Iniciativa del Aire Limpio: Infopool Español
Clean Air Initiative: GlobalClean Air Initiative: AsiaIniciativa del Aire Limpio: América LatinaClean Air Initiative: Sub-Saharan Africa
Advanced Search
Dialogue room
Mailing List
I and M programs

This document only deals with In-use Diesel Trucks, because heavy duty trucks are primarily diesel fueled. However, limited fleets such as refuse haulers operating in a city center could very well use CNG as fuel. In such cases reference is also made to the document "In-Use Buses: I/M Programs".

One of the most effective mechanisms for minimizing emissions from in-service vehicles is periodic testing to identify high polluting vehicles followed by appropriate maintenance. Periodic testing is also important to assure that newer low emitting vehicles still have emissions below the set limit values. Such efforts are most commonly termed 'Inspection and Maintenance', or I/M programs. This is expressed by the European Commission as "An I/M program aims to assure that motor vehicle emission control systems are functioning properly throughout the lifetime of the vehicle".

When possible, an I/M program for verification of exhaust emissions should be combined with a check of the safety of the vehicle.
Until very recently, I/M programs have focused primarily on gasoline-fuelled vehicles. However given the relatively recent public awareness of the health and environmental threats posed by emissions from diesel vehicles, there is now world-wide interest in 'catching up' with equivalent programs and test protocols to reliably and economically identify high-polluting diesel vehicles and get them repaired.

Often a small number of vehicles or a group of vehicles cause a large percentage of air pollution, thereby contributing significantly to poor air quality. When those groups of vehicles are identified as gross polluters, efforts should be made to implement a system to assure appropriate repairs of those vehicles. From a technical perspective the test method used for the I/M program should focus on the pollutants that the program is intended to reduce and should also match vehicle and fuel technology.

In the future it is likely that most attention will be given to reducing the emissions of NOx and particulate matter. This will imply new challenges for the development of modified I/M programs.

The different types of I/M tests are as follows for diesel fuelled trucks:

Measurements of black smoke (Full-load, steady state) or opacity (Free acceleration).

  • Steady-state, loaded test (Acceleration Simulation Mode, ASM).
  • Transient Loaded test (IM-240; Vehicle Mass Analyses System, VMAS).

The two latter loaded tests were originally developed for light duty vehicles (passenger cars) and must be carried out on a chassis dynamometer.
In addition, remote sensing also has the potential to identify high emitting vehicles and therefore may become a part of future I/M programs.
Regarding the technical features of an I/M program, the following must be considered:

  • Technical requirements for the inspection (what kind of test procedure and equipment to be used, manual or "automatic" test lanes to reduce the risk for manipulation of results).
  • Development of a detailed program for the actual inspection of the vehicles (inspection program) to be used during the inspection procedure.
  • Criteria for Pass/Fail decision (to introduce limit values used in other countries or to develop a system for "phase-in" to gradually tighten standards over a period of years, thereby reduce the number of failing vehicles in the beginning of the program).
  • Periodicity of the inspection (Once-twice a year, and the age of the vehicle at the first inspection).
  • System for quality assurance and possible accreditation.
  • Training of vehicle inspectors (Initial and annual).

Steps must also be taken to assure that the repair shop industry is trained to repair and adjust vehicles not passing the inspection.


When it is decided to implement a system for mandatory vehicle inspection, many other issues must also be addressed, such as:
- Type of inspection system (centralized vs. decentralized).
- Enhancement of institutional arrangements (a clear split between regulatory body and operator of the inspection stations, to make way for feed-back from the inspection to the regulators, who will supervise the system etc.).
- Ownership of the inspection stations (independant, private operator).
- Implementation of a vehicle register or enhancement of an existing one (to make it possible to find vehicles not approved at the inspection or not inspected at all).
- Campaigns for making people aware (and supportive) of the I/M program (Important since people must have a confidence in the program and realize the benefits of the program).


Conventional diesel trucks produce significant amounts of pollutant emissions - especially particulate matter (PM) and nitrogen oxides (NOx) - that cause a deterioration of air quality. While properly maintained diesel vehicles tend to have low deterioration, poor maintenance can lead to increased levels of smoke and particulate.

The lack of robust, commercially available equipment for quickly and accurately measuring diesel PM emissions has meant that regulators have been obliged to focus diesel I/M programs on smoke opacity. Smoke is a highly undesirable pollutant in its own right, and reducing opacity levels may also tend to reduce particulate emissions. However, available data indicate that smoke opacity, even when measured under a controlled load on a dynamometer, has a poor correlation with particulate emissions measured under the same conditions. High smoke emissions can result from serious engine problems such as worn out injectors or a dirty air cleaner that affect PM emissions throughout the driving cycle, or they may result from problems such as tampering with the puff smoke limiter that affect emissions only during transient accelerations and thus have much less impact on PM emissions over an entire driving cycle.

The two most commonly run smoke/opacity tests are:
The Lug Down test, which is performed at full throttle, with the dynamometer load gradually increased to pull back engine speed so that the engine is laboring, or "lugging". This test is sometimes run at roadside without a dynamometer by using the vehicle brake. Due to practical reasons such as poor repeatability, the need for experienced personnel to carry out the tests, rather high investment costs when not performing the test on the road, the lug down test has been replaced by the free acceleration test in the EU-countries. This system can not measure NOx.

The Snap-Idle (SAE J1667 Free Acceleration) test simply involves fully depressing the accelerator pedal while the transmission is in neutral, and measuring the maximum smoke. The test requires no dynamometer. The opacimeter however must be rather well specified with regard to time constants and the length of the sampling line and probe design. For older vehicles/engines this test method can possibly cause engine damages on poorly maintained engines. This system can not measure NOx emissions.


In order to meet future demand the "next generation" of I/M programs must be further developed. When the focus is on NOx emissions, loaded tests on a chassis dynamometer must be introduced. This will also make it possible to measure particulate emissions. There are several paths to go such as:
Steady State, loaded test:

  • Possible to measure NOx
  • ASM driving cycles simulates the emissions from a vehicle during acceleration
  • System can be used for testing conventional as well as advanced technologies
  • Special "cut-points" (pass/fail criteria) must be developed
  • Transient loaded test (IM 240, VMAS)
  • Possible to measure NOx
  • Best method to identify high emitting vehicles
  • Offer highest potential for emission reductions
  • Most expensive test method (VMAS some cheaper chassis dynamometer can be used

The short test evaluations have shown that dynamometer-based short tests with transient acceleration segments are considerably more effective than unloaded or steady state tests in estimating "real world" emissions of all regulated pollutants. However, investment costs are high.

Smoke opacity measured during the free acceleration test, or any of the steady-state tests, does not correlate well with particulate emissions generated during the transient dynamometer test cycle. Smoke opacity is basically a measure of visual amenity only. In spite of these reservations, it is possible to define cut points for pass/fail but linked with high values for errors of commission and/or errors of omission.

The emission reduction potential is great, but of course dependant upon the test method used and the criteria for pass/fail. Generally, the potential for emission reduction is higher when a loaded mode test is used than the more commonly used (free acceleration or lug down).
One study carried out in the US estimated that current in use heavy-duty vehicle PM emissions are about 55% higher than their design values. If a program is introduced which fails the worst 25%, and brings about a two-thirds reduction in excess emissions, overall PM emissions would be reduced by almost 25%. Estimating repair costs to be about $250 per failing vehicle would result in a cost effectiveness of about $7,600 per ton reduced. Other studies show a reduction of regulated emissions (CO, HC, NOx and PM) to 20-50% and in addition a reduction of fuel consumption of 15-20%, depending upon engine size and engine technology when a suitable I/M program is implemented.


Diesel engines are very reliable, especially in comparison to newer technologies employing alternative power trains. Diesel engines look back on a comparatively long period of continuous development. For this reason, they have become the technology of choice for commercial vehicles and buses in most regions of the world. If properly maintained they can be expected to perform for several hundred thousand miles before engine overhauls are needed. In reliability tests of alternative fuel trucks, diesel trucks generally serve as reference.

As countries introduce more stringent standards that require the addition of pollution control equipment, more attention may need to be addressed to this issue. Similarly, technologies which are retrofitted to existing in use trucks may require careful attention to assure that they don't deteriorate excessively.
Inspection and Maintenance test equipment is well developed and quite reliable if properly selected and matched to the task.


Compared to gasoline-fueled vehicles, the cost of repair for diesel vehicles tends to be much higher. In one study, repair costs were estimated to be about $250 per failing vehicle, which would result in a cost effectiveness of about $7,600 per ton reduced.


There are a wide variety of diesel bus and truck smoke test programs in place around the world.


It is generally believed that an inexpensive, loaded test that directly measures PM emissions rather than smoke would substantially improve the prospects for a good diesel I/M program. However, even with this, it is likely that the costs of repair would be high making strong implementation difficult in many developing countries.

Related documents
Interesting Websites
General Topics
Inspection and maintenance

Tel: +1 (202) 458-0859 / Fax: +1 (202) 676-0977/8 / E-Mail: [email protected]