Why did WW2 Germany Start Welding Engines Together? DB 600 Double Motors

Throughout the Second World War, as the competition for more power on a smaller scale grew fiercer, Daimler-Benz explored more avant-garde engine designs that they hoped might give them an advantage. Particularly in the early part of the conflict, whichever nation could win the race to gain superior horsepower would likely also hold the upper hand in air superiority and thus have a greater chance of winning the war overall. As such, engineers at Daimler-Benz were driven to research any technological advancements that might increase the power output of an engine while also reducing its overall size. This research encompassed fundamental concepts such as direct fuel injection and turbocharging, all the way to eccentric inventions like the double motor, which was essentially two engines melded together by a single reduction gearbox. And the thing is, unlike many of the other experimental engines laid out by Rolls-Royce and others, these things flew. So stay tuned as we explore what happens if you just straight up weld two engines together in a quest for more power. In 1936, the innovative minds of Siegfried and Walter Gunther set out to construct the Heinkel HE 119, a high-speed light bomber designed for reconnaissance. But one major obstacle stood in their way. No engine on the market could generate more than 2,300 horsepower. To meet this requirement, they had to ask existing engine designers for brand new designs, and to make matters even more complex, this engine would be buried in the aircraft's fuselage, so cross-sectional size was extremely important and needed to be kept to a minimum. Heinkel asked the most renowned aircraft engine manufacturers in Germany to submit proposals, and Daimler-Benz responded by proposing a doppelmotor, or double motor, which was two DB601 V12 engines combined into one 24-cylinder DB606 unit. An ingenious scheme that allowed them to double the power output instantly without having to spend years creating an entirely new motor. Also, I should mention that throughout this video, we'll be making a copious amount of horsepower references, so for context, these numbers are actually Fertigstarke, which is metric horsepower. I don't know if I'm pronouncing that right, but keep that in mind that these are actually metric horsepower. As we said, the engine sections that made up the DB606 were the DB601, which was first conceptualized in the mid-30s with its design based on the DB600. However, unlike its predecessor, which featured a carburetor and geared supercharger, this newer engine, the 601, utilized fuel injection and adjustable speed for the supercharger. This model consisted of two banks of six cylinders each, instead of individual cylinders, and had an inverted liquid cooling system paired with a single aluminum alloy crankcase cover on top to seal it shut. If you've yet to watch my ongoing series about the DB601 and its big brother, the 605, definitely check that out, as it covers everything you'd ever want to know about the specifics of each engine section. In any case, thanks to its pre-existing engine sections, the DB606 was created rapidly and took flight on June 1937 within a Heinkel 119. To deliver power to the 119's four-bladed propeller, the buried engine used a lengthy extension shaft similar to the layout of the P39. Version 1 through version 4 of the DB606 was used for all four 119s made, which initially proved reliable, however, one crash occurred on December 16th due to a defective fuel transfer valve that caused the motor to stop running. Heinkel also chose the DB606 engine to power their long-range heavy bomber design presented in response to the RLM's Bomber A specification. On the 2nd of June 1937, after examining the prototype of this craft they had commissioned, the RLM gave it a designated name, the Heinkel HE-177 Griffin. As modifications to the design imposed by the RLM began accumulating, particularly that of dive-bombing capability, Siegfried was compelled to make adjustments and compromises to meet these demands. Unlike the 119, the 177 was designed with a single DB606 engine installed per wing, which was tucked in close to the main landing gear. This unique design featured two legs on each of its landing gears that retracted toward either the wing root or tip. Unfortunately, this cramped installation resulted in no firewall behind the DB606 engine, which would later prove to be a disastrously bad decision. To make the most of the limited space, right-angle fittings were employed for connections behind the engine. Initially, a surface cooling system was proposed, but it was later changed to annular radiators embedded in each engine nacelle just before the engine assembly. Karl Franke took the first 177 prototype on its maiden flight on November 20th, with engines version 5 and version 6. At that time, before both the aircraft and the engine were even out of prototyping stages, over 800 engine orders were already placed. In December 1940, a significant upgrade was made to the 177 version 6, wherein it received an upgraded 2700 horsepower DB606B1 motor, which was produced by merging two 1350 horsepower DB601E powerplanes. For takeoff and emergency power, the DB606B1 engine could generate 2700 horsepower at sea level. When the engine was run in combat power mode, it ran at 2500 RPM, producing 2400 horsepower from sea level all the way up to 16,000 feet, which was its critical altitude. Maximum continuous power was about 2000 horsepower at 2300 RPM. By the end of the war, this power level would be attained by engines less than half the size of the DB606, but in 1940, these numbers were impressive to say the least. Interestingly, in the same year, Daimler-Benz implemented a lower compression ratio in their right cylinder bank to combat the increased risk of detonation on that side. After some investigating, the detonation was determined to be due to additional oil flinging toward the right cylinders during the crankshaft's rotation and seeping past the piston rings into the combustion chamber. To alleviate this issue, they slightly decreased the compression ratio on that side, improving knock resistance within those cylinders. Also, it's likely that each of the DB601s that made up the entire engine may have been suffering from its own issues, which cropped up later in the conflict, as its valves were likely switched from typical alloys to economy steel, which had less nickel content. They also suffered from corrosion and subsequently caused detonation that would eventually destroy the engine's cylinders on its own. Perhaps unsurprisingly, considering the rushed development of the engine, upon its implementation, the 606 and its installation in the Heinkel 177 proved to be disastrous. As production of the Doppelmotor progressed, it was revealed that both engine sections experienced problematic vibrations of their own, while the combining gear necessitated greater development than was initially envisioned, which would become a theme at Daimler-Benz throughout the war. Also, the DB606 engine experienced significant challenges with its oil circulation, as it would foam up at great heights and lead to inadequate lubrication. While Junkers would begin designing engines that featured an integral oil centrifuge to eliminate this issue, Daimler-Benz would have no such device. As such, these air bubbles caused bearings to cease functioning correctly and pistons to seize, and some of these failures were catastrophic in nature, resulting in connecting rods breaking through the crankcase. However, the most critical issue was caused by the engine installation itself. The annular radiators provided insufficient cooling, which allowed the engines to reach an unacceptable temperature. Even more concerning, exhaust between the two inner cylinder banks became so hot that any fuel or oil dripping from faulty fittings resulted in the engine bursting into flames. With materials scarce and replacements not up to par due to wartime shortages, weeping fittings and seeping seals continually posed a fire hazard. Any spilled fluids would pool in the engine nacelle, only to be set ablaze by hot exhaust radiating heat from behind. In addition, with no firewall between the engine and the aircraft itself, an inferno could quickly spread out of control, engulfing the cockpit, the fuselage, or anything else. Furthermore, due to their cramped layout, the DB-606 engines were challenging to maintain, and any fire that ignited in the wings could cause catastrophic damage within just seconds. The crew seated at the front of the aircraft were often unaware of a blaze until it was way too late. Maintenance crews did their best, but even with diligent effort, sometimes an inferno would break out without warning. In fact, the Heinkel 177 was so prone to engine fires that the Luftwaffe's personnel nicknamed it the cigarette lighter. To tackle this issue, modifications suggested extending the engine nacelle and redesigning the engine's exhaust system. However, since production of the aircraft had already been rushed, modifications were initially dismissed due to fears of delay. So let's talk about the 606's overall design. The two DB-601 engines of the DB-606 were arranged side-by-side with an included angle of 44 degrees, united by a single gear reduction system. This configuration formed an inverted W-shape, where the inner cylinder banks extended only 8 degrees from vertical. The right and left engine sections would be known as the W-motor or the X-motor, respectively. The DB-606's two inner cylinder banks were fitted with exhaust ports in the narrow space between both engines. As opposed to the DB-601, it used a new propeller gear reduction and altered accessory drive, all powering one single propeller without any gun or cannon that could shoot through its hub. Connecting these sections at the rear was an attachable mount for affixing this engine to an aircraft, making them distinct yet conjoined by the gear reduction system and aforementioned stabilizing support. The new gear reduction housing linked the two engine sections and sent it to a single propeller shaft that extended 1.1 meters out in front of the engine. This made it possible to manually disengage and re-engage an engine section, although only when each part was spinning at the same speed. If any engine section suddenly slowed down compared to another, then this would result in automatic decoupling of said component. The engines were coupled together by dogs or claws, as the German literature calls them, on a flange that fit into slots on the crankshaft. To disconnect an engine, you had to pull a lever to move the dog out of the way. By using the levers in the cockpit, operators could pull down on the coupler and disconnect it from both the crankshaft and the reduction housing, yet despite being detached from those parts, it would remain connected to any gears within its own housing. To start the DB-606, each engine section was started one at a time and then once they were brought up to speed and together they were coupled via the reduction gearing system. Rather than each DB-601 being identical to one another, the two engines were mirrored, with the superchargers outbored on the side of each engine. Both the Daimler-Benz DB-606A and B had a 150mm bore and a 160mm stroke, combined for an overall displacement of 67.86 liters. The engine measured 2 meters in length without the extension shaft, 1.6 meters wide and was 1 meter tall from the ground to the top, with its dry weight being about 3,263 lbs or 1,480 kg. Its sibling, the B model, weighed only 110 lbs more than the A version with a total of 3,373 lbs or 1,530 kg when completely dry. While Daimler-Benz was scrambling to resolve issues not only with its 606 engine but with its more traditionally designed engines as well, it was also being continually prodded to simultaneously create engines with even more power. So, in an attempt to both address the issues with the DB-606 and increase the engine's overall power, by late 1942, the Heinkel 177 was revised as the A3 variant. This aircraft would do away with the troublesome DB-606 engines and replace them with DB-610s. The DB-610 was a doppelmotor consisting of two 1,475 hp DB-605A engines. The DB-605 was the development of the DB-601 that operated at a higher RPM, had an increased bore, and had a higher compression ratio. While the DB-610 had a higher power output than before, the new engine only partially eliminated the irksome problems and servicing and maintenance of the engine remained arduous. The DB-610 installation on the Heinkel 177 A3 was extended 200 mm forward and also, fortunately, included a firewall just behind the engine. On March 22, 1943, the Heinkel 177 A3 made its inaugural flight. Despite improvements to dependability, engine fire still occurred with alarming frequency and incurred the same reduction gearing coupling failures that had been previously seen with the DB-606. To say that the doppelmotor program was plagued with problems would be an understatement, and leadership had taken notice. Earlier in May of 1942, Hermann Göring, the commander of Germany's Luftwaffe forces, expressed his opinion about the Heinkel 177 and the DB-606, saying, quote, I have never been so furious as when I saw this engine. Nobody mentioned this hocus-pocus with two welded-together engines to me at all, unquote. By 1944, ideas for a new Heinkel 177 model with four detached engines were forming, an opinion that had been initially mentioned in late 1938 and recommended in mid-1939. However, the RLM eventually cancelled its manufacture and advancement on July 1 of the same year once Allied forces began to approach Europe's shores. German aircraft production shifted towards defensive fighters and attackers rather than bolstering any bombing efforts. Interestingly, Heinkel was not the only aircraft company that would be forced to use the doppelmotors. In the wake of the shortage of 24-cylinder Junkers U-mode 222 inline radial engines, prototypes of the Ju-288 bomber were equipped with the Daimler-Benz DB-606. In the Ju-288, the layout was similar to its predecessors, with each engine installed on its own wing and fitted in a cell along with an annular radiator. To improve the power of the Ju-288, the designers soon switched to the more powerful DB-610 engine. Fortunately, unlike the Heinkel 177s where frequent engine fires occurred, this actually proved not to be an issue experienced with the Ju-288s. Surprisingly, following the conclusion of the Second World War, the DB-610 was featured in a French SNCAC NC-3021 Belphegor high-altitude research airplane. Like its predecessor, the installation featured annular radiators and the engines required an incredible amount of upkeep and SNCAC eventually went bankrupt and the 3021 tests were concluded. The DB-610 had a 154 mm bore and 160 mm stroke and a total displacement of 71.5 liters. For takeoff and emergency power at 2,800 rpm and 20.9 psi of boost, the engine produced 2,950 hp at sea level and 2,700 hp at 18,701 feet. For climb and combat power at 2,600 rpm and 19.1 psi of boost, the engine produced 2,620 hp at sea level and 2,500 hp at 19,000 feet. For maximum continuous power at 2,300 rpm and 16.9 psi of boost, the engine produced 2,150 hp at sea level and 2,160 hp at 18,000 feet. The DB-610 was an equal size to the DB-606, measuring 2 meters long by 1.63 meters wide and standing at a height of 1.06 meters. However, it weighed an extra 130 pounds or 60 kilos. The dry weight for the A model came in at 3,395 pounds, 1,540 kilos, while its B variant had a weight of 3,483 pounds or 1,580 kilos. Although the DB-613 double motor was a lower priority compared to the other models, like the DB-606 and 610, Daimler-Benz saw potential in applying their successful concept to one of their largest single engines, the DB-603. This version had slightly different characteristics. An enlarged bore and an elongated stroke resulted in reduced supercharging and takeoff power, but with a greater compression ratio than that of its predecessors. Ultimately, this transformation would prove beneficial for Daimler-Benz, as it allowed them to create more efficient engines with superior performance capabilities. The external cylinder banks on the 613 had a compression ratio of 7.3, while the inner branks had one of 7.5 to 1. In 1940, the DB-613 was engineered by fusing two 1,750 hp DB-603G engines. The combination gear housing for the DB-613 stood apart from that used on alternate models such as the 606 and 610 due to its H-symmetric design with a propulsion mechanism via the W motor. The Daimler-Benz DB-613A-B had a 162 mm bore, a 180 mm stroke, and a total displacement of a whopping 89.04 liters. For takeoff and emergency power at 27 rpm, it made 3,600 hp at sea level and 3,100 hp at 7,000 meters or 22,966 feet. For climb and combat at 2,500 rpm, it boosted about 19.8 psi. The engine produced 3,150 hp at sea level and 2,860 hp at 23,293 feet or 7,100 meters. For maximum continuous power at 2,300 rpm at 18.4 psi boost, the engine produced 2,790 hp at sea level and 2,650 hp at 21,982 feet. The Daimler-Benz DB-613 was 2.2 m long with its extension shaft and 1.77 m wide as well as 1.14 m tall. The dry weight of the DB-613A was 4,321 pounds and the dry weight of the DB-613B was 4,409 pounds or 2,000 kilos. The DB-613 was proposed for the Heinkel HE-177A-7 variant, but the aircraft was not produced and the engine never progressed beyond the prototype stage. It's not believed that the DB-613 was really ever flight tested. The most powerful double motors the Daimler-Benz ever designed was the DB-613C-D, which had the same increased compression ratio due to it being designed for higher octane fuel. For takeoff and emergency power at 2,900 rpm, the DB-613C-D produced 4,000 hp at sea level. For climate combat power, it could make 3,500 hp and for maximum continuous power, it would produce 2,800 hp. While the Daimler-Benz double motors provided an efficient way to increase engine output while minimizing any increase in cross-sectional area in theory, in practice, due to the constituent engines having their own issues, plus the new problems introduced by conjoining them, the engine largely proved to be a temperamental death trap. Around 1,916 of these engines were manufactured, 820 DB-606s, 1,070 DB-610s, and only 26 DB-613s. The engines had also been tested in the Junkers JU-52 transport planes, as well as 4 Heinkel 119s, 915 Heinkel 177s, and 10 JU-288s. If you enjoyed this video, don't forget to like and subscribe, and as always, thanks for tuning into Flight Dojo.