Alternate Technologies

While SCR looks like the path to be taken by most diesel builders for 2010, there are alternate methods that concentrate on reducing nitrogen oxides in the cylinders but also rely on aftertreatment. Some were described by engine representatives in a well-attended session earlier this year during the annual meeting of the Technology and Maintenance Council of ATA. The methods include homogeneous charge compression ignition (HCCI) and hydrogen enabled combustion (HEC).

HCCI involves mixing engine fuel with air, as with a spark-ignition gasoline engine, but with a greater amount of air. The mixture ignites by compression heat from the piston stroke, as with a diesel. But it burns at a far lower temperature. The reduced temperature plus the high ratio of air significantly reduce NOx. HCCI is being studied in a project involving General Motors, Bosch and Stanford University, as well as by other engine makers, according to Chuck Blake of Detroit Diesel.

One of HCCI's mechanical features is variable compression produced by changing the pistons' stroke; this is accomplished by an eccentrically cradled crankshaft. Low compression ratios at startup and under high load eliminate various problems while still allowing compression braking. Electronically variable valve timing is also a requirement.

HCE uses a reformer to produce a hydrogen-rich gas from the diesel fuel on board the vehicle. The gas enhances combustion, also at lower temperatures, for NOx reduction. The reformer works by injecting minute amounts of fuel into a canister, where it's mixed with air and ignited by a spark plug. The resulting ionized gas, called plasma, glows when burned in the cylinders, helping to preliminarily break down fuel into hydrogen and carbon monoxide, which is sent to NOx traps, where further chemical reactions strip out oxygen to prevent formation of NOx.