What Is an RECD and How Does It Work?

If you own or manage a diesel generator in India, you have probably run into the term RECD in a compliance notice, a vendor’s quote, or a pollution control board circular, usually with a deadline attached. Here is the short answer before we get into the mechanics.

An RECD, or Retrofit Emission Control Device, is an after-treatment unit fitted onto the exhaust line of an existing diesel generator to reduce the particulate matter, carbon monoxide, and hydrocarbons in the exhaust before it leaves the generator’s chimney. It does not touch the engine itself. It sits downstream of the engine, in the exhaust path, and cleans up what the engine has already produced. In India, RECDs are tested and approved against Central Pollution Control Board (CPCB) protocols, and installing one is how most existing diesel generators built before the newest CPCB IV+ emission tier become legally compliant without being replaced outright.

That is the definition. The more useful question, and the one this article actually answers, is what happens physically inside that metal box bolted to your generator’s exhaust pipe. So instead of listing reasons to install one (we have already covered that in detail in why RECD is mandatory in India and in the operational and financial benefits of installing an RECD), we are going to do something more concrete: follow a single breath of exhaust gas on its journey from the combustion chamber to the open air, and watch what happens to it at every stage.

Why the Exhaust Needs Cleaning Up in the First Place

A diesel generator burns diesel to spin an alternator. That combustion is never perfectly complete, and the by-products leaving the cylinder are not just carbon dioxide and water vapour. Raw diesel exhaust also carries:

• Particulate matter (PM), the fine soot particles that make exhaust look black or grey and that are small enough to lodge deep in human lungs.
• Carbon monoxide (CO), a colourless, odourless gas produced by incomplete combustion.
• Unburnt hydrocarbons (HC), fuel that passed through the engine without fully combusting.
• Oxides of nitrogen (NOx), formed when combustion temperatures push nitrogen and oxygen in the intake air to react with each other.
• Trace amounts of sulphur compounds, depending on fuel quality.

None of this is unique to India or to older generators. It is simply what diesel combustion produces. What has changed is the tolerance for it. Cities across the National Capital Region and other major metros already sit well above safe annual PM2.5 levels, and a fleet of diesel generators running during power cuts and peak-demand hours is a meaningfully large, geographically concentrated source of exactly the pollutants that worsen that picture. That is the backdrop against which CPCB, the Commission for Air Quality Management (CAQM), and the National Green Tribunal built the regulatory push toward RECD retrofitting, which we have documented in full in our timeline of CPCB emission norms for generators.

An RECD’s entire job is to intercept that exhaust stream after combustion and strip out as much of the harmful content as possible before it reaches the chimney outlet.

Follow the Exhaust: A Stage-by-Stage Walkthrough

Picture a single pulse of exhaust gas leaving one cylinder of a diesel generator the moment after fuel ignites. Here is everywhere it goes and everything that happens to it before it reaches open air.

Stage 1: Out of the Cylinder and Into the Manifold

The exhaust valve opens, and hot gas, often above 400 to 500 degrees Celsius at this point, rushes out of the cylinder into the exhaust manifold, where it merges with exhaust from the generator’s other cylinders. This is standard for any diesel engine, retrofitted or not. Nothing has been treated yet. The gas is still carrying its full load of PM, CO, HC, and NOx.

Stage 2: Into the RECD Housing

Instead of running straight from the manifold (and, on larger sets, past a turbocharger) into an open silencer and out the chimney, the exhaust is now redirected into the RECD unit itself, a stainless steel or heat-resistant housing that has been fabricated and fitted to match the generator’s exhaust diameter and flow rate. This is the physical retrofit part of “retrofit emission control device.” The housing is sized specifically for the kVA rating and exhaust characteristics of the generator it is installed on, which is why a 25 kVA set and a 1000 kVA set need differently sized RECD units, not the same device scaled up casually.

Stage 3: The Diesel Oxidation Catalyst (DOC)

Inside the housing, the gas first passes through a substrate coated with a catalytic material, typically a washcoat containing platinum-group metals. This stage is called the Diesel Oxidation Catalyst, or DOC.

Here is what actually happens chemically, in plain terms: the catalyst coating lowers the temperature at which oxidation reactions occur. Carbon monoxide (CO) reacts with oxygen already present in the exhaust and converts into carbon dioxide (CO2). Unburnt hydrocarbons (HC) similarly oxidise into CO2 and water vapour. A portion of the soluble organic fraction of the particulate matter, essentially the hydrocarbon-based portion of the soot, is also oxidised at this stage, which has the side effect of reducing the visible smoke and smell coming off the generator almost immediately.

The DOC does not need any external input to do this. It is a passive catalytic reaction driven entirely by exhaust heat and the catalyst surface. No fuel additive, no separate dosing system, is required for this stage in a standard RECD retrofit.

Stage 4: The Diesel Particulate Filter (DPF)

The gas then passes into the second core stage: the Diesel Particulate Filter, or DPF. This is a porous ceramic or silicon carbide honeycomb structure with alternating channels that are plugged at opposite ends, forcing the exhaust gas to pass through the porous walls rather than straight through. As gas pushes through those walls, solid soot particles are physically trapped on the channel surfaces, while the gas itself continues on.

This is the mechanical filtration stage, and it is where the bulk of the particulate matter reduction actually happens, which is why RECD systems built around a DOC-plus-DPF combination can report particulate reductions in the very high nineties under proper operating conditions, a figure we break down technology by technology in RECD vs DPF: what’s the actual difference.

Trapped soot cannot accumulate on the filter indefinitely, or it would eventually choke exhaust flow entirely. So the DPF has a self-cleaning mechanism called regeneration.

Passive regeneration happens automatically whenever exhaust temperature is high enough (generally when the generator is running under a reasonably loaded, steady state) to burn off the trapped soot continuously, converting it to a small amount of ash and CO2, without any driver or operator intervention.

Active regeneration is a deliberate, controller-triggered event used when the generator runs mostly under light load and exhaust temperatures rarely climb high enough for passive regeneration to keep up. In that case, the system’s controller detects rising backpressure (via a differential pressure sensor across the filter) and can, on some RECD models, inject extra fuel or use a small onboard heater to briefly raise exhaust temperature and force a soot burn-off cycle.

We go into filter cleaning, ash accumulation, and when a filter needs manual servicing rather than self-regeneration in our separate RECD maintenance guide; it is worth reading before your first service interval comes up.

Stage 5: Sensors and the Control Loop

Running alongside the physical filtration path is a small monitoring system: a differential pressure sensor measuring the pressure drop across the filter (a proxy for how loaded with soot it is), and temperature sensors positioned before and after the catalyst and filter stages. On systems with active regeneration, a controller reads these sensors continuously and decides when a regeneration cycle is needed. On simpler passive-only systems, these sensors mainly exist to flag abnormal backpressure to the operator, which is the first sign something needs attention.

Stage 6: Out Through the Chimney

By the time the gas exits the RECD housing and continues through the existing exhaust silencer and chimney stack, its particulate matter, carbon monoxide, and hydrocarbon content have all been substantially reduced compared to what left the cylinder in Stage 1. The gas that reaches open air is meaningfully cleaner, not because the engine burned fuel any differently, but because everything it produced was intercepted and treated on the way out.

That is the complete journey. No stage requires modifying the engine’s internals, which is precisely why this is called a retrofit device rather than an engine replacement or rebuild, and why installation, which we walk through step by step in our RECD installation guide, typically takes days rather than weeks.

A Short History: Where RECD Came From

The RECD ecosystem in India did not emerge from a manufacturer’s product roadmap. It traces back to a specific National Green Tribunal order dated 6 August 2019 (O.A. 681/2018), which directed CPCB to build a testing and certification framework for retrofitting emission control equipment onto in-use diesel generators up to 800 kW. Before that order, there was no standardised, government-recognised retrofit pathway for an older diesel generator to reduce its emissions short of replacing the entire unit. The order effectively created the market category that RECD now occupies.

CPCB spent the following two to three years building out the technical testing protocol, publishing procedures such as PCLS/12/2021-22, under which manufacturers could submit their after-treatment technology for formal type approval. The Commission for Air Quality Management, established with statutory footing through the CAQM Act, 2021, layered its own capacity-band framework on top of CPCB’s technology approval once it issued Direction No. 76 in September 2023, which is what actually pins specific kVA ranges to specific compliance pathways (RECD, dual fuel, or both) in the National Capital Region. We cover this full sequence of events, including the intermediate CPCB emission tiers, in our timeline of CPCB emission norms for generators and in our chronological account of the NGT rulings that built this framework.

The practical upshot for anyone buying an RECD today: this is a young, still-maturing regulatory category, not a mature product line with decades of standardised testing history behind it. That is part of why vendor selection and certificate verification, covered in our dedicated guides, matter more here than they might for a more established category of industrial equipment.

RECD on Open vs. Enclosed (Silent/Soundproof) DG Sets

One practical distinction that affects how an RECD gets fitted, though not how it works internally, is whether your generator is an open-frame set or an acoustic enclosure (commonly sold as a “silent” or “soundproof” generator). On an open-frame set, the exhaust run and available space around it are usually straightforward to access, and RECD fitting tends to be more straightforward as a result.

On an enclosed set, the RECD housing, or at least the section of exhaust ducting connecting to it, often needs to be integrated within or routed through the acoustic canopy itself, which constrains the available space and can require modifications to the enclosure’s ventilation or exhaust cut-out. This does not change anything about the DOC and DPF stages described above, but it does mean fabrication for an enclosed unit typically requires closer attention during the site assessment stage than an open-frame equivalent, a point worth raising directly with your vendor if your generator is enclosed. We walk through the full site assessment and fabrication sequence in our step-by-step installation guide.

The Core Components, in One Table

Component	What It Does
Housing / casing	Heat-resistant enclosure sized to the generator’s exhaust flow, redirects gas through the internal stages
DOC substrate and washcoat	Catalytic surface that oxidises CO and hydrocarbons into CO2 and water
DPF (ceramic/SiC filter)	Porous honeycomb that physically traps particulate matter
Differential pressure sensor	Measures backpressure across the filter to track soot loading
Temperature sensors	Monitor exhaust temperature before and after each stage
Controller / ECU (active systems)	Decides when to trigger active regeneration on load-light installations
Regeneration heater (some active systems)	Raises exhaust temperature briefly to force a soot burn-off cycle
Mounting brackets and ducting	Physically integrates the unit into the existing exhaust run

Where DOC, DPF, and SCR Fit Relative to Each Other

A quick clarification, since these three acronyms get used loosely in vendor conversations. DOC and DPF are the two stages inside a standard RECD, as described above; together they are what most CPCB-approved retrofit units for diesel generators are built around. Selective Catalytic Reduction (SCR), which uses a urea-based reagent injected into the exhaust to chemically reduce NOx, is a separate technology more commonly associated with newer CPCB IV+ factory-fitted gensets and heavy vehicles than with retrofit kits for older, in-use DG sets, largely because it needs a reagent supply chain and dosing system that adds cost and operational complexity disproportionate to most retrofit use cases. If you are comparing quotes and one vendor is proposing SCR for a straightforward retrofit and another isn’t, that is worth asking about directly, since it changes both the price and the ongoing consumable requirements.

RECD vs. a Dual Fuel Kit: Different Jobs, Not Competing Products

It is worth being clear that an RECD and a dual fuel kit solve different problems, even though both get installed on the same generators and sometimes get quoted in the same conversation. An RECD treats exhaust after combustion to reduce pollutants. A dual fuel kit changes what the engine burns, blending diesel with a gaseous fuel like CNG or LPG to cut diesel consumption and, as a secondary effect, reduce some emissions at the source. They are not interchangeable, and some CAQM capacity bands specifically require one or the other, or in some bands, either one, as detailed in our full RECD vs. dual fuel kit comparison.

Retrofit vs. Factory-Fitted: Where the “R” in RECD Matters

Every RECD conversation eventually runs into a generator that was manufactured after CPCB IV+ became mandatory for new units. Those newer generators already have equivalent after-treatment (and sometimes SCR) built in from the factory, engineered as part of the engine and exhaust design from day one. An RECD is specifically the retrofit version of that same idea: a bolt-on unit engineered to be fitted onto a generator that was originally built without one, most commonly units manufactured between 2004 and 2023 that would otherwise have no legal path to continued use under current norms. If your generator already carries CPCB IV+ certification from the manufacturer, you do not need a retrofit at all. If it was built before that, retrofitting is the standard path, alongside dual fuel conversion or outright replacement, both of which are usually far more expensive, as we cover in how much an RECD actually costs in India.

How CPCB and CAQM Fit Into the Picture

The RECD is not a generic aftermarket muffler upgrade. To count for legal compliance, it has to be manufactured, tested, and approved through a defined CPCB process, and the generator’s registration paperwork has to reflect that an approved unit is installed and functioning. The CPCB’s own genset notification framework and the Commission for Air Quality Management both govern different pieces of this: CPCB sets and tests the emission-reduction technology standard, while CAQM’s Direction No. 76 sets the capacity-band rules for which generators in the National Capital Region specifically need an RECD, a dual fuel conversion, or both. Buying a device that hasn’t gone through this approval process, however similar it looks, does not satisfy the mandate, which is exactly why choosing a vendor correctly matters as much as the device itself, a topic we cover in full in CPCB-approved RECD vendors: how to choose one.

Frequently Asked Questions

Does an RECD reduce fuel consumption? Not directly. An RECD is an after-treatment device, not a combustion or fuel-delivery change, so it is not designed to improve fuel economy. Some owners do report modest efficiency gains after installation, generally attributed to a cleaner-running exhaust path, but that is a side effect, not the device’s purpose. If fuel cost reduction is your main goal, a dual fuel kit is the more direct tool.

Can an RECD be fitted to any diesel generator? Most in-use diesel generators within the commonly retrofitted capacity range, broadly 25 kVA up to the 1,000 to 2,500 kVA band depending on the specific product line, can be fitted with an appropriately sized RECD. The unit has to be matched to the generator’s exhaust flow, backpressure tolerance, and physical exhaust geometry, which is why a proper site assessment (covered in our installation guide) always precedes fabrication.

Does installing an RECD require modifying the engine? No. The engine itself, its combustion chamber, fuel injection, and internal components remain untouched. The RECD is fitted entirely on the exhaust side, downstream of the engine.

How long does an RECD last? Properly maintained units are commonly rated well beyond 20,000 operating hours, though actual service life depends heavily on load patterns, fuel quality, and whether scheduled maintenance and filter servicing are kept up, which we detail in our maintenance guide.

Is an RECD the same as a silencer or muffler upgrade? No. A silencer manages noise. An RECD manages exhaust emissions through catalytic and filtration stages. Some installations do involve reworking the silencer section to accommodate the new housing, but the two serve entirely different functions.

What happens if the DPF gets clogged? Backpressure rises, which can reduce engine performance and increase fuel consumption if left unaddressed for long enough. This is precisely what the differential pressure sensor is designed to flag early, and it is covered in detail, including warning signs and troubleshooting, in our maintenance guide linked above.

Is an RECD the same thing across every generator brand? No. While the underlying DOC and DPF principle is the same, the actual housing, catalyst loading, and filter sizing are engineered to a specific generator’s exhaust flow and capacity, which is why CPCB approval is granted per tested capacity range and technology configuration rather than as a blanket certification covering every brand and size at once.

Does an RECD reduce noise as well as emissions? Not meaningfully by itself. An RECD is designed around exhaust after-treatment, not acoustic damping, so any noise change after installation is usually incidental rather than a designed feature. If noise reduction is a specific requirement, that is typically addressed through the generator’s silencer or acoustic enclosure design separately.

Where to Go From Here

Understanding how an RECD works is the easy half of this decision. The harder, more practical questions are what installing one on your specific generator actually involves, what it costs at your capacity band, whether any financial relief is available, and how to pick a vendor whose CPCB approval will actually hold up under inspection. We have written a dedicated, detailed guide for each of those questions, starting with our step-by-step RECD installation process. If you already know your generator’s kVA rating and want a quote based on the real specification of your unit rather than a general estimate, you can review Aceget’s CPCB-approved RECD range directly.