Depending on the age of a truck, truck performance can vary greatly.  This has a lot to do with emission controls systems and that were in place at the time a truck was manufactured.  

This is a two-part article on diesel emission controls. This first part will cover how emission control systems eliminate pollutants and the history of emissions standards and control systems. The second part, here, will cover current greenhouse innovations and how the age of your truck can help you understand performance.  

But let's start with Part 1.

The diesel engine, although much more efficient than gasoline engines, is much harder to clean up. The three-way catalytic converter fits perfectly with the gasoline engine and, along with a few other relatively simple modifications, has made it a super-clean engine. But, a three-way catalyst won't work on a diesel.

Three-way catalysts: why they won't work on a Diesel

Nitrogen-oxides (Nox)

Making internal combustion engines clean involves the elimination of a number of types of pollutants. But the entire emission control systems  on a clean engine always revolves around the elimination of one pollutant called nitrogen-oxides. Nitrogen-oxides, or Nox, forms when nitrogen, actually the largest constituent of air, is heated to high temperatures in an engine. The combustion process which burns the fuel also causes some of the nitrogen to combine with the oxygen in the air that the engine takes in to burn the fuel. Nox is a critical element of smog. Since engines thrive on heat, minimizing the formation of Nox and converting it back into its natural state by removing the oxygen from the nitrogen in the Nox emissions is a major challenge. The three-way catalyst takes care of this problem in passenger cars. It uses a tiny bit of leftover unburned fuel to draw the oxygen out of the Nox, and return only nitrogen to the air. But, there can't be any extra oxygen in the exhaust or the three-way catalyst won't work.

A three-way catalyst won't work on a diesel, because of the very different way the fuel is mixed with the air. In a gasoline engine, the fuel has plenty of time to mix, being supplied well before a mixture of gasoline and air is compressed in the cylinder. As a result, this engine can actually use up all the oxygen in the air‚ if the mix is given extra time to burn in the catalytic converter. 

The diesel, on the other hand, although incredibly efficient, needs to supply more air for combustion than a gasoline engine. This is because the diesel compresses only air, and then injects the fuel at the last possible moment, right when the fuel needs to be combusted in order to generate heat and pressure and force the piston down during the power stroke. The only way to compensate for the lack of time to mix fuel and air is to supply extra air. With extra air in diesel exhaust, the three-way converter can't remove the oxygen from the Nox.

Soot

Another part of the problem is that Diesels need a fuel that will ignite by the heat of compression alone, and that means a fuel that is less stable at high temperatures. The diesel's different and slightly imperfect mixing process, together with the use of a less stable fuel, means the diesel also has a tendency to produce soot.

Removing Nox and Soot in Diesels

The addition of both DPF to reduce soot and the SCR system for Nox has finished the cleanup job. But these alterations mostly work against fuel efficiency. 

Tremendous progress has been made in reducing both Nox and soot in diesels with internal modifications. The most potent of the internal Nox reducers is Exhaust Gas Recirculation (EGR). Soot has also been reduced internally via a number of techniques, the most effective of which has been the continual improvement of the powerful diesel injection system. However, it has been the addition of both the Diesel Particulate Filter (DPF) to reduce soot and the Selective Catalytic Reduction (SCR) system for Nox that has really finished the cleanup job. These alterations mostly work against fuel efficiency. 

However, the introduction of the SCR system in 2010, and the introduction of other advances like common-rail fuel injection in recent years, along with dozens of other improvements like better aerodynamics, have made the latest tractors far more fuel-efficient than the trucks of the 1990s.

The introduction of other advances have made the latest tractors far more fuel-efficient than the trucks of the 1990s.

So, truckers, who may run trucks with engines built 20 years ago or even longer, or new trucks, or anything in between, have a number of choices when it comes to the age of the truck they run. Depending on the model year, the complexity of the emission systems their engine will be equipped with, as well as the truck's potential fuel efficiency, will vary greatly. We'll describe these systems one by one so you'll know what you will be dealing with.

Exhaust Gas Recirculation (EGR) 

When the engine runs, the exhaust it produces is a gas that not only contains very little oxygen, but can absorb a tremendous amount of heat. By simply re-breathing a small proportion of the exhaust the engine produces, the mix of fuel and air, and now exhaust, in the cylinder will burn at a lower temperature. The lower temps will reduce Nox.

2002: VG Turbo Eliminates the Turbo-Lag

To meet a lower Nox standard in 2002, diesel makers adopted a new type of turbocharger called a "variable-geometry" or "VG Turbo" that was capable of building just a little extra pressure in the exhaust system prior to the turbo. The performance of the system was improved a lot by adopting an EGR cooler, a heat exchanger just like the truck's radiator‚ something cars don't have. The exhaust would be piped from just in front of the turbocharger (where pressure is highest) through a cooler using coolant at the operating temperature of the engine, and then, through a control valve called, of course, an "EGR Valve" and on into the engine's intake. Reducing the temperature of the recirculated exhaust from over 1,000F to less than 250F really helped it to cool the burning in the cylinders. This meant far less exhaust would be needed to kill the Nox. The extra pressure in the exhaust forced it back into the intake side of the engine in spite of the high intake air pressure created by the turbo.

The VG Turbo nearly eliminated the "turbo-lag" that had always plagued drivers. Now when opening the throttle, the engine responded immediately.

The slower burning that occurs with exhaust mixed in, combined with the higher pressure in the exhaust system, meant a slight reduction in fuel economy. However, the VG Turbo was continually adjustable to help control how much exhaust was recirculated, and a byproduct of this was quick response. The VG Turbo nearly eliminated the "turbo-lag" that had always plagued drivers. Now when opening the throttle, the engine responded immediately.

EGR produced some problems, including increased wear of the cylinder liners. However, liners were soon hardened to cure the problem, extending engine life. Early EGR engines were also plagued with EGR valve failures due to high temperatures, but these valves were eventually relocated to cooler areas of the  system and the problem was largely eliminated.

Diesel Particulate Filter (DPF)

2007: Changes to Diesel Emission Standards

2007 brought a major industry challenge‚ the need to meet both a reduced particulate or smoke standard, and lower Nox at the same time. The result was a more potent EGR system together with the addition of a ceramic filter called the Diesel Particulate Filter (DPF) that would go in the exhaust system after the turbo. The DPF would catch the soot and hold it. The fact that it was made of a ceramic material meant it would hold heat, while also providing an extremely fine filter. The exhaust actually passes right through the nearly solid ceramic material while the soot particles get caught. At highway speeds the hot exhaust, which always contains some oxygen, would burn the soot off, aided by a catalyst at the front end of the DPF, keeping the DPF clear.

Downsides included the fact that the exhaust and therefore the DPF cool off when the truck runs at lower speeds, say in traffic. Under these conditions, a regeneration system that burns fuel to heat up the exhaust needs to operate occasionally to burn off accumulated soot. At times, the engine may even need to operate for a time while the vehicle sits as its operation is modified at a fast idle in order to heat the exhaust up and burn off the soot. This is called an "active regen".

[Lower sulfur levels] greatly benefited engine life in spite of slightly raising fuel costs, because sulfur corrodes engine metals, causing wear.

Also, the sulfur level in fuel had to be reduced to only 15 parts per million to protect the DPF. This was actually a change that greatly benefited engine life in spite of slightly raising fuel costs, because sulfur corrodes engine metals, causing wear. And, a new grade of engine oil, called CJ-4, that minimized the amount of ash in the oil was needed to help keep the DPF clear.

2007 engine makers used various means to strengthen the structure of their engines, raising costs‚ leading to stronger, longer lived engines, provided proper maintenance practices are followed.

2007 engines needed to handle significant levels of EGR plus ingesting enough extra air to keep soot levels low. This meant higher operating pressures in the cylinders. Engine makers used various means to strengthen the structure of their engines, raising costs. Still, such changes, in the end, led to stronger, longer lived engines, provided proper maintenance practices are followed.

The cooler burning and high exhaust pressure meant somewhat reduced fuel economy. And, accumulated ash, which is caught in the DPF but will not burn, needs to be vacuumed out after 200,000 or more miles. Accumulated carbon may also need to be removed from the ceramic filter at this time.

2010: SCR System Helps Restores Fuel Economy

2010 saw the introduction of Selective Catalytic Reduction. SCR is a system that kills Nox after the DPF has removed the soot, using Diesel Exhaust Fluid (DEF). The process occurs in a chamber lined with a catalyst similar to what is used in an automotive three-way catalytic converter, a "noble" metal like platinum that speeds up a chemical reaction. The DEF contains a chemical called urea that quickly turns to ammonia in the catalytic chamber and then converts the Nox into harmless nitrogen and water.

The faster, hotter burning from SCR restored much of the fuel economy that had been lost in 2002 and 2007.

While the SCR system added complexity, it also enabled the engine to be tuned to operate much more efficiently. EGR systems were greatly reduced in size, and the fuel was injected at a more favorable point in the cycle, producing a quicker rise in pressure in the cylinders, hotter burning, and better fuel economy, though engine pressure stresses were actually lowered. The Nox the engine was now permitted to produce greatly helped to burn off the soot the DPF would catch, thus reducing regenerations, often to zero. And, the faster, hotter burning restored much of the fuel economy that had been lost in 2002 and 2007.

Read Diesel Emissions Control: Part 2 where we cover today's innovations and how the age of your truck can help you understand truck performance.

Diesel Emission Controls: Part 2 - Truck Maintenance Downsides