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Understanding Diesel Particulate Filter service and repair

Sunday, October 1, 2017 - 06:00
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The primary products of combustion produced by automobiles and light trucks, whether powered by either gasoline or diesel engines, are carbon dioxide, water and nitrogen. As we all know, it’s the other gases that create a problem. Unlike gasoline-fueled engines, a diesel engine’s power is controlled by the fuel supply instead of throttling the amount of air entering the engine. This causes diesel engines to produce a different array of pollutants than spark-ignited, gasoline-fueled engines. One example is carbon monoxide (CO) emissions. Because diesel engines burn their fuel in excess oxygen, even at full load, their CO production is considerably lower than gasoline engines.

This DPF filter from a Ford 6.4L Powerstroke engine needs to be replaced around 80,000 miles. Once the filter becomes 90% plugged it can’t be regenerated by driving or undergoing a forced regeneration using a scan tool. Replacement cost is around $1,700.

However the lean-burning nature of diesel engines comes at a price, and the high combustion temperatures result in significant production of nitrogen oxides (NOx). In fact, NOx from gasoline engines have decreased 96 percent through the use of catalytic converters, but diesel engines still produce 20 times more NOx if left untreated. Some OEMs reduce their diesel NOx emissions through a method called Selective Catalytic Reduction, or SCR. This process involves injecting urea into the exhaust gases. It reacts with the hot gases and releases ammonia that helps the catalytic converter turn the NOx into nitrogen and water.

The other major exhaust component that separates diesel from gas engines are the particles of soot or particulate matter. The black-colored soot from a diesel engine contains carbon compounds that are formed because of fuel that is not fully atomized, combined with low temperatures within the combustion process. Cylinder walls and large droplets of fuel are relatively cool when compared to the rest of the combustion chamber and carbon deposits (soot) are formed in these areas because of the rich air/fuel mixture and lack of oxygen. To keep particulate matter from being blown out of the tail pipe, modern diesel-powered cars and light trucks use a diesel particulate filter (DPF) to capture and intermittently burn them off. Trapping soot particles is important because of their small size, which can be inhaled and cause serious medical problems for humans.

DPF filtration

Diesel particulate filers can capture around 90 percent of particulate matter and because of the environment they operate in have to be mechanically and thermally durable. Because there is limited space to locate a DPF in a vehicle, their physical size can only be so large and depending on engine operating conditions they can quickly accumulate a considerable volume of soot.

If a DPF collects too much soot, it restricts exhaust flow drastically, affecting engine performance. DPF systems have to provide a way of removing the particulates from the filter to restore its capacity and lower exhaust backpressure. This process is known as filter regeneration and usually takes place continuously as the vehicle is driven. Typically regeneration is triggered when the backpressure in the DPF reaches a predetermined level and the vehicle is operated for a time period long enough to burn off the particulates. Initiated by the vehicle’s PCM, continuous regeneration should be “invisible” to the driver.

DPFs use thermal regeneration where the collected soot particulates are oxidized or burned. The DPF is heated and oxygen/nitrogen (air) is added, causing the soot to burn and transforming it to carbon dioxide or CO2. The DPF must reach a high enough temperature (1100 Degrees Fahrenheit) to oxidize the particles and on some systems the source of heat is the exhaust gas. This type of filter is known as a passive filter and regenerates continuously during normal operation of the engine. Passive DPFs use a catalyst, which lowers the soot oxidation temperature to a level that can be reached by using the engine’s exhaust gas alone. Passive systems may also use fuel additives that allow the soot to be burned off at lower exhaust temperatures.

This diagrams shows how a passive DPF filter operates. Heat from the engine’s exhaust combined with air burns off the particulate matter collected in the filter by transforming it to CO2

Another method for burning off soot is called active regeneration. The “active” process heats the DPF using additional fuel. During the process, exhaust temperatures can be increased by late cycle injection of increased fuel quantities or injection and combustion of fuel in the exhaust system. Exhaust gas combustion consists of fuel burned in a fuel burner or burned in an oxidation catalyst. Another method is the use of electric heating by placing a heating element upstream of the filter substrate or using electrically conductive filter media that acts as both heater and filter. Whatever method is used, regeneration is performed periodically as determined by the vehicle’s on-board computer.

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