Can RECD Efficiency Change Between New and Aged Conditions?

A Retrofit Emission Control Device is usually bought on the strength of one number: its particulate reduction, measured when the device was type-approved and again, perhaps, when it was commissioned on your generator. It is natural to assume that number stays the same for the life of the device.

It doesn’t, quite. An RECD is a working filter and, in most designs, a working catalyst. Both change as they’re used. Some of those changes improve filtration. Others slowly reduce it, or add back-pressure that affects the engine. A few, such as a cracked substrate or a failed seal, can cut performance suddenly.

This article follows an RECD through its life, from the day it’s installed to its later years, and explains what happens to its efficiency at each stage, why, and how you can track it without running an expensive lab test every few months.

The Short Answer

Yes, RECD efficiency can change between new and aged conditions, and the change isn’t always downward.

• In the first operating hours, a filter-type RECD often becomes more efficient as a layer of trapped soot forms and acts as an extra fine filter.
• Over the following months and years, ash from lubricating oil builds up permanently, the catalyst coating slowly loses activity, and mechanical wear can create leak paths.
• Well-maintained devices on healthy engines can hold their performance for a long time. Poorly maintained devices, or those on engines that burn oil, can decline faster.

The practical response is to track simple indicators over time and to retest when they suggest something has changed.

Stage 1: Brand New (Day 0)

What the device looks like inside

A new filter substrate is clean. Its porous walls have no deposits, and its catalyst coating is fresh. Some coatings and binders also go through a short “settling” period in their first heat cycles.

What happens to efficiency

A clean wall-flow filter captures particles mainly through deposition inside the porous wall structure. That works well, but it isn’t the filter at its best. Until a thin layer of soot builds up on the inlet surface of the channel walls, slightly more fine particles can pass through.

A brand-new device may therefore show slightly lower particulate efficiency in its first hours than it will after a short period of use. Its catalyst, on the other hand, is at peak activity, so CO and HC conversion is often at its best.

Why it matters

If the acceptance test is run on commissioning day, the result may slightly understate the device’s settled performance. That is one reason many test plans include a short break-in period before acceptance testing. See RECD trial installation acceptance criteria and how to account for warm-up time when testing an RECD.

Stage 2: The First Operating Hours (Soot Cake Formation)

What happens inside

As exhaust flows through, soot particles collect on the inlet side of the porous walls. Within a relatively short period of running, they form a thin, continuous soot cake. This layer is itself a very fine filter, with pores much smaller than those of the substrate.

What happens to efficiency

Filtration efficiency typically rises as the soot cake forms, then levels off. For a well-designed wall-flow filter, efficiency in this settled state is at its highest.

Back-pressure rises too, because gas must now pass through both the soot cake and the wall. This is normal. The device’s regeneration strategy, passive, active or both, is designed to stop the soot layer from growing without limit.

Why it matters

From this stage on, efficiency depends partly on how much soot is in the filter. Right after a full regeneration, the soot cake is thinner and efficiency can dip for a while until it rebuilds. We cover this in RECD testing during regeneration.

Stage 3: Settled Operation (Months to Early Years)

For a well-matched device on a healthy engine, this is the long, stable middle of its life. Soot loads and clears in a regular cycle, back-pressure moves within a predictable band, and particulate efficiency stays close to its settled value.

Stability isn’t guaranteed. It depends on:

• Duty cycle. Sets that run long enough at adequate load clear soot passively. Sets that run briefly or at very light load may build up soot faster than they clear it. See choosing an RECD for a generator running at low load and RECD selection for DG sets with frequent short start-stop cycles.
• Engine health. Rising oil consumption or worsening combustion feeds more material into the device.
• Fuel quality. Consistent low-sulphur fuel protects the catalyst. See how ultra-low sulphur diesel improves emission control.

Stage 4: The Ageing Years (Slow Changes)

Several slow processes accumulate over thousands of operating hours.

Ash accumulation

Lubricating oil contains additives, such as calcium, zinc, phosphorus and magnesium compounds. When oil burns in the engine, these leave ash, a non-combustible residue. Ash collects in the filter alongside soot, but unlike soot it doesn’t burn off during regeneration.

Over time, ash:

• Raises baseline back-pressure, even straight after a regeneration
• Reduces the effective filter volume, because ash typically collects towards the closed ends of the inlet channels
• Changes how soot deposits, which can affect regeneration behaviour

Ash doesn’t usually reduce particulate filtration efficiency directly. A layer of ash can actually filter well. Its main effects are higher back-pressure and reduced soot capacity, which can lead to more frequent or less complete regeneration. DieselNet’s overview of ash accumulation in diesel particulate filters explains the mechanisms in detail.

The rate of ash build-up depends heavily on oil consumption. An engine that burns oil fills the filter with ash much faster. See engine oil consumption and RECD suitability.

Catalyst deactivation

Catalytic coatings lose activity over time through:

• Thermal ageing. Repeated high temperatures, especially during active regenerations or uncontrolled soot burns, can make the precious-metal particles in the coating grow larger. Larger particles mean less active surface area.
• Poisoning. Sulphur from fuel and oil, and phosphorus and zinc from oil additives, can cover or react with active sites. Sulphur effects can be partly reversible at high temperature. Phosphorus and zinc effects are generally permanent.
• Masking. Ash and other deposits can physically cover the coating.

A less active catalyst converts CO and HC less effectively and makes less NO₂. In passively regenerating designs, less NO₂ means less soot burned at normal exhaust temperatures. So catalyst ageing can affect regeneration, not just gaseous emissions.

Particulate efficiency may also shift. A catalyst that oxidises less of the organic fraction lets more of it pass through and condense downstream as particulate mass.

Mechanical and thermal stress

Every heat-up and cool-down cycle expands and contracts the substrate, housing and mounting materials. Over many cycles, this can cause:

• Cracks in the substrate, often from uncontrolled soot burns or thermal shock
• Degradation of the mounting mat that holds the substrate in the housing, allowing it to shift
• Gasket or flange leaks that let exhaust bypass the filter

These produce sudden rather than gradual losses in efficiency, because even a small bypass path lets unfiltered exhaust through. They often show up as an unexplained drop in back-pressure together with rising downstream opacity.

Good installation practice reduces mechanical stress. See flexible bellows in an RECD exhaust system and thermal expansion planning for RECD exhaust connections.

Moisture and corrosion

Generators that sit idle for long periods, especially in humid or coastal sites, can collect condensation in the exhaust and device. Repeated wetting can corrode housings and degrade some materials. See choosing an RECD for coastal salt-laden environments and protecting an RECD from rainwater and condensation.

How Different RECD Designs Age

Not every RECD ages the same way. The design determines which mechanisms matter most.

Catalysed wall-flow filters with passive regeneration

These rely on the catalyst to produce NO₂ and burn soot at normal exhaust temperatures. Their particulate filtration is mostly physical and tends to hold up well. Their weak point is the catalyst. As it ages or is poisoned, passive regeneration slows, soot builds up faster, and back-pressure and the risk of uncontrolled burns rise. Watch ΔP trends and regeneration behaviour closely, especially on light-load sets.

Wall-flow filters with active (heater or fuel-assisted) regeneration

These depend less on catalyst activity to clear soot, because they raise temperature when needed. They add components that can age: heaters, sensors, controllers and wiring. Frequent active regenerations also mean more thermal cycles, which can stress the substrate. Check controller logs and heater operation as well as ΔP. See RECD controller and heater power supply requirements.

Partial-flow or flow-through designs

Because part of the exhaust passes alongside the filtering media instead of through it, these designs are less likely to block completely. Their particulate efficiency depends more on soot loading and flow conditions, and it can vary more over the device’s life.

Oxidation catalyst stages

Where an RECD includes a separate oxidation stage, its ageing mainly affects CO, HC and the organic part of particulate. A tired oxidation catalyst also reduces the NO₂ available for passive soot burning downstream.

Ask your supplier which mechanisms matter most for your device’s design, and set up monitoring to match. Our guide on what a diesel particulate filter is and how it helps explains the basic filter types.

Efficiency Can Move Up or Down: A Summary

Mechanism	Typical timescale	Effect on PM efficiency	Effect on back-pressure	Reversible?
Soot cake formation	First operating hours	Up	Up	Cycles with regeneration
Regeneration (soot cleared)	Recurring	Temporarily down	Down	Yes, as soot rebuilds
Ash accumulation	Thousands of hours	Usually neutral	Steadily up	By cleaning
Catalyst thermal ageing	Long term, faster with overheating	Slightly down (organic fraction)	Indirectly up (slower regeneration)	No
Catalyst poisoning (S, P, Zn)	Long term	Slightly down	Indirectly up	Partly (S), mostly not (P, Zn)
Substrate cracking	Sudden	Sharply down	Down	No; replace substrate
Seal or gasket leak	Sudden or gradual	Down	Down	Yes, by repair

The pattern to remember: back-pressure rising slowly usually means ash or soot is building up. That calls for maintenance, but filtration usually still holds. Back-pressure falling unexpectedly together with rising smoke usually means a crack or leak, and filtration is compromised.

Worked Example: Tracking an RECD Across Its Life

The figures below are illustrative, for a 160 kVA set with a catalysed wall-flow RECD running about 500 hours a year at moderate load. Differential pressure (ΔP) is read at the same reference load each time. Reduction figures come from periodic gravimetric tests.

Operating hours	ΔP at reference load (mbar)	Downstream opacity (%)	Measured PM reduction	What’s happening
0 (commissioning)	28	1.2	71%	New filter, no soot cake yet
50	41	0.6	78%	Soot cake established
1,000	44	0.6	77%	Settled operation
2,500	52	0.7	76%	Gradual ash build-up
4,000	63	0.8	75%	More ash; cleaning planned
4,050 (after ash cleaning)	42	0.7	76%	ΔP restored
5,600	35	2.9	58%	ΔP falling + opacity rising: suspected crack
5,650 (after substrate replacement)	30	1.1	72%	New substrate, rebuilding soot cake

Three lessons stand out:

1. The commissioning result (71%) was the device’s lowest settled reading until the failure. Judging the device on day 0 alone would have understated its normal performance.
2. Ash build-up raised back-pressure steadily but didn’t reduce particulate efficiency much. Cleaning restored ΔP.
3. The crack showed up first in the simple indicators (ΔP falling, opacity rising) before anyone ran a lab test. Those indicators prompted the investigation.

How to Track Ageing Without Constant Lab Tests

Gravimetric lab tests are the only way to measure particulate reduction properly, but they are too expensive to run often. These simpler indicators, logged regularly, give early warning.

1. Differential pressure at a reference load

Choose one reference load, for example 50% of rated kW, and record ΔP across the RECD at that load, after the set is warm, at every service or monthly test run. Plot it against operating hours.

• Slow rise: ash or soot build-up. Plan cleaning or check regeneration.
• Step down without cleaning: possible crack, seal failure or bypass. Inspect.

2. Downstream opacity

A calibrated opacity meter at the stack gives a quick, cheap check. A filter-type RECD in good condition usually keeps opacity very low. A clear rise at the same load is a warning sign. See smoke opacity vs particulate mass.

3. Engine back-pressure

Record back-pressure at the engine outlet and compare it with the engine maker’s limit. See how exhaust back-pressure affects a DG set.

4. Regeneration frequency (for actively regenerating devices)

If the controller logs regenerations, track how often they happen. More frequent regenerations at the same duty cycle often mean ash is reducing soot capacity, or that the catalyst is less active.

5. Oil consumption

Rising oil consumption speeds up ash build-up and catalyst poisoning. It’s an engine problem that becomes an RECD problem.

6. Periodic gravimetric tests

At intervals that suit your consent conditions and contracts, or whenever the indicators above suggest a change, run a proper particulate test. Compare it with the commissioning baseline using the same method and basis. See how to compare RECD test results from different laboratories.

Our RECD maintenance guide explains how to fit these checks into routine servicing.

What Slows Down Ageing

Practice	Why it helps
Keep engine oil consumption low	Less ash, less catalyst poisoning
Use the oil grade the engine maker specifies	Lower-ash oils produce less filter ash
Use clean, low-sulphur fuel from reliable sources	Protects the catalyst
Avoid long periods of very light-load running	Prevents excessive soot build-up and uncontrolled burns
Follow the supplier’s regeneration and cleaning schedule	Stops soot and ash reaching damaging levels
Fix exhaust leaks and support pipework properly	Reduces mechanical stress and bypass
Protect the device from water ingress during idle periods	Reduces corrosion

Many of these are ordinary good generator maintenance. See the monthly maintenance checklist for diesel generators and how often to change generator engine oil.

What Real-World Studies Suggest

Published research on particulate filters in long-term service generally shows that well-designed wall-flow filters keep high filtration efficiency over long operating periods. The main long-term effects are rising back-pressure from ash, and changes in regeneration behaviour, not a collapse in filtration. One example is a study of particle filter properties after 2,000 hours of real-world operation. Research on ash, such as work available through MIT’s DSpace repository, also links ash build-up to back-pressure and regeneration effects.

These findings match field experience with DG sets: most ageing problems show up first as back-pressure and maintenance issues. Sudden efficiency losses are usually caused by damage or leaks rather than slow wear.

Warranty and Contract Implications

Because efficiency changes over life, contracts should say when and under what conditions performance is guaranteed:

• Is the guarantee for the acceptance test only, or for a defined number of operating hours?
• What maintenance must the owner carry out to keep the guarantee valid?
• What indicators (ΔP, opacity) trigger a supplier inspection?
• Are ash cleaning and substrate replacement included, and at what intervals?

See RECD warranty terms: parts, labour and exclusions, RECD performance guarantees and operating conditions and what a “maintenance-free RECD” claim actually means.

Related Reading

• RECD particle number vs particle mass: what each test shows
• How to interpret uncertainty in an RECD emission reduction claim
• Why RECD test results may differ from day-to-day site readings
• When should an RECD be retested after changes to the DG set?
• Is buying a used RECD a sensible option?

Frequently Asked Questions

Does an RECD get less efficient every year?

Not necessarily. Filtration efficiency in a well-maintained filter-type RECD often stays high for a long time. What changes steadily is back-pressure, mainly from ash. Gradual catalyst ageing can slightly reduce conversion of CO, HC and the organic part of particulate. Sudden drops are usually due to damage or leaks.

Why was my RECD’s efficiency lower at commissioning than later?

A new filter has no soot cake. As soot builds up on the filter walls in the first operating hours, it acts as an extra filter and efficiency rises. A commissioning-day test can therefore understate settled performance.

Can cleaning restore an aged RECD?

Ash cleaning restores back-pressure and soot capacity. It doesn’t reverse catalyst poisoning or repair cracks. If the catalyst is heavily deactivated or the substrate is damaged, replacement may be needed.

How often should an aged RECD be tested?

Track ΔP and opacity at regular service intervals, and run a gravimetric test when they suggest a change, when your consent requires it, or when the DG set changes. See when should an RECD be retested after changes to the DG set.

Does a used RECD perform like a new one?

It depends on its history: operating hours, ash load, any overheating events and the condition of the substrate and catalyst. Ask for its maintenance and test records before buying or transferring one. See can an RECD be transferred to another diesel generator.

Keep Your RECD Performing Through Its Life

An RECD’s performance isn’t fixed on delivery day. It depends on how the engine, fuel, duty cycle and maintenance treat it afterwards. Aceget supplies CPCB type-approved Retrofit Emission Control Devices for DG sets from 25 kVA to 2500 kVA, with guidance on monitoring and maintenance across the device’s life. See our certifications, read about how an RECD works, or contact our team with your ΔP and service records.