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Emission Testing: Periodic Technical Inspection

A deeper look into NPTI

Emission Testing within Periodic Technical Inspection (PTI)

Current Situation

Clean air is important for everyone. As recent observations have shown, air pollution from transport can be quite significant. It has been shown in several studies that air pollution is one of the risk factors for human health and the environment. One path of lowering air pollution from vehicles is to periodically measure the emissions and to identify Diesel vehicles with faulty particulate filters.

In Europe, Germany decided in 2017 to introduce a particle number test to periodic technical inspection (NPTI) for Diesel vehicles by January 1st, 2021. The Netherlands announced the introduction of particle number testing in November 2019 in the Staatscourant with the goal to have it established also in 2021. In preparation for this regulation, several studies have been conducted in the Netherlands, Germany, and Belgium, as well as other European countries.

Based on the experiences gained in these studies — which, in some cases, have not yet published final reports — some common ground was found in the design of a test procedure and a limit value. Overall, a robust, simple, quick and cost-effective test procedure has been identified and is currently being finalized in the respective legal frameworks.


When being faced with the implementation of a new NPTI program all stakeholder involved in the drafting process encounter challenges from present requirements. Additional complexity is added when considering future requirements of geographical, political or technological nature. 

1. Performance-Based Standards vs. Technology-Based Standards

One of the fundamental guidelines in drafting new legislations and standards is that performance-based standards are preferred over technology-based standards to promote competition and free access to markets. The goal is to determine the standard which is best suited to achieve the required performance, reliability, traceability and measurement uncertainty needed for a sound and expandable NPTI regulation provided target price-performance ratios are being met, monopolies are avoided and wide equipment availability is guaranteed regardless of the applied technology.

2. Traceability / Measurement Uncertainty

From a judicial, user and consumer perspective, a traceable reference chain back to national standards is a must. Measurement equipment used in mandatory emission tests has to be verified in a stringent homologation process and needs to be calibrated regularly against traceable reference equipment to guarantee proper functioning before market entry and during its useful life. Equally important is the measurement uncertainty which is directly related to the number of steps in the traceability chain. Current assessments assume an uncertainty budget between 20 to 30% for each step.

3. Cost Effectiveness of the Program

The cost effectiveness of an emission program is determined by three influencing factors: 

  • The duration of the test procedure as it has a direct impact on labor cost as most expensive contributor
  • Limited price target for NPTI measurement equipment 
  • Life-time cost 

Future Requirements 

Next to important considerations about immediate requirements stakeholders also need to consider near and midterm requirements which can already be foreseen:

  • Drafting of an OIML Standard
  • Gasoline direct injection (GDI) vehicles
  • Sub 23 nm measurements
  • Reduction of Limit Value

NPTI Outlook

Test procedures, limit values and equipment performance standards are at an advanced stage in development, with additional progress being made. To protect consumers and regulators while improving air quality the influence of engine technology, fuels and aftertreatment on particle size must be minimized in order to provide reproducibility, traceability and repeatability of the measures undertaken. 

TSI is actively involved 

TSI has participated in the NPTI meetings organized by VERT that have resulted in the Dutch requirements for test equipment and procedures for a particle number measurement in NPTI. As a member of the German Garage Equipment Association (ASA), TSI is further contributing to the development of the German NPTI requirements.

TSI is close to finishing the development of a portable NPTI measurement system based on Condensation Particle Counter (CPC) technology and will submit this instrument for type approval in The Netherlands.

TSI has decided to use CPC technology, as from a metrological point of view, there is no technological alternative for measuring the number of particles in the periodic emission test. This is further supported by the following facts:

  1. CPC-technology is proven and in-use for over 20 years in emission measurement according to PMP and more recently in RDE.
  2. CPC-technology provides direct single particle counting, the CPC-based NPET is the first instrument certified for emission testing
  3. Traceable calibration of CPCs is described in the international standard ISO 27891
  4. CPC-technology therefore provides comparable and reputable results from station to station
  5. The working fluid used in a CPC keeps the measurement cell clean leading to very low maintenance requirements
  6. CPC-technology is fit for the future and ready to measure petrol vehicles (studies are already ongoing at TNO)
  7. CPC-technology can easily handle increased detection limits (e.g. from 23 nm to 10 nm) and stricter error limits

For a more in-depth look at NPTI, please see our recent white paper: Challenges of Introducing PN-PTI in Germany and Other Countries