This article is devoted to the influence of temperature, obstacle and enrichment of the liquid fuel–air mixture with oxygen, as well as the synergistic effect of the combined action of these techniques on the initiation of detonation in a small-sized pulsed detonation engine with a channel of subcritical dimensions (d = 20 mm and L = 500 mm). Explosive limits for detonation of heptane in the frequency mode (f = 1–50 Hz) have been established in terms of the equivalence ratio (ϕ = 0.7–2.6) and in terms of the oxygen-to-air ratio ([O2/air] > 2 for heptane and [O2/air] > 3 for Jet-A1). It was found that the wave velocity in heptane-oxygen-air and Jet-A1-oxygen-air mixtures varies from 500 m/s to 2500 m/s depending on the technique used to accelerate the DDT. Without the use of special techniques, detonation does not occur, and the wave velocity does not exceed 700 m/s. The use of heat, or obstacle or oxygen enrichment leads to the initiation of detonation at distances of 540–660 mm from the ignition initiator, which is 20–26 calibers of engine diameter. A synergistic effect of the combined use of an obstacle, thermal activation and enrichment of the mixture with oxygen was discovered, which made it possible to reduce the pre-detonation distance by almost 60 % and increase the frequency of the detonation mode by also 60 % (up to 80 Hz) compared to each technique separately.