By the 1930s, German aeronautical engineers had a requirement for more than 800 horsepower for the new aircraft designs being developed. But, much like a lot of automotive projects, sheer power wasn’t the only demand. The new line of engines had to meet a strict list of design requirements. The new engine design needed an ultra-compact cooling system that wouldn’t produce aerodynamic losses. It needed to run on low-quality fuels and be easy to repair in austere conditions. It also needed to both provide excellent downward visibility and allow forward placement of armament aligned with the pilot’s line of sight. The answer to all of that was an inverted V12.

Answering that call, Daimler-Benz delivered the DB 600. A 33.9-liter inverted V12 engine that became one of the most important engines of the German manufacturer. But why would someone build an inverted engine? What advantages did it offer?

Why Flip an Engine Upside Down?

Unlike traditional aircraft engines, the DB 600 was completely inverted, with the crankshaft on top and the pistons below. This design offered several benefits over a traditional engine design, checking a lot of boxes on the list. The arrangement allowed for the required downward visibility. The design also created a lower center of gravity,  helping aircraft handle better in high-g maneuvering.

The design also aided engine cooling. With the engine being inverted, the oil fell and remained on the underside of the pistons. This ensured that a large amount of oil touched the pistons cooling them, allowing gravity to perform the job of a piston squirter. The DB 600’s inverted design also meant that the exhaust pipes were in the lower part, ensuring that any oil in the exhaust would not dirty the windshield. This was particularly important during night flights when exhaust flares could interfere with the pilot’s view.

Engineering the DB 600 Inverted V12

Daimler-Benz built the engine block using Duralumin, an advanced alloy of aluminum, magnesium, and copper, which allowed for rapid heat conduction and thus ensured that a uniform temperature was maintained throughout the engine. In addition, this alloy is very light. The cylinder liners contained silicon, making them wear-resistant. The pistons, made of light alloy, featured five rings— three of which were for compression and two oil scrapers.

The DB 600 was also fitted with a supercharger, which increased power at takeoff and then maintained it when it rose and encountered less dense air. With its 33.9-liter displacement and supercharger, the engine produced up to 1,000 horsepower at just 2,800 rpm and weighed less than 700 kilograms or 1,500 pounds.

Dry Sump Lubrication and Advanced Valvetrain

To ensure lubrication in any orientation, the DB 600 used a dry sump system. The valve covers also serve as oil pans, thanks to its inverted design.  In each of these, there are oil pumps that raise the fluid to the tank where the pressure pump is located. This meant the engine could operate even under extreme high-g maneuvers.

The camshafts were gear-driven, with one on each bank rotating at half the speed of the crankshaft. Each cylinder featured two intake valves and two exhaust valves for better airflow.

Inverted V12 Fuel Injection

By 1935, Daimler-Benz introduced the DB 601, which featured mechanical gasoline direct injection, developed by Bosch. Unlike carburetors, which struggled under extreme maneuvers and high altitudes, direct injection delivered precise fuel metering. The pump consisted of 12 inline pistons that raised the fuel pressure to 90 bar (~1,300 psi) and then injected it directly into the combustion chamber.

The introduction of direct injection provided several advantages, such as allowing aircraft to fly inverted without fuel starvation. It also allowed the aircraft to perform all kinds of maneuvers, such as prolonged vertical climbs or negative-g aerobatics, where a carburetor float bowl would simply run dry.

The direct injection also lowered combustion temperatures. by lowering the temperatures, detonation is reduced, allowing the use of low-octane fuels. This also resulted in increased reliability in less-than-ideal conditions. 

Cooling System Innovations and Limitations

While the combustion temperatures were lower, the DB 600-series engines operated at higher engine temperatures. The engine was designed to operate with coolant temperatures in the 250-degree-Fahrenheit (120-degree-Celcius) range.

This allowed for a smaller radiator, improving aerodynamics and reducing weight. However, there was a major flaw. In the event of a system failure any kind of damage to the components, the closed circuit would break and the pressure would drop rapidly. Since the water is at 250 degrees, the result would be an immediate boiling off of all the coolant, causing an engine failure. 

The DB 605 and the Twin-Engine DB 610

Daimler-Benz continued to improve the DB 600-series inverted V12 design, leading to the DB 605, which had a slightly larger displacement at 35.7 liters, an impressive 1,475 horsepower at takeoff.

To generate even more power, Daimler-Benz engineers joined two DB 603 engines in a parallel configuration, creating the DB 610. This 71.4-liter engine produced 2,900 horsepower and was used in bombers like the Heinkel He-177. However, this design was a nightmare to maintain. These engines were so tightly packed that changing an inner cylinder spark plug was impossible without disassembling the exhaust manifolds, which required removing at least one engine.

The DB 600-Series Legacy

The DB 600-series inverted V12 engines powered some of the most iconic German aircraft, including the Messerschmitt Bf-109, Bf-110, and Heinkel bombers. Their compact design, high power output, and pioneering fuel injection system gave German aviation an edge in the early years of World War II. The impact of direct injection and high-temperature cooling lives on — modern automotive fuel injection systems, high-pressure cooling circuits, and compact superchargers all trace their roots back to Daimler-Benz’s radical innovations.

Was the idea of an inverted engine really crazy? If you look at one of the most popular engine designs in aviation, the radial engine, the lower cylinders are inverted. So, producing an engine engine made up of inverted cylinders really doesn’t seem all that crazy. The DB 600-series was ahead of its time, proving that sometimes, thinking upside down really is the best way forward.