Based on the similarity theory

some of important dimensionless numbers in diesel spray and combustion were deduced and discussed by basic conservation equations. The existence of similarity in different diesel engines was proved theoretically in diffusion and premixed combustion processes

and also in spray mixture formation process. On the premise of geometric similarity

scaling rules for some parameters including engine speed

injection pressure and injection duration were established to realize the similarity between large-bore and small-bore diesel engines. In the engine operations where premixed combustion is remarkable

the scaling rule that keeps the engine speed unchanged is more preferable.