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7 Basics of HD Aftertreatment System Explained


In this article, we will be discussing what the heavy-duty aka HD Aftertreatment System does, and why it is important to maintain it for peak performance and a cleaner environment.  

HD Aftertreatment system comes in many shapes and sizes. We cover the most important information for all Class 5-8 Vehicles.

Here are the 7 basic components that make up a HD aftertreatment system!

  1. EGR system - The reason behind why you put exhaust gases back into your engine.
  2. Turbocharger - Pumps air into your engine for more power!  
  3. 7th Injector - The fuel injector for your exhaust – pumping fuel into your exhaust! 
  4. DOC filter – The first of many filters in your exhaust system. Plays a vital role here.  
  5. DPF filter – Takes a lot of the dirty exhaust and collects it all so we can burn it off later. 
  6. DEF Injector – This injects a special water-base formula aka DEF fluid for the SCR System.  
  7. SCR system – Completes the clean air process in a diesel engine. This is found on 2011 and newer diesel equipped vehicles.  

The initial reason for aftertreatment systems was to clean exhaust emissions that have been recognized to damage the environment. After the introduction of EGRs in the 2000s manufacturers learned to use these systems not only for lower emissions but also for more efficient engine operation and better engine reliability when maintained correctly. 

Stemming from the combustion event, the exhaust stream carries harmful emissions through the aftertreatment system. The emissions that are created are NOx, hydrocarbons (unused fuel), particulate matter (heavy carbon and sulfer), and carbon monoxide. Each stage of aftertreatment is designed to clean the exhaust gases with each stage removing specific chemicals end allowing optimal performance.  

If you ever look at a dirty DPF or DOC, you will know what we mean. First, let’s look at the components of the aftertreatment system. Hopefully, by the time we’re finished here, you will be thinking “oh, that’s not so complicated after all!” 

DPF Filter: Diesel Particulate Filter  

Now that we got that covered let's get into the main 7 components that make up this entire heavy-duty truck aftertreatment system. Below you will see an image with the entire heavy-duty aftertreatment system and the flow through from the fresh air to the clean exhaust.

Heavy-Duty (HD) Aftertreatment  System  

Heavy Duty Truck Aftertreatment System

1. Exhaust Gas Recirculation (EGR) system  - The reason behind why you put exhaust gases back into your engine.

The EGR is the first widely accepted industry solution for emissions that were required by the EPA to be reduced in heavy-duty trucks. The EGR works by taking a small amount of gas from the exhaust stream and reintroducing it into the intake of the engine to reduce combustion temperatures.  

While this was initially done to lower NOx emissions from the exhaust stream, later they also took advantage of the fact it introduces “spent” air into the combustion chamber. This unusable air acts as a spring. It keeps peak cylinder pressures lower, and as a secondary effect, it expands and fills volume within the cylinder as combustion pressures drop. This effect keeps the combustion event pushing downward on the piston for a longer amount of time during the power stroke. 

 With engine development now naturally including the EGR system, engine manufacturers have been able to use the lowered internal temperatures and pressures to tighten tolerances. They were also able to change rod-to-stroke ratios due to the spring effect created by the EGR. These things combined make the engine more reliable, consume less oil, operate more efficiently, and run cleaner.  

2. Turbocharger - Pumps air into your engine for more power!  

The turbocharger, or turbo, is primarily used for forcing more air into the intake by using a turbine driven by the exhaust. An alternate role the turbo plays in the aftertreatment system is control heat output.

Turbos on modern trucks typically have an actuator controlling vanes within the exhaust side of the turbo. These vanes are moved to direct exhaust air into the exhaust wheel creating either more or less speed on the wheel of the turbocharger. 

When the actuator changes the position of vanes within the turbo this allows it to simulate a load as if the truck is going down the road. The actuation is primarily used to reduce lag for a wider power output and better driving experience, but for aftertreatment purposes, during regeneration it is used to create a controllable condition that generates specific exhaust gas temperatures.  

These elevated exhaust gas temperatures bring the DOC and DPF up to the correct temperatures that work in conjunction with the 7th injector to do a complete soot burn off. 

Below is a turbocharger attached to the exhaust manifold and attached to it is the turbo actuator.

3. Aftertreatment Injector (7th injector) - The fuel injector for your exhaust – pumping fuel into your exhaust!  

Mounted right off the turbo before the DOC is the aftertreatment injector (or 7th injector). This is used to spray fuel into the exhaust stream after the engine and turbo have worked to bring the DOC/DPF up to temperature.  

Spraying this fuel directly into the exhaust stream causes the hot exhaust gases to react with the fuel and DOC causing a reaction and another rise in exhaust gas temperature as it enters the DPF.  

This is done to truly start the regen process where the soot being collected in the DPF is burned down to ash. This burn off then prevents a restriction from forming, if the 7th injector fails typical symptoms would be forced regens ending sooner than usual or not initiating along with high soot levels and derates.