Diesels are inherently dirty, though modern pollution control systems can clean them up - if used properly.

In the wake of the VW diesel emissions scandal, a series of reports have suggested that other manufacturers might also be cheating, rigging their vehicles to do well on government tests while producing far more pollution in the real world.

The latest is a report in British newspaper The Guardian, its headline blaring, “Wide range of cars emit more pollution in realistic driving tests, data shows.” The paper cites real world test results from a UK lab that supposedly show some European diesel models emit up to 20 times the smog-causing oxides of nitrogen as EU rules permit.

But that’s no surprise, and not necessarily a problem – never mind anything illegal — stressed Nick Molden, the CEO of that lab, Emissions Analytics, in an exclusive interview with TheDetroitBureau.com and NBCNews.

“They picked up the data…which intonate four other manufacturers have been conducting illegal activities,” said Molden, but in reality, “no one has any hard evidence of anyone doing anything clearly illegal.”

(Just following orders? Who was really responsible for VW diesel cheating? Click Here for more.)

The problem, Molden and other experts point out, is that government emissions tests are conducted under tightly controlled lab conditions that generally don’t reflect the way vehicles are driven in the real world. The European tests, in particular, don’t put a vehicle through the aggressive acceleration, uphill climbs and cold weather conditions a motorists is likely to experience.

“You’ve got a very gentle test cycle with a lot of loopholes in it,” said Molden.

In reality, most diesel vehicles produce almost no NOx when the engine and emissions system have been warmed up and they’re cruising smoothly down a flat highway. But that will spike under more aggressive driving, at times – briefly — to up to 20 times the supposed limit.

As a result, tests of about 200 diesel models have found that while the latest European standard permits the emission of 80 milligrams of NOx per kilometer, “on the road it will be four times the limit, or 320 mg,” said Molden, “and all legal.”

That position is echoed by Arvind Thiruvengadam, the research professor at West Virginia University who helped uncover the Volkswagen cheating.

“There would be differences between a certification test and a real world test,” emphasized Prof. Thiruvengadam, who added that, “I would be happy if a pass car is putting out just five to six times the standard. But that would only be under extreme conditions. And cruising down the highway, I would expect to see it producing much less than the standard.”

Diesel engines, both researchers noted, are particularly vulnerable to emissions spikes, especially when it comes to oxides of nitrogen. That has to do with the basic mechanics of the technology, a downside of the fact that diesels tend to deliver more stump-pulling torque, while also getting better mileage than comparable gasoline engines.

(VW says it will fix diesel emissions problem. Click Here for more on its plan.)

The latest diesels – which use turbochargers, high-pressure injectors and urea injection systems – have seen major reductions in emissions, especially NOx and sooty particulates. In fact, British researcher Molden says five of the newest models, which he declined to identify, are matching lab and real-world emissions numbers.

Those more advanced emissions systems are just rolling out in Europe but are mandated here in the U.S. But even then, Prof Thiruvengadam, of WVU, said short emissions spikes will likely be the norm for diesels. What matters most, he said, is that, on the whole, they are much cleaner than ever.

Both Thiruvengadam and Molden would like to see changes to emissions tests to make them better reflect real driving. They also think that revised testing would make it more difficult for a manufacturer, such as Volkswagen, that did try to cheat.

(VW planning to cut diesel line-up in U.S. For more, Click Here.)

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