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. The NTE testing requirements were first introduced as part of the 1998 Consent Decrees with heavy-duty engine manufacturers. NTE testing requirements were then gradually extended on other engine categories, including mobile nonroad engines and marine engines.
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.
Heavy Duty Engines
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):
All engine speeds 15% above the ESC (European Stationary Cycle) speeds:
nlo + 0.15 × (nhi - nlo)
nhi = the highest engine speed on the power curve where 70% of the maximum engine power is still achievable,
nlo = the lowest engine speed on the power curve where 50% of the maximum engine power is still achievable.
- All engine load points greater than or equal to 30% or more of the maximum torque value produced by the engine.
- 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.
- 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.
Figure 1. Basic NTE zone
The NTE cycle also has several temperature exclusions:
- For engines equipped with exhaust gas recirculation (EGR), operation within the NTE control area is not subject to NTE emission limits during cold temperature conditions. Cold temperature operation is defined as engine operating conditions meeting either of the following two criteria (Figure 2):
Intake manifold temperature (IMT) less than or equal to the temperature defined by the following relationship between IMT and absolute intake manifold pressure (IMP) for the corresponding IMP:
IMP = 0.0875 × IMT – 7.75
IMP = absolute intake manifold pressure in bars,
IMT = intake manifold temperature in degrees Fahrenheit.
Engine coolant temperature (ECT) less than or equal to the temperature defined by the following relationship between ECT and absolute intake manifold pressure (IMP) for the corresponding IMP:
IMP = 0.0778 × ECT – 9.8889
ECT = engine coolant temperature in degrees Fahrenheit.
- Intake manifold temperature (IMT) less than or equal to the temperature defined by the following relationship between IMT and absolute intake manifold pressure (IMP) for the corresponding IMP:
- For engines equipped with one or more aftertreatment devices that reduce NOx or NMHC emissions, the NTE NOx and NMHC emission limits do not apply when the exhaust gas temperature measured within 12 inches of the outlet of the aftertreatment device is less the 250°C. For multi-bed systems, it is the temperature at the outlet of the device with the maximum flow rate that determines whether the NTE limits apply.
Figure 2. NTE temperature exclusion zones for EGR engines
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 3).
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.
Figure 3. NTE control area for US 2004 engines:
(a) for C less than 2400 rpm; (b) for C greater than 2400 rpm
(see ESC cycle for definition of speed A, B, and C)
The NTE zones for Category 1 and Category 2 marine diesel engines are shown in the following figures. For constant-speed propulsion engines certified using the ISO 8178 E2 duty cycle, the NTE zone is defined as any load greater than or equal to 25% of maximum power at rated speed, and any speed at which the engine operates in use.
Figure 4. NTE zone for Category 1 propulsion engines certified using ISO 8178 E3 cycle
Figure 5. NTE zone for Category 2 propulsion engines certified using ISO 8178 E3 cycle
Figure 6. NTE zone for variable speed propulsion engines used on non-propeller law vessels and for variable speed auxiliary engines certified using ISO 8178 C1 cycle
Figure 7. NTE zone for recreational marine engines certified using ISO 8178 E5 cycle