In many engine applications, differences in nitrogen oxides (NOx) emissions and smoke concentrations between biodiesel-fuelled and petroleum diesel-fuelled engines may be ascribed to system response issues, where due to differences in the fuels' properties the control of certain engine parameters may be affected. This study explores the effects such system responses have on nitric oxide (NO) emissions and smoke concentrations from a diesel engine fuelled with biodiesel and its 20%-v blend with petroleum diesel. It is observed that, mostly due to system response issues, NO emissions exhibit inconsistent behavior among the studied fuels. On average, the biodiesel fuels tend to emit higher NO emissions; but, the statistical probability of the averages being different is low. Those unique cases, where the biodiesel fuels emit substantially lower NO emissions operate with different engine control parameters; specifically, exhaust gas recirculation is used with the biodiesel fuels, whereas it is not with the petroleum diesel fuel. More consistently, smoke concentrations of this study are generally lower with the biodiesel fuels than with petroleum diesel. Certain isolated cases of the biodiesel blend, where exhaust gas recirculation is used show higher smoke concentrations than petroleum diesel; the increase in smoke concentration is relatively smaller than the decrease in NO emissions, thus creating an opportunity for biodiesel to improve the EGR tolerance of diesel engines in reducing NOx while mitigating the traditional increase in smoke. Due to potential changes in engine control parameters when using a substantially different fuel than that which the engine is calibrated for, "diluting" the base fuel with the different fuel may not necessarily result in a linear or consistent scaling of exhaust emissions. (C) 2012 Elsevier Ltd. All rights reserved.