6.5 Chevy Diesel Rebuild, 6.2L Diesel Engine

LMM Duramax Diesel Emissions System

Written by Joel Paynton Thursday, 15 January 2009 14:13

Article Index
LMM Duramax Diesel Emissions System
Stopping the Fine Particulates
All Pages

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.

Duramax diesel particulate filter particles

Duramax diesel emmissions control system

Duramax diesel particulate filter sensors

(Click on any picture for more information or to enlarge)

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 (NO_x). The engine itself, the head design particularly, was radically changed to help reduce NO_xproduction 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 NO_xwere 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_x(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. NO_xreacts 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, NO_xreacts 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 NO_xand fine particulates. Because of the composition of diesel fuel itself along with high cylinder pressures and the injection strategy, NO_x, 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_x-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!