to become more efficient and have fewer pollutants in their exhaust. For instance, light duty truck must now have NOx emissions less than 0.07 g/mile, and in the U.S., by 2010, NOx emissions must be less than 0.03 g/mile. Moreover, in recent years the United States, Europe, and Japan have extended emissions control regulations from covering on-road vehicles to include farm vehicles and locomotives, marine vessels, and stationary generator applications. Changing to a different fuel (i.e. dimethyl ether, and other bioethers as diethyl ether ) tends to be a very effective means to reduce pollutants such as NOx and CO. When running on dimethyl ether (DME) for instance, particulate matter emissions are near-nonexistent, and the use of diesel particulate filters could even be omitted. Also, given that DME can be made from animal, food, and agricultural waste, it can even be carbon-neutral (unlike regular diesel). Mixing in bioether (or other fuels such as hydrogen) into conventional diesel also tends to have a beneficial effect on the pollutants that are emitted. In addition to changing the fuel, US engineers have also come up with two other principles and distinct systems to all on-market products that meet the U.S. 2010 emissions criteria, selective non-catalytic reduction (SNCR), and exhaust gas recirculation (EGR). Both are in the exhaust system of diesel engines, and are further designed to promote efficiency. Selective catalytic reduction Selective catalytic reduction (SCR) injects a reductant such as ammonia or urea — the latter aqueous, where it is known as diesel exhaust fluid, DEF) — into the exhaust of a diesel engine to convert nitrogen oxides (NOx) into gaseous nitrogen and water. SNCR systems have been prototyped that reduce 90% of the NOx in the exhaust system, with commercialized systems being somewhat lower. SCR systems do not necessarily need particulate matter (PM) filters; when SNCR and PM filters are combined, some engines have been shown to be 3-5% more fuel efficient. A disadvantage of the SCR system, in addition to added upfront development cost (which can be offset by compliance and improved performance), is the need to refill the reductant, the periodicity of which varies with the miles driven, load factors, and the hours used. The SNCR system is not as efficient at higher revolutions per minute (rpm). SCR is being optimized to have higher efficiency with broader temperatures, to be more durable, and to meet other commercial needs. Exhaust gas