RECD Regeneration Temperature: Why There Is No Universal Setpoint

Ask three RECD suppliers what temperature their device regenerates at and you may hear three different answers: 250 °C, 350 °C and 600 °C. None of them is necessarily wrong. They are usually describing different things, measured in different places, under different conditions.

Regeneration is a chemical reaction whose speed depends on temperature, the oxidant available, the catalyst and how much soot is present. Change any of those and the “right” temperature changes too. That is why there is no single setpoint that applies to every filter-based RECD on every DG set.

This article explains what the common temperature figures actually mean, which variables move them, why both too cold and too hot cause problems, and how to write a temperature requirement that is useful on site. For an introduction to the two regeneration modes, see RECD Regeneration Explained: Active vs Passive Regeneration.

Six Temperature Figures You Will Hear, and What Each Means

Figure you may be quoted	What it usually refers to	Why it differs
About 250–300 °C	Temperature at which NO₂ starts to oxidise soot slowly on a catalysed filter	Depends on NO₂ availability and catalyst activity
About 300–400 °C	Typical balance point of a catalysed passive system	Depends on engine soot output and NOx-to-soot ratio
About 400–450 °C	Light-off for faster passive or fuel-additive-assisted burning	Varies by formulation and soot reactivity
About 550–650 °C	Range where oxygen oxidises soot quickly; common target for active regeneration	Depends on soot load and residence time
Above 700 °C	Peak temperatures inside the filter during an exothermic burn	Depends on soot load, gas flow and heat loss
Supplier’s maximum	Highest allowable filter or outlet temperature before damage risk	Depends on substrate, mat, catalyst and housing

The first four are reaction thresholds. The fifth is a consequence. The sixth is a limit. Confusing them is the root of most temperature arguments on site.

Why the Number Moves: Seven Variables

1. The oxidant

Nitrogen dioxide attacks soot at much lower temperatures than oxygen does. A catalysed system that makes plenty of NO₂ can keep a filter clean at temperatures where an uncatalysed filter would load steadily. Our comparison of catalysed vs uncatalysed RECD filters explains this in operating terms.

2. Catalyst formulation and health

The catalyst’s metal loading, coating location and age all change how much NO₂ it produces at a given temperature. A poisoned or thermally aged catalyst needs more heat to do the same work. See catalyst poisoning in an RECD.

3. Engine-out NOx and soot

Passive regeneration depends on the ratio of NOx to soot. An engine that produces a lot of soot relative to NOx, such as one running light or with worn injectors, needs a higher exhaust temperature to keep up. Two engines of the same kVA can therefore have different balance points. We discuss engine-to-engine variation in why same-kVA DG sets can need a different RECD.

4. Soot load

At a given temperature, more soot means more fuel for the reaction and more heat released. Laboratory work summarised by DieselNet shows soot oxidising slowly at 500 °C, faster at 600 °C and very fast at 700 °C, with loading also affecting the rate. A lightly loaded filter needs sustained heat; a heavily loaded one can run away once it lights.

5. Exhaust flow and oxygen content

Gas flow carries heat into the filter but also carries it away. At high flow and moderate temperature, the filter may never reach the target. At very low flow, such as at idle after a load drop, heat released by burning soot is not carried away and the filter can overheat. Oxygen content falls as engine load rises, which also affects reaction speed.

6. Ash and residues

Ash does not burn but does change how heat and gas move through the filter. Heavily ash-loaded filters can show uneven temperatures during regeneration. Background in Soot vs Ash in an RECD.

7. Fuel and oil

Fuel sulphur competes with soot for the catalyst’s attention and forms sulphates at higher temperatures. Oil-derived compounds can coat the catalyst. Both raise the effective temperature needed. Our guide on ultra-low sulphur diesel explains the fuel side.

Where the Thermometer Sits Changes the Answer

Even on the same device at the same moment, “the temperature” depends on where you measure.

• Upstream of the RECD (inlet gas): what the engine delivers. Most passive thresholds are quoted here.
• Between DOC and filter: relevant on fuel-assisted systems, where the DOC releases heat.
• Inside the filter bed: the temperature the soot actually experiences. Rarely measured on retrofit devices.
• Downstream of the RECD (outlet gas): shows exotherm during regeneration. Outlet hotter than inlet usually means soot is burning.
• Housing surface: much lower than gas temperature and lagging behind it. Useful for safety, not for regeneration control.

On a heater-assisted system, the controller may also use a heater element temperature. That figure can be far above the gas temperature and should never be compared with an exhaust reading.

When anyone quotes a number, ask: measured where, with what sensor, at what load? Our article on RECD temperature sensor placement covers the practical side, and installed RECD sensor calibration documentation explains how to make sure the reading is trustworthy.

Too Cold and Too Hot: Two Ways Temperature Goes Wrong

When regeneration runs too cold

• Soot burns slowly or not at all, so ΔP keeps rising
• Active cycles time out without completing
• Partial regeneration leaves soot unevenly distributed, which can create hot spots later
• More fuel or energy is spent for little result

This is the common failure on lightly loaded standby sets. It often appears as cycles that start but do not finish; see RECD Regeneration Not Completing: What Operators Should Record.

When regeneration runs too hot

• Ceramic substrates can crack from thermal stress, especially across steep temperature gradients
• Catalyst coatings age faster at high temperature
• Support mats and seals can degrade
• Ash can sinter and become harder to clean

The most dangerous situation is a heavily loaded filter that ignites while gas flow suddenly drops, for example when the set unloads or stops mid-cycle. The soot burns, the heat is not carried away, and the peak temperature climbs well beyond normal. We look at this case in detail in What Happens When a Generator Stops During RECD Regeneration?.

Substrate material matters here. Different ceramics tolerate heat and thermal shock differently. See silicon carbide vs cordierite RECD filter substrate for how this affects design limits.

Reading a Temperature Trace Through an Active Cycle

If your RECD logs inlet and outlet temperatures, a normal active cycle usually shows five phases:

1. Heat-up. The heater or added load lifts inlet temperature. Outlet lags behind because the filter absorbs heat.
2. Ignition. Outlet temperature catches up with, then exceeds, inlet. Soot has started to burn.
3. Exotherm. Outlet stays above inlet. ΔP falls steadily. This is the productive phase.
4. Burn-out. The temperature difference narrows as soot runs out. ΔP flattens near its clean value.
5. Cool-down. Heat input stops, temperatures settle and the controller records completion.

A trace that never shows phase 2 means the filter did not ignite. A trace with a very sharp, very high phase 3 peak suggests too much soot was present when the cycle started. Both are worth recording; the RECD event log guide shows a practical format.

Why Suppliers Avoid Promising One Number

A responsible supplier will usually describe regeneration temperature as a window with conditions, not a single figure. That can feel evasive, but the alternative is worse: a single number that works on the test bench and fails on your site.

CPCB’s procedure for RECD type approval tests devices on representative engines and duty cycles. It confirms that a device can achieve the required particulate reduction and that its regeneration approach works under those test conditions. It does not, and cannot, guarantee one temperature for every engine in the field. That is a site-matching question. For more on reading approval documents, see how to read an RECD test certificate.

How to Specify Regeneration Temperature Properly

Instead of asking “what is the regeneration temperature?”, ask the supplier to state the following in the technical offer:

Item to specify	Example of a useful answer
Minimum inlet gas temperature for passive regeneration to keep up	Stated with the engine load and duration at which it applies
Share of run time needed above that temperature	For example, a stated percentage of operating hours
Active regeneration target temperature and measurement point	Gas temperature at a named sensor position
Typical active cycle duration	Minutes, at a stated soot load
Maximum allowable outlet or bed temperature	With the action the controller takes if exceeded
Maximum soot load at which active regeneration may start	Expressed as ΔP or estimated grams per litre
Response if gas flow falls during a cycle	For example, heater cut-off, inhibit or alarm
Sensor type, range and accuracy	Thermocouple class and calibration method

This turns temperature from a marketing figure into an operating envelope. Our guides on RECD tender specifications with measurable performance requirements and RECD performance guarantees and operating conditions show how to word it.

Checking Your Own Set Against the Window

Once you know the device’s window, compare it with what your engine actually delivers.

1. Log exhaust temperature upstream of the RECD at each typical load: idle, 25%, 50%, 75%.
2. Combine those readings with your DG load logs to estimate how many hours per month sit above the passive threshold.
3. If that share is low, plan for active regeneration or scheduled loaded runs.
4. Recheck after any engine change, such as an overhaul, injector replacement or turbo change. See when to retest an RECD after DG set changes.

Ambient conditions matter too. Very hot plant rooms raise surface temperatures but have little effect on exhaust gas temperature, while high altitude can reduce engine output and shift exhaust temperature. Our articles on very hot ambient conditions and high-altitude installations go into these cases.

Four Temperature Myths Heard on Site

“If the stack is hot, the filter is regenerating.” A hot stack shows the engine is loaded, nothing more. Regeneration shows up as outlet gas hotter than inlet gas, together with falling ΔP at a steady load. Without both signals, you cannot tell.

“The higher the setpoint, the cleaner the filter.” Raising an active target above the supplier’s window does not remove more soot once the soot is gone. It only adds heat stress, speeds up catalyst ageing and can sinter ash. A complete cycle at the right temperature beats a short cycle at an extreme one.

“A passive RECD regenerates at its brochure temperature.” The brochure figure is usually the lowest temperature at which the reaction begins under good conditions. Keeping up with soot on your engine may need more heat for longer. That is the balance point, and it moves with load, fuel and catalyst health.

“Temperature readings from the panel are always right.” Thermocouples drift, connections loosen and probes can end up in the wrong position after maintenance. A reading that never changes, or that disagrees with a reading at the other side of the device, deserves a check before anyone acts on it. Our guide to separating a sensor fault from an RECD performance problem walks through the checks.

A Practical Temperature Log for the Plant Room

Operators do not need laboratory equipment to understand their device’s temperature behaviour. A simple weekly entry, taken at the same load, builds a useful picture within a month.

Field	Entry
Date, DG hour meter
Load (kW) and minutes at that load
Inlet gas temperature (°C)
Outlet gas temperature (°C)
ΔP across the RECD
Regeneration status shown on panel
Notes (smoke, smell, alarms)

If inlet temperature at your normal load sits below the supplier’s passive threshold week after week, you have early evidence that the set will need active or parked regeneration. If outlet stays hotter than inlet for long periods outside a cycle, ask your service provider to check for slow uncontrolled burning or a sensor issue. The RECD shift handover log shows how to fold these readings into existing routines.

Frequently Asked Questions

What temperature does an RECD regenerate at?

There is no single figure. Catalysed passive systems typically work from roughly 250–400 °C depending on conditions, while active regeneration with oxygen usually targets roughly 550–650 °C. The right value depends on the device, the engine and where temperature is measured.

Why is the outlet temperature higher than the inlet during regeneration?

Burning soot releases heat. An outlet reading above inlet is a normal sign that regeneration is under way.

Can a filter get too hot during regeneration?

Yes. A heavily loaded filter that ignites when gas flow is low can exceed its safe temperature, risking cracks and catalyst damage. Controllers use soot-load limits and flow checks to prevent this.

Does higher exhaust temperature always mean better regeneration?

Up to the device’s limit, higher temperature speeds soot burning. Beyond it, the risk of damage rises. The aim is to stay within the supplier’s window.

Why does my RECD regenerate differently in summer and winter?

Ambient temperature changes intake air, cooling and sometimes load patterns, which shift exhaust temperature and soot output slightly. Load pattern changes are usually the bigger factor.

Think in Windows, Not Setpoints

A regeneration temperature is only meaningful with its measurement point, its load and its duration attached. Ask for the operating window, check your engine against it and log the temperatures that prove regeneration is happening.

Aceget supplies CPCB type-approved Retrofit Emission Control Devices and can review your DG set’s exhaust temperature profile before you choose a regeneration strategy. Explore our DG set emission regulations page or request a quote with your set’s details.