Dieselgate investigator speaks out on Volkswagen emissions

Professor Arvind Thiruvengadam is one of the principals behind West Virginia University’s Center for Alternative Fuels, Engines, and Emissions. If you want to talk engine power densities, catalytic efficiencies, or combustion processes, he’s your expert.

Last year, Thiruvengadam’s group of engineers and scientists uncovered the Volkswagen software “cheat mode” that allowed the automaker’s diesel engines to pass nitrogen-oxide (NOx) emissions tests in the lab then return to nasty polluting ways on the road. Their report led to EPA and Department of Justice investigations, which resulted in civil and regulatory lawsuit settlements of more than $15 billion in damages against Volkswagen.

But does the punishment fit the crime? In an exclusive interview, Thiruvengadam spoke with us about the ongoing Volkswagen “Dieselgate” crisis.

How bad were VW diesel emissions? Your report states that it was often at 40 times maximum regulatory limits. But was that all the time?

People should understand the idea of absolute magnitude, whether it’s 40 times or 25 times or 15 times, the actual magnitude is lost. Even if it exceeded the regulatory maximum by 40 times, it still is low levels of emissions. Because the standard itself is so low, 40 times is not a huge number. The approach that was used to circumvent the emissions testing procedure is the topic of discussion. We don’t know the real reason VW did it, whether it was reliability issues of their diesel engines or cost cutting. The actual magnitude is still pretty low. We do a lot of work with heavy-duty engines, and when you look at those heavy-duty engines at certain operating conditions, they exceed the limits by five or 10 times, but those engines are exempt. Or sometimes the manufacturer was transparent, saying, “We did our best, but we’re still 10 times over the limit.” What do these numbers really mean? If the standard is 0.0001 parts per million, then 40 times that is still virtually nothing.

Why were all three vehicles VWs? Have any other diesel or gas vehicles run through the same test cycle?

When we started the study, it was oriented toward diesel passenger vehicles, and the number of manufacturers selling diesel vehicles in the U.S. was mostly Audi and VW, then BMW and Mercedes. When we started selecting, the lean-NOx trap was only in the VW Jetta. With Selective Catalytic Reduction, we had choice with a Passat, but we also got a BMW from a rental agency. Nobody has looked at the third vehicle we tested. “Vehicle C” is a BMW X5 diesel (a 2014 X5 diesel is shown here), and its emissions are well below the standard. It is perfectly within regulations, and it gets good fuel economy. But it’s also in a different price category. The cost factor to provide a cheaper diesel vehicle could have been a motivator to VW.

But the BMW X5 has a larger engine. Wouldn’t that make it more able to meet emissions standards?

Actually, with larger engines, you typically see more emissions at low load. Diesels are better off working with higher load ranges because at lower loads the catalysts are not up to temperatures. With all these engine downsizings, you have more power density and can work with a higher load. So you would think the BMW’s 3.0 diesel would have a problem. A small diesel is more thermally efficient, so you want it to be working on a higher load. It’s hard to optimize an engine at lower loads. Even heavy-duty manufacturers have tried getting engines calibrated at low loads and high engine speeds so they can have high catalytic activity. If the catalysts are working fine and at temperature, they can cut back on fuel penalties that come from cleaning up NOx inside the combustion chamber.

While we’re on the topic of heavy-duty commercial trucks, how much are modern big-rig engines contributing to pollution?

Heavy-duty trucks use the same technology as light-duty cars. It’s widely known that if you take a long-haul truck on the interstate at 65 miles an hour, we see near zero emissions from the tailpipe. It shows the Selective Catalytic Reduction system is working properly at 200–250 degrees Celsius. Where the problem comes with heavy-duty trucks is with traffic density. When you have trucks queued in ports of Los Angeles and Long Beach, you get idle emissions and traffic congestion, and so the system cools down.

You can’t see NOx, but you can see soot. And there has been some publicity about diesel pickup trucks “rolling coal” and spewing visible emissions. Have you tested those trucks yet?

We have not, but maybe we should. When we start on any emissions work, we try to get a representative vehicle that’s out there. We never tried to compare a badly custom-tuned vehicle. We are from West Virginia, so the irony of the situation is that we have a lot of those rolling-coal vehicles.

Is it bad if you are seeing diesel emissions from rolling coal?

The negative publicity of diesel is visible smoke. That’s why they were always considered dirty. But with modern diesel emissions you don’t see that. In terms of tradeoffs, whenever you see high soot output, NOx is low. Whatever you do to increase soot it will drop NOx. It’s a fundamental of diesel combustion. If you want to advance the injection timing for less fuel consumption, you increase the NOx and the soot drops down. But on other end, if you retard the timing, you are low on NOx but soot goes through the roof. Exhaust gas recirculation is a combustion-based NOx control, but soot goes up as a result. You go richer and produce a lot of soot.

So which is worse, NOx or soot?

Soot is absolutely worse because it’s the one thing that has direct health effects. NOx has a longer residence time in the atmosphere, has photochemical affects, and creates ozone, so it’s a long chain process. But soot is quick. It goes into the atmosphere, and you get direct ingestion into the lungs. The health effects of soot are widely proven.

Let’s talk about your group’s test results. What was the comparison between the non-urea Jetta and SCR Passat vehicles’ emissions?

At max violation, the Jetta was around 40 times the limit, and the Passat was 15 times the limit (a 2011 Jetta TDI diesel is shown here).

In a typical operating environment (say, cruising the freeway at 65 mph), how far were these cars over the limit?

That was the giveaway for us. In Downtown L.A., when we saw 20 times over the limit, I wouldn’t have thought too much about it. There are so many transients and traffic, and none of the systems are operating at optimum, so we know that variation from real world and certification is possible. But driving on the highway is the sweet spot because you have sustained speeds and a balance between fuel consumption and emissions. And we never saw the Jetta and Passat come down (in their emissions levels), and that’s what triggered us saying, “Why is this happening?” In comparing highway vs. urban cycles, yes, highway was lower, but they never exhibited the low emissions we should have seen, especially the SCR Passat.

Your test results show the Volkswagen vehicles were within compliance for CO, CO2, PM, and THC. So this was really a NOx hack, right?

Yes. In diesels, CO and THC is never an issue due to the nature of combustion. THC is only in cold start. NOx is always the concern. And modern diesel cars have diesel particulate filters, so soot is not a concern. The regulation is not separated for diesel and gasoline, which means gas will have higher CO emission than diesel.

We’re going to get technical here. On page 106, your report states that “extended duration of lean exhaust conditions and a lack of frequent enrichment of the exhaust gas (λ < 1) while DPF regeneration was ongoing, leading to an inhibition of necessary LNT regeneration (DeNOx), and thus, causing the NOx storage catalyst to become saturated with NOx emissions that ultimately started to break through.” Would these circumstances apply to any diesel passenger car?

No. That is only for the lean-NOx trap (Jetta). The rest of catalyst systems either does selective catalyst reduction or oxidation reaction. Lean NOx is based on the principle that it will adsorb* NOx onto the catalyst surface, so there’s only a finite amount of NOx it can adsorb. After a while, you have to deplete this catalyst. The engine makes the exhaust slightly rich and sprays diesel into the exhaust, and that creates the reduction reaction to turn it into nitrogen and water. In these more modern cars, it’s slightly more sophisticated, where the injection of diesel is not controlled by the throttle. In heavy-duty engines, they have another injector placed in the exhaust after the turbo. But with light-duty engines, they inject fuel in the exhaust stroke, and they use in-cylinder injectors to do that. [* Editor’s Note: “adsorb” means for matter to adhere to or accumulate on a surface. It is not the same as “absorb,” which means to dissolve matter into a liquid or solid.]

Does doing that affect the long-term reliability of the engine?

If you consider the number of cycles that a lean NOx trap goes through, it is much more demanding than SCR or urea. A lean NOx trap has more temperature swings in every regeneration activity, and that can affect reliability. Plus, the emissions-certification levels were so low that lean-NOx was not looked at as a viable pathway. You also cannot promise certain fuel consumption targets with lean NOx. But for Europe or India or China, a lean NOx catalyst works because of those regions higher-level emissions standards and people buying cheaper cars.

If the VW emissions that triggered these lawsuits and fines were evaluated compared to pre-2008 or pre-2000 regulations levels, would they have been in compliance?

I believe they would be in compliance. The Tier 2 Bin 5 standards went really, really low compared to the previous standard. That’s why it took a lot of time for diesel cars to be 50-state compliant, because it is almost impossible and economically unviable. Automakers had to weigh the payback period to see if it was worth the investment.

We’re seeing European regulators saying numerous other vehicles would fail the same test as Volkswagen? What do you have to say to that?

We are working right now with other models, trying to quantify what the differences are between real-world and certification testing levels. Just because the certification is low doesn’t mean real-world is low. The difference is always going to be there. There are certain differentials where it’s OK and certain magnitudes of difference where it’s because there is something wrong with vehicle—such as with its control systems. The work we are doing now is trying to categorize what these differentials are for both domestic and import models.

For the full-length version of the West Virginia University report, go HERE.

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