Post Injection

Post injection delivers additional fuel after the main combustion event. Its primary purposes are supporting DPF active regeneration (by raising exhaust temperature) and reducing hydrocarbon emissions during engine warm-up.

Overview

The ECU commands post injection when exhaust temperature needs to increase, typically for DPF regeneration or catalyst light-off. The fuel burns in the exhaust manifold or oxidation catalyst, not in the cylinder.

Controlled Signals

• Post injection quantity
• Post injection timing (°ATDC)
• Exhaust gas temperature
• DPF soot load estimation

Maps Involved

• Post Injection Quantity Maps
• Post Injection Timing Maps
• DPF Regeneration Request Maps
• EGT Target Maps

Logic Sequence

DPF Regen Request OR Warm-up Mode
         ↓
Post Injection Enable
         ↓
Quantity & Timing Calculation
         ↓
EGT Feedback Control

Calibration Objectives

• Achieve target EGT for DPF regeneration
• Minimize fuel dilution in engine oil
• Prevent injector coking

Calibration Strategy

• Post injection is emissions-critical — modify with caution
• If DPF is removed, post injection can be disabled
• Monitor oil dilution on high-mileage vehicles

Diagnostics

• DPF regeneration incomplete
• Oil level rise (fuel dilution)
• EGT sensor faults

Best Practices

• Never disable post injection on DPF-equipped vehicles unless DPF is removed
• Check oil condition regularly on tuned vehicles

In Calibration Practice

Late post pulses exist mainly for the aftertreatment: close posts trim soot, far posts carry heat and HC to the DPF or heat the SCR. Their cost is oil dilution — fuel washing past the rings — which is why regeneration frequency and post quantities are calibrated as a pair, and why oil-level rise on short-trip diesels is a signal, not a mystery.

Related: on supported vehicles this system is directly addressed by our DPF Off file service. See also EGT Sensors in the knowledge base.