Complex sentence: Allison's manager, N.H. Gilman, decided to experiment with their own high-power cylinder design, which they presented to the Army in 1928, who turned down the development proposal.
Split into simpler sentences: Allison's manager, N.H. Gilman, decided to experiment with their own high-power cylinder design. The result was presented to the Army in 1928, who turned down their development proposal.