A relic from the early days of using planes as weapons of war, this 11.8-liter WWI-era Hispano-Suiza engine has a 37mm cannon mounted directly between its cylinder banks. In other words, it is perhaps the most badass engine of all time.
It took a few years for people to figure out how to use airplanes as military vehicles. At the start of World War One, planes were only used for reconnaissance. The first fights between planes were pretty improvised, with pilots throwing bricks and bombs at other planes and shooting at each other with pistols.
The next leap forward, mounting large guns on the plane itself, had a serious stumbling block: Since the planes of the era had propellers, it wasn't possible to fire straight ahead without shooting the propeller to bits at the same time.
In 1916, the Barcelona-based aero engine and luxury car manufacturer Hispano-Suiza figured out a solution to the problem. They managed to mount a 37mm gun right in the middle of their popular, 220 horsepower, battle-tested V8. Since the gun fired directly through a hollow propeller shaft, you could shoot in the direction you were flying and the cannon wouldn't just blow apart what was keeping you in the air.
The French-built Puteaux cannon used in the arrangement could be mounted with a rifled barrel for firing explosive shells, or a smooth bore for firing canisters. This basically created a gigantic shotgun, which was fairly effective at close range.
The flaw of the setup was that the Puteaux cannon was only semi-automatic, with the pilot having to close and lock the breech with his fingers after ever shot. While the "motor-cannon" did see combat, it was never very popular.
But what really killed this Hispano-Suiza engine was that it was fast becoming obsolete. In 1917, the Allies began to use something called a synchronization gear. Connecting an independent machine gun to the engine's crankshaft using either gears or hydraulics, the synchronization gear, well, synchronized, the timing of the machine gun fire with the arc of the propeller. A synchronized machine gun fired between the propeller blades, completely superseding the Hispano-Suiza's motor-cannon. Though Hispano-Suiza had been working on a prototype motor-canon fitted with a fully-automatic machine gun, the whole concept of a gun mounted inside an engine was history.
Only Messerschmitt picked up the idea again in 1935, but jet engines ultimately made any effort to not shoot your own propellers very much obsolete. We now remember this Hispano-Suiza engine as just one more strange experiment of the early days of turning motorized vehicles into machines of war.