ACES Phase 2 demonstrates another large step in emissions reduction with 2010 EPA-compliant on-highway heavy heavy-duty diesel engines. The 2010-compliant engines showed substantially lower regulated emissions of NOX, CO, NMHC, and PM relative to the 2010 standard. They also showed a substantial reduction in a very wide spectrum of particle and gas phase unregulated emissions species relative to pre-2007 and 2007 technology engines. The observed reductions in PM (total mass, soot, and number) and unregulated emissions from 2007 to 2010 engines are likely due to differences in active DPF regeneration operation. The 2007 engines triggered multiple regeneration events during the 16-hour cycles, while the 2010 engines did not trigger any events. The improvement/reduction in regeneration was achieved through some combination of lower engine-out PM, increased passive regeneration and improved control strategies. PM emissions during regeneration events have been observed to be higher than normal operation for the 2007 engines used in ACES Phase 1. Unregulated emissions are believed to be higher during regeneration events. Thus, the results from the 2007 engines include emissions measured over both normal (no active regeneration) and regeneration operation, while the results from the 2010 engines only include emissions measured over normal operation.