Complex sentence: The design was the peak of aviation technology in the years shortly before and during World War I and saw use in a few early motorcycles and cars, a very efficient solution to the problems of power output, weight, and reliability.
Split into simpler sentences: The design was used mostly in the years shortly before and during World War I to power aircraft, and also saw use in a few early motorcycles and cars. However, at the time it was a very efficient solution to the problems of power output, weight, and reliability.