For engine owners and operators working towards MSAPR compliance, there may be some confusion surrounding the difference between an engine’s flat limit versus a yearly average.
Depending on your engine fleet and emissions test results, there may be advantages or disadvantages to both options. Until January 1, 2026, engine operators will have to evaluate on an annual basis whether to elect to use a flat limit or yearly average approach. By understanding both, you can make an informed decision about which one best suits your business’ MSAPR compliance strategy.
Currently, most engine operators are electing to use a flat limit approach, which involves the following:
Flat Limit Approach
MSAPR Section: 58-59
Description: Subset of engines ≥ 50% of total rated break power.
NOx Limit: 210 ppmvd15% (4 g/kWh)
Pros: Decrease in testing costs
Cons: Less flexible, all engines must comply by 2026
Yearly Average Approach
MSAPR Section: 60
Description: Yearly average of emissions from all engines in a subgroup (i.e., engines will emit both above and below the NOx limits using the average of all engines for compliance).
NOx Limit: 421 ppmvd15% (8 g/kWh)
Pros: More flexibility with rich-burn heavy fleets, allows for replacement engines
Cons: Administratively burdensome, more testing is required
Per Engine Approach
MSAPR Section: 57
Description: Engines are not assigned to a group (i.e., limit for each engine, no exceedances).
NOx Limit: 210 ppmvd15% (4 g/kWh)
Pros: Relieves administrative burden
Cons: Difficult to meet NOx limits, more testing is expensive
