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Tech Tips: Clean diesel

With so much focus on carbon emissions over the last few years, it is sometimes hard to remember that we all used to be very concerned about smog-causing nitrogen oxide (NOx) emissions.


April 19, 2011
By Patrick Flannery


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With so much focus on carbon emissions over the last few years, it is sometimes hard to remember that we all used to be very concerned about smog-causing nitrogen oxide (NOx) emissions.

diesel 
LEFT: EGR stands for exhaust gas recirculation. A portion of the exhaust is recirculated back into the combustion chamber, which lowers the combustion temperature and reduces the formation of nitrogen oxides. EGR engines have additional manifolds and plumbing. Diagram courtesy of John Deere. RIGHT: Selective catalytic reduction (SCR) engines spray a mist of a chemical catalyst called DEF, or aqueous urea, into the exhaust stream to react with the nitrogen oxides and lower tailpipe emissions. These engines must carry an onboard supply of DEF and will not work if the tank runs dry. Diagram courtesy of John Deere.



 

When fossil fuels are burned, they give off nitrogen dioxide and nitric oxide as byproducts in the exhaust. When sunlight hits these chemicals they react, which produces ozone. That’s OK, because we like ozone, right? Well, not really. Ozone is helpful when it is in the upper atmosphere blocking UV rays. When it is down here where we are it causes a number of respiratory problems and is a prime component of smog.

Because of these concerns, North American environmental authorities started phasing in ever-more-restrictive regulations on skid steer diesel engines way back in 1996. These regulations reduced the amount of particulates and NOx that newly manufactured diesel engines were allowed to emit in stages called tiers. Tier 1 engines were allowed to emit around nine grams of NOx per kilowatt-hour of power produced. Tier 2 came into effect between 2001 and 2003 and dropped the allowable emissions to just over six grams. Tier 3 took us down to four grams in 2006. Now, all 2012 models entering the market must be compliant with the so-called Tier 4 Interim standard, which pegs allowable emissions at two grams per kilowatt-hour. 2014 will mark the final stage, Tier 4 Final, when engines will barely be allowed to emit any NOx at all.

p16_tier4-engine 
Manufacturers are saying their new Tier 4 engine designs will meet or exceed the performance of Tier 3 designs in such areas as cold-weather starting, transient response, power bulge, peak torque and low-speed torque. Photo courtesy of John Deere.


 

The American Environmental Protection Agency developed the standards, but Environment Canada has adopted them and brought them into effect in lockstep with the EPA’s moves. All 2012 model diesel-powered skid steers sold in Canada must comply with the Tier 4 Interim standard. Anyone selling or operating equipment in contravention of Environment Canada regulations can be charged under the Environment Protection Act and fined or even jailed.  

Diesel engine manufacturers were able to meet Tier 2 and 3 standards simply by building better, more sophisticated engines with electronic controls, higher pressure fuel injection systems and turbocharging. Not any more. New engines will either have to recycle part of the exhaust back into the combustion chamber with an exhaust gas recirculation (EGR) system, or mix a chemical catalyst into the exhaust using a selective catalytic reduction (SCR) system. Whatever kind your next new skid steer comes with, you will have some learning to do about how to maintain and troubleshoot these new engine designs.

Here is how the Diesel Technology Forum, a joint think tank made up of five industry associations including the American Rental Association, described the differences between older diesel engines and the new, Tier 4 designs.

The changes most likely to be noticeable are in the packaging and the increased size of the air intake system to accommodate the needs for increased airflow and cooling. New changes to the engine will likely mean that engine compartments may be reworked to manage the new systems. Some OEMs have indicated they will package any new exhaust system configuration inside a reworked sheet metal skin, while others will place the systems in their traditional locations with additional shielding and mounting hardware to accommodate the heavier exhaust system components.

Most Tier 4 engines will be electronically controlled, meaning that a computer will monitor and adjust the fuel and air mixture to optimize emissions and performance for the engine on a real-time basis. In addition, changes in the engine will include new and different systems to accommodate the increased heat rejection of the new engines. For the first time, most off-road equipment will likely incorporate emissions control technology in the exhaust system, such as a catalytic converter and/or particulate filter, probably in place of the existing muffler and exhaust system. Some of these new exhaust aftertreatment systems mean that the pipes and placement of the muffler and exhaust may be different than previous generations of equipment, or potentially larger to accommodate the new functions and (in some cases) hotter temperatures of the exhaust.

Some Tier 4 engines will include use of cooled EGR. EGR is a technique that recirculates a portion of the exhaust gases back into the combustion chamber, which has the effect of lowering the combustion temperature and reduces formation of NOx. This system will add additional manifolds and plumbing around the engine.

One of the biggest changes for engine and equipment dealers is that some engines and machines will utilize a new emissions control technology system known as SCR. This technology is also designed to reduce emissions of nitrogen oxides. Widely used in Europe on heavy duty trucks and in some U.S. stationary industrial and power generation settings, SCR technology is new to the U.S. for mobile on-road and off-road applications in 2010. The majority of heavy-duty truck manufacturers began using SCR technology in the 2010 products, along with a number of light-duty diesel car manufacturers, and some manufacturers will use this in their off-road equipment offerings.

In this SCR system a special catalyst is positioned in the exhaust stream and muffler system downstream from the active spray dosing system that periodically sprays a mist of a chemical reagent (called diesel exhaust fluid (DEF) or aqueous urea) to react with the exhaust nitrogen oxides and lower tailpipe emissions. Depending on its size, a machine will have a storage tank holding up to 15 gallons of liquid DEF. The DEF dosing system, supply and return tubing and control monitoring functions are all integrated into the engine electronic controls. DEF consumption is dependent on equipment utilization, load factors and idle time. Manufacturers are optimizing SCR technology and DEF tank sizes such that DEF tanks need to be replenished in conjunction with key maintenance intervals. Indicator lights on the dash will warn the operator when the DEF supply is running low and should be replenished. If it is not replenished, upon a series of start-ups, the machine will eventually revert to a “limp” mode where engine performance is de-rated until the fluid is replenished and the integrity of the emissions control system is restored. DEF supply has been growing for the on-highway vehicle market. It is generally expected to be more widely available as more engines and vehicles that require it are produced.
Tier 4 engines and machines may have other differences depending on the manufacturer. These could include changes in horsepower ratings, smaller engine displacements, differing power and torque performance and higher fuel economy.

However you feel about lowering NOx emissions, it seems clear that the new systems are going to represent one more thing that can go wrong. To protect your customers’ peace of mind, and your bottom line, you may want to work with your skid steer supplier to become educated about performance and maintenance on the new Tier 4 diesel engine designs.


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