In this paper, a detailed analysis of combustion and emissions is carried out on both metal and optical light duty diesel engines equipped with up-to-date combustion architecture. Both engines were fed with glycerol ethers mixture (GEM) in blend (10% and 20% v/v) within a commercial diesel fuel. The engines ran in significant operating points in the NEDC (New European Driving Cycle) emission homologation area.The results of the experimental campaign on the metal engine show comparable performances between the diesel/GEM blends and the diesel fuel and demonstrate benefits mainly in terms of soot production. The exhaust particles diameters of diesel/GEM blends shift toward smaller dimensions and the total number decreases. Moreover, at lower load conditions, the outputs show a worsening of the unburnt mainly ascribable to the fuel characteristics.In order to characterize and investigate more deeply on the combustion process when using the diesel/GEM blends additional tests were performed on a diesel engine with similar combustion architecture of the metal engine and with optical access through the piston bowl. The collection of images of the injection and combustion processes allowed to characterize the behaviour of the two tested fuels and to support metal engine results. Optical techniques were applied to detect soot concentration and flame temperature versus crank angle. Moreover, soot formation and oxidation rates helped to understand the evolution of PM emission.