This paper deals with the effects on diesel engine combustion and emissions of carbon dioxide and water vapour the two main constituents of EGR. It concludes the work covered in Parts 1, 2, and 3 of this series of papers. A comparison is presented of the different effects that each of these constituents has on combustion and emissions.The comparison showed that the dilution effect was the most significant one. Furthermore, the dilution effect for carbon dioxide is higher than that for water vapour because EGR has roughly twice as much carbon dioxide than water vapour. On the other hand, the water vapour had a higher thermal effect in comparison to that of carbon dioxide due to the higher specific heat capacity of water vapour. The chemical effect of carbon dioxide was, generally, higher than that of water vapour. The chemical effect of water vapour resulted in an increase in the particulate and carbon emissions, but, the chemical effect of carbon dioxide was to decrease these emissions. Similarly, opposite chemical effect were observed on the ignition delay.This is the fourth paper in a series which deals with the effects of EGR on combustion and emissions in diesel engines. The first, second and third papers are SAE 961165, SAE 961167, and SAE 971659 (to be published in 1997 by SAE), respectively.