The engine turns at 3800 RPM and the prop speed is slowed to 2300 RPM for higher efficiency. If you tried to spin the prop at 3800 RPM, you'd have to reduce the diameter to about 54 inches. Tip speed is generally kept below Mach 0.8 because higher speeds cause a loss of prop efficiency and a huge increase in noise. At 2300 RPM, you could swing a prop up to 7.4 feet in diameter without exceeding Mach 0.8, or you could go with a somewhat smaller prop and be a lot quieter. The FAQ section mentions that the engine will swing a 3-blade constant speed prop. Even with only 135 HP, it produces substancially more static thrust than 150 HP Lycoming gas engines and even holds its own against 180 HP engines. Impressive. Unfortunately, the FAQs also mention that the complete installation kit (engine, prop, cowling, electronics, radiator, etc) will cost 40,000 Euros. Ouch!

Reading the engine description, it sounds like a 4 stroke diesel. The clue is the reference to titanium valves - 2 strokes typically don't have valves with the exception of some engines with wooden reed valves.

There are some 2-stroke diesels being developed for aircraft use. I don't think any of them are near production status, though. Teledyne Continential is dragging out development of their 2-stroke turbocharged diesel. It has fantastic specs but no word on when (or if) it'll see production. I'd love to hang one of those engines on my Cessna 172! To give an idea how slow their progress is, they showed the engine in 1997 and just completed ground testing before flight testing the first one last February.

Another company called Delta Hawk has been demonstrating some very impressive diesel engines but they come across as a hobby shop rather than someone who ever intends to build production engines.