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Why don't they just do emissions tests by attaching a probe to exhaust and taking the car on a normal drive. Harder to fake results when you're not performing t

Why don't they just do emissions tests by attaching a probe to exhaust and taking the car on a normal drive. Harder to fake results when you're not performing them in the same lab conditions.

LeifCarrotson 10y ago
The testing process is required to pass a gauge reproducibility and reliability study. See https://en.wikipedia.org/wiki/ANOVA_gauge_R%26R https://en.wikipedia.org/wiki/ANOVA_gauge_R%26R. Essentially, the goal is to analyze the results when a given vehicle is tested in different conditions. In your scenario, each of three operators might take a dozen different makes and models of vehicles on a "normal drive" on each of three consecutive days, resulting in 3 operators x 3 measurements x 12 vehicles = 108 NOx numbers. They would then analyze the results statistically, checking the average value and standard deviation. This should prove that each of the three operators produces the same result ("reproducibility"), and that each day produces the same result ("reliability"). There will be some variation - the vehicle may change a little from one run to another, even if the control inputs are identical. And the inaccuracy of the probe will also produce some small random variation. Those can't be helped - though they should be known. But if you use different operators, lead-foot Larry needs to produce test results that are indistingushable from when slow Sam runs the test, and they both need to produce the same values on windy days and snowy days. This is easy if the only function of the operator is to strap the vehicle to a dynamo, check the HVAC, tire pressure, and so on, plug the control cable into the OBDII port (or wherever it goes), and push the "start" button on the tester. Perhaps the best solution is to collect data on hundreds of actual, reasonable, human drivers and feed those noisy, difficult to distinguish profiles into the test system. But that means expensive dynamos, and lots of hours of testing.
intrasight 10y ago
I'm sure that would be more expensive. And there are union work rules.
closeparen 10y ago
What is a normal drive? It's easier to standardize and precisely specify the test cycle on a dyno.
jpollock 10y ago
It makes it impossible to compare between models or even between model years. Since a key use of the information of for a consumer to compare between two cars to decide which they should purchase based on emissions and fuel economy, a repeatable test is a key requirement.
- oh_sigh 10y ago
  Then put the car on rollers for a fixed speed/acceleration/time profile
  - LeifCarrotson 10y ago
    That's exactly what they do now. Volkswagen and Chrysler (and probably everyone else) have ECU firmware that watches for those profiles and adjusts the engine tuning for emissions differently compared to normal operation. They need profiles that are more difficult to identify and more of them to solve this technically. Or the EPA and other regulatory agencies can bring down the hammer when they catch cheating like this.
    - oh_sigh 10y ago
      Oh, I didn't know that. I thought they just waited til the tech entered the "this is an emissions test" code in the ECU.
    - jpollock 10y ago
      Or else they rewrite the legislation to say "You can't ever exceed X on emissions". Then they do two tests, one for the legislation, which is a random drive over a period of time from a randomly sampled vehicle, and a second which is the dyno test for consumer information.

Why don't they just do emissions tests by attaching a probe to exhaust and taking the car on a normal drive. Harder to fake results when you're not performing them in the same lab conditions.

LeifCarrotson 10y ago

The testing process is required to pass a gauge reproducibility and reliability study. See https://en.wikipedia.org/wiki/ANOVA_gauge_R%26R https://en.wikipedia.org/wiki/ANOVA_gauge_R%26R. Essentially, the goal is to analyze the results when a given vehicle is tested in different conditions. In your scenario, each of three operators might take a dozen different makes and models of vehicles on a "normal drive" on each of three consecutive days, resulting in 3 operators x 3 measurements x 12 vehicles = 108 NOx numbers. They would then analyze the results statistically, checking the average value and standard deviation. This should prove that each of the three operators produces the same result ("reproducibility"), and that each day produces the same result ("reliability"). There will be some variation - the vehicle may change a little from one run to another, even if the control inputs are identical. And the inaccuracy of the probe will also produce some small random variation. Those can't be helped - though they should be known. But if you use different operators, lead-foot Larry needs to produce test results that are indistingushable from when slow Sam runs the test, and they both need to produce the same values on windy days and snowy days. This is easy if the only function of the operator is to strap the vehicle to a dynamo, check the HVAC, tire pressure, and so on, plug the control cable into the OBDII port (or wherever it goes), and push the "start" button on the tester. Perhaps the best solution is to collect data on hundreds of actual, reasonable, human drivers and feed those noisy, difficult to distinguish profiles into the test system. But that means expensive dynamos, and lots of hours of testing.

intrasight 10y ago

I'm sure that would be more expensive. And there are union work rules.

closeparen 10y ago

What is a normal drive? It's easier to standardize and precisely specify the test cycle on a dyno.

jpollock 10y ago

It makes it impossible to compare between models or even between model years. Since a key use of the information of for a consumer to compare between two cars to decide which they should purchase based on emissions and fuel economy, a repeatable test is a key requirement.

oh_sigh 10y ago
Then put the car on rollers for a fixed speed/acceleration/time profile
- LeifCarrotson 10y ago
  That's exactly what they do now. Volkswagen and Chrysler (and probably everyone else) have ECU firmware that watches for those profiles and adjusts the engine tuning for emissions differently compared to normal operation. They need profiles that are more difficult to identify and more of them to solve this technically. Or the EPA and other regulatory agencies can bring down the hammer when they catch cheating like this.
  - oh_sigh 10y ago
    Oh, I didn't know that. I thought they just waited til the tech entered the "this is an emissions test" code in the ECU.
  - jpollock 10y ago
    Or else they rewrite the legislation to say "You can't ever exceed X on emissions". Then they do two tests, one for the legislation, which is a random drive over a period of time from a randomly sampled vehicle, and a second which is the dyno test for consumer information.