Diesel emissions control systems are designed to reduce the harmful pollutants emitted by diesel engines, including particulate matter and nitrogen oxides (NOx). These systems are vital for ensuring that diesel vehicles comply with environmental regulations, but they can present problems if not properly maintained. This section covers the key components of diesel exhaust emissions systems, common issues, and preventive maintenance options.

A) Parts of the Exhaust Emissions System

Modern diesel engines are equipped with several critical components to control exhaust emissions:

1. Diesel Oxidation Catalyst (DOC):
   The DOC is a mandatory system on all diesel vehicles and plays an essential role in reducing emissions. Its primary function is to convert carbon monoxide (CO) and unburned hydrocarbons (HC) into carbon dioxide (CO2) and water (H2O) by oxidation. Unlike other emissions control components, the DOC generally doesn’t affect engine performance or efficiency. It’s a passive system that works continuously without needing frequent maintenance​
2. Diesel Particulate Filter (DPF):
   The DPF captures particulate matter or "soot" produced by incomplete combustion of diesel fuel. Since soot still contains energy, the DPF allows it to undergo a process called regeneration, where the soot is burned off and converted into CO2 and water, which are released as gases. However, if the DPF fails to regenerate properly, it can become clogged, leading to engine performance issues and increased fuel consumption
3. Selective Catalytic Reduction (SCR):
   The SCR system reduces nitrogen oxides (NOx) emissions by converting them into harmless gases like nitrogen and water vapour using a reductant (usually AdBlue) on a catalytic surface. NOx reduction is necessary because controlling NOx at the combustion level alone is no longer feasible with modern diesel engine performance. The SCR system is vital for meeting stringent emissions regulations but can fail if AdBlue is not used correctly or if it crystallises, blocking the system​

B) Exhaust Emissions Issues

Many drivers may not realise their vehicles have these emissions control systems because, when functioning correctly, they do not typically cause noticeable issues. However, when problems arise, they often cascade and lead to more severe engine or performance issues. Notably, exhaust emissions failures are often symptoms of broader issues elsewhere in the vehicle:

1. DPF Continually Filling:
   If the DPF is not able to regenerate (burn off soot) efficiently, it will fill up with soot, leading to back pressure on the engine. This increases fuel consumption, reduces engine power, and triggers more frequent regeneration cycles, which further degrade system efficiency
2. SCR Urea Crystallisation:
   Crystallisation of AdBlue (urea solution) within the SCR system can occur if improper fluid is used or if there is a blockage in the injector. This leads to reduced NOx conversion efficiency and can cause the system to fail altogether, triggering a warning light or reducing the engine’s power

C) Most Common Failure Modes

The emissions system's most common failure modes usually start with engine-related issues that exacerbate the problem:

1. Soot Begets Soot:
   Once an engine starts producing excessive soot—often due to a problem like over-fueling caused by a worn Suction Control Valve (SCV)—the issue escalates. Soot buildup in the intake manifold causes further inefficiencies, and the DPF struggles to keep up with regeneration, leading to more soot. This cycle results in more fuel consumption, higher back pressure, and more frequent DPF regeneration cycles, creating a vicious spiral​
2. Downstream Contamination:
   Any form of contamination—such as oil from a failed turbocharger—can coat the DOC and DPF, reducing their effectiveness. Once coated, these components no longer generate the heat required for proper operation, leading to regeneration failure in the DPF. While cleaning might restore temporary performance, the underlying issue (e.g., a failed turbo) will continue to create problems​

D) Preventive Maintenance Options

Keeping your emissions control system functioning correctly requires regular preventive maintenance, with a focus on reducing soot production and contamination:

1. Engine Servicing:
   Regular oil changes and fuel system maintenance are crucial for preventing soot buildup and contamination downstream. Using the correct engine oil is essential, as improper oil can degrade the DOC and DPF. Additionally, ensuring that the fuel system, particularly the SCV, is functioning correctly can reduce soot production, helping your DPF and SCR systems work efficiently​
2. Periodic Emissions System Checks:
   Regularly checking the efficiency of the DPF and DOC ensures that they are working correctly and can regenerate as needed. If issues like frequent regenerations or back pressure occur, it’s important to address the underlying causes immediately​

E) If Issues Present Themselves – Seek Professional Help

When exhaust emissions issues arise, it’s essential to consult a mechanic who understands the full picture, not just the immediate symptoms. Many mechanics may focus solely on cleaning the DPF without addressing the root cause of the problem, such as over-fueling or a clogged Intake Manifold. This approach can lead to frequent, expensive repairs and ongoing performance issues, leaving you frustrated with your diesel engine. Instead, seek out professionals who can diagnose and resolve the underlying issues, ensuring your diesel engine runs as smoothly and efficiently as possible​

By understanding how these emissions control systems work and the importance of regular maintenance, you can avoid the pitfalls that lead to costly repairs and downtime, all while ensuring your vehicle meets emissions standards.