Knowledge Building: Diesel Emissions Control

Gone are the days when more power from a diesel engine meant supplying maximum air and as much fuel as possible to the engine cylinders. Back then it did not matter if smoke puffed from the exhaust pipe. Needless to say, harmful emissions were not a concern.

Today, we know all too well how exhaust emissions are bad for the environment and bad for our health. Laws have been established to limit the harmful emissions produced by both gasoline and diesel engines –and manufacturers have come up with some ingenious ways of reducing emissions.

While advanced emissions control systems have been part of the gasoline-powered vehicle fleet for decades, major advancements to reduce diesel emissions have only recently come into common usage in North America.

Harmful diesel engine emissions come in many forms. Sulphur, for example, was one of the most potent poisons present in the exhaust. When the sulphur that is present in diesel fuel is oxidized during combustion, it forms sulphur dioxide. This sulphur dioxide, when emitted into the atmosphere, can react with the moisture in the air to form acid rain. (If the exhaust piping on a vehicle was continuously rusting out, it could have been because the sulphur content in the exhaust gas was too high.)

To combat the negative impact of diesel combustion by-products on the environment, starting in 2006, diesel fuel sold in North America was mandated to meet Ultra Low Sulphur Diesel (ULSD) requirements. This ULSD fuel has a maximum sulphur content of 15 parts per million (ppm) or 0.0015%, which was a considerable drop from the previous Low Sulphur Diesel limit of 500 ppm.

Particulate matter is also a very serious concern, especially in diesel engine exhaust. This “diesel soot” is mostly comprised of unburned hydrocarbons resulting from incomplete combustion, and can cause respiratory problems in humans and in animals.

The problem of particulate matter was addressed with the development of the diesel particulate filter (or DPF for short).

The DPF traps the particulate matter in a filter made of paper, metal fibre, cordierite, or silicon carbide. However, if the soot is not removed by some means, the DPF can become plugged very easily.

The DPF regenerates by oxidizing the particulates that are trapped in the filter. A DPF will do this in two ways. It will oxidize the particulate matter while the vehicle is being driven, in what is called self-or passive-regeneration mode, and if necessary while idling, which is termed active regeneration.

In the passive-regeneration process, while the vehicle is in motion the DPF will continuously regenerate as long as the exhaust temperature is high enough and all regeneration criteria are met. The platinum or palladium catalyst in the DPF lowers the oxidation temperature of the particulate matter, making it easier to oxidize. Typically, diesel particulate matter in the presence of air oxidizes at a temperature of about 650 degrees Celsius, but inside the DPF the catalyst reduces this temperature to about 300 degrees Celsius.

This passive mode is not always sufficient to keep the DPF from becoming clogged. However, the engine’s computer system continuously monitors the DPF through a series of temperature and pressure sensors. If the computer senses that the DPF is getting plugged and the passive regeneration process is not enough to burn off all the particulate matter, it will warn the driver with a lamp on the instrument cluster indicating that a park (or active) regeneration needs to be carried out.

Keeping the DPF clean is so important that some manufacturers have programmed the engine computer to shut the vehicle down if the driver ignores the notification to employ the active regeneration cycle.

To run an active regeneration, the vehicle has to be placed in the park position and the regeneration button pressed. Once the button is pressed, the driver does nothing but sit there until the regeneration cycle is completed. The cycle can last up to 40 minutes.

Active regeneration does not rely on the catalytic reaction from the platinum or palladium.

This regeneration process gets its heat from diesel fuel that is injected directly into the exhaust stream or injected into the engine’s cylinder on an exhaust stroke. This fuel ignites in the exhaust system, generating the heat that is needed to oxidize the particulate matter. The use of a diesel oxidation catalyst (DOC), a device similar to a catalytic converter, also raises the exhaust temperature prior to entering the DPF.

To further assist with this, the ECM will reduce airflow into the engine, adjust turbocharger boost pressure, raise engine speed, reduce fuel rail pressure, and retard the injection timing.

In the DPF, unburned hydrocarbons are oxidized, leaving the by-products carbon dioxide and water. However, the exhaust gas entering the DPF contains other materials such as engine oil. This engine oil and other inorganic materials, when oxidized, produce not only carbon dioxide and water but also leave an ash residue. This ash is trapped in the DPF and, for this reason, the DPF eventually will require servicing. This is why low-ash oil is required for engines using a DPF. Depending on the engine’s manufacturer, servicing can be the removal, cleaning, and then reinstallation or replacing the DPF assembly with a new one.

The DPF has to be replaced if it becomes poisoned or saturated with fuel, coolant, or oil. For example, it can become saturated with fuel if an injector tip is blown off, with coolant if the head is cracked, or with engine oil if a turbo charger is passing oil. If the DPF becomes poisoned, it will require replacement.

With the tighter environmental laws now in place and with manufacturers designing systems to meet them while still giving the needed performance, diesels are now running cleaner and greener than ever — respecting the earth we all share.

For more information on automotive technology, visit CARS OnDemand training at: www.cars-council.ca.