Research into how fuel and air mix and burn inside diesel engines could lead engineers at Sandia National Laboratory to devise a low-temperature combustion (LTC) diesel engine with high fuel mileage and low levels of pollution.
It seems that particulate matter (PM) forms in regions of the cylinder with high concentrations of fuel. NOx, or nitrogen oxides, were created within high-temperature flames. Engineers significantly reduce NOx emissions by recirculating exhaust gases back inside the cylinders. This diluted the fuel/air mixture and lowered the flame temperature, thus lowering NOx formation. Engineers also adjusted the timing of the fuel injection so fuel gets sprayed earlier and has more time to mix with air. This makes the fuel/air mixture more uniform and eliminates fuel-rich pockets that lead to PM as well as the temperatures responsible for NOx.
While these two modifications helped cut NOx and PM, they generated more carbon monoxide (CO) and unburned hydrocarbons (UHC). Both emissions are toxic and lower fuel efficiency. Further research revealed that near the fuel injector, fuel had too much air and would not burn to completion, leading to UHC and CO in the exhaust.
Engineers added postinjections — smaller squirts of fuel added after the main spray. The extra fuel extended the region of complete combustion, which lowered UHC and CO while bumping up fuel efficiency by making sure more fuel gets burned.