October 30, 2026
Posted by: editor
Category: RECD & Emission Control Devices

Short answer first. An RECD can cut carbon monoxide and unburned hydrocarbons substantially, but only when two conditions are both true: the device contains an oxidation catalyst (either a separate diesel oxidation catalyst stage or a catalyst coating on the filter), and the exhaust reaching that catalyst is hot enough for it to work. If either condition is missing, the reduction in RECD carbon monoxide and hydrocarbons figures can be small, close to zero, or very different from the number in a brochure.

An uncatalysed particulate filter is a mechanical trap. It is good at holding soot, but CO and gaseous hydrocarbons are molecules, not particles, and they pass through the porous walls largely unchanged. So the first question for any buyer is not “how much CO does it remove?” but “what is inside the housing that could remove it at all?”

The second question is about temperature, and that is what most of this article is about. A catalyst is not an on/off device. Its effectiveness climbs along a curve as the exhaust heats up, and a DG set that spends its life on weekly no-load exercise runs sits on a very different part of that curve from one running a factory at 70% load. Everything else, from test design to how you read a vendor’s percentage, follows from that curve.

What CO and HC are, and where a DG set makes them

Diesel engines run lean, with plenty of excess air, so in a healthy engine at a sensible load CO and HC are low compared with petrol engines. They rise when combustion is incomplete, and a standby generator has more opportunities for incomplete combustion than most engines.

Carbon monoxide (CO) forms when carbon in the fuel is only partly oxidised. In a diesel, that happens in fuel-rich pockets of the spray, near cool cylinder walls, and when combustion is quenched before it finishes.

Unburned hydrocarbons (HC) are fuel and lubricating oil fragments that escape combustion. Some stay as gases in the exhaust; heavier ones condense onto soot or form droplets as the exhaust cools. That heavier fraction is often called the semi-volatile organic fraction, and it counts towards measured particulate mass.

The usual sources in a DG set

Light load. At 10–25% load, cylinder temperatures are low, fuel sprays are short and the mixture burns less completely. CO and HC concentrations typically rise as load falls.
Cold starts. For the first few minutes, cold liners and a cold intake charge quench combustion. Hydrocarbons in particular spike, and the exhaust system itself is cold.
Wet stacking. Long periods at low load leave unburnt fuel and oil in the exhaust, giving black, oily deposits at joints and white or bluish haze.
Poor injection. Worn nozzles, wrong injection timing, dribbling injectors or low injection pressure all leave fuel that does not burn cleanly.
Oil reaching the cylinder. Worn rings, valve stem seals or turbocharger seals add lubricating oil hydrocarbons, which matter doubly because oil also carries additives that harm catalysts (more on that below).

Why they matter

CO is a direct toxic hazard. It binds to haemoglobin and reduces the blood’s ability to carry oxygen; the US EPA’s page on carbon monoxide and indoor air quality describes the progression from fatigue and headache to confusion and, at high concentrations, death. That is why exhaust leaks in enclosed plant rooms are taken so seriously. If your DG sits below ground, the planning points in our post on basement DG room RECD installation apply: a tight exhaust path matters at least as much as any catalyst.

HC matters for three reasons. Some hydrocarbons are irritants and contribute to smog chemistry. Many are responsible for the sharp “diesel smell” people complain about, which we discuss in whether an RECD can reduce exhaust odour. And the semi-volatile part condenses into particulate matter, so reducing HC can also reduce measured PM mass.

The light-off story: how catalyst conversion follows temperature

An oxidation catalyst is a ceramic or metal honeycomb coated with a washcoat that carries precious metals, typically platinum and/or palladium. When CO and HC molecules meet the active sites in the presence of oxygen, they are oxidised to carbon dioxide and water at temperatures far lower than they would need without a catalyst. DieselNet’s overview of the diesel oxidation catalyst describes these reactions along with the side reactions discussed later in this article.

The behaviour with temperature is best pictured as an S-shaped curve.

Cold zone. Below a certain temperature the active sites are too sluggish, and some can be blocked by adsorbed CO and heavy hydrocarbons. Conversion is low, sometimes near zero.
Light-off. As exhaust temperature rises, conversion climbs steeply over a fairly narrow band. Engineers often quote the “light-off temperature” as the point where conversion reaches 50% (sometimes written T50).
Plateau. Above light-off, conversion levels off at a high value, limited mainly by how much catalyst the gas sees and how fast it is flowing through.

CO usually lights off first. Hydrocarbons light off somewhat later, and heavier hydrocarbons later still. Fresh catalysts light off at lower temperatures than aged ones, and formulations differ, so there is no single number that applies to every RECD.

An illustrative light-off table

The table below is illustrative only. It shows the shape of typical behaviour for a reasonably sized, healthy oxidation catalyst on diesel exhaust. Real numbers depend on the formulation, precious metal loading, catalyst volume relative to exhaust flow, ageing, and fuel and oil quality. Do not use it as a specification or a guarantee.

Exhaust temperature at catalyst inlet	Typical CO conversion (illustrative)	Typical HC conversion (illustrative)	What is happening
Below about 150 °C	Low, often under 20%	Very low	Catalyst cold; species adsorb rather than react
About 150–200 °C	Rising quickly; may pass 50%	Low to moderate	CO light-off region for many formulations
About 200–250 °C	High, often 80% or more	Rising; may pass 50%	HC light-off region for many formulations
About 250–350 °C	High plateau, often 85–95%+	High, often 70–90%	Normal working zone
Above about 350 °C	Remains high	Remains high	Also the zone where NO to NO2 conversion is strongest

So a catalyst that delivers “90% CO reduction” at a healthy load could deliver a small fraction of that during a cold start or an idling exercise run. Both numbers are honest; they describe different points on the same curve.

Space velocity: the other half of the curve

Temperature is not the only variable. If the catalyst volume is small relative to the exhaust flow, gas spends less time in contact with the active surface and the plateau sits lower. When reviewing an offer, ask the supplier how the catalyst was sized against your engine’s exhaust flow, not just its rating.

Duty cycle: what RECD carbon monoxide and hydrocarbons results look like in each DG mode

A generator’s real emissions are a blend of the modes it actually runs in. Exhaust temperature varies widely by engine, turbocharging and exhaust layout, so measure your own if you can. The pattern below is typical and illustrative.

DG operating mode	Typical exhaust temperature trend	Likely oxidation catalyst behaviour	Practical note
Cold start, first minutes	Starts near ambient, rising	Little conversion until the catalyst warms; HC slip highest	Keep start counts and run durations in your logs
No-load or idle exercise run	Low and may never reach light-off	Partial CO conversion at best; HC poor	Consider exercising on a load bank or real load
Around 25% load	Moderate; may hover near light-off	Variable; CO often partly converted, HC less so	Results swing with ambient and run time
50–75% load	Comfortably above light-off for most designs	Near-plateau conversion of CO and HC	Best band for comparing before and after
Near full load	Highest	High conversion; strongest NO2 formation and any sulphate formation	Check back-pressure and temperature limits

If your load logs show the set rarely goes above 20–30%, read choosing an RECD for a generator running at low load and our explainer on generator load factor and duty cycle. A short-run, low-load standby set is the hardest case for any oxidation catalyst, and it is also the case where a CO/HC claim measured at 75% load tells you least.

Cold starts deserve their own attention

Every start sends a burst of hydrocarbons through a cold catalyst. On a set that starts many times a day for short runs, much of its total HC output may come from these unconverted minutes. The behaviour of RECDs at start-up is covered in whether an RECD can work reliably during cold starts. Practical levers include combining short test runs into fewer, longer, loaded runs and keeping exhaust pipework insulated between the engine and the RECD, where the installation allows it safely.

What degrades conversion over time

A catalyst that performs well at commissioning can lose activity. The main culprits are well known.

Sulphur in fuel

Sulphur compounds adsorb on the catalyst and compete with CO and HC for active sites, raising light-off temperature. Sulphur also leads to sulphate formation (see the side effects section). Using ultra-low sulphur diesel helps both problems; see how ultra-low sulphur diesel improves emission control. If your site keeps old fuel stock or buys from mixed sources, consider that part of your catalyst risk.

Phosphorus and zinc from lubricating oil

Engine oils contain anti-wear and detergent additives, commonly compounds of zinc, phosphorus, calcium and magnesium. When oil burns in the cylinder, these reach the catalyst. Phosphorus and zinc compounds can form glassy deposits that mask active sites, while the incombustible residue adds to ash. An engine that consumes oil heavily will poison a catalyst faster. Our guide on engine oil consumption and RECD suitability explains how to check this before you buy.

Thermal ageing

Very high temperatures cause precious metal particles to grow (sinter) and the washcoat to lose surface area. The result is a light-off curve that shifts to the right: the catalyst needs a hotter exhaust to achieve the same conversion. Prolonged running at very high temperatures, uncontrolled soot burns in a filter, or engine faults that send unburnt fuel to the catalyst can all accelerate this.

Face plugging and masking

Oily soot from wet stacking can coat the inlet face of a catalyst or filter. Even a thin layer reduces the area that gas can reach. Where a design places a catalyst upstream of a filter, the catalyst face is especially exposed. Visual inspection during scheduled maintenance is worth adding to your routine.

Side effects worth knowing before you buy

Oxidation chemistry does not only remove CO and HC. Three side effects belong in any honest discussion.

Heat release. Oxidising CO and HC releases heat. At normal concentrations the temperature rise across a catalyst is modest, but if a fault sends a lot of unburnt fuel to it, the outlet can get significantly hotter than the inlet. That is one reason why engine faults should be fixed promptly rather than tolerated.

NO to NO2 conversion. The same platinum-rich catalyst that oxidises CO also oxidises part of the nitric oxide (NO) to nitrogen dioxide (NO2). In catalysed filter designs this is useful: NO2 helps burn soot at lower temperatures, a process DieselNet describes on its DPF regeneration page. Total NOx, however, is essentially unchanged, and a higher NO2 share can matter for local air quality near the stack. Our sibling article, do RECDs reduce NOx?, explains what technologies actually reduce NOx.

Sulphate formation. With sulphur in fuel, an active catalyst can oxidise sulphur dioxide to sulphur trioxide, which forms sulphuric acid and sulphate particles as the exhaust cools. With high-sulphur fuel this can add to measured PM, offsetting part of what the device achieves. With ULSD the effect is much smaller.

And a point that comes up often: oxidising CO and HC produces CO2. The quantity is tiny compared with the CO2 from burning the fuel itself, but it means an oxidation catalyst cannot be marketed as a CO2 reduction device. For the full reasoning, see the sibling post does an RECD reduce CO2 emissions?.

Which RECD designs reduce carbon monoxide and hydrocarbons?

Indian RECD designs vary widely. Some combine a separate oxidation catalyst ahead of a wall-flow filter, some coat the filter itself, some use flow-through elements, and some (such as electrostatic or wet scrubber types) have no oxidation catalyst at all unless one is added.

Configuration	Expected CO/HC effect	Questions to ask
Uncatalysed filter only	Little effect on gaseous CO and HC; some condensed HC may be trapped with soot	Is any CO/HC claim made at all? On what basis?
Catalysed filter (coating on filter)	Can reduce CO and HC once hot enough	Which precious metals? Tested at which loads?
Separate DOC plus filter	Typically the strongest and most consistent CO/HC reduction once warm	How is the DOC sized against exhaust flow?
Non-catalytic technologies (electrostatic, wet)	Generally limited direct oxidation of CO; some HC may be absorbed or condensed	Ask for measured gaseous data, not assumptions

The differences in day-to-day operation are covered in our sibling post on catalysed vs uncatalysed RECD filters. Do not assume the presence of a catalyst from the price, the colour of the housing or a generic brochure; ask for it in writing.

Regulatory context, stated carefully

The CPCB procedure for type approval of RECDs on in-use DG engines up to 800 kW centres on particulate matter, with a minimum 70% PM capture efficiency. CO and HC reduction is not the headline requirement for that category at the time of writing. Some state boards have issued their own directions: for example, a May 2023 notification from the Karnataka State Pollution Control Board, as reported in compliance news summaries, referred to testing for a minimum 70% reduction in CO, PM and HC for sets above 800 kW. Requirements can change and vary by state, so confirm the current position with your SPCB, your consent conditions and the Central Pollution Control Board before relying on any figure.

How to test CO and HC properly

A CO/HC reduction figure is only as good as the test behind it. Particulate testing gets most of the attention, but gaseous measurement has its own traps.

Analyser types, in general terms

CO is commonly measured with non-dispersive infrared (NDIR) analysers in laboratory and better portable instruments; many handheld combustion analysers use electrochemical cells, which are cheaper but more sensitive to drift, cross-interference and overload.
HC is measured as total hydrocarbons, typically with a heated flame ionisation detector (FID) in laboratories. Some portable instruments estimate HC by NDIR, which responds differently to different hydrocarbons, so results from the two methods are not directly comparable.
Oxygen should be recorded alongside, so concentrations can be corrected to a common reference if required.

Whatever instrument is used, ask for its calibration certificate, span gas details and zero/span checks on the day. Heated sample lines matter for HC, because heavier hydrocarbons condense in cold lines and simply vanish from the reading.

Same load, same warm-up, same points

The comparison must be like for like.

Warm the engine and the RECD to stable temperatures before recording, and log the catalyst inlet temperature if a port or sensor exists. Our post on warm-up time for RECD testing explains how to decide when “stable” means stable.
Match the load for before and after readings. A 10% difference in load can move CO concentrations considerably at the low end of the range; see why generator load must be matched in RECD comparison tests.
Sample upstream and downstream of the device at proper ports, simultaneously if two analysers are available, or alternately in quick succession if not. Port location affects results, as covered in how exhaust sampling location affects RECD test results.
Test more than one load point, including at least one light-load point if the set spends real hours there.
Record ambient conditions, fuel source and recent run history (a set that has just done a long, hot run will look better than one started cold).

Safety: sampling ports sit on hot pipework, often at height. Follow your site’s permit-to-work rules, use heat-resistant gloves, and keep analyser hoses clear of rotating parts and hot surfaces.

Worked example: RECD carbon monoxide and hydrocarbons over a duty cycle

The following numbers are illustrative, chosen to show the arithmetic. They are not measurements from any particular RECD or engine.

A facility tests its DG set with an oxidation-catalyst RECD at two load points, warm, with matched load:

Load point	CO before RECD (ppm)	CO after RECD (ppm)	Reduction at this point
60% load	400	40	(400 − 40) ÷ 400 = 360 ÷ 400 = 90%
15% load	600	420	(600 − 420) ÷ 600 = 180 ÷ 600 = 30%

The site’s logs show the set spends 70% of its running hours at about 60% load and 30% at about 15% load. What is the real reduction over the duty cycle?

Wrong approach 1: simple average of the two percentages

(90% + 30%) ÷ 2 = 60%. This ignores how long the set spends in each mode and how much exhaust flows in each. It has no physical meaning.

Wrong approach 2: hour-weighted average of the percentages

(0.70 × 90%) + (0.30 × 30%) = 63% + 9% = 72%. Better, but it still treats a percentage at light load as if it carried the same amount of pollutant as a percentage at heavy load.

Better approach: weight the concentrations by exhaust flow and hours

CO mass emitted is roughly proportional to concentration multiplied by exhaust flow multiplied by time. Exhaust flow is lower at light load. As a simplifying illustrative assumption, take the exhaust flow at 15% load as half of that at 60% load (relative flow 0.5 versus 1.0). Real ratios depend on the engine and turbocharger, so use OEM data or measurement where possible; our post on OEM exhaust flow data for specifying an RECD explains where to find it.

Relative CO emitted before the RECD:

60% load: 400 × 1.0 × 0.70 = 280
15% load: 600 × 0.5 × 0.30 = 90
Total before = 280 + 90 = 370

Relative CO emitted after the RECD:

60% load: 40 × 1.0 × 0.70 = 28
15% load: 420 × 0.5 × 0.30 = 63
Total after = 28 + 63 = 91

Duty-cycle reduction = (370 − 91) ÷ 370 = 279 ÷ 370 = about 75.4%.

What if you assume equal exhaust flow?

If you ignore the flow difference and weight only by hours:

Before: (400 × 0.70) + (600 × 0.30) = 280 + 180 = 460
After: (40 × 0.70) + (420 × 0.30) = 28 + 126 = 154
Reduction = (460 − 154) ÷ 460 = 306 ÷ 460 = about 66.5%

What the example teaches

The same device, on the same engine, can honestly be described as achieving 90%, 75%, 67%, 60% or 30%, depending on which number you compute. Always ask which one a claim refers to.
After the RECD, light-load running produced 63 of the 91 remaining units, about 69% of what was left, despite being only 30% of the hours. Light load is where the remaining CO lives.
Improving duty cycle (fewer idle runs, more loaded exercise) can raise the real-world reduction more than any change to the hardware.
These ppm values are not corrected for dilution or oxygen. If the test introduces dilution air or the oxygen content differs between points, apply a consistent correction; see how dilution can mislead before-and-after measurements.

The same method applies to HC, so any claim about RECD carbon monoxide and hydrocarbons reduction should state which calculation it uses. Typically the gap between light-load and mid-load performance is even larger for HC, because HC lights off at higher temperatures than CO.

Turning the light-off story into buying actions

Ask what oxidises the gases (separate DOC, catalysed filter, or none) and get it in the technical offer.
Ask at what loads CO/HC data were measured, and request data at your typical load. Any guarantee should name load, warm-up, fuel and test method; see RECD performance guarantees and operating conditions.
Re-test periodically at the commissioning load point, so drift from ageing or poisoning shows up as a trend.
Never bypass the device to cure a back-pressure or smell problem. Removing or bypassing an RECD is a compliance risk and can lead to sealing and penalties.

FAQ

Does every RECD reduce carbon monoxide?

No. Only designs that include an oxidation catalyst, either as a separate stage or as a coating on the filter, can oxidise CO meaningfully. An uncatalysed filter or a purely mechanical trap does little for gaseous CO.

What is the light-off temperature of a diesel oxidation catalyst?

It is the exhaust temperature at which conversion rises steeply, often defined as 50% conversion. For CO it is commonly somewhere around 150–200 °C for fresh catalysts and higher for HC, but it varies with formulation, ageing, sulphur exposure and flow. Ask your supplier for the figure for their design.

Why is CO high on my DG set at light load even with an RECD?

At light load the engine makes more CO and the exhaust may be too cool for the catalyst to reach its full conversion. Both effects combine. Loading the set more during exercise runs usually helps.

Can an RECD reduce unburned hydrocarbons and odour?

A warm oxidation catalyst can reduce gaseous hydrocarbons substantially, and many odour compounds with them. During cold starts and idle runs the reduction is much smaller.

Does a catalyst that reduces CO and HC also reduce NOx?

No. It converts part of the NO to NO2, but total NOx is essentially unchanged. NOx reduction needs other technologies such as SCR.

Is CO and HC reduction required for RECD approval in India?

The CPCB RECD procedure for engines up to 800 kW centres on at least 70% PM capture. Some states have issued their own requirements for larger sets, so check your SPCB directions and consent conditions.

Want CO and HC Numbers You Can Trust?

Aceget supplies CPCB type-approved retrofit emission control devices for DG sets from 25 kVA to 2500 kVA. Aceget’s product page describes its RECD as using a silicon carbide ceramic filter with a precious-metal coating and an oxidation catalyst stage, which is the kind of configuration this article discusses. Even so, we would rather you see the conditions than take a number on trust: share your DG nameplate, load logs and typical run pattern, and we will explain what to expect at your actual operating points. Explore the Aceget RECD range, review our type approval certifications, or contact our engineering team to plan a properly matched before-and-after test.

Can an RECD Reduce Carbon Monoxide and Unburned Hydrocarbons?