This article, dated Jan 5, 2014, makes a rather vague claim that

Companies such as Volkswagen AG, Chrysler Group and General Motors have invested billions to make that happen, outfitting vehicles with systems that cut tailpipe emissions by about 99 percent from a decade ago.

It seems to me that it is physically impossible to reduce tailpipe emissions by 99% without reducing fuel (and/or air) intake by roughly the same amount. This suggests to me that the claim is probably talking about some specific aspect of emissions (such as particulate matter, or greenhouse gasses, or something).

Did these car companies manage to reduce tailpipe emissions, or some meaningful aspect of them, for diesel engines, between roughly 2004 and 2014?

  • Probably since they were required to by regulation, I don't doubt they did. "In the U.S., all on-road light, medium and heavy-duty vehicles powered by diesel and built after 1 January 2007, must meet diesel particulate emission limits that means they effectively have to be equipped with a 2-Way catalytic converter and a diesel particulate filter."
    – user1873
    Commented Sep 24, 2014 at 1:02
  • @user1873: "Diesel particulate emissions" is quite separate from "tailpipe emissions" -- thus my question. If the article is (implicitly) only about particulate emissions, that's quite a separate claim than the one it actually makes.
    – Flimzy
    Commented Sep 24, 2014 at 1:08
  • Which is why those diesel engines are also required to be equipped with catalytic converters, which convert carbon monoxide to carbon dioxide and unburnt hydrocarbons to water and CO2. I doubt most people consider water and CO2 emissions.
    – user1873
    Commented Sep 24, 2014 at 1:20
  • 2
    @user1873: If it is emitted from the tailpipe, it is tailpipe emissions, by definition.
    – Flimzy
    Commented Sep 24, 2014 at 1:31
  • 1
    @user1873: Do you have a reference for where GM, Chrysler or VW actually say that? It may just be a lazy and inaccurate reporter accidentally generalizing a more specific (and credible) claim.. Commented Sep 24, 2014 at 8:43

1 Answer 1


No they haven't, nor have they claimed so.


The journalist or editor didn't do their job correctly and made a mistake. The figure refers to particulates, not to other important emissions and not to all emissions.

Important emissions

As others have noted, important tailpipe emissions include gases like CO2 which manufacturers quote figures for because governments impose taxes based on those emissions.

At least in the UK, manufactures brochures always give the CO2 emissions in g/km

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Tailpipe emissions

An old 1943 report published by NIH contains this table

enter image description here

It is clear that although 80% of what is emitted through the tailpipe is nitrogen, few of us have much interest in that, it doesn't affect what we breathe.

However you can see that the engine converts oxygen to CO2 at a rate depending on engine load. This CO2 is emitted through the tailpipe and is therefore a tailpipe emisssion. CO2 emission reduction is the subject of International treaties and the subject of government regulation.

In an internal combustion engine, it isn't possible to reduce the total output mass by 99% unless you reduce the input mass by a similar amount.

Diesel Particulate emissions

These are a health hazard and so are of interest to consumers, manufacturers, regulators and to any journalists capable of accurate reporting.

Although not a reliable source, Wikipedia says

Wall-flow diesel particulate filters usually remove 85% or more of the soot, and under certain conditions can attain soot removal efficiencies approaching 100%.

Volvo claim

Wall-flow diesel particulate filters usually remove 85% or more of the soot, and can at times (heavily loaded condition) attain soot removal efficiencies of close to 100%.


This suggests that the reporter who wrote the Autonews article, or their editor, made a mistake and wrote "tailpipe emissions" instead of "tailpipe emissions of diesel particulates under heavily loaded conditions".

This kind of sloppy and innumerate journalism isn't exactly rare.

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