Additionally, uncertainty exists regarding the viability and durability of control technology because prototypes have not yet been designed for many current engine models. Averaging means that an engine family in a manufacture's product line could be certified to an emissions level in excess of the applicable emission standard, so long as its excess emissions are offset by an engine family that is certified to an emissions level below the applicable emission standard. In other words, any emissions in excess of the average emission standard must be balanced by emissions lower than the average emission standard.
Full actual life emissions are taken into account when calculating this balance. The calculation includes:
・ the difference between the applicable emission standard and the engine family emission limit (FEL),
・ sales in the applicable model year,
・ average annual use in hours,
・ the power output of the engine family,
・ the future survival probability of each engine,
