Cooled exhaust gas recirculation (EGR) is a common way to control in-cylinder NOx, production and is used on most modern highspeed direct injection (HSDI) diesel engines. However EGR has different effects on combustion and emissions production that are difficult to distinguish (increase of intake temperature, delay of rate of heat release (ROHR), decrease of peak heat release, decrease in O-2 concentration (and thus of global air/fuel ratio (AFR)) and flame temperature, increase of lift-off length, etc.), and thus the influence of EGR on NOx and particulate matter (PM) emissions is not perfectly understood, especially under high EGR rates. An experimental study has been conducted on a 2.01 HSDI automotive diesel engine under low-load and part load conditions in order to distinguish and quantify some effects of EGR on combustion and NOx/PM emissions. The increase of inlet temperature with EGR has contrary effects on combustion and emissions, thus sometimes giving opposite tendencies as traditionally observed, as, for example, the reduction of NOx emissions with increased inlet temperature. For a purely diffusion combustion the ROHR is unchanged when the AFR is maintained when changing in-cylinder ambient gas properties (temperature or EGR rate). At low-load conditions, use of high EGR rates at constant boost pressure is a way to drastically reduce NOx and PM emissions but with an increase of brake-specific fuel consumption (BSFC) and other emissions (CO and hydrocarbon), whereas EGR at constant AFR may drastically reduce NOx emissions without important penalty on BSFC and soot emissions but is limited by the turbocharging system. (C) 2007 Elsevier Ltd. All rights reserved.