Particulate Problems

PM is any airborne pollutant that is made up of either liquid or solid particles. PM10-2.5 is comprised of particles that are between 2.5 and 10 microns in diameter, while particles less than 2.5 microns in diameter are known as PM2.5.

While particles in both of these categories are considered to be a health risk, PM2.5 is considered to be the most dangerous because the microscopic size of these particles makes it possible for them to be inhaled into the deepest sections of the lung cavities.

PM SOURCES

While there are many natural sources of particulate matter, such as dust and pollen, diesel engine-generated PM is of the greatest concern for the California Air Resources Board (CARB) and the U.S. Environmental Protection Agency (EPA). Diesel PM is mostly made up of particles that are smaller than 2.5 microns in diameter, and has been identified by both California and EPA as being a toxic contaminant with the potential to cause cancer and other respiratory illnesses.

In the past, diesel engines have received limited scrutiny as most of the focus has been placed on controlling emissions from gasoline-powered vehicles. However, diesel engine emission standards have been tightened recently and upcoming regulations will create significant engineering challenges for both the on-and off-highway diesel industry.

PM EMISSIONS CONTROL

There are certain "in-cylinder" techniques that can be used to lower diesel engine emissions. For instance, advancing injection timing and reducing or eliminating exhaust gas recirculation (EGR) flow can reduce PM emissions. This has a tendency to increase combustion temperatures and "burn off" soot in the exhaust gases. Unfortunately, these same measures tend to increase NOX emissions, which can be much more difficult to deal with after the fact.

An alternative approach is to use a "cold combustion" technique: EGR flow is increased to reduce NOX, and the increased PM is then removed downstream using a diesel particulate filter (DPF). According to Makoto Saito, manager of DENSO's ceramic engine department, "Currently, both directions exist, but we think that cold combustion will become the mainstream."

There are many different types of DPFs, but the ones that are being installed in OEM passenger cars and light trucks are "wall flow" designs. The filter itself is a honeycomb structure whose cells are alternately plugged on each end. The walls of the filter are porous, which allow gases to flow through freely but collect the soot in the open channels. The filter is most often made of either silicon carbide (SiC) or cordierite, and its surface is often coated with a catalyst such as platinum. The filter is designed to have high filtration efficiency while limiting backpressure in the exhaust system. These materials also must be able to operate at high temperatures with limited thermal expansion in order to maximize cycle life and durability.

High-sulfur fuel has a negative impact on the operation of a DPF. While sulfur does not permanently damage the DPF, it will "compete" for space on the filter surface and will eventually disable it. In order for the DPF to operate correctly, sulfur content of the diesel fuel must be kept below 15 parts per million (ppm).

These requirements led to EPA mandating the use of Ultra Low Sulfur Diesel (ULSD) nationwide, starting in the fall of 2006. The timing of the availability of this fuel coincides with the introduction of 2007 model-year heavy trucks, which will all use DPFs in order to meet the 2007 EPA emission standards.

DPF REGENERATION

So how does the DPF deal with the collected soot? Obviously, exhaust system backpressure will increase as more soot collects in the open channels of the filter. This will have a negative effect on engine operation and efficiency, and it could lead to engine damage if left unchecked. Eventually, the DPF must be regenerated: The soot is ignited, and the filter is cleaned to the point where normal engine operation is restored. There are many methods that can be used to regenerate DPFs, and it is interesting to note that most OEMs are developing strategies where a number of these are used.

It is possible to use a strategy called passive regeneration, where rising backpressure leads to an increase in exhaust temperature and the soot eventually ignites on its own. This could work if the engine was under a constant heavy load and exhaust temperatures remained high at all times. Unfortunately, most light-duty diesel engines don't work this hard and spend much of their operating time at idle or low load. In these applications, passive regeneration is a very poor strategy and eventually leads to DPF clogging.

The key to successful DPF regeneration in light-duty applications is to lower the ignition temperature of the soot through the use of catalysts and then initiate regeneration by artificial means. This is known as active regeneration: The engine controls intervene when necessary to get the regeneration process started. The introduction of electronic controls on diesel engines has made it possible for these systems to monitor themselves and work well without compromising engine efficiency.

In Europe, a regeneration strategy that has proven successful is to use a fuel-borne catalyst such as cerium. The catalyst is a liquid additive that is metered into the fuel tank from an on-board reservoir when the vehicle is refueled.

The disadvantages of this approach are twofold. First, the reservoir must be refilled from time to time with the liquefied catalyst, which could be neglected by the owner of the vehicle. Second, the catalyst does not burn and leaves an ash deposit in the DPF that must be cleaned out at certain intervals. It is most desirable for these systems to have little or no required maintenance, so other approaches are being considered for the North American market.

THE DIESEL OXIDATION CATALYST

While a DPF can be housed in its own assembly, it is common for the filter to be packaged together with a diesel oxidation catalyst (DOC). This creates excellent opportunities for regeneration of the particulate filter, because the DOC is located upstream of the filter and will increase exhaust gas temperatures (EGTs) as it oxidizes the hydrocarbon (HC) and carbon monoxide (CO) molecules in the exhaust gases. Increased EGTs (up to a point) are good for the DPF because this makes it easier to regenerate the filter. As mentioned earlier, the particulate filter itself will often have its own catalyst coating, which effectively lowers the temperature needed to start the regeneration process.

In situations where very little HC and CO are being generated by the engine and EGTs are low, it also is possible to meter fuel into the exhaust gas stream ahead of the DOC in order to initiate regeneration. This would be similar to using liquid "fire-starter" when building a campfire. This strategy gives a great deal of control over the regeneration process because temperatures in the DOC are monitored and the fuel can be metered in such a way that the optimum temperature for regeneration is achieved.

There are two methods for metering fuel into the exhaust gas stream. The first involves the use of a "post-injection" event where the engine's fuel injectors are pulsed during the exhaust stroke. The unburned fuel is then sent past the exhaust valves and on to the DOC, where EGTs are raised as the fuel is oxidized into carbon dioxide (CO2) and water (H2O). This method requires no extra hard parts and is only dependent on modification to the engine control software.

However, this approach has the potential for oil dilution as the fuel is introduced into the cylinder itself. To avoid this, some OEMs install a fuel vaporizer in the exhaust system between the engine and the DOC; the fuel vaporizer is then used to introduce the fuel to the exhaust gas stream when DPF regeneration is required. The vaporizer utilizes an electric heating device to heat the fuel and is located far enough upstream from the catalyst to allow the fuel vapors to diffuse sufficiently in the exhaust gases before entering the catalyst.

One other strategy that can be used to aid the regeneration process is intake air throttling. Because diesel engines normally do not use throttle plates, combustion takes place in an extremely lean environment as maximum air is always being drawn into the cylinders. This extra air has a tendency to lower EGT and make it more difficult to regenerate a diesel particulate filter. Closing an electronically controlled throttle mechanism on the intake side will slow down airflow through the engine and increase EGT when DPF regeneration is taking place. This is just another trick that can speed up the regeneration process and restore engine efficiency more quickly and reliably.

OBD AND DPF

Diesel particulate filters are monitored by the vehicle's on-board diagnostic (OBD) system and generic scan tools can be used to view data and retrieve any trouble codes that are generated. Two primary pieces of data are used by the engine control module (ECM) to control the regeneration of the DPF and also to diagnose any malfunctions.

First, pressure is continuously monitored at the inlet and outlet of the DPF. The difference between these two measurements is known as the pressure drop (or differential pressure) and this increases as soot fills the open channels of the particulate filter. The other critical piece of data is the temperatures at the inlet and outlet of the DPF. Once a certain pressure drop threshold is reached, the ECM initiates regeneration and looks for a temperature increase across the DPF for feedback on whether the process is working or not. It will also look for an associated decrease in pressure drop across the DPF as the EGTs rise, indicating that regeneration is taking place. Excessive temperatures will lead the ECM to take steps to lower EGT to prevent damage to the DPF assembly.

The ECM also can use the pressure drop measurement to determine the condition of the diesel particulate filter itself. There is a certain amount of normal pressure drop for a clean DPF. This value will increase with higher engine RPMs and loads, but the ECM will have a baseline for what pressure drop should exist for a given operating condition. Any variance from these baseline values can be used to diagnose problems with the particulate filter. For example, a lower overall pressure drop could point to a damaged or missing DPF. Another possibility is a gradual increase in pressure drop values after a regeneration cycle has taken place. This could indicate that ash is collecting in the filter and will have to be removed through manual cleaning of the DPF.

MOTOR OIL SPECS

It is quite possible that a new motor oil specification will have to be formulated for diesel passenger cars with particulate filters. Metallic-based motor oil additives such as phosphorous and sulfated ash cannot be burned and will collect in the DPF.

Future motor oil specifications may call for a reduction in these additives in an effort to reduce DPF maintenance. This is already taking place in the heavy-duty transport industry, as 2007 diesel truck engines will require the use of oils that conform to the API CJ-4 standard. CJ-4 is unique in that it marks the first time that the API has limited the use of specific types of additives in an effort to make the motor oil compatible with DPF systems.

The internal-combustion engine is much more resilient than was thought even just five years ago. Technologies such as common rail injection systems and particulate filters are breathing new life into the diesel engine and will likely keep it vital for some time to come. In the meantime, those in the automotive service industry who are prepared can expect excellent opportunities to arise in the maintenance and repair of these new technologies.