Understanding Your Engine’s Emissions Control System

No one would drink water from a roadside stream without running it through a good filter. While the water itself might be good, it’s usually polluted with silt and chemicals. Like that water, the air we breathe is often mixed with tiny particles and unhealthy gases. 

The diesel engines of yesterday spewed many of these pollutants into the atmosphere, but today’s Tier 4 diesels are much cleaner machines. Since 2014, diesel engines have been built with complex systems that effectively clean their exhaust before it is released into the surrounding air.

You don’t need to be an expert in modern emissions systems to use a diesel engine, but having a basic understanding of how those systems work will minimize  repairs and could prevent costly replacements.  

What are exhaust emissions?

Gasoline and diesel engines have powered modern transportation and industry, but as they create that power, they also discharge exhaust gases and particles into the atmosphere. When it comes to diesel engines, there are several primary pollutants. 

  • Particulate matter (PM) is a mix of fine particles like hydrocarbons that aren’t completely burned. These unburned particles are visible as the black smoke that pours from older diesel engines.
  • Nitrogen oxides (NOx) are forms of nitrogen that are harmful to breathe and are primary ingredients in smog.
  • Carbon monoxide (CO) is a mixture of carbon and oxygen that is harmful to human health.

Tier 4 Diesel Engines

In 1994, because of the long history of diesel pollution, the EPA established a road map for reducing emissions. The various phases of implementation were known as tiers, and each tier reduced the levels of acceptable pollutants. The last step, known as Tier 4 Final, was reached in 2015. To reduce emissions  to the acceptable level permitted by this final stage, manufacturers developed the complete emissions control systems that are found on today’s engines.

System Overview

The emissions systems of most modern diesel engines have three primary filtering components. The first two parts of the system reduce the particulate matter that develops in diesel exhaust, while the final stage of aftertreatment minimizes the NOx level .

  • When exhaust gases leave the engine, the first filter they enter is made of precious metals like platinum. The high temperature of the exhaust allows the platinum catalyst to convert some of the CO and hydrocarbons into CO2 and water. This filter is called the Diesel Oxidation Catalyst (DOC).
  • After leaving the DOC, the exhaust enters a second filter. While the first filter is passive (the exhaust just passes through), this second filter, known as the Diesel Particulate Filter (DPF), is active. Inside this filter, the regeneration process uses very high heat to burn the accumulated soot and other particles. 
  • The last stage of the system is designed to reformulate the nitric oxide (NO) and nitrogen dioxide (NO2) emissions of a diesel engine. This filter, known as the Selective Catalytic Reduction (SCR) catalyst, doses a mix of refined urea and deionized water to the exhaust gases. When this liquid, known as Diesel Exhaust Fluid (DEF), is injected into the hot exhaust gas stream , the NOx in the exhaust is converted to simple nitrogen gas (N2) and water vapor (H2O).

Understanding the Components

Once you have a basic understanding of the system, it’s helpful to understand a little bit more about each component.

Diesel Oxidation Catalyst (DOC)

This first stage of the system is essentially a catalytic converter made specifically for diesel engines. With its honeycomb structure, it has a large surface area which allows many points of contact between the gases and the catalyst. 

This filter, like the one that follows it, relies on high temperatures for peak efficiency. Around 400 degrees, 20% of the carbon monoxide and hydrocarbons are converted to carbon dioxide and water, while around 900, there is nearly a 90% conversion rate. But those extreme temperatures can form undesirable byproducts, so the ideal temperature is around 700. 

It’s important to know that high levels of sulfur can damage the catalyst—that is why modern diesel fuel must be ultra-low sulfur. It’s also important to remember that the system works best under high heat, so an engine that is frequently run at idle or under light load conditions can cause premature clogging of the DOC. As soot accumulates in the filter, fewer  exhaust gases can reach the catalyst, and it becomes less effective.

Diesel Particulate Filter (DPF)

The DPF follows the Diesel Oxidation Catalyst, and just like the filter that comes behind it, the DPF is an active filter. The primary purpose of the DPF is to burn off any materials that have made it through the DOC. This burning process is known as regeneration.

Sensors in the DPF alert it when a regeneration is required. Most of the regenerations take place under active working conditions. When the engine is under load, exhaust temperatures of between 527 and 680 degrees are high enough to burn off the accumulated soot. If these steps are not effective at reducing the soot levels, the system will call for an active regeneration. In this situation, raw diesel fuel is injected into the DPF to raise temperatures to over 1,000 degrees.

It’s important to know that the capabilities of the DPF will be hindered if the face of the DOC gets filled with carbon. Commanding a forced regeneration from time to time will help to keep that surface clear. In addition, the DPF can get rid of soot, but over time ash will accumulate inside, and that ash will remain in the filter until it is taken apart for cleaning. As the ash builds up, engine performance will begin to suffer, and you will notice that the system will require more frequent active regenerations. Higher levels of ash will also increase back pressure in the system, which can cause damage to a turbocharger. When your DPF needs service, you should also consider taking out the DOC and having it cleaned as well.

Selective Catalytic Reduction (SCR) catalyst

Once the first two filters have worked hard to reduce or convert particulate matter and carbon monoxide levels, it is up to the final filter to convert nitrogen oxides into a pure form of nitrogen (N2). To make this conversion, the system needs ammonia added in the form of refined urea. 

This refined urea (DEF) is mixed with the partially cleaned exhaust, and as the two are passed over another precious-metal catalyst, the nitrogen compounds that once contributed to dirty air are now changed into a form of nitrogen that is harmless and can safely be released into the environment.

This system, while it rarely needs maintenance, is highly dependent on the quality and availability of DEF. If the DEF level is too low, engine performance will be intentionally limited until the tank is replenished. And while the urea in DEF is similar to that used in agriculture, it is in a very pure form, and trying to use a product other than DEF will damage the SCR catalyst. Unlike the other components, this catalyst is not easily cleaned or repaired.If it fails, it will need to be replaced.

Now That You Know

Modern emissions systems have added  cost and complexity to diesel engines, but they are here to stay. As you operate your diesel engine powered machine, remember that the systems work best when they are under a load. If they are allowed frequent light loads or idling time, they are more likely to clog or fail prematurely. Be proactive with your maintenance. Don’t wait until you lose engine performance to start paying attention to your system. And keep your DEF tank filled!

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