If propylene oxide used as ignition accelerator additive produce easier starting and smoother running, why nitrates and peroxides (another ignition accelerators) could not produce the same effect?
Propylene oxide is only for use with nitromethane. Nitromethane burns slowly but with great pressure. Sort of like comparing black gun powder to smokeless poweder, the smokeless has more pressure, but without a good primer will burn too slowly. The propylene oxide helps speed up the burn rate of nitromethane thus allowing less advanced timing, and less detonation. Some model fuels have had propylene oxide, but it is not necessary till you get above 20% or so.