Cosworth, an engineering team based in Great Britain, designed a small 3.9L V12 engine for the Gordon Murray T.50 racer long before it came up with a 6.5-liter V12 for the modern Aston Martin Valkyrie. However, it went even smaller once in the past.
In late 1990s, Cosworth was tasked with designing a Formula One-ready V10 and came up with a prototype engine with only one cylinder and mere 300 milliliters of displacement. Despite its modest parameters, the unit produced respectable 90 metric horsepower (89 hp / 66 kW).
Designed and improved in 1998–1999, the prototype motor ultimately provided the basis for the TJ engine that various Formula racers used in the next decade. This included the Jaguar Racing Team car in 2003 and the Red Bull racer in 2005.
The reason for such a tiny engine to exist was because it provided the cheapest and easiest way to test out new components before building a full-scale V10.