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Neat, non-esterified vegetable oils, alternative, locally produced, renewable fuels for diesel engines, have considerably higher viscosity than diesel fuel, even when heated. While mechanical injection pumps with volumetric fuel metering compensate for higher viscosity of the fuel by an increased injection pressure, and possibly longer ignition delay is on some engines compensated by an earlier injection due to higher density and bulk modulus of vegetable oils, newer common-rail type systems do not have such mechanism, and inject vegetable oils and diesel fuel at comparable timing and pressures. The complexity of the newer injection systems also raises the issue of the effects of varying fuel properties. This paper reports on laboratory experiments carried on a four-cylinder, 4.5-liter Cummins ISBe4 engine with a Bosch Common Rail injection system, fitted with an auxiliary heated secondary fueling system, and operated on fuel-grade rapeseed oil heated to 50-60°C.