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One difference between diesel engines and their gasoline counterparts has to do with the way fuel is injected: on gas engines, fuel is injected during the intake stroke; on diesels, fuel is injected during the compression stroke. As a result, gas engines have the advantage of having more time to thoroughly mix the air and fuel before ignition occurs thereby reducing the amount of unburned fuel.

In a diesel, fuel is injected late in the cycle and the air is not as well mixed as in a gasoline engine. As a result of this less homogeneously mixed fuel and air, there are fuel-dense pockets in the combustion chamber. The consequence is that diesel engine exhaust contains incompletely burned fuel (soot) known as particulate matter (PM).

In order to minimize the amount of unburned fuel, modern diesels use high-pressure fuel injection for better fuel atomization and turbochargers to more aggressively mix and force the air-fuel mixture into the combustion chamber. The result is a reduction in the formation of particulates, although some PM is still produced. Barring significant developments in diesel combustion technology, if the mandate is to reduce PM, particulate filters are a proven technology for PM reduction on both light-duty and heavy-duty diesels.

Particulate filter technology has been around for years. It has been proven over and over to be able to reduce PM by 95 percent or more. However, the key to the successful application of particulate filters on diesel engines was the ability to reliably regenerate the filter, or in other words, burn the PM that the particulate filter "traps" or collects.

To understand how a filter regenerates, one must understand how soot or PM burns. Traditionally, combustion of soot is done in an oxygen atmosphere (air). In air, soot will burn at about 450° to 500° Centigrade (840° to 930° F). However, this is not a typical operating temperature for diesel engine exhaust. As a result, in order to burn soot in air, an active system, one that increase the temperature of the exhaust using some external heat source, is required. But if an active system is not carefully controlled, it can often experience an "uncontrolled burn" where the temperature increases to 600° C (1112° F) or more. This will damage the filter element and also pose some potential risk to the vehicle.

An alternate method of burning soot was identified and patented by Johnson Matthey in the 1980's. Johnson Matthey discovered that soot will burn at about 250° C (482° F) in the presence of nitrogen dioxide (NO₂). Typical diesel engine exhaust contains about 5 - 10 percent NO₂, so if this discovery were to have practical application, Johnson Matthey needed to develop a technology that would increase the amount of NO₂ in the exhaust enough to allow for this low temperature combustion to occur. The technology that Johnson Matthey developed was the CRT particulate filter.

The CRT particulate filter is a passive filter using only the heat in the exhaust to combust the soot. It is a dual brick system containing a highly loaded platinum (Pt) catalyst upstream of a filter element. The Pt catalyst serves two functions: first to convert a portion of the nitrous oxide (NO) in the exhaust to NO₂, which allows the soot to be burned at this much lower temperature; and secondly to burn or reduce both carbon monoxide (CO) and hydrocarbons (HC) by over 90 percent. However, this same Pt catalyst will make significant sulfate if used with 500 ppm S fuel. So a requirement for the use of the CRT filter is ultra low sulfur diesel fuel (ULSD) containing no more than 50 ppm Sulfur. And for very low PM levels and reliable operation, 15 ppm S fuel is required.

The requirements for the CRT filter technology to reliably operate are simple.  They are:

1. Use of ULSD
2. An exhaust temperature of 250° C for 40 percent of the operating cycle
3. A NO_x/PM ratio of 20 or more for the proper amount of NO₂ for combustion

When these conditions are met, the CRT filter will operate reliably and will reduce PM, CO and HC by more than 90 percent for many years and hundreds of thousands of miles. In fact there are over 70,000 CRT filters in operation on HDD vehicles around the world. Some of these filter systems have over 1,000,000 miles of reliable service reducing diesel emissions every day.

And technology is not standing still. The next generation CRT filter, recently commercialized and trademarked the CCRT filter, combines a Pt pre-catalyst with a Pt coated filter and is included within the scope of the CRT patent. The CCRT filter requires a reduced exhaust temperature of 220° C (428° F) for proper operation. Field testing has demonstrated that the CCRT regenerates more quickly than the CRT filter and it still reduces PM, CO and HC by 90 percent or more.

Other passive filter technologies include the catalyzed soot filter (CSF) with a Pt catalyst on the filter element itself as well as a Pt additive in combination with a bare or lightly catalyzed filter. Both have their proponents and both have their place in the market.

These filter technologies are the technologies of today, principally applied to diesel retrofits, although the number of OE first fits are increasing. The technology for 2007 and beyond is likely to be a hybrid of today's filters, using the best concepts of coated catalysts and filters with a method of managing the exhaust temperature at all points of the duty-cycle, thereby maximizing emission reductions while optimizing operating parameters. For more information on particulate filters visit www.jmusa.com or www.boschusa.com.