EU: Heavy-Duty Truck and Bus Engines: OCE and ISC PEMS Testing

General Requirements

For European on-road heavy-duty truck and bus engines, PEMS testing is used during type-approval and in-service conformity (ISC) testing. At type-approval, PEMS testing of vehicles equipped with the engine being certified is one part of the off-cycle emissions (OCE) testing; the other component being NTE testing on an engine dynamometer. PEMS testing of a production vehicle after it has accumulated a minimum of 25,000 km is the primary compliance measure for ISC testing.

ISC testing is required within 18 months of the first registration, on a vehicle registered in the EU that has accumulated a minimum of 25,000 km. N3 vehicles shall be tested, when applicable, with a semi-trailer. In the case of Type 1B, Type 2B and Type 3B dual-fuel engines, an additional PEMS test is required in Diesel mode on the same engine and vehicle immediately after, or before, a PEMS test is performed in dual-fuel mode.

Differences in PEMS testing at type-approval and for ISC are primarily in the type of vehicle that is tested, the payload and the accumulated mileage, Table 1. Table 1 also provides the ambient temperature and pressure during PEMS testing as well as coolant temperature requirements at the start of PEMS testing. The phase-in dates and other requirements, including conformity factors, of OCE and ISC tests are summarized elsewhere.

For heavy-duty on-road engines, both type-approval and ISC require that PEMS testing be carried out for CO, HC, NMHC, CH₄ and PN emissions.

ISC Testing Requirements

Testing for ISC is to be repeated at least every 2 years for each engine family. At the request of the manufacturer the testing may stop 5 years after the end of production.

The number of engines to be tested for ISC can range from 3 to 10 depending on the cumulative test results. The sampling procedure is intended to quantify the number of nonconforming engines and with pass/fail decisions made according to Table 2. The procedure has been designed so that the probability of a lot passing a test with 20% of the vehicles or engines defective is 0.90 (producer’s risk = 10%) while the probability of a lot being accepted with 60% of the vehicles or engines defective is 0.10 (consumer’s risk = 10%). While testing three engines is sufficient to reach a fail decision, it is not sufficient to reach a pass decision. Additional engines, up to 10, must be tested and the number of nonconforming engines must be no more than that indicated by the pass decision number for the cumulative number of engines tested before a pass decision can be made.

PEMS Testing Operating Conditions

PEMS testing is conducted over urban (0-50 km/h) followed by rural (50-75 km/h) and motorway (> 75 km/h) driving conditions. The proportion of these conditions depends on vehicle category, Table 3. Urban, rural and motorway portions can be determined by geographical coordinates (by means of a map) or by the first acceleration method.

For the purpose of the assessment of the trip composition, the duration of each share shall be calculated from the moment when the coolant temperature has reached 70°C for the first time or after the coolant temperature is stabilized within ±2°C over a period of 5 minutes whichever comes first but no later than 15 minutes after engine start. The period elapsed to reach the coolant temperature of 70°C shall be operated under urban driving conditions.

The following distribution of the characteristic trip values from the WHDC (World-Wide Harmonized Heavy-Duty Certification, Global Technical Regulation (GTR) No. 4) database may serve as additional guidance for the evaluation of the trip:

• accelerating: 26.9% of the time;
• decelerating: 22.6% of the time;
• cruising: 38.1% of the time;
• stop (vehicle speed = 0): 12.4% of the time.

The test duration shall be long enough to complete between four and eight times the work performed during the WHTC or produce between four and eight times the CO₂ reference mass in kg/cycle from the WHTC as applicable.

If the particle exhaust aftertreatment system undergoes a non- continuous regeneration event during the trip or some OBD malfunctions occur during the test, the manufacturer can request the trip to be voided.

Emissions Evaluation

Methodology

Up to and including Euro VI-D engines, cold start emissions (i.e., while coolant temperature < 70°C) are excluded. For Euro VI-E engines, a portion of the cold start emissions are included but emissions while the coolant temperature < 30°C are excluded. The data evaluation starts once the coolant temperature has reached 70°C for Euro VI-D and earlier engines or 30°C for Euro VI-E engines for the first time or once the coolant temperature is stabilized within ±2°C over a period of 5 minutes, whichever occurs first, but in any event no later than 10 minutes after test start.

Emissions are averaged using the “averaging window” method illustrated elsewhere. It is a moving average process, where the duration of the averaging period (window) is based on the mechanical work or CO₂ emissions that were measured over the WHTC test during type approval testing. For the work-based method, the reference work (W_ref) is equal to the work produced [kWh] by the engine over the WHTC while for the CO₂-based method, the reference CO₂ (m_CO2,ref) equals the CO₂ mass [kg] measured over the WHTC. Window lengths during data evaluation are equal to the time it takes to produce the reference work or reference CO₂ mass. Window length on a time basis can vary during the test depending on the average power output of the engine. After the data has been parsed into averaging windows, valid windows are identified.

Valid Work Windows

Up to and including Euro VI-C engines, valid work windows are those for which the average power exceeds 20% of the maximum engine power (P_max). The percentage of valid windows shall be equal or greater than 50%. If the percentage of valid windows is less than 50%, the data evaluation is repeated using lower power thresholds. The power threshold is reduced in steps of 1% until the percentage of valid windows is equal to or greater than 50% and the average power does not go below 15% of P_max. The test is void if the percentage of valid windows is less than 50% at 15% of P_max.

For Euro VI-D and later engines, valid work windows are those for which the average power exceeds 10% of P_max. The test is void if the percentage of valid windows is less than 50% or if there are no valid windows left in urban-only operations after the 90th percentile rule (see below) has been applied. The latter is intended to ensure that the effect of urban driving on NOx emissions is reflected in the test result.

The conformity factors are calculated for each individual valid work window and each individual pollutant:

CF = e / L(1)

where:
e = the brake-specific emission of the gaseous pollutant [mg/kWh] or PN [#/kWh];
L = the applicable limit [mg/kWh] or [#/kWh].

Valid CO₂ Windows

Up to and including Euro VI-C engines, valid CO₂-based windows are those whose duration does not exceed the maximum duration calculated from:

D_max = 3600·W_ref / (0.2·P_max)(2)

where:
D_max = the maximum window duration, s;
P_max = the maximum engine power, kW.

If the percentage of valid windows is less than 50%, the data evaluation shall be repeated using longer window durations. This is achieved by decreasing the value of 0.2 in Equation (2) by steps of 0.01 until the percentage of valid windows is equal to or greater than 50%. In any case, the lowered value in Equation (2) shall not be lower than 0.15. The test shall be void if the percentage of valid windows is less than 50% at the maximum window duration.

For Euro VI-D and later engines, the valid windows shall be the windows whose duration does not exceed the maximum duration calculated from Equation (3). The test shall be void if the percentage of valid windows is less than 50%.

D_max = 3600·W_ref / (0.1·P_max)(3)

The conformity factors shall be calculated for each individual valid window and each individual pollutant.

CF = CF_I / CF_C(4)

CF_I = m / (m_CO2(t2,i) – m_CO2(t1,i))(5)

CF_C = m_L / m_CO2,ref(6)

where:
CF_I = in service ratio;
CF_C = certification ratio;
m = the mass emission of the gaseous pollutant [mg/window], or PN [#/window];
m_CO2(t2,i) – m_CO2(t1,i) = the CO₂ mass during the i^th averaging window [kg] and t_1,i and t_2,i are the being and end times respectively of the i^th window;
m_L = the mass emission of the gaseous pollutant or PN corresponding to the applicable limit on the WHTC [mg] or [#] respectively.

Final Conformity Factor

For Euro VI-E engines, the final conformity factor (CF_final) for each pollutant using either the work-based or CO₂-based method is:

CF_final = 0.14 × CF_cold + 0.86 × CF_warm(7)

where:
CF_cold is the conformity factor of the period of cold operation of the test, which shall be equal to the highest conformity factor of the moving averaging windows starting above 30°C and below 70°C coolant temperature;
CF_warm is the conformity factor of the period of warm operation of the test, which shall be equal to the 90^th cumulative percentile of the conformity factors.

Up to and including Euro VI-D engines, the final conformity factor is the 90^th cumulative percentile of the conformity factors calculated with either Equation (1) or Equation (4). These exclude data for engine operation below 70°C.

Acknowledgements: Karsten Mathies of TÜV Hessen has graciously provided valuable guidance and background information for this article.