Cars and smoke

Betsy and Billy are on a road trip and are behind schedule. "We're late because you forgot to buy gas last night," Betsy says. "We're late because you couldn't decide what to wear," replies Billy. To make matters worse, the car in front of them is poking along, belching smoke. Betsy suggests that they go around the car, but Billy says he can't see because of the smoke. In an attempt to be polite, Betsy asks, "What causes a car to smoke?" Billy, doing a fair amount of fuming of his own, tells her he doesn't know. While the happy couple engage in a spirited discussion about highway safety, let's get an SwRI Whizard to answer Betsy's question.

"Almost all the cars you see smoking on the road these days have a problem with oil consumption, and the white or bluish smoke is partially vaporized oil. If the oil gets into the combustion chamber and is mixed with air and gasoline, it can be burned quite completely, and you probably won't see much smoke. However, if the oil gets into the exhaust gases after combustion has taken place, or if combustion is far from complete (such as what occurs with misfiring on one cylinder or excessive valve leakage), the oil consumed by the engine is heated to the point of forming a dense, white smoke.

"Although Betsy and Billy didn't mention an unpleasant smell, cars that smoke a lot generally have bad exhaust odors. Some of these odors may result from the oil, but just as often they result from the partially burned fuel that is emitted by engines that are in poor condition. Compounds such as aldehydes come from partially oxidized fuels, and they have strong, pungent odors.

"Depending on where in the engine the control of oil is lost, smoke may come from either the exhaust pipe or from a crankcase vent (under the engine), or both. Since the 1960s, cars have been equipped with Positive Crankcase Ventilation (PCV) systems to prevent pollution from the crankcase vent. Poorly maintained cars sometimes have clogged or disconnected PCV systems, though, and a smoky discharge can result.

"Finally, since smoke looks like 'smog,' it is often commonly accepted that all smoke is a direct smog contributor. This is not necessarily the case. Smog is actually a result of sunlight promoting the reactions of hydrocarbon gases with oxides of nitrogen and other compounds to form ground-level ozone, a respiratory irritant. Oil smoke can certainly reduce visibility through the atmosphere locally, but it is not the major contributor to ozone formation. Smoke particles tend to dissipate, and ultimately coalesce and drop from the air. However, smog (ozone) is mostly formed from gases that are originally clear, and only take on a 'hazy' appearance after the photochemical reactions have occurred."

Thanks to this month's Whizard, Charlie Hare, director of the Emissions Research Department in the Automotive Products and Emissions Research Division. Hare is internationally known for his work in vehicle emissions research.

The Lighter Side SwRI Home

March 25, 2013