Playing It Cool On Emissions

Diesel Progress North American Edition, August, 2001 by Mike Brezonick

Cummins' road to '02 compliance goes through cooled EGR utilizing advanced Holset variable geometry turbocharging

Since the dawn of the emissions era, the engine business has been somewhat like a poker game. When a particular set of regulations is announced, there's a lot of talk and spin about what's going to happen and how the other guy isn't going to be able to bit the standards.

But once those regulations are finalized, there comes a point when the engine builders lay their technological cards on the table and let the industry and their customers know just how they're planning to meet them.

For the signers of the 1998 consent agreement, which accelerated the implementation of planned 2004 on-road emissions standards for some manufacturers to October of 2002, the last few months have been that time. And during that period, we've seen a number of engine builders explain their strategies for meeting the 2.5 g/bhp [NO.sub.x] and NMHC standard mandated by the consent decree.

One of the companies most out front in explaining how it plans to meet the requirements is Cummins. The Columbus, Ind.-based manufacturer recently showed members of the press the engine and component technology it will incorporate into its heavy-duty on-highway engines. And to paraphrase John Wall, Cummins' chief technical officer, cooled exhaust gas recirculation is the only real game in town.

"We've looked at a lot of different technologies," he said. "We've come to the conclusion that cooled EGR is really the only technology we see that is feasible to meet the regulations in the 2002 timeframe.

"EGR is all about controlling temperature,' Wall continued. "If you want to control [NO.sub.x] in-cylinder, you want to control the peak combustion temperatures and keep them from getting too high. So what you do is take some of the exhaust gas, cool it and put it back in with the intake air. Now the fuel that's burning is not only heating up the air and the fuel in the normal process, it's got the extra stuff in the combustion chamber that needs to be heated up as well.

"Because that soaks up some of the energy, you reduce the peak temperatures and you can reduce the [NO.sub.x] formation. [NO.sub.x] is really supersensitive to temperature, so you have a lot of leverage on [NO.sub.x] formation as you control temperature, How you do it is where it gets a little more complicated."

How Cummins plans to do it is through the use of an EGR cooler developea in cooperation with Behr America, which works with an EGR valve and Holset variable geometry turbocharger. But it isn't just a case of slapping a few extra components on and meeting the standards, according to Wall. "Emissions are a big deal, but we really work to keep our eye on the ball for what the customer is asking for," Wall said. "Emissions are pretty low on the list for most customers and we in the research area are often reminded by our marketing colleagues that EPA has never bought a single engine from Cummins. We are driven to a great degree by what our friends at EPA are doing, but they aren't our customer. Our customers are worried about things like fuel economy, power density, reliability, low maintenance, low cost, how well our controls are integrated."

The key is to be able to hit the standards without seriously affecting performance and fuel economy Wall said. "In the past, when you wanted [NO.sub.x] reduction, you used retarded timing," he said. "But if you just do that, you take a very big hit on fuel economy. The piston is already started on the downstroke before you burn the fuel, so you just can't get the energy out of the fuel. By the time you get to 3 grams [NO.sub.x], you're looking at a 10 percent hit in fuel economy And even if you wanted to, you couldn't get to 2 grams just by retarding the timing.

"There has been some talk about uncooled EGR. If you don't cool the exhaust gasses and you just put hot exhaust gas back in the cylinder, it will help you a little bit, but not too much. Since the whole point of putting it in is to reduce the combustion temperature, if you don't cool it, you're missing out on the main reason you're using that technique in the first place.

"If you go to cooled EGR, by the time you get down to 2.5 grams [NO.sub.x], you're looking at fuel economy probably on the order of a 3 to 6 percent reduction. Part of our continuing challenge is to figure out how to minimize this over the operating cycle. Our objective is to get a whole lot better than that."

In operation, an electronically controlled EGR valve will divert a portion of exhaust gas from the exhaust manifold to the cooler. From there, the cooled gas will be introduced to the intake manifold downstream of the compressor. "There are different ways of plumbing EGR systems," Wall said. "In low pressure systems, you just take exhaust, stuff it back in with the intake air and let it go through the compressor again. That requires a lot of extra work with the compressor and you have to worry about things like exhaust gas going through the aftercooler and transient response of the engine. With all that exhaust gas circulating through the intake system, if you decide you want a quick transient, that air can't respond to that.