By: Allen Zeyher
IBM or Mac? Paper or plastic? VHS or Betamax? Republican or Democrat? EGR or SCR? Like a two-pan balance for comparing weights, the human brain seems to be able to weigh two—and no more than two—competing options at a time.
Given a problem, the free market seems to naturally develop two competing solutions. The problem for trucks and heavy construction equipment is cutting emissions of oxides of nitrogen (NOx) and particulate matter from their smokestacks.
The competing solutions for on-highway trucks are exhaust gas recirculation (EGR) and selective catalytic reduction (SCR).
For on-highway trucks, as of Jan. 1, 2010, the maximum NOx level allowed will be 0.2 grams per brake horsepower hour (g/bhp-h). The Environmental Protection Agency (EPA) has allowed engine manufacturers to phase in their compliance with the new standard. Since 2007, engine makers have only been required to have half of their engines meet the 2010 standard for NOx; the other half could meet the old 1.2 g/bhp-h standard.
The standard is the same for all engine sizes. The higher the horsepower, the more NOx they are allowed to emit.
Dichotomy
Another substance in on-highway truck exhaust to be subject to regulation is particulate matter. The maximum allowed level of particulate emissions has been 0.01 g/bhp-h since it took effect in 2007. From 2004 until 2007, the standard for particulates was 0.1 g/bhp-h. The primary factor that made it possible to meet the 0.01 standard was the switch to ultra-low-sulfur diesel fuel.
Off-highway vehicle engines, such as the engines found in dozers, excavators and graders, face similar emission standards, but three or four years later than the on-highway vehicles.
New regulations for NOx and particulates have been staggered, and the reason is that there is a sort of inverse relationship between the two.
“If you have a hotter combustion event, thereby burning more of the fuel, creating less particulates, the direct effect is the creation of more NOx—the classic conundrum of the engine manufacturer and the reason why the EPA typically doesn’t crank both of these things at once,” Tim Shick, director of engine marketing at Navistar, told Roads & Bridges.
Dilemma
Navistar’s solution to the problem of NOx formation is to cool and recirculate some of the engine’s exhaust gas back into the cylinders. The cooled exhaust gas, which is depleted of oxygen, displaces some of the oxygen in the combustion cylinders, so there is less oxygen available to form oxides of the nitrogen. The cooled exhaust gas also cools the combustion temperature in the cylinder, and cooler combustion also reduces NOx formation.
The next problem is that the cooler combustion produces less efficient burning of the fuel, so more of the unburned fuel goes out the stack as soot and other particulates.
Navistar’s solution to the problem of particulate formation is to increase the surface area of the fuel by increasing the pressure at which the fuel is injected into the cylinder. More pressure means smaller droplets of fuel, so a higher ratio of surface area to volume and more complete combustion.
Regular engines can generate high fuel pressure but only at high revolutions per minute (rpm). To get high pressure over the whole rpm range, Navistar uses a common-rail fuel system. The high-pressure common-rail fuel system is a relatively new invention. It is basically a sort of pressure vessel that feeds fuel into all of the cylinders. The vessel, or rail, is big enough that it can maintain its pressure while injecting fuel.
To maintain higher pressure in the fuel system, the parts have to be made to tighter tolerances and hardened for durability.
“These are very finely machined components that are hardened,” said Shick. “And the higher the pressure, the finer the machining, the greater the hardness.”
The preceding description is an oversimplification of the nuts and bolts of the EGR system developed to meet the EPA’s 2010 emissions regulations. Navistar is possibly the only engine manufacturer that will use such an EGR system.
Divergence
The majority of engine manufacturers are planning to use a system that treats the particulates downstream from the combustion chamber.
Cummins is a good example. Cummins Inc. introduced their 2010 on-highway engines in March. The ISX15 engine package, for example, incorporates an enhanced cooled-EGR system—without the high-pressure common-rail fuel injection—an SCR system and a Cummins particulate filter that was introduced in 2007.
The Cummins solution uses a particulate filter to collect and oxidize carbon to remove particulate matter from the exhaust. The SCR system sprays a fine mist of diesel exhaust fluid into the exhaust stream. The fluid is a 32.5% solution of urea in water. It is converted into ammonia through hydrolysis, and then the ammonia works with a catalyst to turn NOx into nitrogen gas and water vapor. Any remaining traces of ammonia are removed before the exhaust exits the system.
SCR is a more mature technology. The Europeans developed SCR years ago and only had access to high-pressure common-rail fuel systems with enough pumping capacity for a truck engine in the past three or four years, according to Shick. SCR has been used for years across the Atlantic in trucks and passenger cars.
“Navistar and Cat developed a rudimentary high-pressure common-rail system in the mid-90s, so we’ve had a lot of experience with it,” said Shick. “We’ve spent our millions of dollars in it, and that’s the key enabler for an EGR system.”
One of the main concerns about SCR in the U.S. is the lack of a serious distribution infrastructure for urea. Users of SCR-equipped trucks will have to periodically top off the urea tank, but the fluid is not currently widely available, such as at truck stops.
But it is only a matter of time before the urea distribution system is established for on-highway trucks as it is for diesel passenger cars.
A more important consideration is the cost to purchase and operate a truck with EGR versus SCR. The manufacturers interviewed by Roads & Bridges had not yet determined the purchase prices for their 2010 engines but expected them to increase a bit because of the added technology, but they expressed their estimates of operating costs.
“Operating costs will decrease, due to improved fuel economy and no active regenerations of the diesel particulate filter,” Jim McNamara, a spokesman for Volvo Trucks, told Roads & Bridges by e-mail.
Cummins claimed that its SCR-equipped engines will provide better fuel economy, better performance and better reliability, compared with the company’s current ISX engine.
McNamara suggested SCR might actually increase the reliability of Volvo’s 2010 engines: “SCR actually reduces the stresses on engines by reducing the rate of EGR used. This produces less thermal stress and reduces cooling requirements.”
“In fairness to the SCR, we will concede that the SCR engines can use somewhat less diesel fuel,” said Shick. “But what we also say is that . . . if you save 2%, or 2 gal for every 100 that you burn, you’ll probably have to add about 2 gal of urea for every 100 gal of fuel that you burn, so it’ll be offset by a like amount. We do not believe that there’s going to be any real cost benefit, even though there will be a fuel benefit to SCR.
“We do view the EGR approach, the approach that we’re taking, as the most customer friendly, because there isn’t the added bulk and weight nor the need for this replenishment of fluid, urea fluid.”
Other quirks of urea include freezing at low temperatures and vaporizing at high temperatures, so the urea tank will require some kind of provision for heating and cooling.
Urea also degrades over time, so it will have to be used before its “expiration date.”
Both EGR and SCR have their advantages and disadvantages. Each has its proponents and critics. Ultimately, the market will decide.
Roads & Bridges contacted Cummins several times about SCR, but was unable to arrange an interview.
