We’ve discussed various engine layouts before — Inline, V, flat, Boxer, horizontally opposed, and even triangular engines (and I’m not talking rotaries). But there’s another, not-so-common layout, known as the X engine. First appearing in the 1920s, the X-layout is not one that proliferated throughout history, but also didn’t die on the vine, with a modern example currently in use today.

The first prototype came from a company now known for its out-of-the-box engine designs, Napier. In 1919, they developed the “Cub.” An almost-asymmetric X design (with three bank angles, of 52.5 degrees, 127.5 degrees, and then 90 degrees), it was a big, 16-cylinder, 60-liter design that produced 1,000 horsepower and powered an early Avro biplane bomber, the Aldershot.

The first automotive X engine came with eight cylinders from the Ford Motor Company 100 years ago. While looking for a space-saving design, the X8 layout was investigated and prototyped. With 90-degree cylinder angles all around and 107 cubes of displacement, the Ford X8 made all of 35 horsepower. Ultimately the Ford Flathead V8 became the compact eight-cylinder engine that revolutionized automobiles. It wasn’t until World War II that the X configuration actually made it into production.

The Napier Cub was the first “X” design, although it wasn’t really much of an X layout. The top bank angle was 52.5 degrees, the bottom bank angle was 127.5 degrees, and the sides were at an even 90 degrees.

Wartime Innovation For The X-Engine

We often see wartime innovation and the X layout was no different, with the design appearing in aircraft and on the sea. The Rolls Royce Vulture — which was an X24 layout made up largely of two Peregrine V12 engines mated together — powered the Avro Manchester bomber. On the ocean, a pair of General Motors two-stroke 16-cylinder diesel engines, the 16-184 (16 cylinders, 184 cubic inches of displacement each cylinder), powered sub-chasers around the world’s seas with 1,200 horsepower and 3,500 lb-ft of torque each. Then, towards the end of the war, a 1,000-horsepower variant called the 16-338 (which had the same displacement as the 16-184, despite the name) two-stroke diesel engine was fitted to the U.S. Navy’s Teng-class of submarines, where they saw much less success than the smaller variant did in the sub-chasers.

In the air, the X platform was looked at as an alternative to V engines, for both its overall size and balance, but was surpassed by the radial engine designs, which were less complicated and were able to pack in more cylinders in a similar amount of space. There were prototypes that never made it to production on both the Allied and Axis sides of WWII.

Known for their unique engine designs, Daimler-Benz built the prototype DB 604 24-cylinder 46-liter square (5.315-inch bore and stroke, as well as even 90-degree bank angles) engine. With a two-speed supercharger, the prototype developed 2,500 horsepower but was cancelled in 1942.

Once The Guns Fell Silent

The X layout wasn’t produced again for another 70 years, but in that time span, it was certainly experimented with. It’s rumored that there was some interest in the design for a Formula 1 effort in the 1960s, and then in the early 2000s, an Australian engine design firm called Revetec experimented with a unique gasoline-powered X4 design. While the modern adaptation was intriguing, it fell short of making it out of the prototype phase.

However, in 2015 an X engine did make it into production — once again as an implement of war. The Chelyabinsk Tractor Plant (or CTZ-Uraltrak) 12N360 A-85-3A turbo diesel engine powers Russia’s newest tank design, the T-14 Armata. It is an X12 design that produces 1,500 nominal horsepower at 2,000 rpm. This modern rendition of the X engine is on the flatter side of square, with 60-degree and 120-degree complementary bank angles, and is force-fed by a pair of turbochargers.

While spanning more than a century, the X engine design can hardly be called one of history’s more successful engine layouts. While initially designed as an aeronautical powerplant, the limited success of the design seems to be centered around diesel applications on the surface of the Earth – be it on land or sea. However, one thing that can’t be questioned is the uniqueness of this interesting internal combustion engine layout.