Several diffusion combustion scaling models were experimentally tested in two geometrically similar single-cylinder diesel engines with a bore diameter ratio of 1.7. Assuming that the engines have the same in-cylinder thermodynamic conditions and equivalence ratio, the combustion models primarily change the fuel injection pressure and engine speed in order to attain similar performance and emissions. The models tested include an extended scaling model, which scales diffusion flame lift-off length and jet spray penetration; a simple scaling model, which only scales spray penetration at equal mean piston speed; and a same speed scaling model, which holds crankshaft rotational velocity constant while also scaling spray penetration.Successfully scaling diffusion combustion proved difficult to accomplish because of apparent differences that remained in the fuel-air mixing and heat transfer processes. A computational investigation revealed that the larger nozzle diameter (relative to the ideal scaled value) used in the small engine experiments caused spray-wall impingement, which altered the mixing and subsequent combustion event. Similar pressure profiles and combustion could be achieved by increasing the injection pressure of the small engine relative to the injection pressure predicted by the scaling models. Large differences in soot and carbon monoxide emissions suggest dissimilarities in fuel-air mixing, late-cycle oxidation, and heat transfer. Computational and experimental results showed that swirl ratio had a large effect on mixing and emissions, and is an important parameter in engine scaling. Finally, a new scaling relation was derived in which engine speed, injection pressure, and injection duration are held constant.