In the early 1920s George Edlin (b. ?, d. ?) and Hector Halhead ('Steam') Stewart (b. 1888, d. ?) developed a new kind of engine.
It was a horizontally opposed two-stroke engine. Two stroke engines are inherently efficient as compared to four-stroke engines, simply because it only takes two piston strokes to deliver the power - so there's less friction, less movement for the same power stroke. However, a two-stroke engine has trouble efficiently flushing the exhaust from the chamber in time for the next intake (that's what the other two strokes in the four stroke are for). This means that, especially at low revs, the two-stroke will always run rough. The Edlin-Stewart engine was more accurate and efficient at getting rid of the exhaust and taking in the new mixture. Two pistons shared a common cylinder, and a sleeve inside the cylinder moved in opposition to the pistons.
Holes in the moving sleeve formed the valves and both the sleeve and pistons were connected to cranks. Tests at Auckland University throughout the twenties confirmed the engine's power-to-weight advantages and during tests it was accidentally discovered that the engine (made to run on petrol) was also a very good diesel engine.