While the military did contribute to primarily large engine development, automobiles including race cars saw the majority of engine development. Superchargers, 4 valve heads, overhead cams and dual spark plugs were first introduced in automotive applications prior to WW1.
Absolutely true, "win on Sunday, sell on Monday." At the bottom, in my opinion most of the significant improvements in engine development have been enthusiast driven.
FWIW I'm a member of the Connecticut Antique Machinery Club, some members have IC engine powered equipment dating back to the 1890's, I'm always amazed at the technology that was employed in some of those early efforts. Unfortunately, it was often sidelined due to cost or manufacturing issues only to be "reinvented" once those issues were resolved.