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Different methods can test emissions |

Different methods can test emissions

| Saturday, October 3, 2015 12:01 a.m

Question: Many people, I think, are like me: totally ignorant of how emissions testing works. I had always assumed, until all the news about Volkswagen broke, that in an emissions test, a technician simply put a hose into a tailpipe, and emitted gases ran into a device that read out the required information.

If this were so, I don’t see how Volkswagen would have been able to cheat the system. There must be more to it. Would you consider discussing the subject in a column?

Answer: Emissions testing programs vary widely from state to state, but most states perform an on-board diagnostic check in lieu of a tailpipe emissions check.

An analyzer is connected to the vehicle’s diagnostic connector, and a technician checks to see whether monitors (self-tests) are completed or whether any diagnostic trouble codes appear.

If the required monitors have run to completion, there are no stored trouble codes, and the check engine light is capable, yet off, the vehicle certifies itself, via inference, that exhaust emissions are within specifications.

Light diesel vehicles, like the Volkswagens in question, may receive an on-board diagnostic test, a simple exhaust opacity (smoke) test, or aren’t tested at all.

Many states defer emissions testing for the first four to six years on a new gas or diesel vehicle.

Some areas still perform tailpipe testing. One test includes a variety of simulated driving conditions lasting 240 seconds, mimicking part of the new vehicle federal certification test.

The driving “trace” is very specific, and a technician needs to carefully manipulate the throttle while driving the trace or the test is terminated.

This test collects all of the exhaust for analysis, looking specifically at five gasses.

A different testing method is simpler and involves the vehicle being run at a steady 15 miles per hour and 25 miles per hour under moderate load. A portion of the exhaust is sampled, and again, five gasses are looked at.

A vehicle’s engine management system is programmed to gather information from its sensors, refer to an instruction set, and execute appropriate outputs to fuel, ignition, and emission control components to maintain optimum drivability, fuel economy, and emissions compliance.

It’s certainly plausible that a vehicle’s control system could be instructed to recognize the unique operational conditions of the complex federal test procedure, or if need be, one of the state-run tests and make changes that differ from normal driving maneuvers.

Reducing nitric oxide emissions during an emissions test would likely involve changes to fuel injection timing and/or duration and exhaust gas recirculation flow.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at; he cannot make personal replies.

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