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Available options for Diesel particulate retrofit programs, Flow-Through Oxidation Catalysts

A flow-through oxidation catalytic converter installed on a vehicle can reduce the soluble organic fraction (SOF) of the particulate by as much as 90 percent and total particulate by as much as approximately 25 to 50 percent depending on the composition of the particulate being emitted.

Smoke emissions from older vehicles can be reduced by over 50 percent and a catalyst can virtually eliminate the obnoxious odor of diesel exhaust.

Furthermore, reductions of 60 percent to 90 percent of CO and HC emissions can be achieved. As a result, the diesel oxidation catalyst has become a leading retrofit control strategy in both the on road and nonroad sectors throughout the world. Using a flow-through oxidation converter on diesel-powered vehicles is not a new concept. Oxidation converters have been installed on off-highway vehicles around the world for over 20 years and have been installed on many urban buses in the US and Europe.


The concept behind an oxidation catalyst is that it causes chemical reactions without being changed or consumed.

An oxidation catalytic converter consists of a stainless steel canister that typically contains a honeycomb-like structure called a substrate or catalyst support. There are no moving parts, just acres of interior surfaces on the substrate coated with catalytic precious metals such as platinum or palladium.

In the case of diesel exhaust, the catalyst oxidizes carbon monoxide (CO), gaseous hydrocarbons (HCs) and the liquid hydrocarbons adsorbed on the carbon particles. The liquid hydrocarbons are referred to as the soluble organic fraction (SOF) and can make up a significant part of the total particulate matter.

The level of total particulate reduction is influenced in part by the percentage of SOF in the particulate. For example, it has been reported that oxidation catalysts could reduce the SOF of the particulate by 90 percent under certain operating conditions, and could reduce total particulate emissions by 40 to 50 percent. Destruction of the SOF is important since this portion of the particulate emissions contains numerous chemical pollutants that are of particular concern to health experts.

Oxidation catalysts are also effective in reducing particulate and smoke emissions on older vehicles. Under the US EPA's urban bus rebuild/retrofit program, several manufacturers have certified diesel oxidation catalysts as providing at least a 25 percent reduction in PM emissions (see below). The certification data also indicates substantial reductions in CO and HC emissions.

Combining an oxidation catalyst with engine management techniques can reduce both NOx and PM emissions from diesel engines. This is achieved by adjusting the engine for low NOx emissions which is typically accompanied by increased CO, HC, and particulate emissions and then using an oxidation catalyst to offset these increases, thereby lowering the exhaust levels for all of the pollutants.

Often, the increases in CO, HC, and particulate can be reduced to levels lower than otherwise could be achieved. In fact, a system which uses an oxidation catalyst combined with proprietary ceramic engine coatings and injection timing retard to provide over a 40 percent NOx reduction while maintaining low particulate emissions has been approved under EPA's urban bus rebuild/retrofit program.

This same system has also been approved as reducing PM emissions to below 0.1 g/bhp-hr. Also, two systems employing catalysts and modified engine components, e.g. camshafts and turbochargers, have also been submitted for approval as providing less than 0.1 g/bhp-hr PM emissions.


The sulfur content of diesel fuel is critical to applying catalyst technology. Catalysts used to oxidize the SOF of the particulate can also oxidize sulfur dioxide to form sulfates, which is part of the particulate.

This reaction is not only dependent on the level of sulfur in the fuel, but also the temperature of the exhaust gases. Catalyst formulations have been developed which selectively oxidize the SOF while minimizing oxidation of the sulfur dioxide.

However, the lower the sulfur content in the fuel, the greater the opportunity to maximize the effectiveness of oxidation catalyst technology. The low sulfur fuel (0.05% wt), which was introduced in 1993 throughout the US and in 1995 throughout Europe has facilitated the application of catalyst technology to diesel-powered vehicles.

Furthermore, the very low fuel sulfur content (<0.005% wt) available in several European countries has further enhanced catalyst performance. Several Asian countries including Hong Kong have also lowered diesel fuel sulfur levels to a maximum of 0.05% and this level is increasingly becoming the global maximum norm. Hong Kong has recently demonstrated global leadership by adopting a tax policy which has resulted in most if not all commercially available diesel fuel meeting a standard of 50 PPM or less.

Catalysts have also been effectively retrofitted to vehicles that run on fuel containing sulfur levels above 0.05% wt. Typical nonroad retrofit applications reduce PM, HC, and CO emissions when fuel containing 0.25% wt sulfur is used. In some instances, CO and HC emissions have been effectively controlled with sulfur levels as high as 0.5% wt.

However, these elevated sulfur levels make it difficult to control particulate emissions and depending on the catalyst formulation and exhaust temperature, may actually increase particulate emissions and other hazardous substances such as sulfates.

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In-use vehicles > Retrofit systems

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