Questions about the Mercedes DIII series

[PreußensGlanz]

So after going thru all threads here I still have some questions left.

1. What part was mainly responsible for raising the PS?
Just the pistons or the carburetor too?
The new carburetor on the aü seems to have helped keeping PS up in greater heights but did the carburetors help in raising the PS at all?

2. When did the PS raising part went into the engine production?
I have a timeline for it but different DIII versions:
1916.08.15 Mercedes D-III(160PS/162,5HP)
1917.02.01 Mercedes D-III(175HP)
1917.09.15 Mercedes D-IIIa(170PS/180HP)
1918.03.01 Mercedes D-IIIaü(180PS/200-217HP)
1918.10.15 Mercedes D-IIIavü(185PS/200-220HP)

but I also have sources that give other dates like this:
AeroScale :: Mercedes D. type motors (http://www.aeroscale.co.uk/modules.php?op=modload&name=SquawkBox&file=index&req=viewtopic&topic_id=86153&page=1)

And production numbers for the DIIIa out of this forum that show it was already coming in in June 1917.

Hopefully someone can say something definitive about when the PS raise occurred.

3. Was the carburetor of the DIII to DIIIa similar?
I heard the DIIIa got a new carburetor but not much different to that of the DIII and it seems to me only the carburetor on the DIIIaü was really improving the PS keeping in greater heights.
Did the aü carburetor come in already with an earlier engine version?

[Bletchley]

Hello PreußensGlanz :)

1. The rise in PS was mainly the result of increase in the Compression Ratio, but there were other incremental improvements that probably contributed. Increase in CR was itself linked to changes in the fuel used - before the Spring/Summer of 1917 the flugbenzin was a 'light' benzin derived from Romanian crude oil that appears to have been low in aromatic content, that would support a max. CR of around 4.7:1. Following the entry of Romania into the war, this supply was cut off and Germany was forced to economise by (a) blending the 'light' benzin with 'heavy' benzin in a 60/40 proportion to create a new 'middle' type of flugbenzin, and (b) rely more heavily on alternative fuels such as benzol and alcohol. The addition of either 'heavy' benzin to the fuel blend (with a higher aromatic content), or the addition of small amounts of benzol to this blend, led to a flugbenzin that was higher in aromatic content and was now very similar to the aviation fuel used by the French and British (derived from East Indian Sumatra and Light Borneo crudes that were naturally very high in aromatic content) that would support a max. CR of around 5.3:1. But from around the Winter/Spring of 1918 Germany started to produce relatively small quantities of a 'special' aviation fuel (in 'winter' and 'summer' versions) that had a very high proportion of benzol added to the blend: called Fliegerbenzin to distinguish them from flugbenzins, these permitted the very high compression ratios of between 6:1 and 7:1 used in the late war high altitude engines. It is likely that there was some chronological overlap in the use of 'Light' and 'Middle' types of flugbenzin, with both still in use up until the end of the war, with the predominant 'middle' type commonly referred to as simply 'benzin'. It has also been speculated on another thread here that the 'F' designation in the Fokker D.VIIF was an indication that only the 'Fliegerbenzin' was to be used in the very high compression BMW IIIa, the 'F' being used to distinguish this from the standard aviation 'benzin'.

3. So far as I am aware, yes, but I would be very interested in any evidence that you may have that there were changes in the carburettor between the D.III and D.IIIa. The aü was essentially a D.IIIa with a raised CR and a new altitude carburettor. This new carburettor did not raise the output (PS) - indeed, it did the very opposite, placing a limit on the output of the engine at lower altitudes by substituting the main fuel jet of the D.IIIa carburettor for a significantly smaller main fuel jet in the D.IIIaü carburettor, forcing the engine to run very lean at low altitude and fully open throttle (so lean that, fully unthrottled, the engine would cut out). This trick of leaning the mixture at fully open throttle and low altitude, to adjust the engine for better high altitude performance, was in use to some extent in most German aviation engines of this period (including the earlier D.III/D.IIIa), but generally to a much lesser extent - the exception being the Maybach high altitude engines introduced around the Autumn of 1917 (used predominantly in high altitude versions of the Rumpler, and in air ships) that used the same technique but in a different design of carburettor. So far as I know the aü type carburettor was used only on the Daimler Mercedes engines, although later Benz engines had a throttle/carburettor arrangement that used the same technique but in a slightly different way. You will find more detailed information and data on this in previouse Aerodrome threads on the altitude compensated engines and petrol octane.

Bletchley

[dirtyshirt]

Good evening,
Thank you PreussensGlanz and Bletchley for your informative posts, but...(there's always a but).....in PreussenGlanz' question 2 we have;

1916.08.15 Mercedes D.III 160 Ps / 162 SHP..
1917.02.01 Mercedes D.III 175 SHP.

Now, these are generally published and accepted output figures and what intrigues me is that we have an unexplained power increase of approximately 12 Ps (7.5%) in what is apparently the same motor. The fuel and carburettor remains the same, the C/R ratio does not increase (except marginally in some reports) and the valve train remains the same.

So where did the extra output come from ?..This question is referring specifically to the D.III motors and not to the subsequent D.IIIa and D.IIIau series.

With regards,
Mike.

[Dan_San_Abbott]

Hi Mike:
Where did you get your data on the D.III with 175Ps? I am curious, it does not sound correct, I thought the 175 (170) Ps came with the D.IIIa?? i am sure Bletchley will sort this out.
Blue skies Mike,
Dan-San

[Bletchley]

There is variation in the published output of almost every engine. There would be about 5 hp/PS difference depending on the engine being 'cold' or 'warm' (a reading taken from cold being higher). The figure could also be taken from the highest reading from one run, or from an average highest reading from a series of several runs. Or the reading might have been back adjusted to standard temp. and pressure at sea level, or simply recorded 'as is' for the altitude and the environmental conditions of the test bench. There might also be variations in rpm, or in the condition and tuning of the engine. Fuel differences might also account for something like a 1% or 2% difference, assuming that there is no detonation or distribution issues. Direct conversion from PS to hp might also raise the figure slightly.

As Dan-San comments, however, I have not seen a figure as high as 175 hp for the Mercedes D.III engine. The date sugests that this may have been a later version with some, but not all, of the modifications later applied to the Mercedes D.IIIa (possibly the higher compression ratio?), or perhaps just a case of mistaken identity. The higher figures for the Mercedes D.IIIaü and D.IIIavü are 'nominal' output figures for this engine at sea level (the lower ones being 'actual', although the output of the D.IIIaü would, I think, have been closer to 160/165 PS at 1400 rpm with flugbenzin, the higher 180 PS figure being attained at around 1500 rpm with the use of Fliegerbenzin to supress detonation at ground level).

Bletchley

[PreußensGlanz]

1917.02.01 Mercedes DIII
Details of that version are:
Similar to the first Mercedes DIII but replaced concave pistons with flat shaped piston head for Albatros D.III.

[Kacey]

So after going thru all threads here I still have some questions left.

1. What part was mainly responsible for raising the PS?
Just the pistons or the carburetor too?
The new carburetor on the aü seems to have helped keeping PS up in greater heights but did the carburetors help in raising the PS at all?

2. When did the PS raising part went into the engine production?
I have a timeline for it but different DIII versions:
1916.08.15 Mercedes D-III(160PS/162,5HP)
1917.02.01 Mercedes D-III(175HP)
1917.09.15 Mercedes D-IIIa(170PS/180HP)
1918.03.01 Mercedes D-IIIaü(180PS/200-217HP)
1918.10.15 Mercedes D-IIIavü(185PS/200-220HP)

but I also have sources that give other dates like this:
AeroScale :: Mercedes D. type motors (http://www.aeroscale.co.uk/modules.php?op=modload&name=SquawkBox&file=index&req=viewtopic&topic_id=86153&page=1)

And production numbers for the DIIIa out of this forum that show it was already coming in in June 1917.

Hopefully someone can say something definitive about when the PS raise occurred.

3. Was the carburetor of the DIII to DIIIa similar?
I heard the DIIIa got a new carburetor but not much different to that of the DIII and it seems to me only the carburetor on the DIIIaü was really improving the PS keeping in greater heights.
Did the aü carburetor come in already with an earlier engine version?

PreußensGlanz,

Where did these numbers come from? The listed numbers for BHP or SHP are converted BACKWARDS. PS is higher than British Unit BHP. PS x 0.9863 = BHP

Regards,

KC

[HoHun]

Hi Kacey,

>PS is higher than British Unit BHP. PS x 0.9863 = BHP

As far as I know, the definition of the two horse power values is as follows:

Metric Horse Power: 1 HP_m = 75 kg * 1 m/s * g = 735.50 W

Imperial Horse Power: 1HP_imp = 550ft * lb(f)/s = 745.70 W

This agrees with the factor you mention, but the Imperial Horse Power is in fact the higher value than the Metric Horse Power.

Regards,

Henning (HoHun)

[Ransom E. Olds]

Another potential source of increased performance that is easily lost sight of is improved bearing materials that would allow the engine to run at higher BMEP levels without producing expensive noises. I know that in the case of the aircooled Volkswagen engines there was a close correlation between main bearing improvements and horsepower increases. Ransom

[dirtyshirt]

Good morning,
All these disparate output figures for the various types of D.III motor have been posted on this forum and other websites over the past 10 years, I suspect PreussensGlanz and myself have been trawling the same ground and come up with the same information. Bletchley has addressed this with his usual acuteness so I think that we can dismiss the existance of a D.III motor delivering 170/175 Ps in January/February 1917. The insinuation of the 170/175 Ps Mercedes D.III motor is that the Albatros D.III of early 1917 had a superior motor to the D.I and D.II types of late 1916.

PreussensGlanz also raises the interesting point of the introduction of the (mythological ???) flat-headed pistons to the Mercedes D.III types. Most online sources concur with the existance of these flat-headed pistons, but certain people who I consider to speak with authority on the subject flatly deny the existance of these pistons but they seem to be a lone voice in the wilderness. Has anybody ever seen one of these pistons or even a photograph of one ?.

Various compression ratios have also been posted viz;
D.III 4.48:1, D.III (transitional ?) 4.64:1, D.IIIa 4.64:1 and 4.9:1 and D.IIIau 5.73:1. I don't have the resources to speak authoritively on the matter but it would appear as if the D.III 4.48:1, D.IIIa 4.64:1 and D.IIIau 5.73:1 are correct, maybe somebody can confirm or deny this.

The last word is a long way from being spoken on these motors, we use the term D.IIIau whilst Daimler called the same motor D.IIIa uberverdichtet. Was the term D.IIIau simply a convenient name given the motor post-war by the English and American evaluators to whom the German titles were untranslateable and if so should we retain the present name for convenience or revert to the correct German name.

Lots of questions.

With regards,
Mike.