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Emission Standards

EU: New Periodic Technical Inspections (NPTI)

This article sponsored in part by TSI Incorporated.


European legislation establishes minimum requirements for a regime of periodic roadworthiness tests of vehicles used on public roads [4757]. Each member state must ensure that vehicles registered in its territory are periodically tested by testing centers authorized by the member state in which those vehicles are registered. Member states may introduce national requirements concerning roadworthiness tests for vehicles registered in their territory, which go beyond the minimum EU-wide provisions.

The EU requirements for exhaust emission tests for periodic technical inspections (PTI) are:

  • For petrol vehicles, the PTI emission test includes an exhaust gas analysis at low and high idle engine speed in which the concentrations of CO2, CO, THC and O2 are measured. The lambda value at high idle speed, calculated based on these measurements, should be between 0.97 and 1.03. Furthermore, there are CO limit values at the two idle speeds.
  • For diesel vehicles, the PTI emission test procedure contains a smoke opacity measurement during a free acceleration test. In this test, the current light duty opacity limit value is 0.7 m-1.
  • EOBD tests: As an alternative for the exhaust gas analysis or the smoke opacity test, vehicles can be checked in the PTI by reading the electronic on-board diagnostic system (EOBD) which might report certain fault codes. EOBD systems monitor the technical status of hardware (sensors and actuators) and monitor some engine behavior with software. In the case of a malfunction, fault codes are generated and stored in the memory of the electronic control unit (ECU).

The above petrol vehicle procedure is a very simple test in a restricted area of the engine map. Fortunately, the emission performance of most petrol vehicles is good because the concept of the three-way catalyst is very robust and stable. However, a small percentage of high emitting petrol vehicles on the road are responsible for very high proportion of the overall pollution from traffic, including high NOx emissions [4347]. A further improvement of the PTI for petrol vehicles could be achieved by implementation of a NOx test and, for vehicles with gasoline particulate filters (GPF), PN emission testing.

The current diesel smoke opacity test has a poor correlation with the PM/PN emission of the vehicle. Most diesel vehicles can even pass this test without a diesel particulate filter (DPF), because the opacity value of the engine-out exhaust is often lower than 0.7 m-1. Removal or failure of the DPF is therefore not detected by the test. Lowering the limit value is not an option because current PTI opacity meters have an accuracy of ±0.3 m-1 and are not suitable to measure low smoke emission levels [4761]. Furthermore, EOBD systems are not able to monitor and control all emissions due to a lack of sensors. Current vehicles have NOx sensors but PM or PN sensors are not yet available.

In 2016, several Swiss, German and Dutch governmental organizations, metrological institutes, scientists and equipment manufacturers recognized the need for new PTI (NPTI) emission tests to identify emissions tampering—such as removing the DPF or disabling the SCR system—or detecting failed aftertreatment components. Through an initiative of the VERT Association and the Netherlands (Ministry for Infrastructure and Water Management, RDW, TNO, NMi) an informal NPTI technical working group was established to develop suitable test procedures.

The NPTI working group issued a white paper [4761] that describes the need and possibilities for a new PTI emission test procedure and is meant to inform policy makers of national and European authorities, equipment manufacturers, type approval authorities and PTI service providers. As a result, several countries are considering a mandatory emission test at the PTI, designed to identify high emitting vehicles.

NPTI Regulations

NPTI Working Group Recommendations

The PTI test should be quick and cost-effective; ideally, the test should be conducted through a steady-state, tailpipe measurement at the idle condition. This means that the use of a chassis dynamometer should be avoided.

PN Emission Testing. In 2017, TNO proposed a new PTI-PN test method for diesel vehicles with a particulate filter [4763][4767]. The test involved PN emission measurement, with the vehicle stationary, at low idle speed. A recommended PN limit value for Euro 5b/6 vehicles, which have to satisfy the type approval PN standard, was 250,000 cm-3. This limit value correlates with Euro 6 type approval data. Euro 3, 4 and 5a vehicles could have a PN limit value of 1,000,000 cm-3.

Such a PN measurement can provide a quick and accurate method for DPF testing—and suitable, cost-effective PN instruments are beginning to be available.

NOx Emission Testing. A PTI NOx emission test for diesel SCR systems remains problematic [4356][4166]. Options that have been considered to warm up the exhaust system to enable urea injection include a load step test, using thermal management tools at idle (intake or exhaust throttle, late fuel injection), or driving the car at high speed on the road immediately before the test. Experiments with Euro 6 vehicles showed that NOx measurements would require 10-15 minutes of driving in order to stabilize the system and ensure the SCR catalyst reaches its operating temperature. Hence, a chassis dynamometer test appears to be necessary for NOx measurement.

National Regulations

The Netherlands is the first EU country to introduce PTI-PN testing requirements for PN emissions [4355]. The Dutch PTI regulation [4762] (English translation), published in November 2019, includes a PN limit of 250,000 cm-3, to be met at low idle. The limit is applicable to diesel vehicles of the Euro 5b or later emission stage.

The effective date of the regulation is 1 January 2020, meaning that from this date there is an official methodology for checking DPFs with a particle counter in the Netherlands. The introduction of mandatory PTI PN inspections depends on the availability of approved PN instruments. If certified PTI particle counters from multiple manufacturers become available on the market in the Netherlands in 2020, NPTI inspections using PN instruments could become mandatory from January 1, 2022.

Other countries that consider adoption of PTI-PN emission tests include Germany, Belgium, South Korea, Japan, Chile, Mexico, Colombia, Peru, and Switzerland. PTI-PN emission tests have also been considered by European-level regulatory bodies:

  • UNECE has proposed to implement the PTI-PN test procedure in their vehicle regulations [4765][4766].
  • The European Commission is expected to come with a recommendation of the PTI-PN test procedure in addition to the PTI legislation [4757] and considers implementation of the PTI-PN test procedure in Euro 7 type approval legislation.

PTI PN Instruments

The PTI PN instruments must be approved/certified by the pertinent national authorities. It is also envisioned that particle counters approved by independent inspection bodies in other countries can also be used, provided that the approval requirements are at least as stringent as the national requirements.

The Dutch specifications for PN testers have been developed by the Dutch metrology authority NMi. The documents are available via the NMi website [4764].

The main characteristics of the PN instrument, as described by the draft NMi specifications, are [4355]:

  • Applicable for diesel and gasoline engines
  • The tester contains a sampling system, a volatile particle remover (VPR) and a PN counter
  • The tester is characterized with particle sizes of 23, 50 and 80 nm
  • The volatile particle remover has an efficiency of more than 95%
  • Particle size for calibration and linearity check is > 50 nm
  • Measuring range: 5,000 – 5,000,000 cm-3
  • Measuring accuracy: ± 25%
  • Stabilization time (T0 - T95) of the PN counter (incl. sample line) is less than 15 seconds
  • Measuring frequency of the PN counter is at least 1 Hz

The NMi authority also recognizes Swiss particle tester approvals issued under the Swiss Regulation SR 941.242 (2014) for nonroad construction machinery.

The approved PTI PN instruments are listed in Table 1.

Table 1. Approved PTI PN instruments
TSI IncorporatedNanoparticle Emission Tester (NPET) Model 3795Swiss Regulation SR 941.242
Test Equipment Nederland (TEN)TEN AEM Particle CounterDutch NMi (2020.07)

The TSI Nanoparticle Emission Tester (NPET) Model 3795 has been approved according to the Swiss Regulation SR 941.242, and the measuring procedure of the TSI NPET fulfills the proposed requirements for the test procedure in the Netherlands. Therefore, DPF checks at Dutch test stations and road side inspections can be performed using the TSI NPET. The TEN AEM Particle Counter was the first PTI PN instrument approved by the Dutch NMi authority.