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

[Bletchley]

Recent posts on the Daimler Mercedes D.III series and BMW IIIa engines have prompted me to look back over previous posts on altitude compensation in the aero engines of the period. Although I think the Allied arrangements for altitude compensation are fairly well understood now, and there seems to be a reasonable consensus here, there still appears to be considerable confusion and some differences of opinion over the German arrangements for altitude adjustment and compensation. In looking back at my previous threads on this subject at Altitude Compensating Carburettors, Pt.1: Allied and Altitude Compensating Carburettors, Pt.2: German I realise that my description of the altitude compensation arrangements in the German aero engines was left somewhat sketchy - although it was usefully filled-out by the many responses from others. At least in part, this was because I was still trying to get my head around the way in which the German engineers approached this problem - and at the time very little information or data was available, and the data that was available was (and still is) somewhat contradictory. I hope to remedy this omission here, pulling together all the information and data that I now have (or have made available to others) to provide an explanation that is not intended to be definitive, but is intended to provide a base-line of information, a conceptual explanation, and a starting point for further discussion.

For me, at least, the story starts with the Allied attempts to gain an altitude advantage over the German Air Service in 1916. Greybeard, in a previous post, has described the 'ideal' carburettor design for optimum engine performance over a range of loads and situations as "a stable idle RPM, fast acceleration (at low RPM), minimum consumption (at middle RPM) and max available power (at max RPM)". Although this is certainly true for the modern normally aspirated piston aero engine, such as the Jabiru with automatic barometric altitude control (Bing carburettor) referenced by mik, it is not necessarily true for all World War One aero engines. It is, however, probably true for the conditions prevailing at the outbreak of war in 1914. By the beginning of the war, the main carburettor manufacturers were nearly all producing twin-jet (e.g. Zenith) or compound-jet (e.g. Claudel) float-chamber carburettors that were capable of delivering automatically, at low altitude and on the movement of a single throttle lever, a relatively rich mixture for idle and for slow speed, a slightly leaner mixture for the middle ranges where max. fuel economy was desirable, a more-or-less stoichiometric (but on the rich side, at an Air Fuel Ratio of around 15:1 ) mixture for full and sustained power at maximum efficiency (full power for climb or level flight), and (at fully open throttle) a full-rich mixture for max. power 'for a few minutes only' for take-off and combat (at an AFR of around 12:1 or 13:1, a throttle setting that was not fuel efficient, as it released unburnt fuel through the exhaust, and could not generally be sustained without risk of damaging the engine). You will find many references to this type of carburettor in the text books of the period.

There were exceptions to this, of course. Rotary engines, for example, were notoriously wasteful of fuel, at least partly because they were usualy run at or near full-rich at any desired throttle setting in order to get maximum power at a particular rpm. There were also some 'fuel-cooled' air cooled stationary engines, such as the Renault and the RAF1a, which were designed to run full-rich at full power (at an AFR of around 12:1 or 13:1) so that excess unburnt fuel would help to cool the engine. This would also give a 'boost' for take-off power of around 10%-12%, although this would be lost immediately as the aircraft climbed. This was fine for the early years of the war (1914/15), when the aircraft generally had a poor power-to-weight ratio and needed either full or maximum power just to get off the ground - and when most air operations were generally flown at 3000 ft or less. As aircraft were forced to fly higher, however - to avoid anti-aircraft fire from the ground - they hit a problem with altitude effects on mixture (the mixture becoming gradually richer as atmospheric air pressure and density fell). A carburettor that is adjusted to give optimum performance at or near ground level will, at full power setting (i.e. more or less stoichiometric, but on the rich side at an AFR of about 15:1), hold full power up to about 2500 ft, if adjusted to sea level, or about 3000 ft if adjusted to ground level (enrichment of the mixture will push it into the max. power mixture setting, and the slight increase in power of around 10%-12% will offset the normal altitude effects - but at the expense of greater fuel consumption). At altitudes above 3000 ft the aircraft will then be hit by a double-whammy of normal altitude effects combined with the effects of a progressively over-rich mixture (the richer mixture will no longer be compensating for the altitude effects, as it will now be contributing). The Allied response to this problem was to develop a new 'vacuum' type altitude compensating carburettor that had a manual control to gradually reduce the fuel content of the mixture as the aircraft went over this 3000 ft limit of normal carburettor efficiency, maintaining a near-stoichiometric mixture control to a height of around 18000 ft. Without this mechanism, British tests indicated that aircraft service ceilings would have been restricted to around 12000 ft. This new 'vacuum' type altitude compensator was probably first used in the Hispano-Suiza engine of the SPAD VII (in a modified Claudel or Zenith carburettor) in the Summer or Autumn of 1916, and it appears to have been in widespread use by the Spring of 1917. Before this, aircraft such as the BE2 and RE8, with their 'fuel cooled' non-compensated RAF engines running on a rich mixture at ground level, were limited to a service ceiling of around 10,000-12,000 ft.

Heron comments that "F.M. Green, who was in charge of design and development of both engines and aircraft at the Royal Aircraft Factory during World War I, states that he initiated work on supercharging at the Factory early in the war. He did so because, "whilst the British aircraft were faster at sea level than the corresponding German aircraft, the British aircraft were slower at altitude" (Heron, S.D. 'History of the aircraft piston engine', Ethyl Corp., 1961). The Allies were aware by mid to late 1916 that German aircraft were apparently performing better at altitude than their non-compensating engines should have allowed. Their Argus, Benz and Mercedes engines, if adjusted to a near-stoichiometric mixture at ground level for full power, as was standard by then for most Allied aircraft, should have restricted their aircraft to an effective service ceiling of around 13,000-15,000 ft without further adjustment or compensation for altitude. But pilots were reporting German aircraft at altitudes of up to 17,000 ft - aircraft that had (from the evidence of captured examples of the engines) no obvious mechanism for altitude compensation. There is, however, an early clue to what was happening in the capture report of a Benz Bz.IV engine from an Aviatik captured in May 1917, where British engineers comment: "the last movement of the throttle ... causes no increase in petrol flow, but on the contrary a slight decrease". This is clearly an aberation from the 'ideal' described by Greybeard, where the final movement of the throttle into the full/max. power range should have been (and in most Allied engines was) matched by a relative increase in the fuel flow to enrich the mixture at full load. The British engineers seem to have dismissed this, however, as an unexplained anomaly with the comment "No compensating arrangement is fitted for high altitude control".

(continued)

Bletchley

[Bletchley]

The German Air Service were well aware, however, that a reduction in fuel to lean the mixture at ground level could give an altitude advantage - but only at the expense of some loss of power at lower altitude. At the 1913 German engine trials, for example, it was reported in the British journal 'Automobile Engineer' that "most competitors ran their engines at about five to six percent below the maximum output", and papers from the secret German wartime technical bulletin 'Technische Berichte' indicate that this trade-off between power and fuel consumption, common knowledge by then, could also be used to sacrifice a limited amount of ground level performance for improved altitude performance, and an increase in service ceiling. Limiting the size of the main carburettor jet to provide a 'lean' air/fuel ratio at fully open throttle, at ground level, could, by sacrificing around 10% of power at ground level, increase the full throttle altitude from approx. 3000 ft to approx. 6000 ft to give an increase in engine power at all altitudes above this - and a 3000 ft extension to an aircraft's service ceiling. Everling, in 1917, writes that "The effects of the explosion... decreases when the engine reaches greater heights; it may, however, at first increase if the mixture were poor in petrol to begin with" and "experience teaches that the speed of the engine, in a few cases, actually does remain constant up to great heights". (Technische Berichte, vol.1, 1917, in an English translation published by the Air Ministry in 1925 as Air Publication 1120). Interestingly, this was written well before development of the new German altitude engines in 1918, and Everling goes on to state that although "the speed falls off... at low heights the deviations are only small... up to 1,700 meters [approx. 5,500 ft]". More evidence that the German engine manufactureres might by now have switched to a 'lean-burn' approach rather than the standard 'rich' mixture for engine operation at low altitude and fully open throttle is provided in a later paper by Bader from the Technische Berichte (vol.2, quoting from the English translation in Air Publication 1121), Bader describes the mechanism for altitude adjustment in more detail: "The small excess air required at ground level... soon disappears at altitude and we get a deficiency of air... With a fixed jet, we can then only obtain best results for one altitude. But this simple device of having a smaller jet will give considerable increase in power as soon as the aeroplane reaches a sufficient altitude. At the ground and at low altiudes there will be a small drop in power... [but] for this jet [a main jet, 2.8 per cent smaller in diameter than the one it replaced] the time of climb to 5000 m is a minimum, i.e., the mean power between 0 and 5000 m is a maximum".

There is evidence for this adjustment from the engine and aircraft performance figures that we have for the Mercedes D.III series of engines. This is a useful series for comparison because, although the dimensions of this engine remained constant from the early D.III of 1914/1916 onwards through to the D.IIIa in the Spring of 1917, and the D.IIIaü in the Spring of 1918, it went through a number of small changes and modifications, including improved cooling, increased compression ratio, and an altitude compensating carburettor. These clearly illustrate the move away from a standard 'near stoichiometric' and towards a 'lean burn' approach in WWI German aero engine design.

Dan San Abbott has provided the official German rated power output for the Mercedes D.III series in a previous thread: "The Mercedes D.III was rated at 160 Ps. The D.IIIa was rated at 170Ps. The DIIIaü was rated at 180 Ps. All were rated at 1400 rpm". There are a number of apparently contradictory output figures flying around that have led us, in the past, to question this - arriving at a consensus that official German figures are often 'nominal' output figures - and that the 'real' output is often different (usually higher). I think now that this is probably not the case, and that the official figure is in most cases a 'real' output at ground level that is in many, if not all cases, held constant to a rated altitude of around 1700-1800 m - or in a few cases to 3000-3200 m.

I think that the Mercedes D.III, rated at 160 Ps, falls into the first class of engines - those rated to an altitude of around 1700-18000 m - and I believe that this was achieved by 'leaning' the mixture to an AFR of around 17:1 or 18:1 at ground level, to produce an output of 160 Ps that remained constant to around 1700-18000 m and then fell off rapidly due to the combined altitude/mixture effects kicking in at heights above this. In an earlier thread dating back to 16 February 2005 mossie posted a useful set of performance figures for the Mercedes D.IIII from 'Die deutschen Militarflugzeuge 1910-1918': nominal power 160 hp, 172 hp at 0 m (I think this should be 162), 162 hp at 1000 m, 152 hp at 2000 m, 115 hp at 3000m (all at 1420 rpm). This engine had a rated power of 160 Ps, but I believe it was 'leaned down' to this at a sea level equivalence (probably to around 10% loss of power, although Duchamps & Kutzbach, 'Prufung, Wertung und Weiterentwicklung von Flugmotoren', 1921) indicate that a similar design, the D.IVa, was leaned down to around a 13% loss of power). It would have retained this power to about 1700-2000 m, so that between 1700 m to 2000 m it had just started to drop power down to 152 hp, followed by a rapid drop to 115 hp by 3000 m (mossie, as above). If we look at a British capture report for the Mercedes D.IIIa ('Report on the 180 hp Mercedes Aero Engine', dated March 1918) there is comparative data for a D.III captured and tested earlier (and published previously in a November 1917 issue of the Automobile Engineer). This test indicated that the Mercedes D.III, with a compression ratio of 4.5:1 (actual figure is 4.48:1 from German sources provided by Pfalz-Scout), gave 162.5 hp, adjusted to a standard 760 mm atmospheric pressure at sea level (Pfalz-Scout gives a German rating of 160 Ps and an average power of 161.6, in a post dated 30 July 2008, obtained either from Huth, 'Motoren für Flugzeuge und Luftschiffe', 1920 or from Kyrill von Gersdorff, 'Flugmotoren und Strahltriebwerke', published 1981). The evidence that this was a slightly 'lean burn' engine at ground level that could maintain this 160 hp to an altitude of 1700 to 2000 m can also be found in the climb performance figures of those aircraft that are known to have used it. If we look at the performance figures for the Roland C.II, for example (from Peter M. Grosz 'The Roland C.II', Profile no.163, 1967), the climb to 1000 m is given as 7 min., and the climb to 2000 m as 14 min., whilst the climb to 3000 m was 26 min. The climb rate from 1000 m to 2000 m was the same as that from 0 to 1000 m, indicating that there was no significant loss of engine power between 1000 m and 2000 m. If the carburettor had been adjusted to give a standard near-stoichiometric mixture at ground level we would expect to see a significant fall in the rate of climb between 1000 and 2000 m (and if fuel-cooled, an immediate fall from ground level). But a carburettor that is 'leaned' back to about 88%-90% power would adjust for the altitude effects of falling air pressure/density at altitudes up to 1700-2000 m, (gradually enriching the mixture) to give exactly this result (or just a marginal drop between 1000 and 2000 m). At an altitude of 2000 to 3000 m we see a sharp decline in the rate of climb (an increase from 7 minutes per 1000 m to 12 minutes per 1000 m) - a result that we would also expect to see as the 'double-whammy' of altitude and mixture effects kicks in. The figures from Pfalz-Scout indicate that the Mercedes D.III also had a maximum power output of 185 Ps at 1660 rpm (a figure confirmed by British tests), to give an 'emergency power' for a few minutes of around 25 hp above the 'normal' power of around 160 hp.

[Bletchley]

If we turn to the Mercedes D.IIIa, we will see a similar picture of an engine, with a slightly higher compression ratio of 4.64:1, 'leaned' to around 90% power at sea level and holding that power up to 1700-2000 m. In the post by mossie, referred to above, the D.IIIa is listed as having a rated power of 160 hp, 180 hp at 0 m, and 130 hp at 5000 m. Dan San Abbott has it rated at 170 Ps. Pfalz-Scout lists a rated power of 160 Ps at 1400 rpm, and an average power of 174 Ps, whilst Dechamps & Kutzbach rate it at 160 Ps. The British tested one and measured an output of 174 hp at 1400 rpm, adjusted to sea level, but noted that in other tests a D.IIIa had produced 179.5 hp. There is clearly a lot of conflicting data here, and it is possible that there was more than one variant of this engine. In a previous thread Dave Watts has, I think commented that the Mercedes D.IIIa probably went through a long process of improvement and development, and the British test report notes a number of small changes from the D.III, many of them 'borrowed' from the D.IVa, which may have been made to the engine incrementally over a long service period (this one was captured from an Albatros D.Va in 1918, and would have been a late version). My best guess is that this engine started out with an official rating of 160 Ps, the same as the D.III, but as a result of a number of small incremental improvements this was then subsequently raised to 170 Ps - as later examples would appear to have had a higher output of around 180 hp at 1400-1450 rpm. There is evidence that the Mercedes D.IIIa engine also held this power constant to around 1700-2000 m, from the performance of a Pfalz D.XII with this engine at the Second Fighter Competition in 1918: the D.XII climbed to 1000 m in 3.8 minutes, and to 2000 m in 4.0 minutes (a climb from 1000 m to 2000 m only marginally slower than that from 0 to 1000 m), with a marked reduction in climb rate to 5.3 minutes between 2000 m and 3000 m (Profile no.199). Pfalz-Scout also reports a maximum power output of 197.5 Ps at 1700 rpm, an additiional 'emergency power' of around 20-25 hp, confirmed by the British tests.

At this point the German engineers appear to have hit a barrier. In order to be viable at all, running an engine at a lean mixture at ground level requires a good power to weight ratio, as the aircraft would need an excess of power, even at 90% full power, to take off and manouevre effectively at low altitude. This excess of power appears to have been there with most of the German inline engines during this period of the war, whilst development of even more powerful engines from the Mercedes D.III onwards, from as early as mid 1916, provided them with a theoretical opportunity to 'lean' the engine even further at ground level - down to around 80% full power or even less - to enhance altitude performance and extend service ceilings. This could be done, without enlarging the engine, by increasing the compression ratio - as we can see happening in the Mercedes D.III series (up from 4.48:1 for the D.III, to 4.64:1 with the D.IIIa). But as the Allies also found (and as KACEY has demonstrated in a previous thread) any increase from around 5.1:1 to 5.3:1 (with the relatively low octane fuel that was then available) pushed the engine over a detonation threshold, with symptoms of pre-ignition and 'knocking' at full throttle and at low altitude - and leaning the mixture further just made this worse. The Allies had continuous access to good quality aviation fuel throughout the war, in particular from oil fields in Sumatra and the East Indies (known as 'Shell A' to the British, and with an octane probably around 60-70). They saw no reason to develop new fuels, and maintained a maximum compression ratio of 5.3:1 up to the end of the war. The German Air Service, on the contrary, had a good supply of high quality aviation fuel only up to mid 1916. According to rammjaeger, in a post dated 3 May 2003, the Fliegertruppe were using two types of petroleum fuel in 1914 - 'Leichte Naphta' and 'Schwere Naphta'. The former 'light' fuel was probably for front-line use, whilst the heavier one may have been for use ground vehicles or for Zeppelins (where a more volatile fuel may not have been desirable). In the late spring of 1916 Romania entered the war, and supplies of high quality aviation fuel from the Ploesti oil fields were then denied to Germany up until the autumn of 1918. Germany now relied heavily on very variable quailty petroleum from the Austro-Hungarian Galician oil fields. German engineers appear to have been concerned about the low volatility of this mostly 'heavy' fuel, and it is from this point that they probably started to experiment by adding either alcohol or benzol (and sometimes both) to the petroleum - probably, at first, in an attempt just to raise the volatility to an acceptable level. They discovered, however, that adding benzol or alcohol had an unintended side effect of lowering the detonation threshold of petroleum (they acted as anti-knock agents). Alcohol lowered the calorific value too, and took up more room in the petrol tank - leading to a double disadvantage of a lower maximum power and reduced endurance (or larger, heavier fuel tanks), and although it appears to have been used from the summer of 1917 onwards - either mixed with petroleum or with benzol and petroleum (Spiritus-Benzol-Mischung) - this was probably reserved for non-combat use in training units or elsewhere on the home front (rammjaeger). The addition of benzol to petroleum, however, did little to reduce the calorific value of the fuel (the Germans had, in 1914, even conducted engine trials using 100% benzol to evaluate it as an alternative synthetic fuel for aviation use) and it did not require a larger petrol tank or significantly reduce endurance. It did have some disadvantages,however - benzol has a higher freezing point than petroleum and will (in higher concentrations at least) seperate out from the petroleum at the low temperatures experienced at altitudes of 20,000 ft or more in winter. It is also corrosive, and will eat its way through rubber seals and tubing. These disadvantages, however, were clearly outweighed by the volatility and the anti-knock qualities of benzol when it was added in a concentration of around 20/80 or higher to the available 'heavy' petroleum fuels from Galicia - and Germany had relatively large stocks of benzol, obtained from liginite or semi-bitumous brown coal, as a by-product of the coking industry. Benzol appears to have been added, at least in low concentrations, from around the summer of 1917 (rammjaeger, as above), but new engines capable of using the anti-knock capacity of the new fuel were not introduced into front-line service until the spring of 1918.

[Bletchley]

The overcompressed D.IIIa, the D.IIIaü, had an increased compression ratio of 5.73, a new type of altitude compensating carburettor, and a number of other small improvements over the D.IIIa then in widespread use. The high compression ratio made it one of the first 'high altitude' engines adapted to or designed to run on a benzol-petroleum mixture. Without this fuel, they would not have worked to their designated limits - as the British engineers discovered when they tested one on 'normal' aviation petroleum. This engine was still rated at 160 Ps at 1400 rpm (Dechamps & Kutzbach), although it could comfortably achieve an output of around 180 Ps at an rpm 0f 1500-1600 for at least a few minutes, and possibly longer. When the British engineers tested it with a petroleum fuel, however, they could only run it up to a maximum of 1500 rpm with the D.IIIaü carburettor fitted - at all engine speeds above this the AFR became too weak for the fuel to support combustion, and the engine cut out. The addition of benzol to the fuel meant that German engineers could, however, break this AFR barrier, to 'lean' the mixture down to an AFR of around 20:1 and a power reduction of 18% with the throttle fully open - although the throttle was only supposed to be fully open at altitudes above the full-throttle height of 1800 m (a full-throttle height given by Dechamps & Kutzbach). At low altitudes the throttle was advanced to it's maximum for low altitude working, to a point on the throttle quadrant marked by an new instruction to pilots to go no further until higher altitude was reached. At 1400 rpm the engine had an output of around 160 Ps (measured at 164 hp adjusted to sea level by the British, using a petroleum fuel) and was 'leaned down' to approximately 90% 'nominal' power - this 'leaning' is indicated by the British attempt to get more power from the engine, replacing the D.IIIaü altitude compensating carburettor with it's small 250 cc capacity main jet with the 'standard' D.IIIa carburettor with it's main jet 'beefed up' from 350 cc to 450 cc. With this 'standard' carburettor the engine produced it's otherwise 'nominal' 180 hp at 1400 rpm (an increase of approx. 10%). Using the D.IIIaü carburettor and maintaining a 1400 rpm, pilots could maintain 160 Ps up to 1800 m (the full throttle height) without any further movement of the throttle - just as with the D.III or D.IIIa, the carburettor 'adjusted' for altitude by letting the mixture go from a 'lean' mixture of just under 18:1 AFR at ground level to a full-rich mixture of around 12:1 or 13:1 AFR at 1800 m. At this point the pilot could advance the throttle into the final 'altitude' section of the throttle quadrant, and this would then uncover additional air holes in the throttle barrel that weakened the mixture even further (up to an AFR of just under 20:1 ground level equivalence, when fully open) but added no (or very little) extra fuel as the small 250 cc jet was by now supplying fuel at full capacity (the D.III/D.IIIa carburettor, by contrast, had a larger 350 cc capacity main jet, and this would support engine speeds of up to 1700 rpm). This 'leaning' in the D.IIIaü carburettor can be seen clearly on the relative fuel consumption graph in the British test report which, instead of having a standard 'dish' shape (increasing from a lean-ish setting to full rich as the throttle is increased from half-throttle) it is down-hill all the way. From altitudes of around 1800 - 2000 m the pilot could continue to 'lean' the mixture by advancing the throttle, manually maintaining a more-or-less constant AFR up to greater altitudes - power would still decline, as normal altitude effects started to have an impact from 1800 m, but the 'double whammy' of both altitude and mixture enrichment effects would be delayed up to much higher altitudes of 4000 m or more. Although the D.IIIaü was rated at 160 Ps at 1400 rpm, British tests indicate that it ran well at up to 1600 rpm. At 1500 rpm the British measured an output of 171 hp adjusted to sea level, and it is therefore likely that the engine could comfortably achieve an output of 180 Ps at ground level, and it is often seen rated at this higher value.

Bletchley

[Kacey]

Hugh,

Thanks for the extremely detailed explaination of German AC-carbs and their approach to achieving such!

You have a gift for text explaination, mine on the other hand is almost all numeric.

Your numbers and explaination fit quite closely to a table I have for altitude performance of these aeroengines up to 6km.

As I explained, in another thread, I use a air density factor, a volumetric efficiency (breathing) factor and a carburetor efficiency (mixture) factor, to explain reported performance at altitiude.

The sea-level (ground-level) leaning factor shows up just like you are surmising!

EXCELLENT PRESENTATION!

Thanks,

KC

[Bletchley]

Thank you KC, it is good to have some independet corroboration I have been looking at some other German engines: the Daimler Mercedes D.IVa, for example appears to have been 'leaned down' by around 13% power - using a 'light' aviation fuel this appears to be the most that could be achieved in the way of a 'lean burn' engine before they started to use benzol mixtures.

Dechamps & Kutzbach list it as a 'High Altitude' engine, although I have not found any evidence that it had an altitude compensating carburettor (they list it as a D.IV, but describe it as a 6 cylinder 160x180, so I think it must be the D.IVa). I would guess that it is the extra 'leanng' by 3% power that must have pushed it into this category.

Going to Dechamps & Kutzbach again, they list a 'nominal' power of 286 Ps 'leaned down' to 250 Ps at ground level, but rate it at 260 Ps. This amount of 'leaning' would have put the AFR beyond 18:1 at ground level, which might have led to problems with rough running - but looking at the British capture report for a D.IVa taken from a downed Gotha in 1917, the relative fuel consumption graph is not 'dish' shaped (as it is for the D.III/D.IIIa and most other engines), but flattens out at high rpm instead of going up (and even dips very slightly at 1400 rpm). I guess this means that if, in some conditions, the engine started to run rough at full throttle, then they could throttle back slightly to enrich the mixture and bring the engine back to smooth running. The British report also confirms the power output of around 250 Ps (they measured 251 hp at 1400 rpm, adjusted to sea level). Leaning the engine by 13% power should have given aircraft with the D.IVa, potentially, a slightly higher ceiling than otherwise (enough, I guess to class it as a 'high altitude' engine, although the increase is not likely to be more than a few hundred meters). If it had been 'leaned' by 10% power, it would have had a ground level power output at the rated 260 Ps, which suggests that the extra 3% might have been a slightly later 'modification' rather than part of the original design. The compression ratio was 4.9 to 1 (higher than the D.IIIa), as measured by the British engineers. The maximum output is listed at 300 Ps by Dechamps & Kutzbach (1500 rpm), although the British report suggests that the movement of the throttle (at least in the example they had from the Gotha) was limited to 1400 rpm.

There is a 'bis1' version listed by mossie (in his post 16/2/2005), from 'Die deutschen Militarflugzeuge 1910-1918', which lists the power output at 267 hp maintained to 3000 m at 1450 rpm. I don't know anything more about this version - although possibly a later, overcompressed, version with the same altitude compensating carburettor used on the overcompressed D.IIIa?

Bletchley

[YavorD]

Excellent review, Bletchley!
It is a very good explanation indeed and just in time for one of my projects. I need a bit more time to read carefully all the details.
Thank you very much!
Yavor

[Kacey]

Originally Posted by Bletchley

Thank you KC, it is good to have some independet corroboration I have been looking at some other German engines: the Daimler Mercedes D.IVa, for example appears to have been 'leaned down' by around 13% power - using a 'light' aviation fuel this appears to be the most that could be achieved in the way of a 'lean burn' engine before they started to use benzol mixtures.

Dechamps & Kutzbach list it as a 'High Altitude' engine, although I have not found any evidence that it had an altitude compensating carburettor (they list it as a D.IV, but describe it as a 6 cylinder 160x180, so I think it must be the D.IVa). I would guess that it is the extra 'leanng' by 3% power that must have pushed it into this category.

Going to Dechamps & Kutzbach again, they list a 'nominal' power of 286 Ps 'leaned down' to 250 Ps at ground level, but rate it at 260 Ps. This amount of 'leaning' would have put the AFR beyond 18:1 at ground level, which might have led to problems with rough running - but looking at the British capture report for a D.IVa taken from a downed Gotha in 1917, the relative fuel consumption graph is not 'dish' shaped (as it is for the D.III/D.IIIa and most other engines), but flattens out at high rpm instead of going up (and even dips very slightly at 1400 rpm). I guess this means that if, in some conditions, the engine started to run rough at full throttle, then they could throttle back slightly to enrich the mixture and bring the engine back to smooth running. The British report also confirms the power output of around 250 Ps (they measured 251 hp at 1400 rpm, adjusted to sea level). Leaning the engine by 13% power should have given aircraft with the D.IVa, potentially, a slightly higher ceiling than otherwise (enough, I guess to class it as a 'high altitude' engine, although the increase is not likely to be more than a few hundred meters). If it had been 'leaned' by 10% power, it would have had a ground level power output at the rated 260 Ps, which suggests that the extra 3% might have been a slightly later 'modification' rather than part of the original design. The compression ratio was 4.9 to 1 (higher than the D.IIIa), as measured by the British engineers. The maximum output is listed at 300 Ps by Dechamps & Kutzbach (1500 rpm), although the British report suggests that the movement of the throttle (at least in the example they had from the Gotha) was limited to 1400 rpm.

There is a 'bis1' version listed by mossie (in his post 16/2/2005), from 'Die deutschen Militarflugzeuge 1910-1918', which lists the power output at 267 hp maintained to 3000 m at 1450 rpm. I don't know anything more about this version - although possibly a later, overcompressed, version with the same altitude compensating carburettor used on the overcompressed D.IIIa?

Bletchley

Hugh,

You will see there are actually two versions of this engine (three if you count the BIS version).

In Dechamps & Kutzbach on page #6 you will see a listing for the:

D.IVa: inline 6 cyl, 160/180mm (b/s), rated 260ps@1450rpm@msl, with 4.80:1CR.

then on page #193 you see a listing for:

D.IV*: inline 6 cyl, 160/180mm (b/s), rated 260ps@1400rpm@msl, with 5.60:1CR

I call this the D.IVaü which it should be. This, for sure, had an AC (altitude-compensating) carb on it! However, not a very good one since the D.IIIaü had the same 5.60:1 CR and could hold its output up to 1800m where this D.IVaü could only hold it up to 1300m. Breathing (vol) efficiency and carb efficiency (mixture) were controlling these outputs. As you well know!

This may also be the BIS version I'm not sure about that yet.

Respectfully Submitted,

KC

[mik]

I have never given a lot of thought to exactly how carbs work before. I have the Roland C11 profile somewhere. What you have written has made what I am trying to do a waste of my time (not your fault). I have tried to calculate rates of climb using available data without a lot of success. I came to the conclusion someone is lieing. In the case of the Roland if it takes 7 mins to 1000m then even allowing for fuel burn the second 1000m must take longer than 7 mins as the engine loses power due to altitude. If I had been asked the question I would have said the compensating carb could not make up for the loss of power. However now I feel perhaps the Germans were not lieing and there is a whole lot I don't know about WW1 aero engines.
I can calculate what a 180hp engine gives at any altitude but not with a compensating carb. Therefore I cannot calculate rate of climb.

[mik]

I just had another thought If these compensating carbs work as you say and I believe you. Perhaps the allies who printed the data I use are lieing. I have always thought the albatross DVa's performance should be nearer an SE5a.