Clean Diesel or Costly Diesel?

by Jim Anderton

SONOMA, Calif. – “Clean” diesel was everywhere at this year’s Michelin Bibendum Challenge, an annual rally and exposition of alternate and environmentally friendly vehicle technologies ranging from hydrogen fuel cells to cleaner conventional piston engines. This was the trucking industry’s first year participating in the international sustainable mobility event, and participate they did, with Freightliner, Cummins Westport (entering a Peterbilt) and Volvo Trucks North America entering diesel and natural gas powered vehicles.

“Diesel: Good Clean Fun” boomed a Bosch exhibit. But behind the test-drives and the camaraderie was the knowledge that emissions regulations are tightening up everywhere, not just in California and Europe. “Clean” diesel and engines that burn diesels more cleanly were being touted as a possible solution.

Here’s how some of these solutions work: Diesel and gasoline have one important commonality – they’re hydrocarbons. Burning hydrocarbons, whether they’re gasoline, diesel, or Jersey Milk chocolate is all about combining the carbon in hydrocarbons with oxygen from the air. Mixing those two ingredients makes heat, but it also creates carbon monoxide (CO), carbon dioxide (CO2), and when the process is incomplete, unburned hydrocarbons, which the engineers label “HC”. Unburned hydrocarbons are less of an issue with diesels, which burn more efficiently than gas engines.

Still, what we used to call soot, now “particulate matter” is enough of an issue that trapping it has become a growth industry among engine parts suppliers like Bosch and Delphi. Traps are literally sieves and like any filter will clog when enough particulates load them up.

Current technologies get around this problem – they “regenerate” the traps by burning off the particulates. This happens spontaneously at temperatures above 550 C, so boosting combustion temperature in the trap requires a little upstream technology. A little post-combustion fuel injection into the cylinder can add as “afterburner” effect which boosts exhaust temperatures, as does a traditional oxidation catalyst just ahead of the trap, which burns any hydrocarbons left over from the cylinders, creating heat. “Regeneration” happens automatically, with no driver input needed.

CO, or carbon monoxide, is the familiar deadly invisible gas that lives in closed vehicles with faulty exhaust systems. CO is also deadly in a less dramatic way, mainly by combining with another emissions product, NOx, to form smog. Catalysts work here too, converting the CO to the slightly more palatable greenhouse gas CO2, but the difficult pollutant is NOx.

Unfortunately, NOx, which is “chemicalese” for “oxides of nitrogen,” is a by-product of efficient combustion. Air is 78 per cent nitrogen, but it’s the oxygen that we want to burn with the fuel. At the high combustion temperatures needed for economical power, some of the nitrogen combines with the oxygen to generate the smog-producing stuff. Reducing it is a mainly a matter of reducing combustion temperatures, and the best way to do that without killing engine efficiency is to feed back some of the oxygen-depleted exhaust gas into the cylinders.

This is EGR, or exhaust gas recirculation, and contrary to many rumours in the industry, it’s a proven technology that’s not excessively complex. Computer-controlled EGR valves simply divert a small portion of exhaust into the intake. A typical example is Mack’s “C-EGR” system, which cools the recirculated exhaust gases for better efficiency, and uses a variable geometry turbocharger to assist in moving the gases thorough the system. Oil contamination issues are handled with an improved filtering medium and an oil pan that holds an extra seven litres of oil to maintain current drain intervals. Cooled EGR works best in “steady state”, highway driving conditions, but for vocational engines like construction and refuse trucks, it’s possible to use a simper EGR strategy that times the valves to leave a portion of the exhaust gas in the cylinder before the intake stroke, achieving EGR without valves.

Maintenance issues appear to be few with conventional EGR, which has been adopted by Cummins, Volvo, Mack and Detroit Diesel, with CAT a notable exception. CAT’s ACERT technology uses internal EGR combined with a diesel oxidation catalyst and internal engine changes to achieve similar emissions levels as conventional EGR. Caterpillar will EPA certify all the firm’s on-highway truck engines by the end of the year, so the debate about ACERT vs. EGR will likely continue, with Detroit Diesel as a major protagonist.

While many technologies will reduce emissions, operating costs and engine longevity in actual fleet use remains to be seen. Despite extensive testing by engine manufacturers. EGR and ACERT engines will be more expensive to be sure.

But with ever-tightening emissions regulations, clean diesel isn’t a matter of choice. Ultimately, clean diesel economics will likely boil down to “Who pays?”

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