- October 15, 2026
- Posted by: editor
- Category: RECD & Emission Control Devices
Most Retrofit Emission Control Devices fitted to diesel generators in India include a particulate filter. A filter does one simple thing: it traps soot. Trapped soot can’t stay there forever, so at some point the device has to clear it. That clearing process, regeneration, is good for the device and bad for anyone trying to take a clean, repeatable emission measurement.
If a sample happens to overlap a regeneration event, or is taken just after one, the result can differ sharply from what the device delivers the rest of the time. Sometimes the difference is a few percent. Sometimes it’s enough to turn a pass into a fail.
This article is built around one practical question: what should be written down so that regeneration can be identified, explained, and either excluded from or reported alongside the main test result?
First, What Kind of Regeneration Does Your RECD Use?
Before planning what to record, find out how the device clears soot. The answer changes what you watch for.
Passive (continuous) regeneration
Many RECDs combine a catalyst with the filter. The catalyst converts some NO in the exhaust to NOâ‚‚, and NOâ‚‚ oxidises soot at temperatures well below what soot needs to burn in plain oxygen. As long as exhaust temperature stays above the device’s working threshold for long enough, soot is cleared continuously and in small amounts.
What it looks like during a test: usually nothing dramatic. A slow, steady balance between soot arriving and soot being burnt. Differential pressure across the device stays roughly flat at a given load.
When it causes trouble: at light load, the exhaust may be too cool for passive regeneration. Soot builds up, differential pressure climbs, and then when the set is loaded heavily the accumulated soot may burn quickly.
Active regeneration
Some devices raise exhaust temperature on purpose to burn off accumulated soot, usually with an electric heater, sometimes with a fuel burner or dosing, or by asking the operator to run at higher load for a period. These events are distinct: they start, last for a defined time, and stop.
What it looks like during a test: a clear rise in temperature at or after the filter, falling differential pressure, and often a controller status message or log entry.
Heater-assisted devices need a proper electrical supply. See RECD controller and heater power supply requirements.
Operator-triggered or maintenance regeneration
Some systems need the operator to start a regeneration cycle, or to send the filter out for cleaning when ash builds up. The RECD maintenance guide and the guide on what a “maintenance-free RECD” claim actually means explain how these routines work in practice.
For a deeper technical background, DieselNet’s overview of diesel filter regeneration is a useful reference. For the basics of filter design, see our explainer on what a diesel particulate filter is and how it helps.
Why Regeneration Moves the Numbers
Regeneration affects emission results through several separate mechanisms that overlap.
The clean-filter effect
A wall-flow filter traps soot on the inlet side of its porous walls. Once a thin layer of soot (the soot cake) has built up, it acts as an extra fine filter on top of the substrate, and overall efficiency goes up. When regeneration burns that layer off, the filter is back to the substrate alone, and efficiency can dip for a while until the cake rebuilds.
Effect on the test: a PM sample taken during or shortly after a full regeneration can show lower efficiency than the device normally achieves.
The sulfate effect
At regeneration temperatures, sulphur dioxide from fuel and lube oil oxidises more readily to sulphur trioxide, which forms sulfate particles. Sulfate counts as particulate mass on the filter paper.
Effect on the test: post-RECD PM can rise during hot regeneration, more so with higher-sulphur fuel. Background reading: how ultra-low sulphur diesel improves emission control.
The gas-phase effects
During regeneration:
- CO can rise or fall, depending on how completely soot is oxidised and how the catalyst responds to the higher temperature
- HC usually falls if the catalyst is hot, but can briefly rise with fuel-based active systems
- NOâ‚‚ share of total NOx can change noticeably on catalysed systems, because NOâ‚‚ is being consumed by the soot
Since an RECD must reduce PM without adversely affecting other emissions, these gas changes need to be captured, not ignored.
The back-pressure effect
As soot burns, the restriction through the filter falls and the engine breathes more easily. Exhaust flow and engine behaviour shift slightly. At the start of an active event the temperature rise can briefly raise back-pressure as the gas expands, before the soot clearing takes over. See how exhaust back-pressure affects a DG set.
The release effect
Loose soot, condensed organics or flakes from upstream pipework can be blown off during temperature transitions. This shows up as short spikes in opacity and PM.
How Do You Know Regeneration Is Happening?
Not every device reports regeneration on a display. These are the signals that, taken together, reliably show a regeneration event:
| Signal | What you’ll see | Reliability |
|---|---|---|
| Controller status / event log | “Regen active”, heater ON, cycle counter increments | High, if the device has a controller |
| Differential pressure (ΔP) across the RECD | Steady fall at constant load | High, the best single indicator |
| Temperature rise across the RECD | Outlet hotter than inlet | High for active regeneration; smaller for passive |
| Heater current draw | Rise on the heater supply | High for heater-type systems |
| Downstream opacity | Short-lived spikes or a gradual rise | Moderate, a useful early warning |
| CO/HC/NOâ‚‚ trends | Step changes at constant load | Moderate, needs a gas analyser |
| Visible stack plume | Brief haze or white vapour | Low, subjective |
If you can only add one measurement to your test kit, make it differential pressure across the RECD, logged every minute. A falling ΔP at steady load and steady temperature almost always means soot is being cleared.
The Record Sheet: Before, During and After
This is the core of the article. It is a structured record you can adapt into your test plan. The aim is that anyone reading the report later can answer three questions: was the device regenerating during sampling, how loaded was the filter, and what did regeneration do to the emissions?
Before testing begins
| Item to record | Why |
|---|---|
| Regeneration type (passive / active heater / active fuel / manual) | Sets which signals to watch |
| Date, time and duration of the last known regeneration | Indicates soot load at test start |
| Operating hours since last regeneration or cleaning | Same, for devices without logs |
| ΔP across the RECD at a reference load, before sampling | The soot-load baseline |
| Device age / operating hours since new | Ash build-up and break-in state |
| Fuel batch and sulphur content (supplier certificate) | Explains sulfate contribution |
| Lube oil consumption trend | Ash and organic PM source; see engine oil consumption and RECD suitability |
| Any regeneration inhibit or force settings on the controller | Shows whether events could occur during sampling |
During each sample
Log these every minute, from the start of stabilisation to the end of sampling:
| Item | Where measured |
|---|---|
| Engine load (kW) and power factor | DG controller or power analyser |
| Exhaust temperature before the RECD | Gas thermocouple |
| Exhaust temperature after the RECD | Gas thermocouple |
| ΔP across the RECD | Differential pressure transmitter or manometer |
| Engine back-pressure | At the turbo or manifold outlet |
| Regeneration status / heater state | Controller display or log |
| CO, HC, NOx (and NOâ‚‚ if possible), Oâ‚‚ / COâ‚‚ | Gas analyser at the downstream port |
| Opacity | Downstream opacity meter, if available |
| Start and end time of each PM filter | Lab sampling log |
The PM filter’s start and end times are the key link. They let you overlay the sample window on the ΔP and temperature traces and see exactly whether a regeneration overlapped it.
After the test
| Item | Why |
|---|---|
| ΔP at the same reference load at the end of testing | Shows net soot loaded or cleared during the test |
| Any regeneration events logged by the controller | Confirms or rules out overlap with samples |
| Visual notes (plume, vapour, smells) | Supporting evidence |
| Lab notes on filter appearance (colour, uniformity) | Sulfate-rich filters can look different from soot-rich ones |
Three Ways to Handle Regeneration in an Acceptance Test
Once you can see regeneration, you need to decide what to do about it. There are three sensible strategies. Choose one and write it into the test plan before testing day.
Strategy 1: Exclude regeneration from the steady-state result
Sample only when no regeneration is occurring and the filter is at a known, moderate soot load. If a regeneration starts during a sample, void that sample and repeat it after the device has re-stabilised.
- Best for: routine site acceptance, where the question is “what does the device achieve in normal operation?”
- What to agree in advance: the ΔP band or operating-hours window that counts as “normal soot load,” and how long to wait after a regeneration before resampling.
Strategy 2: Test regeneration separately and report it alongside
Run the steady-state test as in Strategy 1, then deliberately capture a regeneration event, either triggered or waited for, and record the full sheet above throughout. Report the regeneration findings as a separate section.
- Best for: larger installations, critical sites, or devices where regeneration is frequent
- What you learn: peak temperatures, ΔP recovery, how long the event lasts, emissions during the event, and whether the “no adverse effect” condition holds throughout
Strategy 3: Time-weighted reporting
Some regulatory regimes for vehicles and non-road engines take infrequent regeneration into account by weighting emissions during regeneration according to how often it occurs. A site test can borrow the logic:
Weighted PM = (PM normal × fraction of time in normal operation) + (PM during regeneration × fraction of time regenerating)
This needs a reasonable estimate of how often and for how long the device regenerates on your duty cycle. That is easier if you have DG load logs and controller history.
- Best for: sites that want one representative figure and have good operating data
- Caution: state clearly that this is a site-specific weighted figure, not the same as the type-approval result
Special Situations Worth Planning For
Light-load sets that rarely regenerate passively
A standby set that runs mostly at 20–30% load may never get hot enough for passive regeneration. Its filter loads steadily, and its ΔP may be quite high at the time of testing. If the acceptance test then runs at high load, a large regeneration can start mid-test.
What to do: check ΔP before testing. If it is high, agree on a controlled regeneration before the test, then allow time for re-stabilisation and some soot cake rebuild. Read choosing an RECD for a generator running at low load for the selection side of this problem.
Brand-new devices
A new filter has no soot cake, so its initial efficiency may be lower than it will be after a few hours of use. It also hasn’t regenerated yet. Agree whether acceptance testing happens on commissioning day or after a break-in period. Our article on how to account for warm-up time when testing an RECD discusses break-in alongside thermal stabilisation.
Ash-loaded devices
If a device has many operating hours and shows high ΔP even right after regeneration, ash may be the cause. Ash doesn’t burn, so regeneration won’t clear it. Testing an ash-loaded device gives you a true picture of its current state, but the report should note it, and cleaning may be due.
Sets with frequent short runs
Sets that run for 10–15 minutes at a time may rarely complete a regeneration. Their soot load patterns differ from continuously running sets. See RECD selection for DG sets with frequent short start-stop cycles.
Worked Example: One Test Day, Four Samples, One Regeneration
The figures below are illustrative, but the pattern is typical. A 160 kVA DG set with a heater-assisted RECD is tested at a matched 96 kW using upstream/downstream ports. The upstream (engine-out) PM is steady at about 48 mg/Nm³ all day, corrected to the same reference basis. Before testing, the ΔP across the device reads 62 mbar at this load, which is towards the high end of its normal band.
| Sample | Time window | Controller status | ΔP (mbar) | RECD outlet temp (°C) | Downstream PM (mg/Nm³) | Apparent reduction |
|---|---|---|---|---|---|---|
| 1 | 10:40–11:10 | Normal | 62 → 63 | 405 | 12.0 | 75.0% |
| 2 | 11:20–11:50 | Regen started 11:32 | 63 → 41 | 410 → 520 | 19.2 | 60.0% |
| 3 | 12:30–13:00 | Normal (post-regen) | 41 → 42 | 408 | 14.9 | 69.0% |
| 4 | 14:30–15:00 | Normal | 46 → 47 | 406 | 12.4 | 74.2% |
What the records show:
- Sample 1 is a clean steady-state result at a moderate-to-high soot load. The soot cake is well established and the device performs at its best.
- Sample 2 overlapped an active regeneration. ΔP fell by about a third and outlet temperature jumped by more than 100 °C. Downstream PM rose sharply, from a mix of the clean-filter effect, sulfate formation at high temperature, and loose material being released. On its own, this sample would have failed the device.
- Sample 3 was taken after the regeneration finished and temperatures had settled, but the filter was still almost clean. Efficiency was lower than in Sample 1 because the soot cake hadn’t rebuilt. Without the ΔP record, this result would have looked like a borderline device.
- Sample 4, two hours later, shows ΔP creeping back up and the reduction returning close to Sample 1.
If the lab had taken a single sample at 11:30, the report would have said 60%. If it had sampled only at 12:30, it would have said 69%. Both numbers are real, and neither describes normal operation. Only the ΔP, temperature and controller records make it possible to tell which samples represent steady-state performance (1 and 4), which show regeneration behaviour (2), and which show the recovery period (3).
For a steady-state acceptance decision under Strategy 1, Samples 1 and 4 would be averaged. Sample 2 would be reported separately as regeneration data, and Sample 3 would be noted as post-regeneration recovery.
Reading a Report That Doesn’t Mention Regeneration
Many RECD test reports say nothing about regeneration. That doesn’t automatically make them wrong, but it leaves a gap. Ask the lab or supplier for:
- ΔP and temperature traces covering each PM sample window
- Controller regeneration logs for the test day
- The time since the last regeneration at the start of testing
- Confirmation that no PM sample overlapped a regeneration event, or an explanation if one did
If none of this exists, you can’t judge whether an unexpectedly high or low result was caused by regeneration. For more on reviewing paperwork, see how to read an RECD test certificate.
Turning the Records Into Ongoing Monitoring
The same parameters that make an acceptance test defensible are also the best early-warning system once the device is in service:
- ΔP at a reference load, logged at every service, shows soot and ash build-up over time
- Regeneration frequency shows whether duty-cycle changes are affecting the device
- A sudden drop in ΔP that doesn’t recover may mean a cracked substrate or a bypass leak, and calls for an inspection
- Downstream opacity gives a cheap cross-check between gravimetric tests
Keep these logs with your compliance records. Our environmental compliance checklist for generator owners shows where they fit, and they are worth having during a Pollution Control Board inspection.
Regeneration in the Context of the Full Test
Regeneration is one of five linked variables that determine whether an RECD comparison is valid:
- Why generator load must be matched in RECD comparison tests: load sets the exhaust temperature, which drives passive regeneration
- How to account for warm-up time when testing an RECD: a regeneration can start as the device heats up
- How exhaust sampling location affects RECD test results: the port’s position determines whether regeneration spikes are fully captured
- How dilution can mislead RECD before-and-after measurements: very low post-RECD readings are most sensitive to sampling artefacts
Frequently Asked Questions
Can regeneration make an RECD fail its acceptance test?
It can, if a PM sample overlaps a regeneration event or is taken just after one, because filtration efficiency and sulfate formation change during and immediately after the event. That is why regeneration status should be recorded and, for steady-state acceptance, samples overlapping regeneration should be repeated.
What is the single most useful measurement for spotting regeneration?
Differential pressure across the RECD, logged every minute. A steady fall in ΔP at constant load and temperature is the clearest sign that soot is being cleared.
Should regeneration be prevented during the test?
For the main steady-state result, samples should be taken outside regeneration. Preventing it entirely isn’t always possible or wise. It is better to see it happen and record it, then sample again once the device has settled.
Does a passively regenerating RECD need these records?
Yes, though the signals are subtler. Passive regeneration can speed up when load increases, and light-load operation can let soot build up. ΔP and temperature logs show both.
Do I need a gas analyser to record regeneration?
Not strictly. ΔP, temperatures and the controller log identify regeneration. A gas analyser adds valuable information about CO, HC and NOâ‚‚ during the event, which supports the “no adverse effect on other emissions” condition.
Get Regeneration Into the Test Plan From Day One
A regeneration event during testing isn’t a failure. It’s information, but only if someone writes it down. Aceget supplies CPCB type-approved Retrofit Emission Control Devices for DG sets from 25 kVA to 2500 kVA, and we help clients define regeneration recording, ΔP reference points and sampling windows before acceptance testing. See our certifications, learn how an RECD works, or talk to our team about your DG set.