Because the internal combustion engine is an inherently dirty design. But with the right controls and add-on devices, it can run exceptionally clean -- not as clean as a zero emissions vehicle like an electric car, but clean enough so as not to pose a serious threat to the environment.
The automobile that created our mobile society and gave us the freedom to travel where we want, when we want and with passengers of our choice, has now become the object of environmental scorn. The internal combustion gas and diesel engines that power almost all vehicles today are often blamed for everything from air pollution to global warming. Consequently, the internal combustion engine has become a prime target for regulatory crucifixion.
We hear a lot of so-called "facts" about the automobile's role in pollution. We hear statements like "nearly two-thirds of the total carbon monoxide and half the hydrocarbon and nitrogen oxides polluting our atmosphere comes from motor vehicles." We also hear alarmist statements like, "three out of five Americans face lung damage from ozone-polluted air."
We can't argue with the evidence that says automotive emissions are a major contributor to urban air pollution. You don't need a Ph.D. in chemistry to experience firsthand the nasty effects of exhaust pollution in a traffic jam. But there aren't many Americans dropping dead in their tracks from breathing polluted air. It may be a contributing factor to lung disease and respiratory ailments in some people. But deadly? Not unless you're running an engine in a closed garage.
It's simply not true that all motor vehicles are polluters. The fact is that thanks to all that "pollution junk" on your engine today's cars produce far less pollution than ever before.
Pre-1963 vehicles were the worst because they lacked any emission controls whatsoever. In 1963, positive crankcase ventilation (PCV) was added to recycle the blowby vapors in the crankcase back into the intake manifold so they could be reburned. This virtually eliminated crankcase emissions as a source of air pollution. Sealed fuel systems and charcoal canisters showed up in 1971, which reduced evaporative emissions to zero. In 1973, exhaust gas recirculation (EGR) was added to reduce oxides of nitrogen (NOX) in the exhaust.
An even greater change appeared in 1975 when the catalytic converter (which requires unleaded gasoline) was introduced. The converter greatly reduced unburned hydrocarbons (HC) and carbon monoxide (CO) levels in the exhaust by "reburning" the pollutants. Switching to unleaded gas was necessary because lead poisoned the catalytic agents, which also eliminated lead as a significant exhaust pollutant.
In 1981, onboard computers for closed-loop running and three-way oxidation/reduction converters were added to most cars, which cut CO and NOX emissions by another 50%. By the mid 1980s, fuel injection arrived. And today we have engines that can meet even the toughest low emission standard yet conceived -- except for zero emissions which the internal combustion engine cannot meet.
The tailpipe emissions from late model cars (1981 and later) with computerized engine controls and three-way catalytic converters are only a fraction of the older pre-emissions controlled cars. Today's cars produce 96% less HC and CO, and 76% less NOX than their pre-emission counterparts.
Additional reductions being phased from 1994 through 1998 model year cars and light trucks are further reducing emissions. The federal standards up through 1993 allowed no more than 0.41 grams per mile (gpm) of HC, 3.4 gpm of CO and 1.0 gpm of NOX for the first 50,000 miles. The new standards slash these limits by almost half again: CO is cut to 0.25 gpm, while HC and NOX drop to 0.4 g/m.
So what do all these numbers really mean? It means a 1975 to 1979 model car puts out roughly as much HC and CO pollution as four new cars; a 1972 to 1975 model year car produces as much of these pollutants as seven new cars; a 1968 to 1971 model year car produces pollution equivalent to 10 new cars; and a '63 or earlier pre-emissions controlled car pumps as much crud into the atmosphere as 25 or more new cars! These comparisons assume that a vehicle produces no more pollution that it did when new -- which is usually NOT the case once an engine accumulates a lot of miles. Emissions go up with age. It's not unusual to find high mileage engines in older vehicles (1981 and earlier) that are belching out the pollution equivalent of 100 new cars!
So be glad your engine has all that pollution junk on it. We can all breathe easier as a result of it.