Clean Diesel

Clean diesel technology represents a significant achievement in cleaning up diesel emissions to meet new federal and European standards while at the same time exploiting diesel's fuel economy advantage and improving the public perception of diesel.

You've probably heard the term “Clean Diesel” used in the automotive industry recently. That's because vehicle manufacturers, the petroleum industry and many trade organizations have invested heavily in developing and promoting this new technology. The goal is to clean up diesel emissions to meet new federal and European standards while at the same time exploiting diesel's fuel economy advantage and improving the public perception of diesel. Clean diesel technology represents a significant achievement in meeting these goals.

Let's take a look at Clean Diesel: what it is, and what it is not.

The Technology

Clean Diesel is not one particular technology or component, it's a system of multiple pieces working together to clean up diesel emissions. This includes combustion technology, exhaust after-treatment, fuel reformulation and advanced electronics. First, let's look at the problem with diesels that Clean Diesel Technology is designed to address.

Diesels are the most efficient internal combustion engine available. However, two things have stood as obstacles to diesel; Particulate Emissions and Nitrogen Oxide Emissions, better known as NOx. Particulate emissions are characterized by that black sooty plume of smoke you see coming out of the exhaust. NOx emissions cause harmful ground-level ozone (smog) and acid rain.

NOx Emissions

NOx emissions are created when fuels are burned at high temperatures. There are two primary methods to reduce NOx emissions.

The first method, which addresses the control of combustion temperatures, is the use of Exhaust Gas Recirculation (EGR). EGR reduces combustion temperatures by recirculating exhaust back into the intake system where it is drawn back into the combustion chamber. At first, it would seem this would increase combustion temperature because the temperature of the exhaust gas is much higher than the intake air charge. However, EGR works in another way. Exhaust gas has less oxygen content than air, which contains about 21% oxygen. Because of this, the exhaust gas is considered inert to the combustion process. When exhaust gas is added to the combustion chamber it consumes volume that would otherwise be occupied by oxygen rich air. By removing oxygen from the combustion chamber the temperature of the combustion process is reduced, thus reducing NOx emissions.

The second method to reduce NOx is the use of exhaust after-treatment devices such as a catalytic converter. Prior to 2007 catalysts could not be used on diesels because of the high sulfur content in diesel fuel, which would quickly contaminate the catalytic converter. In 2007 the federal government mandated that diesel fuel be reformulated to contain less than 15 parts per million of sulfur content. Prior to this federal mandate, diesel contained up to 500 parts per million of sulfur. This new fuel is referred to as Ultra Low Sulfur Diesel fuel or ULSD.

Particulate Emissions

Diesel engines inject fuel directly into the combustion chamber. The time that the fuel has to mix with the air is very short compared to gasoline engines, where the fuel is injected into the intake manifold. This can result in inconsistent air fuel mixtures within the combustion chamber resulting in incomplete combustion, which causes particulate emissions. In order to reduce particulate emissions, manufacturers utilize a Diesel Particulate Filter (DPF). The filter works by trapping the carbon particles present in the exhaust on a substrate within the filter. Over time, the substrate will build up with particulates and begin to restrict the exhaust gas flow, causing decreased engine performance. In order to prevent this, the DPF is regenerated on a periodic basis. This process heats the filter's substrate to temperatures that allow the soot to be burned off. Regeneration is controlled by the vehicles Engine Control Unit (ECU) which monitors particulate build up using sophisticated software models and exhaust system pressure readings. Regeneration must be carried out approximately every 200 to 500 miles, but this varies significantly based on vehicle operating conditions. A typical regeneration cycle can last between 10-15 minutes. The regeneration process begins with the ECU retarding the fuel injection event while adding an injection event after the main injection pulse. This increases the exhaust gas temperature to the necessary levels for regeneration. The high exhaust temperature heats the filter substrate and the soot is burned off and turned to ash. The DPF stores the ash for the life of the vehicle.

Clean Diesel Technology results in a 25%-50% reduction in NOx emissions and a 90% reduction in particulate emissions, solid proof that diesels are cleaning up their act.

To learn more about Clean Diesel Technology visit the Diesel Technology Forum at


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