Can an RECD Reduce Diesel Exhaust Odour? A Suspect-by-Suspect Look

Short answer: yes, an RECD can reduce some of the smell that comes out of a diesel generator, but it will not remove every odour, and it will not fix smells that never travel through the exhaust pipe in the first place. How much you notice depends on what is causing the smell, how hot the exhaust runs, and what fuel and load the set sees.

That answer may sound less exciting than a sales brochure, so this guide takes a slower route. Instead of listing benefits, we will line up the five usual “suspects” behind a diesel smell and check, one by one, whether a retrofit emission control device (RECD) touches them. By the end you will know which smells to expect to soften, which ones point to an engine or site problem, and how to measure the change instead of relying on memory.

If you are new to the device itself, you may want to read what an RECD is and how it works first. Here we stay focused on one question: odour.

First, What Exactly Is “Diesel Smell”?

People use the word “smell” for several different experiences, and mixing them up leads to wrong fixes.

Some people mean the raw, oily, fuel-like smell that hangs around a generator during start-up. Others mean an eye-watering, sharp and slightly sweet smell that shows up when the set is worked hard. Some mean a rotten-egg or acrid note. A few mean a hot, burnt-oil smell. And some are actually smelling the generator room, the fuel tank vent or a leaking joint, not the exhaust at all.

Exhaust smell has one main root: fuel and fuel-derived compounds that leave the engine without burning completely. A study on stationary direct-injection diesel engines published in Applied Energy on ScienceDirect notes that unburned and partially burned fuel are mainly considered responsible for exhaust odour. That framing is helpful, because it tells us where to look. If a device can finish the job of oxidising those leftover compounds, odour should fall. If the smell comes from somewhere else, the device has nothing to work on.

Most RECDs pair an oxidation catalyst with a particulate trap, which is the design described on many CPCB-approved product pages, including Aceget’s RECD page. The catalyst half is the part that matters for smell. Its job is to convert hydrocarbons and carbon monoxide into carbon dioxide and water. DieselNet’s technical page on diesel oxidation catalysts states that oxidising hydrocarbons results in a reduction of diesel odour, and that aldehydes, which are a non-regulated but smelly group of compounds, can also be oxidised to harmless products.

So the mechanism is real. What matters now is whether it applies to your particular smell. Time to meet the suspects.

The Smell Line-Up: Five Suspects

Suspect 1: Raw and Part-Burnt Fuel

What it smells like: Oily, heavy, “fuel-in-the-air”, often strongest at start-up, at idle, and at light load.

Why it happens: A cold engine does not burn every drop of fuel. Small injection quantities at idle or light load can leave more fuel partly burned. Long periods of running at low load make this worse, and the residue collects in the exhaust system.

What an RECD does: This is where a catalyst helps most. Once the exhaust is hot enough for the catalyst to become active, hydrocarbons passing through the device are oxidised instead of leaving the stack as vapour. The result should be a noticeably less oily smell during normal running.

The catch: The first minutes after a cold start are the weak spot, because the catalyst has not yet reached working temperature. A generator that starts, runs for four minutes at no load, and shuts down may never get the device fully warm. In that pattern, an RECD helps less than owners expect.

Verdict: Likely to improve, with limits at cold start and very light load.

Suspect 2: Aldehydes and Other Partial-Oxidation Products

What it smells like: Sharp, pungent, sometimes sweetish, and often linked to stinging eyes when the exhaust is close by.

Why it happens: Aldehydes form when fuel partly oxidises during combustion instead of burning to the end. They are more likely when the flame is quenched, for example during cold running or heavy transient loading.

What an RECD does: Oxidation catalysts are designed to deal with exactly this family of compounds. In the DieselNet reference mentioned above, aldehydes are listed among the compounds that a diesel oxidation catalyst can convert to harmless products.

The catch: Same temperature dependence. Also, the catalyst treats what reaches it, so an engine that is producing far too many partial-oxidation products because of a fuel or air problem will still smell, only less so than before.

Verdict: Likely to improve, especially for the sharp, irritating character of the smell.

Suspect 3: Sulphur Compounds

What it smells like: Acrid, “burnt match”, or at worst a rotten-egg note.

Why it happens: Sulphur in the fuel burns to sulphur dioxide. The more sulphur in the diesel, the stronger this component. India moved to BS-VI fuel with a maximum of 10 ppm sulphur, down from 50 ppm in BS-IV, according to a PIB release on fuel quality. Fuel bought in bulk, stored for a long time, or supplied by uncertain sources may not always be as clean as you assume.

What an RECD does: It does not remove sulphur. Worse, if fuel sulphur is high, an oxidation catalyst can convert some sulphur dioxide into sulphate particles, which is why the US EPA’s Diesel Oxidation Catalyst General Information sheet says these catalysts work best with ultra low sulphur diesel (15 ppm or lower in the US context). Our own article on ultra-low-sulphur diesel and emission control explains the fuel side in more detail.

Verdict: No help, and possibly a slight penalty with poor fuel. Fix the fuel, not the device.

Suspect 4: Burning Lubricating Oil

What it smells like: Hot, burnt, slightly sweet oil, often with pale blue-grey smoke, more noticeable after long idling or on older engines.

Why it happens: Worn piston rings, valve stem seals, a turbocharger seal leak, or an overfilled sump can push oil into the combustion chamber or exhaust.

What an RECD does: Very little for the cause. In fact, oil ash reaching a particulate trap over time is one of the reasons filters need servicing. An engine that burns oil should be repaired before, or at least alongside, RECD installation. Our guide to blue smoke from generators walks through the common causes.

Verdict: Not an RECD job. Treat the engine.

Suspect 5: Smells That Never Pass Through the Exhaust

What it smells like: Diesel fumes inside the generator room, near the fuel tank, near the canopy, or at ground level next to the set even while the engine is off.

Why it happens: Several sources sit outside the exhaust system entirely:

• fuel tank vents and filler caps
• weeping fuel line joints, injectors or filters
• crankcase breather fumes on older engines
• oil-soaked lagging or insulation on hot surfaces
• spilled fuel or oil on the floor or in the drip tray
• exhaust joints that leak upstream of the RECD

What an RECD does: Nothing for the first five. For the last one, a leaking joint before the device allows raw exhaust to escape, and the device cannot treat what it never receives.

Verdict: Not an RECD job. A careful walk-around with a torch usually finds the cause.

The Verdict Table

Smell source	Can an RECD help?	What actually fixes the rest
Raw and part-burnt fuel	Yes, once the catalyst is hot	Load the set properly, service injectors, avoid long idling
Aldehydes (sharp, stinging)	Yes, usually the clearest gain	Correct air supply, healthy injectors, warm running
Sulphur compounds	No	Buy clean BS-VI diesel from a reliable source
Burning lube oil	Little to none	Repair oil consumption, check turbocharger seals
Non-exhaust smells	No	Fix leaks, vents, lagging and housekeeping

Read across the table and a pattern appears. An RECD is a specialist for one group of smells: incomplete-combustion products in the exhaust gas. For the other groups it stays quiet.

Why Temperature Decides How Much Smell You Lose

A catalyst is not a filter that works from the first second. It needs heat. Below a certain temperature the reactions are slow, and hydrocarbons slip through unchanged. Once the exhaust has warmed the catalyst, conversion improves quickly.

That temperature dependence explains a common complaint: “The smell is better during the long run, but still bad for the first few minutes.” That is expected behaviour, not a defect. Several patterns keep the device cooler than it should be:

1. Short runs, such as a two-minute weekly test at no load.
2. Long idle periods between load pick-ups.
3. Very lightly loaded sets, for example a large generator that carries only a small office load.
4. Cold, damp or winter conditions with a long exhaust run that loses heat before it reaches the device.

Cummins advises in its maintenance guidance for diesel generator sets that a set should be exercised at least once a month for at least 30 minutes, loaded to no less than one third of the nameplate rating, and that no-load running should be limited because unburned fuel tends to accumulate in the exhaust system. That is good engine advice on its own, and it also happens to be good odour advice, because a hotter exhaust gives the catalyst more to work with. If your set runs only now and then, our companion article on whether an RECD suits a DG set that runs only occasionally covers that pattern in detail.

What the Evidence Does and Does Not Tell Us

It is worth being careful here, because odour claims are easy to overstate.

What is well supported: incomplete combustion products are the main source of diesel exhaust odour, and oxidation catalysts convert those products. That is the basis of the expected improvement.

What is less well supported: a specific percentage of “odour reduction” for a given RECD. The CPCB-recognised laboratory testing for RECDs, which the Maharashtra Pollution Control Board circular summarised by TeamLease RegTech describes as requiring at least 70 percent reduction in particulate matter, is built around emission measurements, not smell panels. So the certificate on your unit will tell you about particulate performance. It will not promise a smell score.

The ScienceDirect study mentioned earlier tested a different technology, a water-scrubbing and air-dilution arrangement, and found roughly a 70 percent odour reduction with that system. We cite it for what it says about the cause of odour, not as a claim about RECD performance. If a vendor quotes you an odour reduction percentage for an RECD, ask for the test method and the report behind it.

In practice, owners typically describe the change as “less oily, less sharp, less lingering” rather than “no smell”. That matches the mechanism.

The First Week After Installation

New exhaust hardware can behave a little differently at first. Protective coatings, insulation and mounting materials on new components may release a burning smell as they heat up for the first time. This usually fades within the first few hours of running, but confirm with your installer what is normal for the unit you have chosen, and ask them to check joints and mounts after the first proper load run.

Signs that deserve a call to the vendor rather than patience:

• A smell of raw fuel that gets stronger, not weaker, over several runs
• White or grey smoke that appears where none was seen before
• Visible exhaust leaks at flanges, marked by black sooting around a joint
• A hot, sharp smell from the device housing itself
• Rising exhaust temperature or a loss of power together with any of the above

Those can point to a joint that was not sealed properly, a restriction that is higher than the engine tolerates, or an engine problem that the retrofit has simply exposed. Our note on how exhaust back pressure affects a DG set explains why restriction matters.

Build a Smell Log Before You Decide Anything

Memory is a poor instrument. People forget how bad a smell was two months ago, and they tend to judge a change by their mood on the day. A simple log turns “I think it is better” into something you can show a vendor, a facility manager or a neighbour.

Keep the conditions the same each time: same person, same spot, same time of day if possible.

Paste-ready smell log

Date	Load (%)	Run time before check	Engine warm or cold	Spot (5 m behind stack / at gate / in plant room)	Oily (0 to 5)	Sharp or stinging (0 to 5)	Acrid (0 to 5)	Notes

How to use it:

1. Record five entries before the RECD goes on, covering cold start, warm running and shut-down.
2. Record the same five entries after installation, under similar load.
3. Compare the columns. A drop in “oily” and “sharp” with no change in “acrid” tells you the catalyst is working and the remaining smell is sulphur or something outside the exhaust.
4. Add a photo of the exhaust tip and a note on wind direction if a neighbour complains.

This takes ten minutes per session and is far more persuasive than opinion.

Five Cheaper Fixes That Also Cut Odour

If smell is your only reason for calling a vendor, try these first. They cost little and are sensible whether or not you ever fit a device.

1. Load the set properly. A generator running at 10 percent load for hours smells worse than one at 50 percent. Reviewing your load factor and duty cycle may show that the set is oversized for its normal use.
2. Keep the air path clean. A choked air filter reduces the air available for combustion and pushes smell and smoke up. Follow the air filter maintenance guide.
3. Service injectors and fuel filters on time. Worn injectors spray poorly and leave more fuel partly burned. The monthly maintenance checklist covers the routine items.
4. Use fresh, clean fuel. Stale or contaminated diesel changes combustion and adds smell. Keep the tank topped up and drain condensation.
5. Look at where the exhaust goes. Fumes that drift into an air intake, a window or a ground-floor gathering area feel worse than they are. Pollution control rules include stack height requirements linked to the generator set and the building, so discuss routing and height with your installer and check the current requirement with your State Pollution Control Board. A well-placed, correctly sized stack can matter as much for complaints as any device.

If smoke is also part of the problem, our guides to reducing smoke from DG sets, black smoke and white smoke help you sort out the cause first.

Should Odour Alone Justify Buying an RECD?

Usually no, and it is better to say so plainly.

For most owners, the reason to fit an RECD is compliance. Older diesel sets are covered by the retrofit requirement in many states, and running without a certified device can invite penalties or sealing. We explain the regulatory background in why an RECD is mandatory in India and the wider gains in the benefits of installing an RECD.

Seen that way, odour improvement is a useful side benefit rather than the main purchase reason. It is a reasonable tiebreaker when you are comparing options, or when neighbours have complained and you need one more argument to move a budget through. It is a weak reason on its own if your set is already compliant and the smell comes from a leak or a sulphur-heavy fuel batch.

One more point: maintenance affects long-term odour. A particulate trap that is overloaded with soot or ash raises exhaust restriction, and a restricted engine runs hotter and less cleanly. Following the schedule in our RECD maintenance guide keeps the device doing its job. Kirloskar’s RECD product page, for example, lists servicing about once every six months or 500 hours, which shows the kind of interval to expect from a well-designed unit.

Questions to Put to a Vendor About Odour

Bring these to any RECD conversation. They protect you from vague promises.

1. Does your device include an oxidation catalyst, and what is its size relative to my engine displacement?
2. At what exhaust temperature does it become effective, and does my duty cycle reach that temperature?
3. What is the expected back pressure at my set’s rated load, and how does it compare with the engine maker’s limit?
4. Can you share a test report that covers hydrocarbons and carbon monoxide, not only particulate matter?
5. Have you seen odour complaints resolved on sets like mine, and what else did you change besides fitting the device?
6. What fuel quality does the device expect, and what happens if I use higher-sulphur diesel?
7. What should I look for in the first week, and who do I call if smell increases?

A vendor who answers these clearly is more likely to give you a device that suits your set.

Frequently Asked Questions

Will an RECD remove diesel smell completely?

No. Expect a reduction in the oily and sharp components of exhaust smell, especially at normal operating temperature. Some smell will remain, because sulphur compounds, cold-start emissions, leaks and non-exhaust sources are outside the device’s reach.

Why does my generator still smell after RECD installation?

Common reasons are short runs that never warm the catalyst, very light load, high-sulphur fuel, a leaking joint upstream of the device, or oil consumption. Check the smell log, then look at load pattern, fuel quality and joints in that order.

Does an RECD reduce black smoke as well as smell?

The particulate trap in an RECD is designed to capture soot, which is what makes black smoke visible. Smell and smoke are related but not identical, so you may see a bigger change in one than the other. Persistent black smoke usually signals an engine or air supply problem worth fixing regardless.

Is diesel exhaust smell harmful?

Smell is a warning sign, not a measurement. Exhaust contains particulate matter and gases that are unhealthy even when you cannot smell them, so a lack of odour does not prove clean air. Follow site safety rules and keep exhaust routed away from people and air intakes.

Can a bad RECD cause more smell?

A blocked or badly installed device can raise back pressure and worsen combustion, which may increase smoke and smell. Leaks at joints can also let raw exhaust escape. That is why installation quality and a first-week check matter.

The Bottom Line

An RECD can take the edge off diesel exhaust odour, mainly by oxidising the unburned and partly burned compounds that give exhaust its oily and sharp character. It cannot fix sulphur-heavy fuel, oil burning, leaks or fumes that come from anywhere other than the exhaust. Its benefit shows up most once the exhaust is hot and the set carries a decent load.

The practical plan is simple: work out which suspect you are dealing with, fix the cheap causes first, keep a smell log before and after any change, and choose a device on compliance and engineering grounds, treating odour relief as a welcome extra.

If you would like help matching a certified device to your set, or checking whether the smell you are seeing is an RECD matter at all, contact the Aceget team with your generator nameplate details, or read more on the Aceget RECD page.