Exhaust system fuel injection is required for many diesel engine aftertreatment systems including diesel particulate filter (DPF) systems, fuel reforming systems and lean NOx trap (LNT) systems. The design objective of this work was to develop exhaust system fuel injectors that promoted high aftertreatment conversion efficiencies with low fuel usage. A fuel injection system utilizing a pulse-width modulated (PWM) pressure swirl atomizer was first evaluated in a spray laboratory using drop size and spray patternation diagnostic equipment. The fuel system was later evaluated as part of an aftertreatment system consisting of fuel reformer, LNT, DPF and selective catalytic reduction (SCR) catalysts. Three system fuel injectors with high turndown ratios (16.5:1 to 25:1) were developed to provide fuel flow ranges of 10 to 250 grams per minute, 30 to 500 grams per minute and 50 to 1000 grams per minute. The fuel sprays consisted of droplet Sauter mean diameters (SMD) that ranged between 55 to 70 μm within a fuel supply pressure that ranged between 4.8 and 7.0 bar. A higher pressure of 8.3 bar was used for the highest flow injector resulting in a SMD range of 60 to 75 μm. The medium flow injector (30 to 500 grams per minute) was demonstrated on an aftertreatment system that was sized to meet EPA 2010 emission standards for an on-highway heavy-duty diesel engine. The fuel flow range was required to operate the fuel reformer in both lean and rich modes. Low fuel flow rates were used during lean mode operation for catalyst heating and active DPF regeneration. During rich mode operation, diesel fuel was converted to hydrogen and carbon monoxide in the fuel reformer for LNT regeneration and desulfation. The fuel injection and aftertreatment systems were evaluated using a heavy-duty diesel engine during dynamometer and on-highway vehicle testing. Test results showed the ability to meet EPA 2010 emission levels with low fuel usage rates.