By Paul Abelson
senior technical editor
Coming to a truck near you – EPA 2010. That may sound like the trailer pimping the next big Hollywood sci-fi thriller. Science it is, but fiction it’s not.
It’s the EPA-mandated regulations package that dramatically limits nitrogen oxide (NOx) in diesel exhaust. And, as with the first two installments of the EPA emission trilogy, a debate is raging among engine makers about the best way to meet EPA 2010 emission standards.
Navistar International is promoting the use of enhanced exhaust gas recirculation (EGR for short), termed the in-cylinder solution. All others, Cummins, Detroit Diesel, Mack, Paccar and Volvo, will use selective catalytic reduction – or SCR – a chemical treatment of the exhaust downstream of the engine.
In order to better understand these alternative solutions, you have to understand the nature of the problem.
EPA proposed regulation of diesel exhaust in 1988, and the first regulations applied to 1994 engines. The 2004 engine standards, first enforced in October 2002 because of civil action against some engine makers, severely limited NOx.
Exhaust gas recirculation was adapted by most engine makers to limit NOx, which contributes to smog and is a respiratory irritant. In 2007, particulate matter, primarily soot, was the next target. Particulate matter is made up of fine solids that damage lungs when inhaled. Diesel particulate filters control particulate matter.
Difficulties arose because what reduces particulate matter during combustion increases NOx, and what reduces NOx increases particulate matter. NOx is created when nitrogen and oxygen, the main components of air, are subjected to high pressure and high heat during combustion.
Particulate matter is formed when hydrocarbons (diesel) burn incompletely, leaving a residue. To control soot, a hotter flame is needed, but this increases NOx formation. Cool the flame to control NOx and more soot is left.
For 10/02, to meet 2004 standards, all engine makers except Caterpillar started re-circulating exhaust gas. By drawing some of the already burned – and therefore inert – gas from the exhaust, cooling it and injecting it into engine intake, they lowered the oxygen concentration, which slowed the combustion process. They had to regulate NOx formation to 1.2 grams per horsepower per hour.
Caterpillar’s process is ACERT, using high pressure, finely tuned fuel injection and intake air management to reduce NOx.
For 2007, emphasis was on controlling soot. Allowable particulates were limited to 0.01 g, about 1/60th of pre-regulation levels. DPFs physically remove solids from exhaust. Particulate matter standards remain unchanged for 2010.
Reducing NOx is the objective of the 2010 standard – limiting it to 0.20 g.
And, once again, engine makers have alternative strategies to achieve the mandated levels.
International’s in-cylinder solution calls for up to 45 percent EGR without SCR. With advanced EGR, no additional hardware is required. An SCR system can add several hundred pounds to a truck and, according to an announcement made by Volvo, cost close to $10,000.
SCR requires diesel exhaust fluid – DEF for short – which is a mix of purified urea and deionized water. The driver has the burden of keeping the DEF tank full. Frequency of topping off DEF depends on fuel consumption and tank size, but projections are that DEF will be needed every other fill-up.
While simpler and less expensive, EGR also has disadvantages. It raises under-hood temperatures. Cooling systems must be beefed up, offsetting some of EGR’s weight and cost advantages. Excess heat caused reliability problems for early 2004 engines.
EGR reduces combustion efficiency, cutting fuel economy and requiring larger displacement engines to maintain power ratings. International will introduce a 15-liter MaxxForce engine based on Caterpillar’s C15 block with International’s combustion technologies. Caterpillar will not offer its own 2010-compliant engine.
The technology other engine makers use, SCR, has been in Europe for most of this century. It converts the urea in DEF to ammonia that reacts with a catalyst and NOx, which changes NOx to nitrogen and water vapor.
DEF has 32.5 percent high purity urea. About one gallon of DEF is injected into the SCR assembly for every 50 gallons of diesel used. According to Material Safety Data Sheets from one DEF producer, DEF is considerably less toxic than windshield washer fluid.
DEF is expected to be priced lower than diesel when dispensed in bulk. It will be available in jugs and bulk. Major truck stops, truck dealers and distributors have committed to having DEF available by next year.
Advantages of SCR are improved fuel economy, cleaner combustion and longer maintenance intervals. SCR allows engine makers to dial back the amount of EGR needed, and initial testing indicates a 4 to 5 percent mpg improvement compared with 2007-compliant engines. Also, less soot will be in cylinders, so engines may last longer and require less frequent oil and filter changes.
Regeneration of the DPF is reduced, contributing to improved fuel mileage. The more soot is in the exhaust, the faster the DPF will clog. To free exhaust flow through the porous ceramic filter element, fuel is injected over a catalyst and into the DPF. Temperature rises close to 1,200 degrees, converting all the trapped soot to carbon dioxide gas. While efficient for removing soot, regeneration burns fuel without contribution to motive power.
With a pair of 120 gallon fuel tanks, you’ll use 4 gallons of DEF between fill-ups. A 10 gallon tank, holding a bit more than 92 pounds of DEF, will take you to your second fill-up with fluid to spare.
If you run out of DEF, the EPA has mandated power reduction. First, you’ll get a series of warnings that the DEF tank is running low. When it gets to 5 percent of tank volume, torque will be reduced by 20 percent. If, after all the warnings, you manage to run out, you can still drive, but at only 5 mph.
DEF turns to slush, and freezes solid when cold-soaked to minus 20. Engines will operate with slushy DEF. Tanks have heaters to keep the fluid flowing when the engine runs. It will start and run until enough DEF liquefies. EPA allows it because NOx formation is greatly reduced if the engine isn’t at operating temperature, by which time DEF will flow.
Rumors about DEF becoming toxic above 118 degrees are inaccurate. Concentrated urea is often stored or shipped at temperatures to 140 degrees. The amount of free ammonia released from DEF at 122 degrees would take two years to equal the concentration in household ammonia.
Those are the facts.
What we don’t yet know is how these claims will be quantified. If heavier cooling systems are needed with EGR, what will be the net overall weight savings? If fuel economy is indeed better, by how much? Will it be enough to offset the added cost of the SCR system? That will depend on fuel prices over the life of the truck. Will fuel prices rise again? By how much? Will there be supply problems for DEF, or, as others state, will DEF be readily available at a reasonable price?
These all remain to be resolved – not by claims and counterclaims, but in real-world experience. Stay tuned. 2010 will be an interesting year. LL
Paul Abelson can be
reached at firstname.lastname@example.org.