Engine Tests Reveal Opportunities for Improvement in Biodiesel Emissions: An investigation into engine performance and exhaust emissions in a late model year production diesel engine operated on biodiesel fuel under both steady-state and transient conditions was conducted at the National Traffic Safety and Environment Laboratory (NTSEL) and New Advanced Combustion Engineering (NACE). A production 4-liter, inline-four, turbocharged, DI that complied with Japan's 2005 emissions standards was employed for the investigation. The engine was equipped with a variable geometry turbocharger, a cooled EGR system, and a diesel particulate-NOx reduction (DPNR) aftertreatment device. Based on the experimental results, researchers recommended several approaches to improve exhaust emissions of a production engine operating on biodiesel fuel. The chapter includes 12 pages, 2 tables, and 9 figures.