One promising oxygenate additive being considered as a fuel component for diesel engines is 1-butanol. However, since 1-butanol is characterized, like many other alcohols, by poor autoignition properties and, consequently, a low cetane number, the introduction of this additive into diesel fuel naturally worsens the autoignition properties of the blend so obtained. It is usual to consider a proportion of 1-butanol no higher than approx. 30% alcohol by volume. Thus, when considering the addition of 1-butanol to diesel fuel, it is necessary to improve the autoignition properties of such a blend. One such additive may be 2-ethylhexyl nitrate (2-EHN). This article determines the effect of the 2-EHN additive on the autoignition properties of a blend of 1-butanol and diesel fuel at an alcohol content of 30% (v/v). The tests were carried out using a constant volume combustion chamber method, which additionally made it possible to determine the effect of ambient gas temperature on the ignition delay (ID), combustion delay (CD) and derived cetane number (DCN), among other factors. The study showed, among other things, that with an increase in the mass proportion of 2-EHN in the 1-butanol–diesel blend (BDB) tested, the ignition and combustion delay were shortened, which resulted in an increase in the value of the derived cetane number.