Appendix 1: Types of I/M programs and tests

Programs

Conventional I/M programs come in three types. In a decentralized I/M program (``test and repair''), motorists must have their cars inspected at a certified garage that can also perform emissions repairs. Proponents claim that decentralized I/M programs are more convenient because they provide more test locations and because both the test and the necessary repairs may be done at the same location, avoiding additional trips to a remote test centre.

In a centralized I/M program (test only), the motorists must drive their cars into a test centre that is completely independent of any repair garage. Proponents, including the US EPA, claim that this separation between test and repair is necessary because there is widespread collusion between mechanic and consumer that results in fraud that reduces the potential benefits of the program by half. British Columbia's AirCare is centralized.

A hybrid I/M program provides some centralized and some decentralized testing facilities. Some hybrid programs allow consumers to choose what type of facility they will use; other programs assign the type of testing facility based on criteria such as a vehicle's model year or the place where it is registered.

Which program is best?

The debate about centralized and decentralized programs has important consequences for competing interest groups. Millions of dollars in test fees will either go to one or more testing contractors, or the test fees will be paid to various certified repair garages. Studies indicate that there is little difference between the effectiveness of the two types of program in reducing emissions--or between either type and no program at all (Lawson et al. 1996, sample = 44,000 vehicles).

Tests

There are three basic I/M tailpipe tests. The simplest and cheapest is the idle test which involves testing the concentration of exhaust gases as the vehicle's engine is running at its normal idle speed and, in some programs, also at a ``high-idle'' speed, typically 2,500 rpm. This test is designed to identify excessive emission of CO and VOCs but not of NOx.

To identify excessive emissions of NOx, the vehicle's engine must be under load, as it would be if accelerating or climbing a hill. During a loaded-mode test, tailpipe emissions are measured as the vehicle is running in gear on a roller that provides resistance to the drive wheels. In steady-state loaded-mode tests, the wheels of the vehicle are turning at a single speed (e.g., 25 mph) throughout the test. AirCare combines this type of loaded-mode test with an idle test.

A more sophisticated transient loaded-mode test requires that the vehicle's engine goes through a pattern of accelerations and decelerations under load. The most common version, the IM240, was once required in the US EPA ``enhanced'' programs and is still recommended in the CCME Code of Practice. The IM240 tests cars under more driving conditions than the other tests but at a very high price--around $250,000 per unit--so that only relatively few test centres can afford it.

According to the US EPA, VOCs leaking from fuel tanks, fuel lines, and the fuel delivery system are often greater than excess emissions of VOCs from the tailpipe (US EPA 1991a). Therefore, the US EPA had required evaporative tests to be performed in its enhanced I/M program. A partial description of the test is as follows:

The inspector must locate the evaporative canister, remove the vapor line from the fuel tank, and hook up the pressure test equipment to the vapor line. The system is automatically filled with nitrogen using computer controls and hardware flow controls. The pressure supply system is closed off and any loss of pressure is monitored by the computer. (US EPA 1991b: s. 4, 7)

This is supposed to be done in a couple of minutes for every make and model of vehicle by an inspector who is not a trained mechanic and who is typically earning little more than the minimum wage. The test was so poorly devised that it has now been abandoned virtually everywhere, primarily because of the damage it caused to consumers' vehicles.

Which test is best?

Test equipment can vary by an order of magnitude in cost from the least to the most expensive. The more expensive test equipment is claimed to measure emission levels more precisely. However, it is not measuring but identifying vehicles producing excessive emissions that is the object of the inspection. All three types of tailpipe test, if properly performed, should be able to identify vehicles that are polluting excessively at the time of the test while none of them can reliably identify all the vehicles that have serious but intermittent excess emissions. The more expensive equipment more reliably identifies marginally excessive emitters, but such identifications provide no significant environmental benefit because subsequent repairs provide little or no emission reduction, and are just as likely to make emissions worse as better (Lawson 1995: 474). In addition, the precision of the expensive I/M240 used in some I/M programs is hardly impressive. One study, which compared the results of IM240 and idle tests to the results of the much more elaborate Federal Test Procedure (FTP), the official American vehicle certification test, found that the I/M240 mistakenly failed up to 18 percent of vehicles producing low emissions and mistakenly passed up to 33 percent of the vehicles with high emissions (Gallagher et al. 1997).

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Last Modified: Wednesday, October 20, 1999.