The paper deals with a calculation method of ignition delay period ϕ ig.d. in diesel engines using its momentary values ϕy. Starting from beginning of the injection ϕ in.b. and going along with the compression, or in backwards direction from combustion beginning ϕc.b., we calculate the variable ϕy for every equal small increment of crank angle Δϕ(1°CA or less). For the full ignition delay, the sum of the Δϕ/ϕy reaches the value 1. Using simple computation we obtain calculated curves ϕ ig.d. = f(ϕin.b.), ϕ in.b. = f (ϕ c.b.) etc.The ϕ y' s are calculated from the empirical equation where py, Ty are the momentary pressure and temperature resp. and a,b,c are the constants. There are difficulties in determination of these constants as well as of the mean polytropic coefficient (during the ignition delay) needed for the calculations.Experimental data for comparisons with the calculated values are taken from a D.I. high speed laboratory diesel engine.