NTE (Not-To-Exceed) Testing

The US EPA has introduced not-to-exceed (NTE) emission limits and testing requirements as an additional instrument to make sure that heavy-duty engine emissions are controlled over the full range of speed and load combinations commonly experienced in use. NTE testing does not involve a specific driving cycle of any specific length (mileage or time). Rather, it involves driving of any type that could occur within the bounds of the NTE control area, including operation under steady-state or transient conditions and under varying ambient conditions. Emissions are averaged over a minimum time of thirty seconds and then compared to the applicable NTE emission limits.

The NTE approach establishes a control area (the “NTE zone”) which represents engine speeds and loads expected to be encountered in normal vehicle operation and use by diesel heavy-duty engines. It consists of the following engine speed and load points (Figure 1):

1. All engine speeds 15% above the ESC (European Stationary Cycle) speeds:
   n_lo + 0.15 × (n_hi - n_lo)

   where:
   n_hi - the highest engine speed on the power curve where 70% of the maximum engine power is still achievable,
   n_lo - the lowest engine speed on the power curve where 50% of the maximum engine power is still achievable.

2. All engine load points greater than or equal to 30% or more of the maximum torque value produced by the engine.

3. All operating speed and load points with brake specific fuel consumption (BSFC) values within 5% of the minimum BSFC value of the engine. The manufacturer may petition to exclude any of these speed and load points where the engine is not expected to operate in normal vehicle operation. Engines equipped with drivelines with multi-speed manual transmissions or automatic transmissions with a finite number of gears are not subject to this requirement.

4. All speed and load points where the power produced by the engine is less than 30% of the maximum power produced by the engine are excluded.

For Consent Decree engines meeting 2004 EPA standards and subject to NTE requirements, a PM carve out zone was defined at high speed and low load. PM emissions in this zone did not need to meet NTE requirements (Figure 2).

For 2007 and later model year engines, the PM carve out zone was eliminated. Instead, a manufacturer can petition the EPA to:

• have those speed and load points excluded from the NTE zone where the engine is not capable of operating and
• limit the amount of NTE testing in a single region of speed and load points if these operating conditions account for less than 5% of all in-use operation. This region should be generally elliptical or rectangular in shape and share some portion of its boundary with the outside limits of the NTE zone. Testing would not constitute more than 5% of the time-weighted operation in this region.