Engines Exposed: Vehicle Emissions – The Desolation of Smog

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Los Angeles freeways

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I recently had to go through the joyous annual experience of having my car inspected. This practice – which, amazingly, is not required in every state – helps to ensure that your vehicle is safe on the road and performing well. For all but a handful, this inspection requires some manner of proof that your car is not polluting the environment (any more than necessary, that is). With the Volkswagen Dieselgate scandal showing no signs of going away anytime soon, it seems like this is a perfect opportunity to explain the emissions created by cars and the history of testing them.

Gasoline and diesel are hydrocarbons, which means that they are made up of hydrogen and carbon atoms. It’s not easy to assign the fuels a chemical formula given that they’re usually blends of several different hydrocarbons. However, the formulas would look something like C8H18. When a hydrocarbon like gasoline or diesel is combusted (burned), it is combined with air and heat and it releases heat and waste products, following this formula:

C8H18 + _O2 -> _CO2 + _H20

The blank spaces in the formula indicate that we might need more of one or more of the components to ensure the reaction is balanced. The problem with the equation above, at least as it relates to vehicle emissions, is that it assumes two things for simplicity. First, it assumes that air is pure oxygen. As I discussed in my article on nitrous oxide, the air that we breathe is only 23% oxygen. The rest of the stuff in the air changes the reaction. Second, it assumes that the combustion occurs under ideal conditions. As we all know, cars are subjected to various conditions, and most are far from ideal. This changes the equation and the result looks more like this:

C8H18 + _O2 + _N2 -> _HC + _CO + _CO2 + _NOx + _H20

In the first (ideal) equation, all of the fuel is converted into carbon dioxide (CO2) and water (H2). In the second equation, the fuel is combined with air and the non-ideal combustion results in unburned hydrocarbons (HC), carbon monoxide (CO), carbon dioxide (CO2), oxides of nitrogen (NOx), and water (H2O). Obviously, it’s a bit more complicated in the real world, but these additional terms give us a better sense of the true challenge.

The origin of emissions testing

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As early as the 1940s, vehicle emissions posed a serious problem in populated areas, especially in areas that were ringed by mountains, such as Los Angeles. The emissions from the rapidly growing number of cars produced a white haze that was called “smog,” which is a combination of “smoke” and “fog.” This smog caused eye irritation and respiratory challenges for people in the affected communities. In Los Angeles county in 1962, pollution levels were high enough that eye irritation was reported on 212 days out of the year. According to Martin V. Melosi’s paper, “The Automobile in American Life and Society,” the average car made in 1963 “discharged 520 pounds of hydrocarbons, 1,700 pounds of carbon monoxide, and 90 pounds of nitrogen oxide for every 10,000 miles traveled.” The issue of vehicular pollution was made even more complicated by leaded gasoline, which was used to boost octane ratings and to protect valve seats.

The Motor Vehicle Air Pollution Act of 1965 imposed federal emissions standards on vehicles produced starting with the 1968 model year, specifically relating to unburned hydrocarbons (both from the tailpipe and other sources on the car) and carbon monoxide. The first technologies designed to reduce emissions were charcoal canisters, which capture crankcase and fuel tank emissions, and exhaust gas recirculation valves (EGRs), which recirculate some of the cars exhaust into the intake. The presence of exhaust gas, which cannot be easily burned, reduces the power of the combustion, thereby lowering its temperature and reducing NOx. In 1976, Volvo introduced a new sensor on its 200-series cars: the oxygen sensor. This sensor, which is mounted in the exhaust, measures the amount of oxygen that is exiting the engine. That allows the Powertrain Control Module (PCM) to adjust the air/fuel ratio.

The challenge is that affecting the air/fuel ratio can have multiple effects. To elaborate on that, here’s a video explaining the importance of having the right air/fuel ratio:

Choosing the stoichiometric (meaning that the combustion equation is balanced) air/fuel ratio reduces CO and unburned HC, but causes NOx to increase. Catalytic converters, which were first introduced in 1975, were added to reduce NOx, but the lead that was in the gasoline damaged them and rendered the catalyst inactive. This led to widespread adoption of unleaded gasoline. But removing the lead from the gasoline reduced the octane, which could cause pre-ignition, which could increase combustion temperatures and cause more NOx. So the next big step was the introduction of three-way catalytic converters in the early ’80s that were designed to combat all three exhaust pollutants (HC, CO, and NOx).

Car in the shop

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If you take a car that doesn’t have the second generation of the On-Board Diagnostic System (OBD-II, which was introduced in 1996) to a shop for an emissions inspection, they will put the car on a dynamometer, hook a five-gas analyzer to it by inserting a “sniffer” probe into the tailpipe, and run it through a pre-determined set of conditions, established by the EPA to replicate real-world driving (called the Urban Dynamometer Driving Schedule or UDDS). It would measure the amount of each pollutant that was made to determine if your car was functioning properly. Cars with OBD-II systems simply need to be plugged in to the testing computer, which checks each emissions control system to ensure it is functioning properly.

VW’s trick, and the source of their current woes, was to create a mode in the car that changed how the engine operated if it sensed that it was being tested. This practice is not that unusual. Many cars have a test mode that will override certain issues that the car might detect as a problem, such as only two wheels spinning on a dynamometer. The difference with Volkswagen is that its “test” mode had significantly more conservative settings than the car would employ in normal operations. This meant it was down on power, had poor fuel economy, and was sluggish, but it passed the emissions tests. Their numbers may have looked great on paper, but with the alarming amount of pollutants the Volkswagens belched out in the real world, these “clean diesels” only made our smog problems worse.

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