A phenomenological approach is developed to calculate the velocity of flame propagation and to estimate the value of pressure peak when igniting gaseous combustible mixtures in a congested space. The basic idea of this model is afterburning of the remanent fuel in pockets of congested space behind the flame front. The estimation of probable overpressure peak is based on solution of one-dimensional problem of the piston (having corresponding symmetry) moving with given velocity in polytropic gas. Submitted work is the first representation of such phenomenological approach and is realized for the simplest situation close to one-dimensional.