91.1307—Credit calculation.
For each participating engine family, emission credits (positive or negative) are to be calculated according to the following equation and rounded, in accordance with ASTM E29-93a, to the nearest gram. ASTM E29-93a has been incorporated by reference. See § 91.6. Consistent units are to be used throughout the equation. The following equation is used to determine the credit status for an engine family whether generating positive or negative in-use emission credits:
Where:
S(t)=cumulative fraction survived at time t;
μlife=average useful life in years, specific to the power rating and the application as given below.
Power = the average power of an engine family in kW (sales weighted). The power of each configuration is the rated output in kilowatts as determined by SAE J1228. This procedure has been incorporated by reference. See § 91.6.
t = time in model years
max useful life = maximum useful life specific to the power rating and the application; max useful life = 2μlife
sales = the number of eligible sales tracked to the point of first retail sale in the U.S. for the given engine family during the model year.
FEL = the family emission limit for the engine family in grams per kilowatt hour.
CL = compliance level of the in-use testing in g/kW-hr.
μuse = mean use in hours per year. For outboard engines, μuse=34.8 hrs /yr. For personal watercraft, μuse=77.3 hrs/yr;
AF = adjustment factor for the number of tests conducted
| No. eng. tested | 2*, 4 | 6 | 8 | 10 |
| Adjustment factor | .5 | .75 | .9 | 1 |
| * Small volume manufacturer | ||||