Findings from performance tests

The engine power (P) and BSFC at full load of the same engine running with market diesel (Do) and biodiesel B5 (B5) are given in Fig. 23.20. It is observed in Fig. 23.20 that engine power is higher and BSFC is lower with B5 fuel at all measuring points, although the improvement is not much due to low percentage of biodiesel in the blend. The average engine power was increased 1.34% while averaged BSFC was reduced 1.29%. The detailed explanation of this effect will be shown together with impacts of B5 on exhaust emissions below.

Impacts of biodiesel B5 fuel on exhaust emissions in comparison with the market diesel can be observed in Fig. 23.21.

It is depicted in Fig. 23.21 that using B5 fuel the HC, CO and PM were reduced 12.29%, 8.60%, 2.25% respectively, while NOx was increased 1.93%. The reduction of HC, CO and PM, and the increasing of NOx emissions with biodiesel fuels have already been mentioned by many researches. Those shown in Fig. 23.22 by U. S. Environmental Protection Agency (EPA)40 are an example of these effects.

Average emission changes found by the EPA for B20 (a blend of 20% biodiesel with conventional diesel) also showed significantly lower levels of emissions of specific toxic compounds for biodiesel and biodiesel blends, including aldehydes, PAH (polycyclic aromatic hydrocarbons), and nitrated-PAH.40 However, a number of factors such as different fuel system designs, engine calibrations, fuel

23.21 Comparison of exhaust emissions as the same engine was run with market diesel and biodiesel B5 fuel.

23.22 U. S. Environmental Protection Agency evaluation of biodiesel effects on pollutant emissions for heavy-duty engines.22

quality and blending rate can cause biodiesel emissions to differ significantly from the average values.

The increasing of NOx was shown to be related to a small shift in fuel injection timing caused by the different mechanical properties of biodiesel relative to conventional diesel. Because of the higher bulk modulus of compressibility (or speed of sound) of biodiesel, there is a more rapid transfer of the fuel pump pressure wave to the injector needle, resulting in earlier needle lift and producing a small advance in injection timing.41

Of the testing case, 5% of biodiesel in the blend did not affect much in the reduction of the energy content comparing with that of the market diesel, while the structural oxygen content of a biodiesel fuel improved its combustion efficiency due to an increase in the homogeneity of oxygen with the fuel during combustion. Because of this the combustion efficiency of biodiesel is higher than that of petrodiesel. The results were that, with the biodiesel B5 fuel, the engine power was increased, CO, HC and PM emissions were reduced. Because of low energy content of the biodiesel, higher biodiesel blends may lead to lower engine power and higher fuel consumption.