EPA 2007 NOx limits might be possible to reach without NOx trap, nor HCCI: engine researcher

Diesel Fuel News, Sept 16, 2002 by Jack Peckham

San Diego--Engine makers might be able to buy a few years extra development time (between 2007-2010) for high-efficiency nitrogen oxides (NOx) traps through advanced combustion management, a conventional lean-NOx catalyst and a diesel particulate filter (DPF), according to a Southwest Research Institute (SwRI) investigator.

Speaking to the Diesel Engine Emissions Reduction (DEER) workshop here, SwRI diesel engine research manager Jeffrey Leet showed that such a scheme might make it possible to hit U.S. EPA's s interim 1.1 grams/brake horsepower-hour NOx average limit for 2007-2009, which is replaced by a final 0.2 g/bhp-hr NOx limit in 2010.

However, it's still uncertain whether such a scheme would cover all the EPA test modes including the "not-to-exceed" (NTE) limits. If not, then perhaps a first-generation (even if not 90% efficient) NOx trap might have to be included for 2007 engines. (Note: Hitting the 0.2 grams NOx limit in 2010 will require over 90% reductions from the 2.5 g/bhp-hr NOx HC EPA limit on highway diesels that begins (for most diesel makers) next month, then hits everyone else in 2004).

Unless there's a quick breakthrough on homogenous charge compression ignition (HCCI), it won't be possible to hit the ultra-low NOx limits without exhaust aftertreatrnent, Leet showed.

One of the major obstacles to HCCI is that conventional diesel fuel hasn't been proven as capable of making HCCI practical. Something like a mixture of gasoline and diesel seems more appropriate, as fellow SwRI researcher Tom Ryan explained here. Problem: Re-inventing the entire fuel production/distribution system for such a radical fuel--just for HCCI--probably isn't likely any time soon.

Meanwhile, if NOx traps take too long to develop, or seem too bulky or costly for 2007 compliance, then engine makers could turn to a variety of alternative tools, Leet explained.

Among these are variable geometry turbochargers (VGT), higher boost levels and increased peak cylinder pressures (>200 bar), all enabling higher exhaust gas recirculation (EGR) for NOx control, Leet explained. The DPF will handle the extra soot likely to be produced from such a scheme.

So-called "partial premixed controlled compression ignition" (PCCI), a sort of limited version of HCCI, also could be employed to reduce NOx further--without requiring more change to diesel fuel. Fuel-air mixing also can be improved via high injection pressure and smaller injector hole sizes, Leet showed.

These techniques--higher EGR, modified combustion schemes--could take a baseline 2004 diesel engine down to about 1.28 grams NOx, very close to the 1.1 grams EPA average target for 2007-09. Adding variable valve actuation--an emerging technology--could cut NOx to 1.15 grams, he said.

Engineering always requires some over-compliance head-space, so the "real" target would be about 1 grams NOx, rather than 1.1. So, if the conservative estimates on these engine/EGR schemes prove out, then adding a durable, high-temperature lean-NOx catalyst (as a last resort) could bring such an engine within the engineering target of about 1 gram NOx, he said.

Such a scheme not only could "buy time" for further NOx trap development for 2010 limits, but also avoid excessive costs in the "early years" of post-2007 EPA requirements, Leet showed. Engine makers adopting such a scheme not only might be able to cut total costs for 2007-09 compliance, but also might be able to offer a more durable, compact package, as NOx traps (today's state of art) are quite large and durability hasn't vet been proven.