California Air Resources Board

Infrequent Regeneration Adjustment Factors (IRAFs) are used to calculate and apply emission adjustment factors for engines using aftertreatment technology with infrequent regeneration events that may occur during testing. These adjustment factors are typically calculated based on measurements conducted for the purposes of engine certification, and then used to adjust the results of testing related to demonstrating compliance with emission standards.

The term ‘‘regeneration’’ means an intended event during which emission levels change while the system restores aftertreatment performance. Also, ‘‘infrequent’’ refers to regeneration events that are expected to occur on average less than once over a transient or ramped-modal duty cycle, or on average less than once per mode in a discrete-mode test.

For example, exhaust gas temperatures may increase temporarily to remove sulfur from adsorbers or to oxidize accumulated particulate matter in a trap.

If active regeneration does not occur during a test segment, apply an upward adjustment factor (UAF), which will be added to the measured emission rate for that test segment. If active regeneration occurs or starts to occur during a test segment, apply a downward adjustment factor (DAF), which will be subtracted from the measured emission rate for that test segment.

The current requirements for IRAFs are contained in the Code of Federal Regulations at 40 CFR 1065.680 and in the California test procedures for on-road heavy-duty diesel engines and off-road compression-ignition engines.