One of the primary incentives for expanding the production and use of biofuels worldwide is the potential environmental benefit that can be obtained from replacing petroleum fuels with fuels derived from renewable biomass resources. The use of straight vegetable oil (SVO) in diesel engines is one of the available alternatives, but its use in existing vehicles usually requires modification of engine or fuel system components. In order to find out the trends in changes of different exhaust emission components using fossil diesel and pure rapeseed oil fuel, the car VW GOLF and the truck MAN 19,464 were modified using one-tank and two-tank conversion kits respectively. To ensure stable driving characteristics, a Mustang Chassis Dynamometer MD-1750 was used and for the determination of the content of different exhaust gas components, the AVL SESAM multicomponent exhaust gas measurement system was used. The analyses of obtained results show that the content of NO_x and SO₂ using rapeseed oil fuel in comparison with fossil diesel decreased with both one-tank and two-tank systems. The content of CO and mechanical particles was higher using rapeseed fuel, but the content of unburned hydrocarbons differs depending on the used engine modification system. Since in the one-tank system original engine nozzles were replaced, the pilot studies of the influence of ignition timing on vehicle power characteristics and exhaust emissions were carried out. It was found out that changing the ignition timing from 10.5 to 18.5 degrees decreases the content of CO, mechanical particles and unburned hydrocarbons by up to 70% without losses in power and torque.