Increasingly strict emissions regulations along with man-made global warming has peaked interest in clean burning diesel engines. Renewable biofuels such as biodiesel are being investigated to reduce carbon monoxide (CO), unburned hydrocarbon (HC), and smoke opacity, while slightly increasing nitrogen oxides (NOₓ). Two modern diesel engines, a light-duty engine and heavy-duty engine, were investigated with various biodiesel blends. The heavy-duty engine was a Cummins 4-cylinder direct injection (DI) diesel engine, which was run at three idling conditions: low, medium and high idling states operated at 800 revolutions per minute (rpm), 1000 rpm, and 1200 rpm respectively. The engine was fueled with biodiesel-diesel blends with two additives. Two additives, ethanol and diethyl ether (DEE) at 5% and 15% were mixed with biodiesel-diesel blends B20, B50 and B100. B100 was produced from canola oil. The engine was tested from cold start to warm up in real world conditions. The light-duty engine was a HATZ 2-cylinder diesel engine, which was fueled with biodiesel blends with dissolved expanded polystyrene (EPS) and fuel stabilizer additive acetone. The light-duty engine was tested at three speed conditions 1000 rpm, 2100 rpm, and 3000 rpm. Each speed condition had 4 load conditions: 0%, 20%, 50%, and 80% load. EPS was dissolved at 50g/l of biodiesel and the acetone additive was tested at 100ml/l of biodiesel and 250ml/l of biodiesel. Emissions analysis was conducted for carbon monoxide (CO), carbon dioxide (CO₂), nitric oxide (NO), nitrogen dioxide (NO2), oxides of nitrogen (NOₓ), smoke opacity and unburned hydrocarbons (HC). Investigation results demonstrate that for the heavy-duty engine at idle conditions, diesel-biodiesel blends with additives produce lower CO emissions than neat diesel. Ethanol and DEE additives can also reduce NOₓ emissions in diesel-biodiesel blends, and increasing biodiesel content reduced HC emissions. For the light-duty engine at all loading conditions biodiesel-diesel blends produced lower CO emissions, higher NOₓ emissions and higher smoke opacity. EPS content overall decreased CO and NOₓ emissions, but increased smoke opacity. At 100ml/l of biodiesel acetone decreased CO emissions, acetone at 250 ml/l of biodiesel increased CO emissions. Acetone increased NOₓ emissions and decreased smoke opacity.