Monday, June 15, 2009

LMM Duramax Diesel Emissions System

Some time ago, there was a love-hate relationship with the Clean Air Act and many mechanics and owners who tended to view diesel engine emissions controls as evil. That was because most carburetors and vacuum control systems were not terribly reliable: as a result, fuel economy and survivability suffered. Entering the 80’s and the era of fuel injection, emissions systems suddenly became more reliable and usually improved the overall performance of an engine; at the very least, they did not hinder it. Computer controls allowed for much more precise engine operation. Despite this, diesel engine emissions controls sometimes still have that original stigma attached to them. In the early years, people often solved their drivability problems by removing all the emissions controls, retuning the engine’s performance – and pollution output – to pre-emission levels. This was illegal then, of course, and remains illegal today even though some people still remove EGR valves and catalytic converters.

In heavy-duty pickup trucks equipped with emissions controls, the diesel engine has avoided virtually any emissions control equipment for many years. In 2001, the Federal Emissions-equipped Duramax (the original LB7) did not have a catalytic converter, EGR valve, or even a PCV system. Crankcase oil vapors were dealt with by a separator and a road draft tube, similar in design to tractors! That produced many complaints of a hot-oil smell after prolonged engine operation due to the fact that some oil residue ended up leaking out of the road draft tube. This brings up an interesting point in emissions systems: believe it or not, your sense of smell is one of the best judges of noxious truck emissions. Testing equipment is necessary to determine quantities of pollutants and to tune modern engines, but the final result can often be assessed with our nose.
The diameter of a single PM2.5 particle found in the LMM exhaust (pre-filter) is 2.5 microns (represented by the smaller, red circles), such that it takes 28 of these particles, lined up end-to-end, to equal the diameter of a human hair (green) which is approximately 70 microns. The larger, white circles represent PM10, or 10-micron, particles. The above graphic, based on an EPA figures, is magnified approximately 625 times actual size. In other words, it would take 625 2.5-micron particles, lined up end-to-end to equal the diameter of ONE of the red circles.

The most obvious addition to the LMM’s emissions control equipment is the Diesel Particulate Filter (DPF). The Diesel Oxidation Catalyst (DOC) and the DPF work together to reduce harmful particulate emissions.

These sensors help the ECM to monitor the restriction in the DPF as well as the effectiveness of the regeneration cycle: A. Differential Pressure Sensor Fittings; B. Differential Pressure Sensor; C. Exhaust Gas Temperature Sensors; D. Diesel Particulate Filter (DPF).
All that laissez-faire approach ended in the 2004 model-year. Federal law tightened regarding diesel engine emissions. For Duramax LLY equipped trucks, a catalytic converter was added to deal with unburned hydrocarbons (HC) – mostly remnants of diesel fuel not consumed in the combustion process. An EGR valve was introduced to reduce oxides of nitrogen (NOx). The engine itself, the head design particularly, was radically changed to help reduce NOproduction in the combustion chamber and to allow for a PCV system. The Duramax diesel no longer produced the hot-oil smell when warm and was cleaner running overall. Changes included moving the injectors from inside to outside the cylinder heads. The primary reason for this change was not to make them more serviceable but to ensure that diesel fuel leaks would not fill the crankcase with fuel. With a closed-loop PCV system, diesel fuel leakage into the crankcase can get pumped out of the PCV system and into the air intake. That is bad news for a diesel, as raw fuel ingested into the intake manifolds will auto-ignite in the engine and cause a runaway condition.
Moderate improvements in NOwere made with the LBZ Duramax in 2006 with a bigger EGR valve and a lower compression ratio, but the real quantum-leap occurred mid-2007 with the introduction of the LMM Duramax. Federal law mandated a huge reduction in particulates (90% reduction) and NO(50% reduction) over 2004 regulations. Particulates are the visible portion in black smoke, thus named because they are very small particles of carbon (soot). These particulates are a component of smog and have also been proven to cause respiratory problems.
Most fine particulates are not always that visible. Diesel engines under normal “non-smoking” operation will still emit a large quantity of small particulates – around 2.5 microns (about 1/28th the diameter of a human hair) across and even smaller. These fine particles do not just contribute to smog, they also pose other health risks. Because they are so small, they can get right into our lungs, bypassing all of the natural processes in our body designed to filter out particulates. At the very least, they can cause irritation of the airways and coughing. They can also cause more serious issues, like chronic bronchitis, or aggravate existing health concerns like asthma. In the worst-case, long term exposure to fine particulate matter can cause premature death in those with lung or heart disease.
Of further concern are the oxides of nitrogen themselves. NOreacts chemically with other pollutants in the atmosphere, creating its own particulates, including smog, which contribute to respiratory problems. And that is besides the damage done to our surroundings by acid rain. If that wasn’t enough, NOreacts to create ground level ozone, which can also cause respiratory problems. Smog is just the visible portion of the fine particulate matter that can contribute to health problems. The noxious chemicals that we can not see are just as dangerous. In the future, even more aggressive diesel engine emissions controls will be mandated to reduce these nasty group of pollutants.
The bad news is that the diesel engines we love are among the worst contributors to these kinds of pollutants. The very way that a diesel engine operates makes it prone to producing NOand fine particulates. Because of the composition of diesel fuel itself along with high cylinder pressures and the injection strategy, NOx, fine particulates, unburned hydrocarbons and sulfur dioxides are naturally produced by the combustion cycle in relatively large quantities. Improvements have been made by changing to common-rail fuel injection and introducing an EGR valve, but there is a physical limit to what can be done with the diesel engine combustion process. The only remaining option is to clean up what comes out. Duramax owners – welcome to the wonderful world of exhaust after treatment.
It is worth noting that all truck manufacturers are in the same position relative to diesel emissions. Ford and Dodge both have completely redesigned engines and exhaust systems. GM has only had to modify the existing Duramax engine to work with an exhaust after treatment system, a complete redesign of the engine was not necessary for now. In fact this new standard (modified slightly for larger engines and trucks) even applies to highway tractors. Caterpillar, Cummins, Isuzu, Mercedes, and others have all engineered exhaust after treatment systems. All of these systems have similar operation and limitations to what is described further in this article. The exception is the Cummins Bluetec diesel, which already meets the 2010 standard and has an additional NO-reducing catalyst in the exhaust. This additional component and the reductant (fluid) required for it’s use will be fully implemented on all highway diesels by 2010.
Stopping the Fine Particulates
No matter what anyone does, a diesel engine will produce fine particulates, so fine we can not see them in the exhaust. Just take a look at any conventional diesel exhaust system – there is always a coating of black carbon on the inside of the pipe: clear evidence of passing particulates. Particulates can be limited somewhat by the way a diesel engine is set up, but to reduce particulate output by 90% requires the installation of something that is able to trap these particles.
Enter the Duramax Diesel Particulate Filter (DPF). It literally is a filter, designed to handle the extreme temperatures of diesel engine exhaust. It is made out of a porous ceramic substrate. The basic design is similar to the air filter on the LMM Duramax, known as a wall-flow filter. Exhaust gases enter one end of the filter and run down long chambers. The wall between the chambers are porous, and the exhaust gases migrate through the side-walls between chambers and then exit the other side of the filter. Soot accumulates at the far end of the chamber which is blocked. As soot accumulates, the filter becomes more and more restrictive. It does not take a great deal of time to begin to restrict the DPF to the point where it could begin to adversely affect engine operation; in fact this threshold can be reached in less than 375 miles!
In order to clean the soot out of the filter, the engine control module commands a special cycle known as regeneration. The regeneration cycle is designed to reduce the soot down to fine ash. By heating the particulate filter to around 1,100°F, the soot is burned down leaving only a white ash residue. Over time, this ash does accumulate in the filter and will need to be removed during the service life of the vehicle. GM’s initial service-life expectations for the DPF were around 100,000 miles.
In order to heat the exhaust to that incredible temperature, the Engine Control Module alters the injection timing to occur much later and adds post-injection, that is, injection events that occur during the exhaust stroke. These events send unburned diesel fuel down the exhaust pipe. After going through the manifolds and the turbocharger, this unburned fuel, along with excess oxygen, is consumed in the catalytic converter. This raises the temperature of the catalyst to 1,100°F or more. Not far behind the catalyst sits the DPF, which gets superheated by the catalyst reaction. It takes approximately 20 minutes of straight highway driving for the DPF to regenerate.
A view of the working media of the Diesel Particulate Filter. Every second port one end is closed at one end and open at the other end, forcing the exhaust gases through the wall of each chamber as illustrated below.

The temperatures coming out of the DPF are also quite high. In fact, exhaust temperatures get so high on the LMM Duramax that the entire exhaust system had to be redesigned with much better materials in order be reliable. Stainless steel runs throughout and much of it is double-walled. The exhaust tip is also designed with a cooling venturi, a device to bring in outside air to help cool the exhaust gases. Even so, if you were to stand right at the tailpipe during a regeneration cycle, you could get seriously burned. Tailpipe temperatures can still exceed 500°F.
In order for the ECM to effectively monitor and control the regeneration cycle, additional sensors had to be placed in the exhaust system. There are two Exhaust Gas Temperature sensors, one in front of the DPF and one after it. These sensors monitor how well the catalyst is heating the DPF to burn down the soot. They are also used in diagnostics in order to determine the effectiveness of the catalyst itself and if it has been damaged. There is also a differential pressure sensor, which measures the restriction of the DPF. Higher restriction means that it is loaded up with soot and will require a regeneration. This sensor also can be monitored to determine the effectiveness of the regeneration cycle.
The operation of this system is entirely dependent on the catalytic converter. In technical terms it is known as a Diesel Oxidation Catalyst, or DOC. Its primary function is to produce the extreme temperatures required to regenerate the DPF. When it is provided raw fuel and oxygen from the late-injection process, it converts them to carbon-dioxide and water vapor. In the process, it produces a great deal of heat. This catalyst also provides a secondary benefit: it reduces unburned hydrocarbon and carbon monoxide during normal engine operation. The DOC looks very similar to any typical catalytic converter, with a flow-through, honeycomb-style, ceramic substrate. This ceramic structure is coated with a layer of materials including Platinum and Palladium. They provide a catalyst for the chemical reaction that occurs within the DOC.

Reducing Oxides of Nitrogen

Oxides of nitrogen are formed under high temperatures in the combustion chamber of any internal combustion engine. The higher combustion pressures of diesel engines, relative to gasoline engines, make them even more prone to the production of NOx: as pressure goes up, so does temperature. The audible knock characteristic of a diesel engine is a sign of the presence of a high spike in combustion pressures which creates localized zones of very high temperatures in the combustion chamber. The high temperatures cause an endothermic reaction between free Nitrogen (N) in the incoming air and Oxygen (O2), producing varieties of chemicals which are known as Oxides of Nitrogen (NOx).
With the introduction of the LMM Duramax, several changes were made to help reduce the formation of NOx. Fuel injectors went from a seven-hole to a six-hole nozzle, presumably to reduce the number of flame fronts in the combustion chamber, which can reduce the local pressure and temperature spikes where the flame fronts collide. Injection calibrations were changed to minimize peak cylinder temperatures to slow down NOproduction. Essentially, a smoother peak in the combustion process reduces the formation of NOx. As an additional benefit, the engine itself runs quieter.
An Exhaust Gas Recirculation (EGR) valve reduces peak combustion temperatures even further. Exhaust gas is inert, or non-reactive, and composed primarily of carbon dioxide and water vapor. As a result, they quench the combustion process slightly, reducing the percentage of oxygen available for combustion and absorbing some of the heat produced by the combustion process. The EGR system on the LMM Duramax was modified from previous models with a larger-capacity cooler to allow for more EGR flow.
Another interesting change in the LMM was the addition of an air intake valve, which acts as a throttle valve. It stays wide open most of the time, but is used to manage airflow during regeneration and EGR operation. During regeneration, there is a narrow temperature window to shoot for, and the catalytic converter needs to be controlled to that temperature. Think of the air intake valve as the damper on a wood stove – it regulates oxygen available for combustion. The DOC is basically like that wood stove – it needs regulated amounts of oxygen and fuel to burn at the correct temperature. That is where the air intake valve comes in, primarily as a damper valve to help regulate the temperature of the DOC during a regeneration event. The air intake valve is also used to provide a pressure differential between the air intake and the exhaust system so that the EGR system can precisely regulate the exhaust gases that are recirculated into the engine.
As if the emissions system on the LMM diesel does not seem complex enough, keep in mind that future versions of the Duramax will get even more complex. The EPA has mandated further reductions in NOproduction to be phased in for 2010: another 80% reduction in NOwill be required over and above the 2007 standard. As mentioned earlier, this will necessitate the addition of a urea-based, selective catalyst reduction system to the exhaust system, with further sensors, controls and diagnostics.

Changes Made to Accommodate the After Treatment System

If this new exhaust system is starting to look like a complex chemical conversion plant, that is because it is. In order for the system to remain healthy, care needs to be taken to insure that the chemicals coming into this system are compatible with all of the materials used. The new catalytic converter is very sensitive. It can be compromised by deposits that can form on the materials due to the sulfur content of diesel fuel. This is the primary reason that a change was made in the fuel requirements for the LMM Duramax. Ultra-Low Sulfur Diesel (ULSD) fuel is mandatory, which has a maximum sulfur content of 15 parts per million. LMM owners will notice special labels both on the instrument panel and the fuel door, specifying the need to use ULSD fuel. It is important to differentiate Low Sulfur Diesel Fuel and Ultra-Low Sulfur Diesel Fuel. Low Sulfur Diesel fuel has a maximum sulfur content of 500 parts per million. In the US, Low Sulfur Diesel Fuel can still be obtained until December 2010, at which point ULSD will be mandatory to provide at the pumps. In Canada, ULSD has been mandatory since September 2006.
The only way one can really differentiate ULSD and LSF is by the labeling at the pumps. Off-highway, or dyed fuel, can be either ULSD or LSF. Prolonged use of LSF fuel will harm the catalyst, which compromises the operation of the rest of the system. Trouble codes will set and the DPF will eventually plug up with soot, causing drivability problems.
A special venturi tailpipe draws in outside air to help cool superheated exhaust gases that are produced during the regeneration cycle.

Inside the Diesel Oxidation Catalyst, unburned Hydrocarbons (HC) and Carbon Monoxide (CO) are converted to Carbon Dioxide (CO2) and Water (H20). This process also creates the heat required for the DPF regeneration cycle when excess fuel and Oxygen are supplied to the DOC. Particulate Matter (PM) passes through the DOC and is trapped by the DPF.
The LMM Duramax requires Ultra-Low Sulfur Diesel (ULSD) which has a maximum sulfur content of 15 parts per million.
Some people are concerned that reducing the sulfur content of diesel fuel will adversely affect the lubricity and cause damage to fuel system components. Much research has been performed by engineers to ensure that this new fuel is backward compatible with older diesel fuel systems. The LMM Duramax is specifically designed to run ULSD fuel and there is no concern with running ULSD on older Duramax engines either, despite some marketing attempts to convince people of the harmful effects of lower lubricity of ULSD. In fact, back in 2005, ASTM-D975 came into effect which in part regulated the lubricity of diesel fuel. Fuel manufacturers have been obligated to meet that standard despite moving to ULSD. That means that other minor additives are included in ULSD to restore its lubricity, allowing GM engineers to recommend that ULSD be used on all GM diesels including the 6.2, 6.5 and all 6.6 Duramax engines. As an additional benefit, it has been noted by EPA studies that switching an older diesel engine to ULSD will in itself reduce particulate and NOemissions by 10 to 20 percent.
Another necessary change was the introduction of low-ash diesel engine oil. All engines consume a small amount of oil, and DPF equipped engines need oil formulated to minimize the possibility of damage to the exhaust after treatment system. API CJ-4 has a maximum ash content of one percent and is the engine oil specified for the LMM Duramax. Remember, the DPF loads up with ash over time. If the engine oil contributes to the ash-loading, that will reduce the service life of the DPF. It is important to use CJ-4 oils to maximize the DPF's service life.

The Duramax LMM in Real World Driving

All this complexity is sure to result in some compromises in real-world use, and it does. Due to the fact that the regeneration cycle uses diesel fuel, fuel economy suffers significantly. Compared to the prior generation LBZ Duramax, the LMM’s fuel economy is down approximately ten percent. That can be a bitter pill to swallow for LMM owners depending on the price of diesel. Also factor in that the initial cost of buying a diesel is now slightly higher due to the added complexity of the exhaust. Diesel owners are accustomed to recovering the traditionally higher initial cost of a diesel vehicle over time by having lower operating cost – at least that is the way it has been. At the current cost of fuel, a diesel engine is generally more expensive to operate than a gasoline engine, and that is without factoring in the loss in fuel economy created by the exhaust after treatment system. Essentially, the diesel engine just became more expensive to purchase and more expensive to operate. There is a positive result though - less damage done to the environment and our health.
What about the additional carbon dioxide produced by the LMM? If the engine consumes more fuel than the prior generation then it has to produce more carbon dioxide, a greenhouse gas. The answer is yes, it does. There is a trade off: with 90 percent less particulates and 50 percent less NOcomes ten percent more carbon dioxide, roughly speaking. By the numbers, that is a more than acceptable trade off: especially in areas with higher density populations that struggle with smog.
Another unexpected challenge that some customers face is the apparent unpredictably of the regeneration cycle. It requires that the truck run for 20 to 30 minutes at speeds greater than 30 MPH. For some delivery truck or service truck operators, those conditions are simply not practical. They are running at slower speeds most of the day, with the engine idling much of the time. Often the Clean Exhaust Filter message will appear on the driver information center. Continuing to run the truck without being able to do a regeneration can set a trouble code and put the truck into limp mode. Not a particularly practical result for individuals using their truck for this kind of work.
GM did foresee this condition on medium-duty trucks – they are equipped with a push button on the dash which will command a manual regeneration cycle while the truck is stationary: the process takes around 20 to 30 minutes to complete. However, the Chevy Silverado and GMC Sierra pickup trucks do not have that feature. There are only two alternatives to regenerate the DPF for Silverado/Sierra owners: take it for a 30 minute drive on the highway – which may or may not work - or take it to the dealership where a technician will perform a procedure with the Tech II scan tool. GM does cover a service regeneration at the dealer under warranty – but I am fairly confident that they will not reimburse you for your time.
Additionally, extreme cold can create problems. It has been my experience that it is possible for exhaust water vapor to freeze in the DPF under certain conditions and completely plug it off. The engine will either not start or barely pull the truck around. With all the idle time that trucks see in cold conditions, the DPF gets plugged with soot far sooner and has to regenerate far more, which crimps fuel economy even more than usual.
These compromises, built into the LMM, should make customers carefully evaluate their needs and determine whether or not they really need or can use these new diesels in a practical way. We have built up a diesel is cheaper mentality since the Duramax came out because of its lower total cost of operation over the long haul. The Duramax boasts awesome torque, good horsepower and great fuel economy compared to gasoline-powered vehicles. Yes, a diesel engine costs more to buy, but many buyers considered that the higher initial cost was well worth the benefits. The Duramax is still the king of torque compared to gasoline engines and still returns reasonable fuel economy. However, the threat of an increased cost for diesel fuel, along with the inconveniences of expanded truck emissions systems may push some traditional diesel customers to consider smaller diesel engines or even gas-powered vehicles. Consider that the Duramax/Allison option adds $8,395 to the sticker price of a pickup truck.
That last paragraph may sound like sacrilege to readers of this magazine – and I do not like saying it – but it is realistic. I have been with the Duramax since the beginning, and this is the first time, because of the additional costs associated with the new diesel engine emissions controls, that I would seriously consider not purchasing a new one.
What about throwing the exhaust system away, replacing it with a turbo-back aftermarket system, and retuning the ECM to shut down anything related to the DPF? I can honestly say that this would make a considerable improvement in driver convenience and fuel economy. But there are three serious things to consider when making a decision like that.
First and foremost, this defeats a system that works extremely well to reduce chemical pollutants that have a very negative effect on people and the environment. Remember the nose test mentioned at the outset? Stand behind a running LMM Duramax, and you will find that it is significantly better than any older diesel. In fact, they are not even in the same league. The new diesel is so much cleaner that you will generally not even see black on the tailpipe. Some may argue when in a low population density environment that running a diesel without the DPF system is not likely to cause any real harm and may even reduce carbon dioxide production - greenhouse gases. Perhaps there is a small element of truth to that, but there can be no question that the DPF system is a benefit to the overall environment.
Secondly, to do so violates Federal Law. Some owners will remove their stock exhaust system and store it, just in case they are forced to reinstall it at a future date. If they are willing to overlook the obvious increase in diesel engine emissions that this causes.
Finally, to do so violates the terms of GM’s warranty agreement. As the previous issue of maxxTORQUE points out, GM has the technology in place and is beginning to enforce this more and more.

Looking Forward, Not Backward

It is easy to see where some may begin to take a dim view of these new truck emissions systems. The old stigma of diesel engine emissions controls has found new life with the new DPF equipped diesels. Diesel forums often hum with owners intent on removing the DPF systems in modern diesel trucks. It will likely take some time to really smooth out their operation, much as gasoline emissions controls were a little bumpy starting off in the ‘70’s. As it is right now, GM has released a considerable number of software updates to improve the operation of the LMM engine. Looking into the future, the complexity of exhaust after treatment will only increase. However, engineers are now optimizing the overall design of the engine to include these systems. GM is also working hard on a redesign for the 6.6 Duramax – 2010 is coming and emissions regulations will get tighter yet.
One thing is certain though – these new diesel engine emissions systems will pay good dividends with the environment and public health. The bottom line: make sure that you evaluate the cost/benefit ratio for yourself before buying a new Duramax diesel. If you do, and are prepared for the changes in how the engine operates, you will be quite satisfied.


  1. what happens if you take the stock tip off

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