Aftertreatment Fuel Pressure: Maintaining Optimal Engine Performance

Aftertreatment fuel pressure is an important consideration for vehicle powertrain systems, particularly in diesel engines, where it plays a crucial role in reducing emissions and ensuring the proper functioning of the engine. The aftertreatment system typically consists of components like the diesel oxidation catalyst (DOC), diesel particulate filter (DPF), and selective catalytic reduction (SCR) system. These components work together to reduce nitrogen oxides (NOx) and particulate matter (PM) in diesel engine emissions.

The aftertreatment fuel pressure should be maintained at optimal levels to ensure the efficient operation of the system. However, the ideal fuel pressure can vary depending on factors such as the specific aftertreatment technology used, engine specifications, and emissions standards. It is important to consult a certified technician or refer to the vehicle's manual to determine the appropriate fuel pressure for a particular system.

The role of diesel oxidation catalysts

Diesel oxidation catalysts (DOCs) are catalytic converters designed to reduce emissions from diesel engines. They are the first step in the aftertreatment process and are installed in the engine exhaust pipeline. DOCs are simple, inexpensive, maintenance-free, and suitable for all types and applications of diesel engines. They are also known as particle oxidation catalysts (POCs).

DOCs oxidise the organic parts of carbon monoxide (CO), hydrocarbons (HC), and particulate matter (PM) in engine exhaust emissions to form harmless water (H2O) and carbon dioxide (CO2). They also oxidise a small part of nitric oxide (NO) in exhaust gas to nitrogen dioxide (NO2). The oxidation of NO to NO2 is essential for the operation of modern diesel emission control systems, as it supports the performance of diesel particulate filters (DPFs) and selective catalytic reduction (SCR) catalysts used for NOx reduction.

The working principle of DOCs is similar to that of the three-way catalyst of a gasoline engine. When the high-temperature exhaust gas passes through the purification device, the purification agent in the catalyst enhances the activity of CO, HC, and other gases and promotes the oxidation reaction. The catalyst exhibits a very high activity in the oxidation of the organic fraction (SOF) of diesel particulates. Conversion of SOF may reach and exceed 80%.

DOCs are usually washcoated onto ceramic monoliths. Modern catalytic converters consist of a monolith honeycomb substrate coated with platinum group metal catalyst, packaged in a stainless steel container. The honeycomb structure with many small parallel channels presents a high catalytic contact area to exhaust gases. As the hot gases contact the catalyst, several exhaust pollutants are converted into harmless substances: carbon dioxide and water.

The diesel oxidation catalyst, depending on its formulation, may also exhibit some limited activity towards the reduction of nitrogen oxides in diesel exhaust. NOx conversions of 10-20% are usually observed. The NOx conversion exhibits a maximum at medium temperatures of about 300 °C.

The diesel oxidation catalyst is designed to oxidise carbon monoxide, gas phase hydrocarbons, and the SOF fraction of diesel particulate matter to CO2 and H2O:

Diesel exhaust contains sufficient amounts of oxygen, necessary for the above reactions. The concentration of O2 in the exhaust gases from diesel engines varies between 3 and 17%, depending on the engine load. A minimum exhaust temperature of about 200°C is necessary for the catalyst to "light off". At elevated temperatures, conversions depend on the catalyst size and design and can be higher than 90%.

A formation of the sulfate (SO4) particulates occurs, outweighing the benefit of the SOF reduction. In reality, the generation of sulfates strongly depends on the sulfur content of the fuel as well as on the catalyst formulation. It is possible to decrease DPM emissions with a catalyst even at high temperatures, provided suitable catalyst formulation and good quality fuels of low sulfur contents are used. On the other hand, diesel oxidation catalysts used with high sulfur fuel will increase total DPM output at higher temperatures. This is why diesel catalysts became more widespread only after the commercial introduction of low sulfur diesel fuel.

The use of low sulfur diesel fuel can minimise the effect of sulfate formation. Modern DOC formulations are being developed and designed to be selective oxidation to obtain a compromise between the sufficiently high CO and HC activity, the excellent oxidation of NO to NO2, and the minimum SO2 oxidation activity.

The development of diesel oxidation catalysts can be divided into three stages:

• The first is the start of using DOCs with noble metals as active components, such as Pt, Pd, and Rh. They have lower ignition temperatures and better oxidation capacity for HCs.
• The second is to reduce the oxidation of SO2 by DOCs and decrease the emission of sulfate.
• The third is to improve the low-temperature oxidation performance of DOCs and reduce the ignition temperature of catalysts to harmful pollutants.

The improvement of DOCs can be studied from the following aspects:

• Improve the anti-aging performance of DOCs by adding second noble metals, such as Pd, or adding additives such as rare earth oxide CeO2.
• Reduce the oxidation of SO2 by DOCs and decrease the emission of sulfate by using low S or S-free diesel.
• Improve the low-temperature oxidation performance of DOCs and reduce the ignition temperature of catalysts by adding Ce-based oxygen storage material to DOCs and applying HC low-temperature capture technology.

Selective catalytic reduction systems

Selective catalytic reduction (SCR) systems are an advanced post-combustion NOx control procedure that can reduce harmful nitrogen oxide emissions from mobile and stationary sources. SCR technology is applicable to both large utility boilers and diesel engines found in automobiles, ships, and locomotives. SCR systems have been shown to reduce NOx emissions by up to 95%

The evolution of exhaust aftertreatment

The evolution of the diesel exhaust system has been a challenging process for technicians to keep up with. This evolution has been driven by the need to manage the byproducts of the combustion process in the diesel internal combustion engine, specifically oxides of nitrogen (NOx) and particulate matter (what we see as "soot").

The Early Generation of Exhaust Aftertreatment

The early designs of exhaust aftertreatment systems (around 2007-2009) focused on managing NOx by reducing cylinder temperatures through the use of exhaust gas recirculation (EGR). This involved reintroducing spent/inert exhaust gases into the combustion chamber, effectively reducing the size of the combustion chamber and lowering the temperature. While this was effective in reducing NOx, it had the negative effect of increasing particulate matter. To address this, aftertreatment systems were introduced, including a diesel oxidation catalyst (DOC) to oxidize post-combustion gases and a diesel particulate filter (DPF) to trap and incinerate particulate matter.

Aftertreatment Technologies Advance

In 2010, selective catalytic reduction (SCR) was introduced, using an ammonia-based urea chemical called diesel exhaust fluid (DEF) to reduce NOx. This was a more fuel-efficient method than the previous technology and involved adding more components, such as NOx sensors, a storage tank, a dosing mechanism, and a decomposition tube. The EGR strategy also changed, with DEF becoming the primary system for NOx management and EGR becoming a backup.

The Fourth Evolution

In 2017, a significant change occurred, with many manufacturers adopting a "one-box" system. This system houses a small DOC and a small DPF, as less particulate matter is created due to reduced EGR dilution. By using more DEF, less diesel particulate occurs, leading to a smaller and more reliable unit with smaller temperature spans. Post-2021, these one-box units have been further enhanced with the addition of soot sensors, an NH3 sensor, and other sensors to ensure proper performance.

The role of diesel particulate filters

Diesel Particulate Filters (DPFs) are an essential component of the exhaust system in diesel-powered vehicles. They are designed to capture and store the soot and other harmful particulate matter produced during a diesel engine's combustion process. These particles can harm human health and the environment, so it's essential to have a mechanism to capture and remove them.

DPFs are typically made from a ceramic material with tiny, precisely engineered pores that allow the exhaust gases to flow through while trapping the particulate matter. As the particulate matter accumulates on the filter, it can reduce the flow of exhaust gases and increase back pressure, leading to decreased engine performance and increased fuel consumption.

To prevent these issues, DPFs undergo a process called regeneration, which involves burning off the trapped particulate matter to keep the filter clean and free-flowing. There are several types of regeneration methods, including active and passive regeneration.

Passive regeneration relies on the heat generated by the engine to burn off the trapped particulate matter. The filter is made from a ceramic material with small, precisely engineered pores that allow the exhaust gases to flow through while trapping the particulate matter. Once the filter becomes clogged with soot, it must be cleaned or replaced. Passive DPFs are typically less expensive than active DPFs and are generally suitable for most driving conditions.

Active regeneration, on the other hand, uses an external source of heat to burn off the trapped particulate matter. This can be accomplished by misting fuel upstream of the Diesel Oxidation Catalyst (DOC) by a doser valve. Active regeneration is generally more efficient than passive regeneration because it allows for more precise control over the regeneration process. Active DPFs are typically more expensive than passive DPFs but may be necessary for vehicles that operate under a wide range of driving conditions.

The choice between passive and active DPFs depends on several factors, including the driving conditions, the vehicle's operating requirements, and the cost. Original Equipment (OE) manufacturers have designed diesel vehicles with the appropriate DPF suitable for its use.

DPFs are critical to meeting emissions standards, and their use is legally mandated in many countries and regions, including the European Union, California, and throughout North America. By capturing and removing these harmful particles, DPFs help to improve air quality, comply with regulations, and minimize the harmful effects of diesel emissions on human health.

Overall, DPFs are essential to modern diesel technology, and their importance cannot be overstated. Understanding the role of DPFs in reducing harmful emissions and maintaining their proper function is critical for ensuring that diesel vehicles operate safely and efficiently while minimising their impact on the environment.

The campfire analogy

Now, let's consider what happens inside the combustion chamber of a diesel engine. The air charge, consisting of oxygen and nitrogen, is forced into the cylinder under pressure, creating greater cylinder pressure. The fuel is injected at specific times directly into the cylinders through a common rail fuel injector, and the engine control unit (ECU) manages both the fuel injection system and the turbocharger system.

The combustion process begins when the fuel and heat from the compressed cylinder air charge interact, resulting in a chemical reaction that produces fire. However, this chemical reaction does not completely convert all the components of the air-fuel charge, leaving leftovers. The main leftover component is nitrogen, which doesn't typically react to combustion but can be oxidized at temperatures above 2500 degrees Fahrenheit, forming harmful oxides of nitrogen (NOx).

The early generation of exhaust aftertreatment systems aimed to manage NOx by reducing cylinder temperatures through exhaust gas recirculation (EGR). This involves reintroducing spent or inert exhaust gases into the combustion chamber, effectively reducing the size of the combustion chamber and lowering the temperature of the combustion event. While this method was effective in reducing NOx, it had the negative effect of creating more soot or particulate matter.

To address this issue, the aftertreatment systems evolved to include a diesel oxidation catalyst (DOC) and a diesel particulate filter (DPF). The DOC oxidizes the post-combustion gases rich in hydrocarbon and carbon monoxide due to the cooler in-cylinder temperatures, while the DPF incinerates the particulate matter. To function effectively, these components require careful management of their internal temperatures.

Over time, the DPF becomes overloaded with particulate matter, and a regen cycle is induced to burn off the particulates and clean the filter. This is achieved by introducing diesel fuel, either through a standalone hydrocarbon doser in the exhaust stream or by injecting additional fuel during the exhaust stroke, creating the heat needed to regenerate the DPF.

While this system successfully managed NOx and particulate matter, it was not very fuel-efficient. As emissions standards became stricter, a more efficient solution was needed. This led to the introduction of selective catalytic reduction (SCR) technology in 2010, which used an ammonia-based urea chemical called diesel exhaust fluid (DEF) to reduce NOx. This system provided greater fuel efficiency while serving the same purpose of NOx reduction.

In summary, the campfire analogy helps illustrate the challenges of managing NOx and particulate matter in diesel exhaust aftertreatment systems. By understanding the trade-offs between in-cylinder NOx reduction and particulate matter creation, technicians can effectively maintain and troubleshoot these complex systems.

Frequently asked questions