This work investigates the potential of RCCI concept to achieve ultra-low NOx and soot emissions over a wide range of engine speed and loads. For this purpose, a detailed experimental methodology has been defined and applied in a heavy-duty single-cylinder engine fueled with diesel and gasoline. In addition, to assess the influence of the engine compression ratio on RCCI capabilities two different compression ratios, 14.4:1 and 11:1, have been tested. Results suggest that a low compression ratio allows to fulfill all the self-imposed constraints (maximum cylinder pressure rise rate of 25 bar/CAD, NOx < 0.4 g/kW h and soot* < 0.01 g/kW h) from idle to full load and engine speeds from 900 to 1800 rpm. However, the use of higher compression ratio requires a delayed injection strategy to avoid excessive knocking levels, which results in unacceptable soot emissions at loads higher than 50%, even when gasoline fractions around 90% are used. (C) 2015 Elsevier Ltd. All rights reserved.