This study examines the composition and combustion performance of biodiesel produced from waste cooking oil. Six fuel batches produced from waste oil used in dining-hall fryers were examined to determine their physical and chemical properties, including their elemental and fatty acid methyl ester composition. Oleic and linoleic methyl esters accounted for more than 70% of the fuel composition, while the oxygen content averaged 10.2% by weight. Exhaust emissions were monitored for 5–100% biodiesel blends using two off-road engines: a 2007 Yanmar diesel generator and a 1993 John Deere front mower. Increasing biodiesel content resulted in reduced emissions of partial combustion products from the diesel generator but a rise in NOx, with the greatest changes occurring between 5 and 20% biodiesel content. For the riding mower, biodiesel content up to 50% had little effect on emissions, while NOx and total hydrocarbon emissions decreased with 100% biodiesel. The difference in NOx emissions is attributed to the two different fuel injection control designs used in the two engines. These results indicate that the effects of biodiesel use on nonroad engine exhaust emissions may be substantially lower in older engines optimized for performance over emissions control. 1. Introduction Rising fuel costs and energy demands, combined with growing concern over greenhouse gas emissions, have led to increased interest in the use of renewable fuels to help meet increasing worldwide fuel demand and reduce atmospheric CO2 emissions from transportation sources [1–3]. Biodiesel is an oxygenated diesel fuel composed primarily of fatty acid methyl esters that can be produced from a variety of vegetable oils and animal fats [4]. It can be used directly in existing diesel engines, either as a fuel replacement or as an additive to improve combustion processes [5]. In addition, biodiesel fuels may help to reduce emissions of some toxic air pollutants [6, 7]. Biodiesel can be produced from a range of vegetable oils and animal fats. The use of soybean oil and other high-quality food-grade vegetable oils presents economic difficulties because of competition with use for food products. One more economically feasible source for biodiesel is waste cooking or frying oils, also known as yellow grease. As a waste product, used cooking oil is a potentially cheaper feedstock than edible vegetable oils [8], and does not directly compete with the growth of food crops. Estimates for potential biodiesel production from waste cooking oil in the United States range from 100 to 200 million gallons per
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