The paper presents results of experimental verification of a theory which predicted the possibility of combustion similarity in differently sized diesel engines. Combustion similarity means that the flow, pattern and flame distribution develop similarly in differently sized engines. In the experiment, observations were made of the similarity in fuel jet distribution in a model apparatus, and comparisons of thermal efficiency, heat release rate, and emissions were made for engines varying from 260 to 400 mm in bore size. Comparisons were also made of combustion patterns of differently sized engines by a three-dimensional computer simulation. All of the results showed good agreement with the theoretical prediction. The establishment of the theory of size effects can provide model experiments and a method to correlate the vast amount of independent data that are available, and offer significant opportunities for diesel engine research and design.