What Is 7th Injector?

Understanding the Role of the 7th Injector in Engine Aftertreatment System

Before explaining what the 7th injector is, it’s important to understand the engine’s aftertreatment system. With growing environmental awareness, exhaust emissions have become a major concern. The aftertreatment system is designed to reduce harmful gases released by the engine. Its main sequence generally follows:

Turbo → Fuel Injector (7th Injector) → DOC → DPF → DEF Doser → SCR

Among these components, the 7th injector plays a key role in helping the aftertreatment system meet strict emission standards.

What Is 7th Injector?

A “7th injector” in 6-cylinder diesel engines is an extra injector located in the engine aftertreatment system, not in the cylinders. The 7th injector is a fuel doser; its function is to add diesel fuel into the exhaust gas to raise the exhaust temperature for DPF regeneration. Its main role is emissions reduction.

Before the DPF, there is a DOC (Diesel Oxidation Catalyst) that oxidizes hydrocarbons (HC). Regeneration in the DPF occurs only when the temperature conditions are favorable, when exhaust temperatures are high enough to burn away soot. The 7th injector (fuel doser injector) is therefore used to increase the exhaust temperature.

Note: Some may mistakenly think the 7th injector functions as a Diesel Exhaust Fluid (DEF) doser, since both the fuel doser and the DEF doser operate in the engine aftertreatment system. However, they are two different parts.

What’s the Difference Between Fuel Doser and DEF Doser?

A frequent source of confusion is between the 7th injector (fuel doser) and the DEF doser. While both are part of the aftertreatment system, they have different functions:

• 7th Injector (Fuel Doser): Sprays diesel fuel after the turbo and before the DOC to raise exhaust temperatures for DPF regeneration.
• DEF Doser: Sprays Diesel Exhaust Fluid (DEF) after the DPF and before the SCR (Selective Catalytic Reduction). The DEF turns into ammonia (NH₃), which reacts with exhaust (NOx) inside the SCR catalyst, converting it into harmless nitrogen (N₂) and water (H₂O), achieving zero emissions.

Where Is the 7th Injector Located?

The 7th injector is typically mounted after the turbocharger and before the DOC in the exhaust system. This location ensures that the injected diesel fuel effectively increases exhaust gas temperatures before entering the DPF for regeneration.

Common Materials for 7th Injector Gaskets

Now we know the 7th injector works with exhaust, meaning it operates in a high temperature environment. Therefore, its gaskets must be made from durable high-temp materials.

1. Metal with MoS₂ Coating

The rigid steel base, combined with the excellent heat resistance and low-friction properties of MoS₂, ensures that the 7th Injector Seal can withstand continuous exposure to exhaust heat and pressure.
Example: Cummins ISX15 4963857

2. MLS with MoS₂ Coating

Multi-Layer Steel (MLS) construction offers superior mechanical strength and sealing reliability. With the added MoS₂ coating, the 7th Injector Seal achieves enhanced thermal stability.
Example: Detroit Diesel DD15 A4721420580

3. Graphite

Graphite is well-recognized for its resilience in high-temperature and high-pressure applications. Its flexibility allows it to conform to sealing surfaces effectively, making it a dependable option for the 7th Injector Seal exposed to exhaust gas and thermal cycling.

4. Rubber Coated Metal (RCM)

A strong steel core combined with an elastic rubber coating provides both rigidity and flexibility. This structure allows the 7th Injector Seal to maintain reliable sealing under vibration while resisting heat and exhaust gas exposure.
Example: Navistar Maxxforce DT 3018016C2

Quick Q&A for 7th Injector

What Is 7th Injector?

An extra fuel injector in the aftertreatment system that sprays diesel into exhaust to raise temperature for DPF regeneration and emissions reduction.

7th Injector (Fuel Doser) vs DEF Doser

7th Injector (Fuel Doser): Adds diesel before the DOC for DPF regeneration.
DEF Doser: Adds DEF before the SCR to convert NOx into nitrogen and water.

Where Is the 7th Injector Located?

After the turbocharger, before the DOC.

What are common materials for 7th Injector?

See sections above for details on Metal with MoS₂, MLS, Graphite, and RCM materials.