EU: Periodic Technical Inspections (PTI)

Introduction

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, in which an existing European PTI control point is tested according to a better test method.

Among a number of safety tests—for brakes, steering, lighting, tires, suspension, and others—PTI inspections include checks of exhaust emissions. PTI emission checks have the potential to identify high emitting vehicles that can be responsible for the majority of air pollution from traffic in urban areas. These high emitting vehicles include vehicles with technical problems, as well as vehicles with tampered emission systems.

Emission tests conducted during a PTI check are performed quickly, usually within minutes, using simple and relatively inexpensive emission testers. Many of the test methods and instruments were developed years ago, for the testing of higher emission levels that were typical for older vehicle technology. As modern engines became cleaner, some of the tests used in PTI programs are no longer sensitive enough to detect emission failures. One example is the diesel smoke opacity test that is not suitable to detect the failures of diesel particulate filters (DPF) on modern diesel engines.

PTI-PN Programs. To address this problem and to better prevent emission tampering, several European countries have adopted particle number (PN) measurements as part of the PTI emission checks. These PTI-PN tests are conducted at idle, using a portable particle number counter. The PTI-PN test has been also referred to as a particulate filter check (PFC).

The interest in a PTI-PN test is not limited to Europe; the approach has been considered by several countries worldwide.

PTI Emission Testing

The EU requirements for exhaust emission tests for periodic technical inspections are [4757]:

• 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. The measurement of NOx emissions is not required.
• For diesel vehicles, the PTI emission test procedure includes a smoke opacity measurement during a free acceleration test. In this test, the light-duty opacity limit value for most modern diesel cars is 0.7 m^-1. The measurement of NOx or other gaseous emissions is not required.
• 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 three-way catalyst technology is robust and durable. 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][5169]. 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 diesel smoke opacity test has a poor correlation with the PM/PN emission of the vehicle. Many diesel vehicles can pass this test without a diesel particulate filter, because the opacity value of engine-out exhaust in modern engines 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, the OBD system is often manipulated when removing particulate filters, and EOBD systems are not able to monitor and control all emissions due to a lack of sensors. Most production vehicles are equipped with NOx sensors, but not with PM/PN sensors.

PTI-PN Programs

NPTI Working Group

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 published a white paper [4761] that described the need and possibilities for a new PTI emission test procedure, and was meant to inform policy makers of national and European authorities, equipment manufacturers, type approval authorities and PTI service providers. As a result, several countries have been considering a mandatory emission test at the PTI, designed to identify high emitting vehicles.

The PTI test should be quick and cost-effective, without the need for a chassis dynamometer. Ideally, the test should be conducted through a steady-state, tailpipe measurement at the idle condition.

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 already 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 PTI-PN Regulations

While the PTI-PN particulate filter check is voluntary for EU member states, a growing number of countries have been adopting a mandatory PTI-PN check that replaces the smoke opacity test. However, as the individual countries define their own PTI criteria, the particle test, the configuration of the particle counter, and the threshold limit value (TLV), the national PTI-PN programs are not fully harmonized.

The first EU country to introduce PTI-PN testing requirements has been the Netherlands [4355]. The first Dutch PTI-PN regulation, published in 2019 [4762] (English translation), included a PN limit of 250,000 cm^-3 for Euro 5b or later vehicles. This limit has been relaxed to 1,000,000 cm^-3 for all diesel vehicles under a PTI-PN regulation published in 2021 [5018] (English translation).

The Dutch PTI-PN testing program enters into force on 1 July 2022. It is applicable to all diesel cars and commercial vehicles fitted with a DPF at the manufacturing stage (essentially, Euro 5 and later passenger cars and vans, and Euro VI trucks). Transitional provisions for 2016 and older passenger vehicles and for 2011 and older vans allow vehicle owners to avoid repair costs by voluntarily reporting a defective or removed DPF. After deregistration of the particulate filter, a surcharge on top of the annual tax must be paid.

Other European countries that adopted mandatory PTI-PN particulate filter checks include Belgium, Germany, and Switzerland, Table 1.

The adoption of PTI-PN emission tests has also been considered by a number of countries outside of the EU—including Switzerland, South Korea, Japan, Israel, Chile, Mexico, Colombia, and Peru—as well as by European-level regulatory bodies:

• UNECE has proposed to implement the PTI-PN test procedure in their vehicle regulations [4765][4766].
• The European Commission has published a recommendation to improve the periodical technical inspection of vehicles by implementing PN emission testing as part of PTI inspections [5914]. To ensure a coordinated approach across the EU, the Commission’s guidelines define requirements related to the PN measuring equipment and procedure as well as metrological and technical requirements. The guidelines recommend a pass/fail PN limit of 250,000 cm^-3.

PN Instruments

The PTI PN instruments must be approved/certified by the pertinent national authorities. It has been 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. However, the degree of harmonization between the various specifications for particle counters varies between countries. For instance, the Dutch metrology authority NMi also recognizes Swiss particle tester approvals issued under the Swiss Regulation SR 941.242 (2014) for nonroad construction machinery. On the other hand, the German metrology authority PTB requires that all PN counters to be used in Germany be approved according to the German specifications [5163].

The technical requirements for PN testers can be illustrated by the Dutch requirements that have been developed by the NMi and TNO. The documents are available via the NMi website [4764]. The main characteristics of the PN instrument are [4355][5419]:

• Applicable for diesel 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

PTI PN instruments available on the market—approved and in the process of certification—are listed in Table 2.

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 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.

Acknowledgements: Gerrit Kadijk of Emission Training Services (ETS)—who maintains the ParticlesMatter.com website—has provided valuable feedback on this article.