzol mixture. The British and French, testing the engine during the war, appear to have used just the standard Shell aviation fuel ("Shell A") from Sumatra, a high quality petrol for the time (and slightly better than the 'X' grade gasoline), but less resistant to detonation. In the British tests the engine was run up to only 1500 rpm, rather than the 1600 rpm indicated by both German and American sources, which indicates that they were probably experiencing a drop in power from pre-ignition or mild detonation at engine speeds and throttle openings above this. Using a petrol/benzol mixture, they would probably have achieved a 5% or more power increase at the higher rpm (although when adjusted to run very lean as a high-altitude engine at an rpm of around 1500-1600, at or near ground level, the air fuel ratio would probably have been reduced by the carburettor to the lower limit of explosability, which would have resulted in misfiring and rough running) See: Gardiner, Arthur W. & Schey, Oscar W. 'The comparative performance of an aviation engine at normal and high inlet air temperatures'. NACA Report no.277: "A gain of 5 to 6 per cent in the full-throttle power of the Liberty 12 aviation engine, 5.4:1 compression ratio, at speeds giving the maximum volumetric efficiency is obtained by using a fuel mixture consisting of 30 per cent benzol and 70 per cent aviation gasoline instead of plain aviation gasoline".

Bletchley