Diesel Emissions

When you unlatch the hood of a newer diesel-powered pickup it seems as though you are staring at a sea of swirling steel lines, rubber hoses, wires and plastic! Is there even an engine under all of this mess? What happened to the good ole days when you could stand beside the engine in the engine bay and work on it? The real question to be asked is why the engine bay is getting more and more cluttered and less and less room is available to work on the engine? There can be several explanations to this question, but one to consider is the rapid accumulation of emission systems on current diesel pickup trucks. A lot of the extra componentry which has been added to standard diesel engines has to do with the larger amount of emission equipment installed on the engine. This article will focus on emission standards over the past 20 years on diesel vehicles and the componentry associated with the standards.

In Review

It is important to review what has happened over time with emission requirements. For several decades, vehicle manufacturers and consumers did not have to worry about tailpipe emissions. It was not until December of 1970 at the initial persuasion of the Nixon administration that the Environmental Protection Agency was formed. There may not be any other agency known to motor enthusiasts which can stir up more emotion when mentioned than the EPA. The overarching goal of this agency initially was broken up into five areas: water quality, air quality, pesticides, solid waste and radiation.

At the helm of the air quality movement of 1970, the EPA passed an act through Congress known as the Clean Air Act. The Clean Air Act was the first of its kind in North America to monitor and regulate tailpipe emissions on passenger cars, light-duty pickups and heavy-duty pickups. Between 1975 and 1990, the Clean Air Act was amended several times and emissions were tightened more and more each time, specifically in the light industry.

Tier 1

It wasn’t until 1990 that Tier 1 was introduced. The birth of this new emission standard was one of the first to affect diesel pickup trucks. Beginning in 1994 the CAA mandated Tier 1 and it was effective until 1999. The goals for this first tier or level of emissions focused on reducing several exhaust propelled emissions such as hydrocarbons, particulate matter, carbon monoxide, formaldehyde, and oxides of nitrogen. Specifically, Tier 1 emission standard focused on two key areas for diesel-powered vehicles which are namely NOx (oxides of nitrogen) and PM (particulate matter).

Tier 2

In 1999 the CAA Introduced Tier 2 and mandated it in 2004. The clamp was pinched even tighter in restricting NOx levels and particulate matter. Since then, several changes have been made in emission standards for on- and off-road diesel-powered vehicles. We have seen the birth and passing of Tier 3, Tier 4 interim, Tier 4, and the introduction to Tier 4 Final. Each tier will focus on less Nox and particulate matter. It is a great paradox due to the fact that as particulate matter emissions decrease in an engine, NOx levels increase. So many components have to be adjusted, altered, or reengineered in order for the engine to emit less of both of these emissions. Each topic or component which we will review in this article could very easily have several pages of information in and about itself, but we will only discuss what each component has to do with the reduction of exhaust-driven emissions. The following will be analyzed: EGR, Particulate Filters and Selective Catalytic Reduction.

EGR

The term EGR stands for Exhaust Gas Recirculation. In essence a portion of the exhaust gasses are taken from the engine after leaving the combustion chamber, cooled down, and then reintroduced into the intake stream. The real question that is always asked is why it would be positive to introduce hot exhaust into the intake of the engine. Exhaust gasses by nature are inert, meaning they are a dead gas. Exhaust gas is very corrosive and the soot contained in it is similar to the hardness of rubies. They do not add power to the engine when inhaled by the intake. In a lot of ways they are very hard on the engine oil in the engine because when EGR is used it is similar to dusting the engine slowly over time. Soot contamination destroys engine oil. Then what is EGR good for? One thing. If an inert, cooled-down gas is introduced into the intake of an engine, combustion chamber temperatures decrease. When combustion temperatures decrease then NOx levels also decrease. It is sad, but this is the only reason.

Particulate Filter & Catalytic Converter

Adversely, when NOx levels decrease, particulate matter increases. Now we have to control particulate matter and that is where particulate matter filters come in. Particulate filters are exactly what their title describes. They act as a large particulate trap in the exhaust and trap particulate matter as it is moves through the exhaust pipe. They look similar in appearance to a catalytic convertor on the outside, but are very different on the inside. Most diesel pickups still utilize a catalyst upstream of the particulate trap and this is for good reason. A catalytic convertor utilizes a precious metal or precious metals as a catalyst. As exhaust gasses pass over the catalyst, there is a reaction and the gas is changed or converted into a less harmful gas. The particulate trap then takes these less harmful emissions and filters the particulate from them. Particulate matter in essence is truly the tiny, dark particles which make up the black smoke we see exiting an over-fueled diesel truck. It is very fine particles which are rich in acid and are often called soot. One may ask why particulate matter can be harmful to the environment. The EPA has established several studies proving that the increased amount of particulate matter entering the atmosphere has increased the amount of acid rain occurrences in North and South America and Europe. Also, particulate matter has proven to be one of the largest known carcinogens in Europe to date. So EPA demands we reduce it.

As this soot enters the soot trap or particulate filter, it is trapped in very small micro cells and the other gasses pass around it and through the trap. When enough soot is trapped, the cells inside become clogged and gasses cannot pass through the filter. The vehicle must renew or regenerate the particulate filter or go into an active or passive regeneration.

During regeneration several things occur. The vehicle needs to increase exhaust gas temperatures over 1000 degrees Fahrenheit. This can be done in several ways including Doser Injection, which is raw diesel fuel sprayed past an open flame in the exhaust to increase EGT. (Essentially a flame thrower, Caterpillar likes this method.) It can also be done by increasing engine rpm and altering electronic fuel injection parameters to increase EGTs. (This method is utilized by all pickup truck manufacturers) This increase in EGTs results in the burning of soot and converting it into ash. The resulting ash is only 1/100 of the size of the initial soot and the particulate filter is regenerated.

Over time the particulate filter can become clogged from the increasing amount of leftover ash and will need to be cleaned in a heated, ultrasonic cleaner or even replaced. Several factors can affect how often a particulate filter must be regenerated. These factors include—but are not limited to—high amounts of idle time, high sulfur diesel fuel, less time spent towing a load, and altered fuel parameters or modified engine tuning. Several other factors affect the reduction of particulate matter internally on the engine including turbo design, fuel injection type and componentry, electronic fuel injection pulse width programming and timing, and even what type of engine oil is being used in the engine. (Next time you pick up a bottle of 15W-40 for your diesel you will notice the new “LE” label on it, which stands for low emissions.)

SCR & Urea Injection

Due to the steeper emission requirements even less NOx levels are now permissible and another solution needed to be implemented aside from EGR. This new system is known as Selective Catalytic Reduction or SCR. As emissions pass from the engine into the first catalyst and then the particulate filter, they then enter into another type of catalyst. This one is made of a flow-through material which has been catalyzed with copper Zeolite. In front of the copper zeolite catalyst is a spray nozzle which doses the catalyst with a solution of water and ammonia. Downstream from the Urea catalyst is usually an ammonia slip catalyst which can convert raw ammonia which may be saturating through the ammonia catalyst into nitrogen and water vapor. This solution is in liquid form and known as Urea. The combination of Urea sprayed on a hot catalyst reduces NOx levels significantly, so significantly that engine manufacturers can now use less EGR because the Urea injection can control NOx levels so well. This does mean that an additional tank needs to be added to a vehicle to hold Urea solution. Urea injection also adds another fluid which needs to be added to a vehicle at oil change intervals or fuel fill intervals.

Busy Under The Hood

So many additional components seem to cluster up the engine bay, but also add to the great capabilities of finite control and clean-burning characteristics, which these new diesel engines now have. We will save these for additional tech columns which may include common rail fuel injection, multiple turbo chargers, variable geometry turbo chargers, large electronic engine processing capabilities, massive engine cooling system componentry, and many other parts which all add together to produce a very clean-burning, powerful diesel pickup truck.

In conclusion, it is a well-known fact that new diesel pickup trucks are very busy under the hood. It may not be a well-known fact that if these vehicles are driven through smog-infested areas such as San Francisco, Calif., or Salt Lake City, Utah, the air which exits the tailpipe may very well be cleaner than the air entering the air cleaner! So next time you open the hood on a newer diesel pickup and you are upset about the clutter, you might want to think twice about it. If the truth were told, diesel engines have progressed a lot from their progenitors 30 years ago and much of the reason comes from emission requirements.    

About The Author

Levi Perkins is the head instructor for the diesel program at the College of Southern Idaho located in Twin Falls, Idaho. The diesel program has had 100 percent placement in the last three years and provides future diesel mechanics and engineers a bright future in the diesel industry. For more information visit www.csi.edu or call 208-733-9554.