This paper is a complementary to previous investigations by the authors (1,2,3,4) on the different effects of EGR on combustion and emissions in DI diesel engine. In addition to the several effects that cold EGR has on combustion and emissions the application of hot EGR results in increasing the inlet charge temperature, thereby, for naturally aspirated engines, lowering the inlet charge mass due to thermal throttling. An associated consequence of thermal throttling is the reduction in the amount of oxygen in the inlet charge. Uncooled EGR, therefore, affects combustion and emissions in two ways: through the reduction in the inlet charge mass and through the increase in inlet charge temperature. The effect on combustion and emissions of increasing the inlet charge temperature (without reducing the inlet charge mass) has been dealt with in ref. (1). In the present work, the effect of reducing the inlet charge mass, (without increasing the inlet charge temperature) and the associated reduction in inlet charge oxygen were experimentally investigated.In a first series of tests where the overall effects of intake charge throttling were demonstrated, only air was used as the inlet charge and cylinder number one was throttled isothermally from 10 g/s to 8.5 g/s in steps of 0.5 g/s. A second series of tests was then run by throttling the inlet charge over the same range of mass flow while maintaining the oxygen mass flow rate (and mass of oxygen trapped in the cylinder) constant. The specific heat capacity of the inlet charge for this series of tests was also kept constant and equal to that of air. This was achieved by injecting into the inlet manifold calculated amounts of oxygen and helium, whilst throttling the inlet charge air.Comparison of the results from the two series of tests enabled the following question to be answered: when the inlet charge mass is reduced, is the reduction in the charge mass itself that is important, or is the consequent reduction in the amount of oxygen mass trapped that controls the emissions?It was found that the NOx emissions increased somewhat due to the reduction in total inlet charge mass. However, CO, UHC, and smoke emissions increased substantially due to the reduction in the amount of oxygen trapped.