Identifying DB605AS vs.
As the air war progressed through 1942 and 1943, newer variants of the 109 were introduced in an effort to maintain a competitive advantage. From an aerodynamic standpoint, incremental changes were introduced with the original G series, and a seminal change became evident with the introduction of the G-6 variant in early 1943. This model introduced higher-caliber cowl guns (13mm MG131s vs. 7.7mm MG17s on earlier models), but substantial modifications were required to the airframe in order to accommodate this change. Aerodynamically, the aircraft suffered from the addition of two large circular fairings covering the feed chutes for the MG131s, and the increased weight of the weapons and their ammunition led to a further slight decline in performance. Clearly, more power was required.
Naturally, efforts were underway to remedy this situation. Daimler-Benz engineers had been working since late 1942 on an improved version of the DB605, the DB605D, but numerous teething troubles were encountered in the development process. Some measure of improvement was found by the relatively simple expedient of marrying the larger DB603 supercharger to the existing DB605A engine, resulting in the DB605AS (S= Sonder, or special). This combination proved worthy of concurrent development alongside the gestating DB605D, as this allowed for the use of existing DB605A major componentry thereby reducing the development time and production interruptions an entirely new engine would bring. In fact, development of new AS variants continued even after the service introduction of the DB605D, with Daimler Benz factory documentation discussing the emerging DB605ASB and ASC in early 1945 (source: Mermet, p.11).
The DB605A Family
It is instructive to begin this story by looking at the original DB605A engine, to better understand the improvements offered by the DB605AS and D engines.
The following technical data is taken from the 1945 Jane's "All The World's Aero Engines", and based directly on wartime evaluations by RAE engineers.
The Daimler Benz DB 605 A, B, and C.
The DB605 was a development of the DB601 and was very similar in basic construction to that power-unit. The main improvements were an increase in the permissible r.p.m.; altered valve-timing, which increased the inlet period and improved the scavenging to give greater volumetric efficiency at the higher r.p.m.; complete re-design of the cylinder block to obtain the maximum possible bore with the existing cylinder centres; and re-positioning of the sparking plugs. The big-end bearings were also modified.
TYPE.- Twelve-cylinder inverted Vee liquid-cooled.
CYLINDERS.- Bore 154mm., Stroke 160mm., Capacity 35.7 litres. Compression ratio 7.3:1 (left block) 7.5:1 (right block). Two inlet and two exhaust valves per cylinder.
BIG-END BEARINGS.- Roller bearing big-ends discarded in favour of flanged lead/bronze-lined bearings with tin-flash coating.
FUEL SYSTEM.- Bosch direct-injection pump mounted between the cylinder blocks. Injectors on inside of cylinder blocks. Special slow-running system which by-passes the main throttle. 87 octane fuel.
SUPERCHARGER.- Centrifugal impellor with 16 blades. Hydraulic coupling drive. The coupling is automatically regulated by a control capsule subjected to atmospheric pressure which varies the supply of oil to the hydraulic coupling. This form of coupling has been retained from the DB601, but has been considerably modified in order to reduce the operating temperature of the coupling. High temperature was a fault which occurred in the DB601 supercharger drive. Gear ratio (no slip) 10.07:1.
IGNITION.- Dual Bosch magnetos mounted at the top of the rear cover.
LUBRICATION.- Pressure pump delivers oil to each of the seven main bearings, which feed pressure oil to each crankpin and big-end bearing surface. Pressure oil is also supplied by the pump to the hydraulic supercharger gear, and to the interior of the camshaft. The oil is scavenged from each camshaft cover.
STARTER.- Bosch hand/electric inertia starter.
AIRSCREW DRIVE.- Spur gear type. The detachable airscrew shaft of the DB601 has been discarded for a longitudinally splined gear shaft extension. Gear ratio A= .594:1, B= .534:1, C= .497:1. (Ed. note: The B and C versions were rarely seen)
DIMENSIONS.- Overall length (including starter and airscrew shaft) 2158.5 mm., Width 760mm., Height 1,037 mm.
WEIGHT (with starter).- 756 kg (1,663 lbs).
PERFORMANCE.- Take-off and emergency 1,475 h.p. at 2,800 r.p.m. at 1.42 ata at sea level, 1,355 h.p. at 2,800 r.p.m. at 1.42 ata at 18,700 ft. Climbing 1,310 h.p. at 2,600 r.p.m. at 1.3 ata at sea level, 1,250 h.p. at 2,600 r.p.m. at 1.3 ata at 19,000 ft. Maximum cruising 1,075 h.p. at 2,300 r.p.m. at 1.15 ata at sea level, 1,080 h.p. at 2,300 r.p.m. at 1.15 ata at 18,000 ft. Fuel consumption .473 lb/h.p./hr. maximum cruising, sea level.
Janes' description of the DB605AS is somewhat more succinct, focusing on the performance differences between the standard A motor and the AS:
Daimler Benz DB 605 AS.
Similar in general construction to the DB605A, but fitted with a supercharger of increased diameter. *
PERFORMANCE.- Take-off and emergency 1,435 h.p. at 2,800 r.p.m. at 1.42 ata at sea level, 1,200 r.p.m. at 2,800 r.p.m. at 1.42 ata at 26,200 ft. Climbing 1,275 h.p. at 2,600 r.p.m. at 1.3 ata at sea level, 1,150 h.p. at 2,600 r.p.m. at 1.3 ata at 25,600 ft. Maximum cruising 1.075 h.p. at 2,400 r.p.m. at 1.25 ata at sea level, 1,050 h.p. at 2,400 r.p.m. at 1.15 ata at 25,200 ft.
* Ed. note: The hydraulically-coupled supercharger was that fitted to the DB603, as noted in Mermet's book, and offered a 25% greater volumetric capacity than the standard unit.
The larger supercharger installation was seized upon as a way to improve the high-altitude performance of the DB605A motor, and was successfully tested on a Bf109G-5, W.Nr. 26108 (SL+RR) (source: Monogram Luftwaffe Interiors, p. 155).
The addition of the larger supercharger necessitated new engine mounting methods. A new port engine bearer had to be fitted which curved up and over the new supercharger; the lower attachment point of the bearer arm also required an offset flange to be fitted, as shown in the photos below:
Bf109G-6 with DB605A
Bf109G-6/AS with DB605AS (photo of G-10/ DB605D, bearer arm similar)
As noted previously, development of the AS engine proceeded alongside that of the D, with the following major sub-types being listed:
- DB605ASM: Provisions for the use of MW50 additive with 96 octane C3 fuel. It was possible to use standard 87 octane B4 fuel with this engine, in which case the use of MW50 was absolutely required to obtain the best possible power and avoid engine damage. The compression ratio of the engine was raised as well, to 8.3:1 (left) and 8.5:1 (right), giving 1,800 h.p. at 1.7 ata at takeoff. Other changes introduced with this variant included a larger capacity oil cooler (Fo987) and redesigned cylinder head covers, both of which were fitted to the DB605D as well. (source: Mermet, p. 9/10) The Fo987 was first evaluated on G-6/AS W.Nr. 16550 (KT+DX) in June 1944 (source: Monogram Luftwaffe Interiors, p. 155).
- DB605ASB/ASC: Appearing in the beginning of 1945, this engine series resulted in performance on a par with that achieved in the DB605D, and were created by "modernizing" (Daimler-Benz' term) existing DB605AM, AS, and ASM engines using a fuel selection device developed for use on the DB605D (see below). The original small 38.6 liter oil tank from the DB605A was used in conjunction with the larger Fo987 oil cooler and larger cylinder head covers shared with the DB605D. The compression ratios remained the same as with the DB605ASM. The difference between the two designations is a subject of some debate:
In the JaPo book on the Bf109K (p.81), Janda and Poruba mention the differences between the DB605DB and DC as being dependent not on fuel compatibility, but on maximum boost pressure. The maximum manifold pressure figures quoted are as follows:
- DB605DB: 1.80 ata at 2,800 r.p.m., giving 1,850 h.p. (no performance difference noted between B4 and C3 fuel usage)
- DB605DC: (with MW50) 1.98 ata at 2,800 r.p.m., giving 2,000 h.p
(without MW50) 1.80 ata at 2,800 r.p.m., giving 1,850 h.p
However, J.C. Mermet (p. 14, 15) quotes an official Daimler-Benz factory manual dated 5 December 1944 concerning the different designations. The B designation indicated the engine was capable of using 87 octane B4 fuel WITH MW50, or 96 octane C3 fuel WITHOUT MW50 whereas the C designation indicated the use of 96 octane C3 fuel WITH MW50. Interestingly, the engine could be converted from a B to a C model and back again by the simple expedient of adjusting a screw valve which regulated the flow of MW50 to the engine. This would seem to be the more proper explanation, especially given the chaotic fuel situation in the Reich from 1944 onwards. It can also be seen how the different fuel configurations would account for the maximum permissible boost ratings as noted by Janda and Poruba. There is still discussion on this point, however, and further documentation may yet refine this point.
The maximum power figures between the two sources do agree, although Mermet points out that the 1.98 ata figure of the C motor was attainable only with MW50 at 110% emergency power, and operation of the B motor without MW50 would be limited to a maximum manifold pressure of 1.45 ata, and 100% power was not available anywhere within the flight regime (Note: these restrictions did not apply if MW50 was used with the B motor)
The ASB/ASC engines also shared the hydraulic and barometric supercharger regulation with the original DB605A (see table above).