Since the 1980s, there has been increased interest in low concentration blending of ethanol with diesel fuel. Ethanol/diesel blends are commonly referred to as E — diesel. They generally contain from 10% to 15% ethanol and are used for many of the same reasons that ethanol/gasoline blends are used (decreased petroleum need and decreased emissions). Ethanol and diesel blending is more complicated than ethanol/gasoline blending, because of the low solubility of ethanol in diesel at low temperatures and the high flammability. At temperatures below 10°C, ethanol and diesel will separate [39]. The solution is either to add an emulsifier or a cosolvent. Boruff et al. has shown that approximately 2% surfactant (emul­sifier) is needed for every 5% of ethanol added to diesel fuel (40). The addition of the surfactant to the ethanol/diesel blend led to transparent solutions with no visible separation down to -15.5°C (40). Ethyl acetate has been studied as a co­solvent. Researchers have shown that adding 2.5% ethyl acetate for every 5% ethanol will ensure no separation down to 0°C (41). Cosolvents have been more popular than surfactants. The second issue with e-diesel is the increased risk of fire and explosions compared to plain diesel fuel. The National Renewable Energy Laboratory recommends solving this problem by equipping all fuel tanks with vents, better electrical grounding, and employing safer fuel tank level detectors (42). The physical properties of E-diesel compared to ethanol and diesel are shown in Table 7.5.

Blending ethanol with diesel fuel decreases emissions in a similar way to ethanol/gasoline blends. E-diesel has achieved reported 20% to 30% decreases

in carbon monoxide emissions and 20% to 40% decreases in particulate matter emissions (43). Miyamoto et al. showed that these improvements in emissions depend directly on the oxygen content (44). However, minimal decreases in NOx emissions have been reported (43) and an increase in hydrocarbon emissions have been reported (45-46). Table 7.6 shows the vehicle emissions from the use of 10% and 15% ethanol in diesel.

As far as engine use is concerned, the decrease in fuel viscosity and lubricity have been investigated for ethanol blends with diesel, but they do meet diesel specifications (42). Materials compatibility has also been investigated. E-diesel was found to have similar corrosive properties to typical diesel (42).

It is important to note that E-diesel fleet demonstrations have shown no fire or explosions incidents and no mechanical failures associated with the fuel system 43). Many studies of engine wear have been conducted and have shown no abnormal wear or deterioration due to the blending of ethanol with diesel at low concentrations (10-15%). E-diesel does shows a reduction in engine power, but this reduction is small and equivalent to the reduction in energy content of the ethanol versus diesel (39). The main engine performance issue with E-diesel is the leakage of fuel from the fuel injection pump due to slight decrease in viscosity of the blended fuel. Studies of engine power loss have shown decreases in power from 4% to 10% for ethanol/diesel blends ranging from 10% to 15% ethanol (46-48). Therefore, ethanol is a good choice as an oxygenate for diesel. It has minimal effect on engine power while dramatically decreasing particulate matter and carbon monoxide emissions.

TABLE 7.6

Vehicle Emissions from the Use of 10% to 15% Ethanol in Diesel

Emissions Range
(% ratio of blend/diesel)

73-80 59-70 96-100 95-100 80-160 73-140 171-200 175-210

Source: Data compiled from Hansen, A. C., Lye, P. W. and Zhang, Q., ASAE Paper No. 01-6048, Aug. 2001; Waterland, L. R., Venkatesh, S. and Unnasch, S., NREL/SR-540-34817, Sept. 2003; Spreen, K., Final Report for Pure Energy Corporation prepared at SRI, San Antonio, TX, 1999; Kass, M. D., Thomas, J. F., Sto­rey, J. M., et al., SAE Technical Paper 2001-01-2018, 2001.

CONCLUSIONS

Ethanol can be blended with gasoline to produce an oxygenated fuel with lower hydrocarbon emissions. Ethanol can also be blended with diesel to decrease carbon monoxide emissions and particulate matter emissions. Although green­house gas emissions are decreased with ethanol-blended fuels, emissions of certain aldehydes are increased, which could cause health issues. Automobiles can be operated on ethanol/gasoline blends from 5% to 25% and ethanol/diesel blends from 10% to 15% without need for any alterations in engine equipment or settings and with no effect on engine lifetime.