As General Motors prepares to defend itself against a lawsuit alleging cheating on its diesel engine emissions, scientists are keying in on three graphs that provide the basis for the claim.

The lawsuit – filed by the Hagens Berman Sobol Shapiro law firm, a law firm that specializes in class actions – accuses GM of installing “multiple defeat devices” to deceive federal regulators and emissions tests.

Duramax scandal small quote

The allegations take aim at the 6.6-liter Duramax turbodiesel V8 engines in the 2011 to 2016 Chevrolet Silverado and GMC Sierra heavy-duty pickup trucks. Since the so-called defeat devices are not physical units but rather lines of software and code – and therefore, invisible – there is no physical proof that GM is guilty of wrongdoing.

GM said the lawsuit is without merit.

“These claims are baseless and we will vigorously defend ourselves. The Duramax Diesel Chevrolet Silverado and GMC Sierra comply with all U.S. EPA and CARB emissions regulations,” the automaker said in a statement.

Hagens Berman conducted its own emissions tests and included its findings in the complaint. The firm has expertise after participating in similar class-action cases against automakers including Volkswagen, Mercedes-Benz and Fiat Chrysler Automobiles.

updated graph diesel The lawsuit hinges on the data collected by Hagens Berman, illustrated in three graphs amidst a 184-page document, according to two vehicle emissions scientists. David Cooke, Ph.D., senior vehicles analyst for the Union of Concerned Scientists, and John German, senior fellow at the International Council on Clean Transportation reviewed the data and shared their findings with Trucks.com.

1. The Stop and Go Test

NoX emissions graph GM diesel scandal

(Source: HBSS Law)

Hagens Berman outfitted one 2013 Chevy Silverado 2500HD with sensors and over the course of 3,500 miles conducted tests to monitor its emissions under different conditions. In the first graph presented by the law firm, the x-axis measures ambient temperature, and the y-axis represents nitrogen oxide, or NOx, emissions.

“They’re in the real world, driving the vehicle and measuring the actual tailpipe emissions output,” Cooke said.

What’s important here is the horizontal red line showing the acceptable level of NOx emissions according to federal standards, and the vertical blue lines marking 68 degrees and 86 degrees Fahrenheit. This represents the temperature range within which federal regulators normally conduct emissions testing.

What the graph allegedly shows is that the Silverado tested by Hagens Berman operated normally within the standard federal testing zone, but produced much higher emissions than allowed outside of it.

Assuming the test conditions are representative of the vehicle’s performance and the data is correct, “then I think that would actually confirm the presence of a defeat device,” German said.

Here’s why:

  • The blue dots represent measurements showing higher than normal NOx emissions when the truck was tested at temperatures lower than the standard testing zone. (Cooke advised to ignore the encircled “Active regens” data as those are viewed as outliers in such studies.)
  • The green dots represent measurements showing acceptable levels of NOx emissions within the standard testing zone.
  • The red dots represent higher than normal NOx emissions when the truck was tested at the high end or above the standard testing zone.

“Outside of that range of temperatures, the system is not operating in the same way,” Cooke said. “For that reason, they’re observing higher nitrogen oxide emissions.”

Hagens Berman alleges that this data proves the existence of at least two “defeat devices” – one that allows the engine to pollute heavily at temperatures above the range of testing, and another that allows it to pollute heavily below

2. The 40 MPH Test

40 MPH diesel Scandal Genera motors graph

(Source: HBSS Law)

The second set of data measures the level of pollution emitted over time. For this test, Hagens Berman drove the Silverado at 40 mph for roughly 15 minutes.

This is significant because federal regulators usually conduct emissions testing between 200 and 600 seconds – or 3.3 to 10 minutes – of operation. The truck is expected to measure an acceptable level of NOx pollution within that time frame.

But what about outside it?

Hagens Berman and Cooke both agree that the higher NOx levels occurring before 200 seconds are negligible, and to be expected. The truck is burning more energy directly after being started and emits more pollution as it gets up to speed.

Once it’s reached 40 mph, the pollution levels off. Within the 200 to 600 second range, the levels are acceptable.

“At steady operation, it falls back to what would be more typically steady driving behavior,” Cooke said.

But almost immediately after, the NOx emissions shoot up.

This, according to Hagens Berman, proves the existence of a third defeat device. It allegedly allows the engine to release higher emissions “after the vehicle has been run for 200 to 500 seconds.”

Cooke said that emissions are typically cleaner the longer the vehicle operates. The 40 MPH Test, he said, appears to contradict that

“It does seem to me surprising that you would see an onset of these large spikes in nitrogen oxide emissions,” Cooke said. “It does seem abnormal to my understanding of how the emission system operates. It seems odd.”

3. The 50 MPH Test

50 MPH diesel Scandal General motors graph

(Source: HBSS Law)

Hagens Berman also released a third graph to bolster its case. This time, the speed has been increased to 50 mph. Once again, the truck releases relatively low emissions until about the 500-second mark. It then spikes up, signaling more pollution coming from the diesel engine.

“It does suggest there’s an improper calibration,” German said.

But the scientists said that all of the graphs included in the lawsuit require further data and analysis. For instance, it’s unclear whether the data points in the Stop and Go Test represent specific one-time readings or averages over a certain number of tests. Why didn’t Hagens Berman test more than one vehicle? Were the tests conducted on flat ground or did they include hills?

“These are questions I would like to have answered,” German said.

What Happens Next?

Cooke and German speculated that the Hagens Berman lawsuit may be the first step toward a forthcoming investigation from the Environmental Protection Agency. Diesel emissions cases with VW and FCA have followed a similar pattern.

As it stands, the data alone is not enough to conclude GM acted knowingly, Cooke said. But it could well be enough to open an investigation, if it hasn’t already.

“The proof that they’ve provided is not as overwhelming as the data around the VW scandal,” Cooke said. “It could be an indication that something is wrong and if I were an investigator that’s how I would view it.”

In 2014, German was an instrumental researcher in the investigation that ultimately proved VW’s installation of defeat devices in its vehicles.

Of all automakers, GM should understand the emissions laws “backward and forward,” since its truck engines are made in America for the U.S. market, he said.

“Both the temperature data and the speed data are potentially troubling,” German said.

About The Author

Ryan ZumMallen

Ryan ZumMallen is a Trucks.com staff writer who covers the light truck market. A Long Beach, Calif. resident, his automotive writing has appeared in Car and Driver, Road & Track, The Verge, AutoBytel and elsewhere. He can be found on Twitter: @Zoomy575M

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