Early 100 hr tests on transesterified sunflower oil initially showed the improved properties of esters for use in a diesel engine by reducing the viscosity of vegetable oils and solving engine problems (29-31).

Table IV compares the essential fuel properties of some esters. In all cases the viscosity decreases dramatically and is only about twice that of DF2. The CNs are also improved, now being higher than that of DF2.

The methyl and ethyl esters of soybean oil generally compared well with DF2 with the exception of gum formation which leads to problems with fuel filter plugging (20). Another study reports that methyl esters of rapeseed and high-linoleic safflower oils formed equal and lesser amounts of deposits than a DF standard while the methyl ester of high-oleic safflower oil formed more deposits (55). Methyl and ethyl esters of soybean oil were evaluated by 200 hr EMA (Engine Manufacturers Association) engine tests and compared to DF2. Engine performance with soybean esters differed little from that with DF. In that work, also a slight power loss was observed, together with an increase in fuel consumption due to the lower heating values of the esters. The emissions for the two fuels were similar, with the exception of NOx which are higher for the esters as discussed above. Engine wear and fuel-injection system tests showed no abnormal characteristics for any of the fuels. Deposit amounts in the engine were comparable, however, the methyl ester showed greater varnish and carbon deposits on the pistons. Operating DI engines with neat soybean oil esters under certain conditions produced lubricating oil dilution which was not observed with an IDI engine (109). Lubricating oil dilution was estimated by Fourier-Transform infrared spectroscopy combined with a fiber optic probe when using rapeseed methyl ester as a fuel (110). The carbonyl absorption was used for quantitation.

Low-temperature Properties. As discussed above, one of the major problems associated with the use of biodiesel, including methyl esters, is its poor low-temperature properties, documented by relatively high cloud point (CP) and pour point (PP) (Tables III and IV). CPs and PPs of vegetable oils and their esters are a result of these materials being mixtures of various compounds. For example, as seen in Table I, saturated fatty compounds have significantly higher melting points than unsaturated fatty compounds and in a mixture they therefore crystallize at higher temperature than the unsaturates.

The CP, which occurs at a higher temperature than the PP, is the temperature at which a fatty material becomes cloudy due to formation of crystals and saturates solidifying. These solids can clog fuel lines. With decreasing temperature, more material solidifies and the compound approaches the pour point, at which it will no longer flow.

Besides CP (ASTM D2500) and PP (ASTM D97), two test methods exist for examining the low-temperature properties of diesel fuel (as discussed briefly in the section on “Biodiesel Standards”), the Low-Temperature Flow Test (LTFT; used in North America; ASTM D4539) and the Cold-Filter Plugging Point (CFPP; used in Europe). These methods have also been used to evaluate biodiesel. Low-temperature filterability tests are necessary because they correlate better with operability tests than CP or PP (111). Recent results showed that for fuel formulations containing at least 10% methyl esters, both LTFT and CFPP are linear functions of the CP (112). Additional statistical analysis showed a strong 1:1 correlation between LTFT and CP (112).

Five possible solutions to the low-temperature problems of esters have been investigated: blending with conventional DF, additives, branched-chain esters, bulky substituents in the chain, and winterization. Blending of esters is currently the preferred method for improving low-temperature properties and is discussed in the next section.

Numerous additives have been synthesized and reported mainly in the patent literature, which allegedly have the effect of lowering CP and PP. These additives are usually a variety of viscosity-modifying polymers such as carboxy-containing interpolymers (113), styrene-maleic anhydride copolymer (114), polymethacrylates (114-115), polyacrylates (113-115), nitrogen-containing polyacrylates (113), poly[alkyl (meth)acrylates] (116), ethylene-vinyl ester (acetate) copolymers (117-120), fumarate or itaconate polymers and copolymers (comb polymers) (117-118), polyoxyalkylene compounds (113). Polar nitrogen compounds (117) have also been reported as additives. Similar additives have also been tested for conventional diesel fuel (7). The beneficial effect of some additives appears to be limited, however, because they more strongly affect the PP than the CP, and the CP is more important than the PP for improving low-temperature flow properties (121).

Another route is the synthesis of fatty compound-derived materials with bulky substituents in the chain (122). The idea associated with these materials is that the bulky substituents would destroy the harmony of the solids which are usually oriented in one direction. However, these materials had only slight influence on the CP and PP.

The use of secondary alcohols in the transesterification reaction provides branched — chain esters such as isopropyl and 2-butyl instead of the methyl esters (95, 123) . These esters showed a lower crystallization onset temperature (Tco) as determined by differential scanning calorimetry (DSC) for the isopropyl esters of SBO by 7-11 °С and for the 2-butyl esters of SBO by 12-14°C (123). The CPs and PPs were also lowered by the branched-chain esters. However, economics would only permit the isopropyl soyate appear attractive as branched-chain ester, raising the price of a biodiesel blend containing 30% isopropyl soyate by $0.02/L while lowering the Tco by 15 °С.

In the winterization method (121, 124), solids formed during cooling of the vegetable oil are removed by filtration, thus leaving a mixture of more unsaturated fatty compounds with lower CP and PP. This procedure can be repeated to achieve the desired CPs and PPs. Saturated fatty compounds, which have higher CNs, are among the major compounds removed by winterization. Thus the CN of the biodiesel decreases. The Tco of typical methyl soyate was lowered from 3.7°C to -7.1 °С by winterization (124), but the yield was low (26%). Winterization of low-palmitate methyl soyate, however, gave a Tco of -6.5 °С with a yield of 86%. Winterization of typical methyl soyate diluted in hexane gave a Tco of -5.8°С with 77% yield. In the latter method, crystal formation was greatly affected by the nature of the solvent, with acetone and chloroform being unsuitable for winterization.

In a paper on fatty acid derivatives for improving ignition and low-temperature properties (125), it was reported that tertiary fatty amines and amides were effective in enhancing the ignition quality of biodiesel without negatively affecting the low — temperature properties. In that paper, saturated fatty alcohols of chain lengths C,2 and greater increased the PP substantially. Ethyl laurate was weakly decreased the PP.