<div class="section abstract"><div class="htmlview paragraph">Reactivity Controlled Compression Ignition (RCCI) is an engine combustion strategy that utilizes in-cylinder fuel blending to produce low NO_x and PM emissions while maintaining high thermal efficiency. The current study investigates RCCI and conventional diesel combustion (CDC) operation in a light-duty multi-cylinder engine over transient operating conditions using a high-bandwidth, transient capable engine test cell. Transient RCCI and CDC combustion and emissions results are compared over an up-speed change from 1,000 to 2,000 rev/min. and a down-speed change from 2,000 to 1,000 rev/min. at a constant 2.0 bar BMEP load.</div><div class="htmlview paragraph">The engine experiments consisted of in-cylinder fuel blending with port fuel-injection (PFI) of gasoline and early-cycle, direct-injection (DI) of ultra-low sulfur diesel (ULSD) for the RCCI tests and the same ULSD for the CDC tests. At the selected engine load, a step speed change was commanded and both combustion modes were compared for performance and emissions using fast response HC, NO and PM instruments. Optimized intake conditions (i.e., intake pressure, temperature and exhaust gas recirculation (EGR)) were used to explore the robustness of RCCI using real-world operating conditions. It was found that the engine was able to operate in the RCCI combustion mode using production level engine hardware with significantly lower engine-out PM and NO_x emissions than CDC over the specified transient engine operating conditions.</div></div>