Fuel, Exhaust and Emission Control Systems
Fuel delivery and aftertreatment
Follow the fuel path: tank, supply pump, water separator and filter, high-pressure pump, rail, injectors, and return circuit. Air in the fuel shows as hard starting, rough running and low rail pressure; water shows as corrosion and injector damage. Know cetane, ULSD sulphur limits (15 ppm), cloud point and gelling, winter blending, and the consequences of misfuelling - gasoline in a diesel damages the high-pressure pump and injectors.
Aftertreatment: the diesel oxidation catalyst (DOC) oxidizes hydrocarbons and carbon monoxide and generates heat for regeneration; the diesel particulate filter (DPF) traps soot; selective catalytic reduction (SCR) uses diesel exhaust fluid (DEF, 32.5 % urea) to convert NOx into nitrogen and water; exhaust gas recirculation lowers combustion temperature. DPF regeneration is passive when exhaust temperature is high enough and active when the ECM doses fuel to raise temperature. Ash from engine oil is permanent and requires cleaning or replacement; soot burns off. Regeneration is blocked by low speed, low load, low temperature or an active fault.
Worked example. A truck derates and logs high exhaust backpressure. Check DPF soot load and differential pressure, confirm regeneration conditions are met, then look for a cracked filter, failed sensor, or an engine problem making excessive soot - leaking injector, restricted intake, faulty EGR. Never add DEF to the fuel tank or fuel to the DEF tank.