Diesel fuel is a lot less refined than traditional gasoline which means it burns dirtier and releases more harmful exhaust gases. Starting around 2007, diesel emissions standards started to become a huge focus point. Today, all diesel engines have a number of emissions systems, including exhaust gas recirculation (EGR), diesel particulate filters (DPF), selective catalytic reduction (SCR), and various specialized catalytic converters.
This guide will discuss the different harmful gases that diesel engines produce along with every emissions system used by the 6.7 Cummins and its purpose for reducing emissions. Since deleting emissions systems is common on diesel engines we’ll include some brief commentary at the bottom about the benefits and downsides of 6.7 Cummins emissions deletes.
6.7 Cummins Exhaust Byproducts
Before we dig into the various components of the 6.7 Cummins emissions equipment, we need to cover the primary byproducts of diesel engine combustion. These are the harmful exhaust gases that are produced as a result of burning diesel fuel.
Carbon Dioxide CO2
Carbon dioxide is a greenhouse gas. As humans, we release carbon dioxide when we exhale. So is it really that bad? Carbon dioxide traps heat within the atmosphere and is believe to be one of the leading causes of global warming. Because humans only exhale the carbon dioxide that we inhale, we don’t actually increase the amount of carbon dioxide in the atmosphere.
However, when we burn fossil fuels, such as diesel fuel, it creates carbon dioxide that otherwise wasn’t present in the atmosphere. Therefore, reducing the amount of it that is released to the atmosphere via diesel exhaust gases is important.
Nitrous Oxides – NO, NO2, NOx
NOx is nitrous oxides which includes nitric oxide (NO) and nitrogen dioxide (N02). NOx gases are produced as a chemical reaction between nitrogen and oxygen during diesel engine combustion. Nitrous oxides create smog, which is air pollution that reduces air visibility. It is also dangerous to people who have preexisting breathing conditions.
Diesels release particles or particulate matter through their exhaust gases. These are very small particles that are considered carcinogens. When breathed in by a human they can get stuck to the lining of the lungs and have various health effects. Don’t worry, this doesn’t mean owning a diesel will be harmful to your health.
Components of the 6.7 Cummins Emissions System
To reduce the amount of harmful byproducts that are released into the atmosphere, the 6.7 Cummins is packed with a number of advanced emissions systems. These systems have gotten a bad rap over time and are frequently deleted or removed from the vehicle. First of all, they are extremely expensive. Since the majority of this equipment is some form of catalytic converter, it is packed with expensive rare earth metals making the products expensive to replace.
Additionally, they create a lot of backpressure within the exhaust system which is bad for the turbocharger and increases exhaust gas temps. Furthermore, they are all prone to clogging which further exacerbates the backpressure and heat issues. And lastly, they are detrimental to engine performance and gas mileage.
However, they are all required and are crucial components for reducing harmful emissions produced by the 6.7 Cummins.
Exhaust Gas Recirculation (EGR): 2007.5-Present
EGR isn’t a diesel-only concept, virtually all engines use some form of exhaust gas recirculation. Beginning in 2007.5 model years, the 6.7 Cummins added an EGR system which includes an EGR valve and an EGR cooler.
The system works by taking exhaust gases and recirculating them through the intake manifold back into the combustion chamber. This allows the exhaust gas to be re-burned in the combustion chamber which reduces the amount of harmful byproducts.
Once the exhaust air leaves the combustion chamber it passes through the diesel particulate filter (DPF). Then, a portion of gases flow to the rest of the exhaust system and a portion flow into the EGR cooler. The EGR cooler cools down the air that is going to be passed back into the intake system. After the cooler is an EGR valve that is connected to the intake throttle body. This valve opens and closes, controlling the amount of exhaust gas that is sent back to the intake system.
Diesel Oxidation Catalyst (DOC): 2007.5-Present
A diesel oxidation catalyst, or DOC, is simply a catalytic converter designed specifically for diesels. For 2007.5-2012 model years the DOC is integrated into the 6.7 Cummins downpipe, which bolts directly to the turbocharger. 2013+ models years simply used a flex pipe as a downpipe.
The DOC in later model years is integrated into the DPF system and does not sit within the downpipe which is a common misconception.
NOx Absorption Catalyst (NAC): 2007.5-2012
The 6.7 Cummins SCR system wasn’t integrated until 2013. Prior to the SCR system, a NOx absorption catalyst was used. The exhaust system for 2012 and prior goes like this: DOC to NAC to DPF and then out the tailpipe. In 2013, with the addition of SCR, NAC was removed, and therefore the exhaust systems flows: DOC/DPF to SCR and then out the tailpipe.
The NAC catalyst captures nitrogen oxide and converts it into pure nitrogen and oxygen gases. Again, it is similar to a traditional catalytic converter but specific to diesel NOx emissions. This system does require undergoing a regeneration process to remove sulfur deposits that buildup in the catalyst.
Diesel Particulate Filter (DPF): 2007.5-Present
As we discussed above, diesel engines release particulates which is also commonly called diesel soot. The DPF system is a honeycomb filter, similar to a traditional catalytic converter, that captures and collects particulate soot.
The 6.7 Cummins has had a DPF system since halfway through the 2007 model year. 2007.5-2012 models had a completely separate DPF system that sat behind the DOC system. In 2013+ models the DOC and DPF were integrated together.
DPF systems require “regeneration cycles”. As the DPF system collects soot it eventually fills up and needs to burn the soot or clear it out to be able to capture more particulates. Once the DPF has built up too much particulates, the backpressure experience in the exhaust system increases. This triggers a regeneration cycle.
There are two regeneration cycles: passive and active. Regeneration is the process of burning the particulate matter. Doing so requires high temperatures and essentially incinerates the built up soot into harmless ashes which need to be cleaned out from time to time.
Passive regeneration occurs naturally as you drive. Once exhaust gas temperatures reach 950 degrees Fahrenheit or higher, the soot naturally burns into ash. However, reaching these EGTs doesn’t happen from just driving your 6.7 Cummins to the store and back and therefore active regeneration is also required.
6.7 Cummins Active Regeneration
Since high enough EGTs aren’t always achieved, active regeneration needs to take place. During active regeneration, diesel fuel is injected into the exhaust stream. The fuel flows over the oxidation catalyst (DOC) which oxidizes the fuel and creates heat. This heat then burns the soot into carbon dioxide.
Selective Catalytic Reduction (SCR) & DEF Fluid: 2013-Present
The SCR system sits after the DPF system. Since the DPF system burns diesel particulates into carbon dioxide, the SCR system is in place to reduce the produced carbon dioxide.
The selective catalytic reduction system required diesel exhaust fluid, or DEF fluid. DEF is a fluid made of urea and water. This fluid gets injected into the SCR catalyst where a chemical reduction reaction takes place. The reduction reaction converts the carbon dioxide into water and nitrogen gas.
Deleting 6.7 Cummins Emissions Systems
As discussed above, these emissions components are not only expensive but are also prone to failure. If you checkout our 5 most common 6.7 Cummins engine problems, the DPF and EGR cooler are two of the most common issues. DPF clogging and backpressure created by these emissions system can also cause turbo failure.
Outside of the costly repairs they can cause, they also limit performance by creating a lot of backpressure. In order to bring more air into the engine and create more power through higher turbo boost levels, backpressure needs to be minimized. Backpressure inhibits a turbos ability to spool quickly and creates resistance against the turbine wheel.
All that said, deleting the DPF and EGR systems is very common. However, it is also very illegal.
Benefits of 6.7 Cummins Emissions Delete
- Improved reliability
- Removes components that are costly to replace
- Decreases turbo backpressure / more free flowing exhaust
- Increases performance potential
- Improves mpg slightly
Why You Shouldn’t Delete Your 6.7 Cummins Emissions
- It is illegal
- It voids warranty
- Dealers won’t touch your car, a lot of independent shops won’t anymore either
- Hurts resale value
- Creates problems with vehicle registration and renewal
There are really two reasons people delete these systems: they don’t want to pay $5k for a new SCR, or they don’t want to restrict performance. I get it, the cost of these parts are expensive which makes breaking them concerning. However, from a performance perspective. Unless you are chasing crazy performance numbers, the 6.7 Cummins is completely capable of punching out some extra power with some simple legal mods like an intake and a tune.