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Based on the similarity theory and conservation equations, some of the important dimensionless numbers in diesel combustion are deduced and discussed. Existence of similarity is theoretically proved in diffusion (or mixing-controlled) combustion and premixed combustion as well as in spray mixture formation processes in different size diesel engines. With the prerequisite of geometric similarity, scaling rules for some parameters including engine speed, injection pressure and injection duration are established to realize the similarity between large-bore and small-bore diesel engines. To verify the similarity theories, the computational fluid dynamics (CFD) simulation are conducted, and three scaling rules, which keep the engine speed, injection pressure and lift-off length constant, respectively, are compared under the conditions of the light load (0.3 MPa IMEP) and high load (1.55 MPa IMEP) operations.