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One of the most effective mechanisms for minimizing emissions from in-service vehicles is appropriate maintenance in combination with periodic testing to identify high polluting vehicles. 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. According to the European Commission "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. This is of special importance when, for example, existing heavy diesel buses are converted to operate on natural gas and the modification of vehicle implies installation of gas tanks and piping from the tanks to the engine.
Until very recently, I/M programs have focused primarily on gasoline-fuelled vehicles. However, given the increasing public awareness of the health and environmental threats posed by emissions from diesel vehicles, there is an growing interest to introduce equivalent programs and test protocols also for diesel vehicles in order to ensure reliability and to identify high-polluting diesel vehicles.
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. When new alternative fuels will be used, especially for city buses and fleets of trucks operating in city centers, the I/M programs must be modified accordingly.
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:
- No-load short test for gasoline fuelled vehicles (2-stage idle; calculation of air-fuel ratio).
- For diesel fuelled vehicles, measurements of black smoke (Full-load, steady state) or opacity (Free acceleration).
- Steady-state, loaded test (Acceleration Simulation Mode, ASM). Can be used for all vehicles regardless the fuel used.
- Transient Loaded test (IM-240, Vehicle Mass Analyses System, VMAS). Can be used for all vehicles regardless the fuel used.
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 (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 reducing the number of failing vehicles in the beginning of the program).
- Periodicity of the inspection 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, the supervision of the system etc.).
- Ownership of the inspection stations.
- 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 confidence in the program and realize the benefits of the program).
While properly maintained diesel vehicles tend to have low deterioration, poor maintenance can lead to increased levels of pollutant emissions of which particulate matter (PM) and nitrogen oxides (NOx) are of special concern.
The lack of robust, commercially available equipment for quick and accurate measurements of 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 PM measured under the same conditions. (1) 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. (2)
The two most commonly run smoke/opacity tests are:
The Lug Down test, which is performed at full throttle with a gradually increasing dynamometer load in order to pull back the engine speed as it 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 test, rather high investments cost 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 emissions.
The Snap-Idle test (SAE J1667 Free Acceleration) 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 well specified with regard to the time constant and the length of the sampling line and probe design. For older and poorly maintained vehicles/engines this test method can possibly cause engine damage. 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 (however, that needs special arrangements)
- 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 criteria 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 for a loaded mode test as compared to the more commonly used free acceleration or lug down tests.
One study carried out in the US estimated that the 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 PM. (2). 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.
As a general matter, diesel vehicles emitting excess smoke are operating too rich, i.e., are burning excess fuel. Repairs of vehicles which reduce this excess fuel consumption would therefore be expected to not only reduce urban air pollution but also to reduce the emissions of carbon dioxide (a potent greenhouse gas) and an improved fuel economy.
Diesel engines generally show a good operational stability and durability, 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 vehicles, diesel vehicles generally serve as the 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 buses may require careful attention to assure that they don't deteriorate excessively.
If, in the future, alternative fuels play a more important role, it is obvious that more focus must be given the development of suitable I/M programs for engines operated on these other fuels.
Inspection and Maintenance test equipment is well developed and quite reliable if properly selected and matched to the task.
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.
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