Diesel Exhaust Filters
How do they work, what do they do?
By 2007 the EPA requested all manufacturers to turn to an exhaust
after-treatment system for the particular matter (soot) and NOx
emissions. This was to lower the overall emissions and to bring the
diesel engines into compliance with the new regulations.
After countless hours of research and development the DPF
systems was initiated. The DPF is actually a ceramic filter that
has thousands of tiny channels or honeycomb shaped openings that
trap the soot onto the channel walls and prevents the particulate
matter (down to 1 micron), from exiting out the tail pipe. The honeycombed inner structure is covered with a layer of a chemical catalyst that contains small amounts of precious metals, usually platinum or palladium.
These DPF (Diesel Particulate Filters) have been fitted to diesel cars and trucks for over a decade now. The particulates that are filtered out are the unburnt fuel residue left over from combustion or what is more commonly referred to as the soot or black smoke coming from the tail pipe.
The modern diesel is a much more sophisticated affair than its early 1980’s predecessor and with these filters and a few other bits and pieces we’ve got a smooth, powerful, clean diesel engine in our cars and trucks than ever before. In fact, a modern diesel engine running with a properly tuned system can actually vent cleaner air than what it inhales through the intake.
There is a problem the manufacturers had to overcome in order to lower the emissions to a level that the government was happy with. To reduce the PM (Particulate Matter) or soot from the exhaust you need to raise the combustion chamber temperature high enough to reduce the PM’s from forming. NOx is formed when the combustion temperatures exceed 3200°F (1,800°C) and the amount of oxide formed depends not only on the temperature but the length of time the heat is applied. However, raising the combustion chamber temperature higher inadvertently raises the amount of NOx that’s formed. Which is even worse for the environment than the soot they originally were trying to remove. An alternative method had to be developed.
Most manufacturers have gone with an Exhaust Gas Recirculation (EGR) system as a method of reducing the NOx formation. This works by re-introducing the relatively inert exhaust gasses (Already burnt non-combustibles with little to no O2 present) back into the intake at a metered percentage amount. Basically, weakening the air to fuel ratio just enough to cause the combustion chamber temperatures to be reduced. Which has the effect of slowing down the combustion process and reducing the peak NOx development. Unfortunately it too has a side effect that doesn’t meet government standards. We’re back to an even higher amount of unchecked PM’s (soot) coming out of the exhaust.
Now, Diesel cars and trucks manufactured after 2009, feature a device called a Diesel Particulate Filter (DPF), and a Selective Catalysts Reduction (SCR) system. All of which work together to reduce, and when working properly, remove all of the harmful NOx and soot from the exhaust. Not only does this help the environment but it also creates a much cleaner engine. Engine oil doesn’t get as dirty or as fast as it would have without all of these systems in working order. Less carbon being mixed with the engine oil means less of a chance of that gritty carbon to form in the oil galleys and other moving parts. Thus increasing the life of the engine at the same time cleaning the air.
The drawback to all of this is that the DPF needs to be cleaned regularly. The soot particles attach themselves to the lining of the DPF while the engine is running but at the same time the filter itself is slowly clogging with the very particulates that it’s designed to remove from the exhaust. This process of cleaning the DPF is done through a process called regeneration. There are several different methods used by various manufacturers to clean the DPF. Passive, active or stationary (parked) regeneration.
Passive self-regeneration is completely transparent to the operator and does not affect the machine’s operation or performance. The only indication when a passive regeneration cycle has been activated is either an exhaust temperature warning light indicating the exhaust temperature is higher than normal or a message stating that a regeneration is in process or both.
Active self-regeneration occurs when there is not sufficient heat in the exhaust to convert the PM being collected in the DPF. The active regeneration is self-activated by the PCM based on various inputs. The PCM sends a command to raise the exhaust temperatures by adding a small amount of injected raw fuel upstream of DPF. The chemical reaction of the precious metals in the DPF and the raised exhaust gas temperatures oxidizes the PM from the filter.
Stationary (Parked) regeneration
Stationary, or parked, regeneration is the same as active regeneration but takes place while the equipment is not being operated on the road. This is either driver induced or by use of a scanner. There are times when the driver will need to perform a manual or “parked” regeneration on the side of the road. This may be because they cancelled an earlier regeneration, or an automatic regeneration had started, but was interrupted. In some cases, the regeneration is “forced” onto the driver for ignoring early requested to perform a parked regeneration by causing the vehicle to go into limp mode. A lot of times a warning light and or a message will tell the driver to pull over and begin a parked regeneration. This usually involves the driver setting the parking brake and engaging a switch to start the process.
Stationary (Parked) regeneration precautions
Due to the high heat created during a regeneration, when performing a parked regeneration or a scanner induced regeneration's follow these simple rules to avoid any outside interference.
Stay clear of combustibles and people. Avoid crowded worksites, fueling stations, and tunnels. Dry fields or grain elevator areas should be avoided. Use common sense and clear the area around and below the vehicle before starting the parked or scanner induced regeneration process.
Some DFE failures are a result of not allowing the regeneration to take place. This will inadvertently clog the DPF to the point that replacement is the only option. Although it can be cleaned to some degree a portion is still lost due to the severity of the restriction. Another problem is when it is in regeneration and the excess heat combined with the clog causing the metal casing of the DPF to expand and rupture. Which of course, means the only solution is to replace the DPF. Generally, the DPF requires professional cleaning every 150,000 –250,000 miles or 5000 hours.
On some vehicles the monitoring is done by way of a pressure sensor that measures the input and output pressures of the DPF. Others use mileage, while others use an engine hour counter. On most vehicles there is a way to shut off the regeneration process if you are in a situation where raising the exhaust systems temperature could cause a fire. But don’t leave it off or permanent damage can be caused to the DPF.
Regeneration can only happen when the conditions are within the preset specifications for that motor and manufacturer’s needs. In general, most regeneration cycles are handled even without the driver knowing it. Depending on the conditions of course. The regeneration process occurs by raising the temperature of the DPF to around 1100°F (600°C) and enough oxygen is providing directly to the DPF.
Some systems will inject extra fuel into the cylinder on the exhaust stroke which effectively send hot gasses into the oxidation catalyst of the DPF, raising its temperature sufficiently to cause the carbon to react with the excess oxygen that was also provided. Other systems rely on a heating element just in front of the DPF to raise the temperature.
The regeneration process will continue until the pressure differential across the DPF (input and output) drops to an acceptable level. Should the driving circumstances change, for instance the car comes to a halt, the regeneration is abandoned until the conditions once again become suitable. Regeneration can be a noisy affair as the engine revs up to 4,000rpm’s for 4 minutes or more, then to 2,000rpm’s for a further 4 minutes or more. When the regeneration has completed the vehicle will return to its normal idle and the service light will go back off.
Problems arise when successive regenerations are abandoned and the soot levels build to a point where the DPF clogs and can’t be regenerated on its own, such as cars that drive around in stop and go town traffic or only on short trips are the most susceptible to this. When this happens, the driver is notified by a flashing DPF warning light. If the warning light is ignored, then a second warning appears which can cause the vehicle to go into limp mode. In limp mode the vehicle will not operate over 5 or 10 miles an hour and will remain that way until it has been properly serviced using a scanner to perform the regeneration process.
Selective Catalytic Reduction (SRC)
SCR is an alternative to the EGR which addresses basically the same issue of reducing the NOx contaminates. This system uses a solution of 32.5% urea and 62.5% of denatured water called DEF (Diesel Exhaust Fluid). This blue fluid is contained in a separate holding tank that is injected into the exhaust. When the urea mixture meets the hot exhaust gasses it decomposes to ammonia (NH3) and CO2. The ammonia then reacts with the oxides of Nitrogen in a second catalytic converter to form a harmless output of Nitrogen and water. The advantage is not only a reduction in NOx, but a reduction in the use of the EGR, which means a more efficient combustion, and a reduced PM output along with improved fuel consumption.
DEF and SCR common problems
A few of the problems generated by the SCR can lead back to the owner/operator or outside temperatures. One of the most common issues is letting the DEF to run low in the storage tank. This can cause a no start situation. Which seems to be a very common issue with some drivers who are not accustomed to the need of adding DEF in their vehicle. Even though a warning light will come on stating the level is too low some drivers still ignore it as just another one of those silly warning lights.
Another common problem is using a cheaper grade of DEF that does not meet the 32.5% mixture level.
For most manufactures there is about a 5% leeway in the amount of urea to water mixture allowed. This can easily be checked by using a refractometer to check the actual mixture level. Also, it’s better to purchase the DEF in one gallon containers or even smaller quantities. Larger bulk storage DEF containers that sit out in the hot sun can actually have a lower water content than the jugs of DEF kept on the store shelves. This is due to the evaporation of the water leveling and a higher concentration of urea.
One other problem is in severe cold temperatures where the DEF can freeze in the lines or in the injector for the DEF. In some cases thawing it out will work sufficiently, but in rare cases the cold will expand the water to a point where it can crack the plastic housings of the injector or holding tanks.
Newer systems are being developed even now that may reduce the use of the EGR system all together. Some systems may incorporate new sensors and new processes for cleaning the environment that may eliminate the need for the regeneration process. But there’s thing for sure, the diesel is here to stay.