The paper qualitatively describes processes during the internal flow, discharge of two-phase mixture as well as spray formation with a particular focus on the energy transfer during effervescent atomization of light heating oil. The near nozzle spray visualization elucidates the liquid breakup process at different operation modes. Numerical results illustrate the forms of energy involved in the atomization process, their values and the influence of operational conditions on relations between the energy forms. The main part of the paper focuses on the atomization efficiency. A simple method for estimation of the atomization efficiency of pneumatic atomizers is proposed; surface tension energy of created droplets, obtained from phase-Doppler anemometry data, is compared with the energy required for atomization. The atomization efficiency of effervescent atomizers is found to be in fragments of per cents for atomizing pressures ranging between 0.1 and 0.5 MPa, and gas-to-liquid ratios (GLRs) between 0.02 and 0.1, and it is inferior, by about one order of magnitude to the efficiency of simple pressure and pressure-swirl atomizers for a comparable droplet size. The efficiency declines with both the pressure and GLR with approximately logarithmic tendency. (C) 2013 Elsevier Ltd. All rights reserved.