Engine Tests Demonstrate Need for Optimized EGR, Aftertreatment, and Rich Spike Injection to Control Emissions with Biodiesel: Biodiesel has been introduced to reduce consumption of petroleum-based fuels, and conventional diesel fuel blended with biofuel of up to five percent is generally available in the market. Increasing the fraction of biofuel ultimately to 100 percent is desirable to accomplish this objective. However, operating the latest diesel engine designed to comply with current exhaust emissions regulations on 100-percent biodiesel fuel may deteriorate exhaust emissions characteristics. Therefore, it is necessary to investigate the combustion and exhaust emissions characteristics of the latest diesel engine operated on 100-percent biodiesel fuel. Researchers at the National Traffic and Safety Laboratory conducted engine tests to characterize the effects that biodiesel fuel has on combustion and exhaust emissions. A 4.009-liter, inline four-cylinder, turbocharged diesel engine equipped with a diesel particulate and NOx reduction aftertreatment device was operated on biodiesel fuels containing various fractions rapeseed methyl ester under JE05 transient test conditions and steady-state conditions. The chapter consists of 11 pages including 10 figures and 2 tables.