Nasty Gases, and Other Things You Don’t Want to Breathe

What is the stuff that is actually coming out of the tailpipe of your car? Is there really any cause for concern or are those that say the pollution from cars will kill us all just crazy? As with most controversial topics, the truth is usually somewhere in the middle. The stuff that comes out of the tailpipe is formed as part of a simple chemical reaction. We need to have a look at this simple chemistry.

The gasoline that is burned in the engine is made from a combination of hydrogen and carbon. Gasoline typically has a molecular formula somewhere in the neighborhood of C7H16 to C8H18. These are heptane and octane. In order to burn the gasoline we must also have oxygen which is O2. This oxygen is in the atmosphere and is mixed with the air that makes its way into the combustion chamber. The other thing that is in the air is nitrogen which is known as N2. Nitrogen isn’t required to make combustion happen but since it’s there when this chemical reaction is occurring, it will have an impact on the byproducts of the combustion process.

So we have hydrogen, carbon, oxygen, and nitrogen, along with trace amounts of other things that don’t have too much of an impact on the combustion process or the results thereof.  The combustion process is going to rearrange all of these things since that is what happens with a chemical reaction takes place. When we burn the hydrocarbons, we split them apart into the H and the C. Ideally a couple of H’s join up with an O from the O2 to form H2O, and the C’s from the HC’s join with an O2 to form CO2. In a perfect world the only byproducts of combustion would be H2O and CO2. We do not live in a perfect world and according to some people the CO2 that results is something akin to poison but that’s a topic for another day.

Remember we also have nitrogen present during the combustion process and this will lead to a bond with oxygen as well. Nitrogen doesn’t usually stick to anything in a reactionary way, but at high temperatures it will, and it will very easily stick to oxygen since oxygen easily bonds with all sorts of different things. Combustion chamber temperatures that are consistently north of 2500° F will cause a lot of bonding between nitrogen and oxygen. These molecules are referred to as oxides of nitrogen or NOx. The N is for nitrogen, the O is for oxygen, and the x is the variable amount of these atoms that can be found together as a result of the combustion process. Usually the molecules that we are dealing with in this situation are NO, nitric oxide, and NO2, nitrogen dioxide.

Particulate matter is another thing that comes from the tailpipe of cars and trucks. Particulates are small microscopic particles that are usually carbon if they come from the tailpipe. These particulates are very minimal in the exhaust from a gasoline powered car or truck but can be quite substantial from the exhaust pipe of a diesel powered vehicle. When you see black plumes from the exhaust pipe of a diesel powered vehicle, these are particulates, usually referred to as soot. Particulates are usually measured in micrometers or microns. One micron is one millionth of a meter. One human hair is about 70 or 80 microns thick. The particulates that are commonly measured coming from a tailpipe are referred to as PM 10’s for the big ones, and PM 2.5’s for the smaller ones. The numbers in these designations refer to diameter of the particle. 

This picture from the DOE puts PM 2.5's into perspective

Other things that come out of the tailpipe can vary a little from one engine to another. Some of these gases are normal trace gasses that are part of the atmosphere such as argon, and some of them result from trace elements found in the fuel such as sulfur. These trace gases are very minute and not much of a concern. Sulfur can be a problem with diesel engines but this has been taken care of through fuel reformulations that eliminate the sulfur. A fair amount of oxygen also comes out the tailpipe because it did not all get used up in the combustion process.

So the things that come out of the tailpipe of a gasoline powered car that are tracked and monitored in some form or another are HC, CO, NOx, CO2, and O2. An important diagnostic tool used by automotive technicians is the 5 gas analyzer that is made to analyze the exhaust for the content of these 5 gases. Of these the ones that are considered hazardous are HC, CO, and NOx.

Carbon Monoxide

Carbon Monoxide is a problem because it is a powerful asphyxiant. This means that it displaces oxygen in the blood stream. The CO itself does not cause tissue damage but considering that it crowds out the oxygen that is normally carried to all of the parts of the body by the red blood cells; it might as well be poison. Prolonged exposure to CO above a level of 50 ppm will lead to a feeling of general flu-like symptoms. Above 200 parts per million and an extreme headache will result after a few hours. Over 1000 ppm and a person would be dead in a few hours. Over 10,000 ppm and a person would be dead after a few breaths. Understanding that this is not a gas that would be good to have coming out the tailpipe is easy.

Carbon monoxide results from incomplete combustion or partial oxidation of the HC’s found in the fuel. You might say that it is fuel that only burned halfway. CO will be very abundant if the engine management system is dumping too much fuel into the engine, and there is not enough oxygen to properly burn it all. This condition is also associated with an extreme amount of carbon buildup in the combustion chamber. When spark plugs are fouled black, chances are the CO emissions from the tailpipe are very high, engine output is low, and fuel economy is down.

If a vehicle is found to have a high amount of CO in the exhaust then the fuel metering system must be examined. Sometimes the problem might just be something like a bad sensor. If the O2 sensor that the engine computer uses to gain feedback about its performance is giving an incorrect reading to due loss of calibration, the computer may dump more fuel into the engine than it needs too. The computer must maintain the air/fuel ratio at 14.7:1, if it adds more fuel than needed the ratio will go down, and the mixture will be too rich. Something like this could also be caused by a plugged fuel injector that is leaking too much gasoline into the intake manifold. The computer might try to lean out the mixture but the leaking fuel is so abundant that the computer can do nothing to correct the situation.

Hydrocarbons

Hydrocarbons which are sometimes referred to as volatile organic compounds (VOC) are linked to the formation of photochemical smog. HC’s react with UV sunlight to form smog which is full of all sorts of lung irritants and respiratory poisons. This smog formation is very common in dry, sunny, climates where big cities are found. HC emissions result anytime HC based fuels are burned. This includes fuels such as gasoline, diesel, and coal.

Hydrocarbon emissions are simply the result of not being able to burn all of the fuel in the combustion chamber. This usually happens when there is too much oxygen and not enough fuel to go around. This causes the combustion to be so weak that much of the fuel that is there doesn’t even get touched by the combustion event; it’s almost as if the fuel is just getting pumped right through the engine and out the exhaust pipe. HC emissions also result when a car is refueling. When the fuel tank is empty and the pump nozzle is dispensing fuel down the filler neck, the tank is full of fumes that get displaced by the liquid as it fills the tank. These fumes get pushed up and out the filler neck opening. All new vehicles have an onboard vapor recovery system that prevents these fumes from escaping but it can never capture all of these evaporative emissions.

One cause of high HC readings is a lean condition.  As is the case with a rich condition, a lean condition could be caused by a sensor that is giving an incorrect reading. The problem may result from the sensor being bad, or the sensor being acted upon in a false manner. For example, the mass airflow sensor measures the amount of air coming into the engine. This sensor usually uses a small wire that is heated by the computer. As the air rushes past the wire it cools it down. The more abundant the airflow, the more the computer must work to keep the wire heated. When the sensor wire gets coated with dirt or oil as might happen when the air filter is not properly maintained, the dirt acts to insulate the wire and the sensor ends up telling the computer that there is less air coming in than there really is. This causes the computer to inject less fuel into the engine so that there is an air/fuel ratio greater than 14.7:1.

If a car has high HC readings, the most likely culprit is the ignition system. High HC readings most commonly result from a misfire of the ignition system. The ignition system must deliver a spark to each cylinder at just the right time in order to set off the combustion process. If the spark doesn’t get delivered because the spark plug is bad or the ignition coil isn’t working right then a misfire occurs. When this happens the engine stumbles a bit and the air and fuel that were supposed to be burned in the combustion chamber get pumped straight into the exhaust. Many times this will look the same to an emissions analyzer as a lean condition because not only will the HC level be elevated but so will the O2 levels because neither one reacted in the cylinder as it should have. To make matter worse, the O2 sensor in the exhaust will see all of the oxygen and think that the engine is lean so the injectors will just pump even more fuel into the engine making the HC readings at the tailpipe even higher.

If your gas cap has a bad seal HC emissions will result from the expanding fumes leaking out of the tank past the cap seal. Modern cars are smart enough to know if the gas cap is physically compromised, or if you have left it loose. These cars and trucks will illuminate the check engine light in such an event, and some of the newest cars out there will illuminate an indicator to alert the driver that the fuel cap is loose or missing.

Oxides of Nitrogen

Like hydrocarbons, NOx gases react with UV light from the sun to form the components of photochemical smog. One of the worst things that results from this reaction is ground level ozone, or O3. Ozone in the stratosphere is good because it blocks harmful UV rays from the sun and lets the non-harmful rays through, but ozone at ground level is bad because it is a respiratory irritant that can make you sick, especially if you already have respiratory troubles such as asthma.

To form ozone, the NO2 molecule loses one of its O’s and that free O will quickly join with an O2 to form O3. The NO that remains mixes with rain water to form a very weak nitric acid. This is what they call acid rain and it has been known to cause problems in some urban areas because it can be very corrosive.

High NOx emissions are only a problem with some engines; much of this depends on the overall design of the engine. Historically if a car had high NOx emissions the problem may have been improper adjustment of the ignition distributor. Since NOx forms when combustion chamber temperatures are high, overly advanced ignition timing caused by misadjustment of the distributor results in burning the fuel under higher pressure which also means higher temperatures. Engines that have a tendency towards high combustion chamber temperatures will usually have a system that is meant to specifically reduce NOx formation. This is the Exhaust Gas Recirculation system (EGR).

EGR reduces combustion chamber temperatures by circulating inert exhaust gases from the exhaust manifold into the intake manifold. These exhaust gases displace oxygen, and because they are inert, or already spent, they don’t affect the combustion process. With less oxygen in the combustion chamber, the burn will not be as intense and temperatures will go down.

Newer vehicles do not have distributors so there is nothing to be misadjusted and cause an increase in NOx emissions. Most of them do have EGR systems however, so if NOx levels at the tailpipe are too high then the problem is almost guaranteed to be related to a malfunction of the EGR system. Sometimes an overheated engine will produce too much NOx for reasons that are obvious, but if you have an extremely overheated engine you likely have much bigger issues to deal with than high NOx from the tailpipe.

Oxygen, Carbon Dioxide, and Water

None of these gases are pollution. O2 and CO2 are monitored by a 5 gas analyzer because they give clues about the overall efficiency of the combustion in the engine that is being tested. CO2 output should be about 14% and O2 output should be around 1%. If oxygen is higher than that, a lean condition is present. If it is lower than that, then a rich condition is indicated.

Water is the most abundant byproduct of the combustion process. This is very visible when a car is started and driven with very low ambient temperatures. The white plumes that come from the exhaust on a cold morning are the condensation of this water vapor. Many times it is possible to see water dripping from the tailpipe of a car that is idling. This water is also the reason that exhaust systems always seem to rust out eventually. With so much water coming out of that tailpipe it’s amazing they don’t rust out sooner than they do.

Normal Readings



Emissions testing that invovles simulated driving of the vehicle

 Looking at the results of emissions testing in a heavily populated area that requires such testing annually, one might routinely see the following readings on newer cars and trucks: CO 0.00%, HC 0 ppm, and NOx 0ppm. Does this mean that these new cars don’t pollute? No, it just means they are so clean now that the emissions analyzers cannot read that low. Modern cars are very clean compared to what they used to be, but they can probably get even cleaner and will likely continue to do so. In the old days the three nasty gases mentioned above (CO, HC, NOx) were responsible for the major pollution in the air of big cities around the country. While air pollution is still a problem in some places, things are much cleaner than they used to be.

One of the biggest pollutants from vehicles now is particulate matter. As stated previously the exhaust of a gasoline powered car doesn’t contain much in the way of particulates. The particulates that come from cars and trucks now comes from things like the tires wearing down, or normal dust and dirt being kicked up from the road surface as the car passes over it. Cars really are so clean now that pollution from bits of tire is now one of the biggest concerns.

CO2 is and likely will remain the biggest controversies in relation to tailpipe emissions. The issue lies in the fact that many, not all, and maybe not even most, scientists link CO2 emissions to global warming. CO2 is said to be a greenhouse gas and the fact of the matter is that a significant portion of car exhaust is CO2. The problem here is that CO2 is also in the air we exhale as we breathe. So are human beings just living and breathing also contributing to global warming? Some of the extremists on the left would say so. If 14% CO2 from the exhaust pipe causes global warming, why doesn’t anyone address all of the water vapor that comes out of the tailpipe? Water vapor in the atmosphere is such a powerful greenhouse gas that it is almost singlehandedly makes this planet inhabitable. Is 14% CO2 really contributing to global warming more than the 86% water vapor?

No matter what happens with the theory of global warming or climate change, air pollution problems are real, burning fossil fuels has led to sickness and death in the past, emission controls are very important and emissions laws do have a place. The perceived right of one party to spew whatever they want into the air does not trump the literal right of others to breathe air that is untainted. These problems are all local problems that apply to small areas, meaning certain cities or counties, and not entire contents as a whole. Any attempt to push such things onto larger, all-encompassing areas becomes problematic. Cause and effect cannot be proven, so for now we should all keep our vehicles running properly so we don’t pollute.

If your check engine light is on it means that your vehicle is polluting and you should get it fixed as soon as possible. If your vehicle is putting out excessive emissions, it is also likely to be using more gas then it should, and not producing as much power as it ought to be able to produce. Surely those are things that everyone can understand.