There have been a number of studies on fuel economy and emissions concentrating on biodiesel and bioethanol. Biofuels have different energy content from the fuels that they replace, and therefore this will affect their fuel economy. Fuel economy can be meas­ured in terms of volumetric fuel consumption, brake-specific fuel consumption (BSFC, kg/kWh fuel flow/power) and brake thermal efficiency by measuring the torque and fuel consumption. In other studies, fuel consumption was determined from the carbon dioxide emissions and fuel carbon content but a more accurate value can be obtained when carbon dioxide emissions and fuel consumption are combined. The fuel con­sumption is normally measured at various power outputs (kW) and loads (Nm).

Biodiesel

The effect of the concentration of biodiesel used in blends on BSFC measured as MJ/ kWh is shown in Fig. 8.1. Although the histogram shows a drop in BSFC as the

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biodiesel concentration increases, the final value is only reduced by 4%, and with a standard deviation of 1.5%, it appears that fuel composition has little effect on fuel consumption. The type of oil used to produce biodiesel has also been investigated, and little difference can be seen in the fuel consumption using a variety of biodiesel (Fig. 8.2). This is perhaps not surprising as the calorific value of biodiesel made from various oils varies very little.

The change in engine load will also affect fuel consumption. A number of studies have been carried out and Fig. 8.3 shows a comparison of diesel with a 50% mixture of sunflower oil biodiesel blended with marine diesel tested in a single cylinder,

indirect injection diesel engine. In the case of the marine diesel when compared with a blend of 50% sunflower biodiesel, there was little difference in BSFC over the loads imposed apart from an increase in fuel consumption from around 400 g/h to over 900 g/h. In a similar study on the effect of load on BSFC using a biodiesel made from waste olive oil compared with diesel and oil, there was an increase of 7.5% in fuel consumption compared with diesel (Fig. 8.4).

A number of studies using a variety of biodiesel blends and engines have shown that increases in BSFC can vary from 0 to 13.8% fuel consumption increase over diesel (Table 8.1). Biodiesel showed higher fuel consumption of 13.8% measured as BSFC using soy oil biodiesel in a four cylinder, four-stroke turbocharged John Deere engine (Monyem and Van Gerpen, 2001). The same result was found with biodiesel

from waste olive oil run in a three cylinder, four-stroke Perkins diesel engine where the BSFC was 7% greater (Dorado et al., 2003). The reduction in fuel consumption may be a consequence of the lower energy density of biodiesel (33-42 MJ/kg) com­pared with diesel (46 MJ/kg). The variation in the reduction in BSFC may also be due to the different combustion conditions found in the different engines.