can an RECD for DG Set be added to a generator that already has DPF or SCR?Modern diesel generators can already have sophisticated exhaust after-treatment systems such as a Diesel Particulate Filter (DPF), Selective Catalytic Reduction (SCR), or both. This creates an important question for owners of older or imported equipment:

In most cases, adding another emission-control device should not be treated as a simple exhaust modification. The existing system, engine configuration, certification, exhaust back-pressure, and applicable regulations must all be evaluated before making any change.

First Understand What the Existing System Does

DPF and SCR perform different functions in a diesel exhaust system.

A DPF is primarily designed to capture particulate matter and soot. Depending on the system, accumulated soot must periodically be removed through regeneration.

SCR, on the other hand, is designed primarily to reduce nitrogen oxides (NOx). It uses a reagent such as urea solution to facilitate a chemical reaction within the SCR catalyst.

A typical RECD retrofit package can combine oxidation and particulate-control functions. This means that installing an RECD in front of or behind an existing DPF can potentially duplicate part of the exhaust-treatment system.

CPCB's official RECD testing procedureThe establishes the framework for emission-compliance testing of retrofit devices for diesel power-generating-set engines up to 800 kW.

A Factory-Fitted DPF Is a Different Situation

If a generator was manufactured with a DPF as part of its original emission-control system, the DPF is already part of the engine manufacturer's engineered configuration.

The same applies to a factory-fitted SCR system.

In such a case, adding another device simply because someone recommends an "RECD" may create more problems than it solves. The engine manufacturer has designed the exhaust system around particular temperature ranges, pressure limits, sensors, regeneration strategies, and control software.

Changing that exhaust system without engineering approval can affect how those systems operate.

For newer generators, the first step should therefore be to identify exactly what after-treatment equipment was installed by the OEM and what emission standard the generator was designed to meet.

What If the Generator Already Has a DPF?

Suppose an older generator has a DPF installed by a previous owner or third-party workshop.

Do not immediately assume that the DPF is equivalent to a certified RECD.

There is an important difference between a particulate filter that physically traps soot and a retrofit emission-control system that has been tested and documented for a particular application.

CPCB's current published list of certified RECD manufacturersThe identifies manufacturers and the engine-power ranges for which their certified devices apply.

Therefore, check the device's manufacturer, model, serial number, test documentation, and applicable engine range.

If the existing device is already a properly documented and applicable RECD, installing another RECD would generally be unnecessary. If it is an uncertified or unknown filter, the better approach may be to assess or replace it rather than stack another device behind it.

What If the Generator Has SCR?

The situation becomes even more complex when SCR is already installed.

SCR is designed to control NOx, whereas a typical RECD is primarily concerned with particulate and certain gaseous emissions. These systems therefore do not simply perform the same job.

However, that does not mean an RECD can automatically be added to an SCR-equipped generator.

The complete exhaust system needs to be considered because changing exhaust temperature, pressure, flow, or chemical conditions can influence the performance of downstream equipment.

For example, an SCR system relies on suitable exhaust conditions for its chemical reactions. Adding another restrictive component upstream or downstream could change those conditions.

Follow the Exhaust From the Engine to the Stack

Before installing anything, map the complete exhaust path.

A typical modern system may contain:

Engine → oxidation catalyst → DPF → urea dosing → SCR catalyst → silencer → stack

The exact order varies by manufacturer and engine model.

An RECD added to this chain becomes another component that affects exhaust flow and thermal behaviour. That is why physical installation space alone is not enough to determine compatibility.

CPCB framework for RECD testingThe should be considered alongside the manufacturer's specifications and documentation for the existing system.

Back-Pressure Is a Critical Check

Every diesel engine has a maximum permissible exhaust back-pressure.

Adding another filter, catalyst, or silencer can increase resistance in the exhaust system. If the final configuration exceeds the engine manufacturer's limit, the consequences can include increased exhaust temperature, reduced engine performance, increased fuel consumption, or other operating problems.

This is especially important if the existing DPF is already partially loaded with soot or ash.

A second device cannot solve a restriction problem created by the first one.

Before installation, determine:

Existing exhaust back-pressure
Engine manufacturer's maximum allowable back-pressure
Pressure drop across the existing DPF or SCR system
Expected pressure drop across the proposed RECD
Exhaust flow at relevant engine loads
Final pressure drop of the complete system

The final configuration should be evaluated as a complete system rather than adding pressure-drop figures without considering actual operating conditions.

Four Common Situations

It helps to divide generators with existing after-treatment into four categories.

1. Factory-fitted DPF/SCR on a modern generator

The generator was designed and approved with the existing system. An additional RECD is normally not the starting point. Consult the OEM and verify the applicable regulatory requirement first.

2. Older generator with a certified RECD already installed

If the device is properly documented and applicable to the engine, adding another RECD would generally provide no obvious benefit.

3. Older generator with an unknown aftermarket filter

This requires investigation. Identify the device and verify its documentation before deciding whether it qualifies as an acceptable emission-control system.

4. Imported or modified generator

This requires additional caution because the engine may have been designed for another country's emission requirements. If its original DPF or SCR has been modified, removed, or bypassed, restoring the original engineered system may be more appropriate than simply adding another retrofit device.

retrofitting an RECD to an imported diesel generatorAceget's related guidance on also highlights the importance of understanding the engine's original emission configuration before selecting a retrofit.

Certification Matters as Much as Hardware

One of the biggest mistakes is looking only at the physical device.

An emission-control device may fit an exhaust pipe perfectly and still not provide the documentation required for the particular generator.

CPCB certified RECD manufacturer listThe shows that certified devices have defined applicability ranges rather than being universally interchangeable.

Ask for:

RECD manufacturer and model
Serial number
Certification/test documentation
Applicable engine-power range
Certification agency
Installation requirements
Maintenance requirements

If the existing DPF or SCR has its own OEM documentation, keep those records together with the generator's engine and emission-system information.

Do Not Remove the Existing System Without Approval

If someone suggests removing the factory DPF or SCR to "make space" for an RECD, stop before making the modification.

Factory after-treatment can be integrated with sensors, electronic controls, regeneration strategies, dosing systems, and engine-management software. Removing one part can therefore affect the complete system.

The correct solution may be to repair the existing equipment, replace a failed component, or obtain an OEM-approved modification rather than replacing the factory system with a different retrofit arrangement.

Diagnose the Actual Problem First

Sometimes the request for an RECD comes from a completely different problem.

If the generator is producing black smoke, investigate combustion, injectors, air supply, loading, and the condition of the existing DPF.

If there is an SCR fault, investigate the dosing system, sensors, reagent quality, catalyst condition, and associated controls.

If the concern is regulatory compliance, examine the generator's existing certification and documents before modifying the exhaust.

If the concern is odour, identify whether the smell is coming from exhaust gases, fuel, lubricating oil, or a leakage point.

Adding another device without diagnosing the original problem can make troubleshooting more difficult.

Questions to Ask Before Installing an RECD

Before approving an installation, ask the OEM or qualified technical supplier:

What emission system was originally fitted to the generator?
Is the existing DPF or SCR factory-fitted or aftermarket?
Is the existing system still functional?
Is it certified for this particular engine?
What is the engine's maximum permissible back-pressure?
What is the pressure drop across the existing system?
What pressure drop will the proposed RECD introduce?
Will the additional device affect regeneration or SCR operation?
Will the modification affect the generator's warranty?
Is the complete configuration recognised for the applicable compliance requirement?

Get important answers in writing, especially when the generator is under warranty or used at a site subject to regulatory inspection.

A Practical Decision

The safest decision path is straightforward:

Existing DPF/SCR → Identify system → Check documentation → Confirm applicable regulation → Consult OEM → Check back-pressure → Evaluate complete exhaust configuration → Install only if technically and compliantly justified

If the existing system already provides the required emission control, an additional RECD may simply add cost and exhaust restriction without delivering a meaningful benefit.

If the existing system is uncertified, damaged, or unsuitable for the applicable requirement, replacement with an appropriately certified solution may be more sensible than stacking multiple devices.

Conclusion

can an RECD for DG Set be added to a generator with an existing DPF or SCR?So, It may be technically possible in some configurations, but it should never be assumed to be necessary or appropriate.

A generator with factory-fitted after-treatment has already been engineered as a complete emission-control system. An older generator with an aftermarket filter requires a different assessment. In both situations, certification, compatibility, exhaust back-pressure, operating temperature, and OEM requirements need to be considered before any modification.

Can an RECD Be Added to a Generator With an Existing DPF or SCR?

RECD For DG Set