When evaluating a Retrofit Emission Control Device (RECD) on a diesel generator set, emission testing is often treated as a straightforward process: start the DG set, allow it to warm up, reach the required load, and begin sampling. However, the actual thermal condition of the RECD can have a major influence on the reliability of the test results.

An engine may reach its normal operating temperature relatively quickly, while the RECD installed downstream can take considerably longer to stabilise. Its housing, substrate, insulation, exhaust piping and internal components all absorb and retain heat. If emission measurements are taken before these components reach a stable operating condition, the results may not accurately represent the device's normal performance.

thermal stabilisation should be treated as an important part of RECD emission testing rather than simply waiting for a fixed number of minutes.This is why

Why RECD Thermal Stabilisation Is Important

A DG set's coolant temperature is commonly used as an indication that the engine has warmed up. While this is useful for monitoring engine operation, it does not necessarily confirm that the downstream RECD has reached its intended operating condition.

An RECD may contain a catalyst, filter or other emission-control components that respond to exhaust temperature. During initial operation, these components continue absorbing heat from the exhaust stream. As a result, the temperature at the RECD outlet can continue increasing even after the engine coolant has already stabilised.

This thermal lag can influence emissions readings, back-pressure and the behaviour of the emission-control system. Therefore, a valid test should consider the condition of the complete exhaust-treatment system rather than relying only on engine warm-up.

Aceget guide on accounting for warm-up time during RECD testingThe explains this process in greater detail, including the different thermal phases that can occur between DG start-up and a stable sampling condition.

Engine Warm-Up Is Not the Same as RECD Stabilisation

One of the most common mistakes during emission testing is assuming that an engine is ready for sampling as soon as its coolant and lubrication temperatures become stable.

The exhaust system continues to experience thermal changes after this point. The RECD may still be heating up, and its outlet temperature may continue to rise gradually.

For example, if a test begins while the RECD is still significantly cooler than its eventual operating temperature, the emission-control system may not yet be operating under representative conditions. A reading taken during this period could therefore describe the transition phase rather than steady-state performance.

engine-side and RECD-side temperaturesFor this reason, testing teams should monitor both before declaring a test point ready.

What Happens During the Stabilisation Period?

The warm-up process can broadly be understood in several stages.

Initially, the engine and exhaust system begin warming from ambient conditions. Exhaust temperature rises rapidly, while the colder RECD absorbs heat.

The second stage occurs when the engine is already warm but the RECD continues catching up thermally. During this period, outlet temperature may continue changing and other readings can also shift.

Eventually, the inlet and outlet temperatures begin to level off. Back-pressure becomes more stable and emission concentrations stop showing a significant trend. This is the point at which the test condition becomes much more representative of steady-state operation.

The duration of these stages can vary depending on DG capacity, RECD size, exhaust-pipe length, insulation, ambient temperature and especially the operating load.

Load Conditions Can Change Stabilisation Time

Load is another important factor that should not be overlooked.

At higher loads, exhaust temperatures are generally higher, allowing the RECD to reach its operating temperature more quickly. At very light loads, however, the exhaust may remain comparatively cool and the device can take much longer to stabilise.

This means that applying one universal warm-up period to every test condition may not provide consistent results.

A test plan should therefore consider the actual load points being evaluated. After changing from one load to another, the RECD may again require time to reach a new equilibrium.

Why a Fixed 15- or 30-Minute Rule May Not Be Enough

A stopwatch-based approach appears simple, but it can create problems when site conditions vary.

A 30-minute warm-up may be sufficient for one DG set under one set of conditions but inadequate for a larger system operating at a low load in cold ambient conditions.

“Has the DG been running for 30 minutes?”Instead of asking only, , the more useful question is:

“Has the RECD reached a stable and repeatable operating condition?”

A practical test plan can define measurable stability criteria using parameters such as load stability, RECD outlet temperature and exhaust back-pressure. The specific acceptance values should be agreed between the relevant parties and should not be confused with regulatory limits.

Measurements That Can Support a Reliable Test

A useful thermal-stabilisation record does not necessarily require an excessive number of measurements. Some of the most useful parameters include:

DG load in kW
Engine coolant temperature
Exhaust temperature before the RECD
Exhaust temperature after the RECD
Exhaust back-pressure
Ambient temperature
CO and HC readings, where applicable
Time of each load change
Time at which sampling begins

Recording these values over time creates a much stronger technical record than simply writing “DG warmed up” in a test report.

Thermal Stabilisation After Load Changes

Stabilisation should not be considered complete for the entire testing session.

Whenever the generator load changes, the thermal balance of the exhaust system changes as well. Increasing load can raise exhaust temperatures, while reducing load can leave the RECD temporarily hotter than its normal condition at the new lower load.

Therefore, sampling immediately after a load transition may produce results that do not represent the actual steady-state condition of that load point.

A better approach is to repeat the agreed stability assessment after every significant load change.

Importance for RECD Comparison Testing

Thermal stabilisation becomes particularly important when comparing baseline emissions with emissions after RECD installation.

If the baseline test is performed after the engine has stabilised but the post-installation test is conducted while the RECD is still warming up, the two datasets are not truly comparable.

The testing methodology should therefore aim to keep operating conditions, load, measurement procedures and stabilisation requirements consistent between the relevant test stages.

Central Pollution Control Board (CPCB) CPCB RECD Type Approval & CoP ProcedureThe official documentation on RECD type approval and conformity procedures provides an important regulatory reference for understanding how RECD systems are evaluated and associated with certified engine families.

Practical Checklist Before Sampling

Before beginning an RECD emission measurement, the testing team can verify:

The DG has reached the intended test load.
The load remains within the agreed tolerance.
RECD inlet and outlet temperatures are being monitored.
Outlet temperature has stopped showing significant movement.
Back-pressure is reasonably stable.
Any agreed minimum operating period has been completed.
The same stabilisation approach is being applied consistently.
The time and readings supporting the start of sampling are recorded.

This approach makes the test more transparent and helps both suppliers and buyers understand why a particular sample was considered valid.

Conclusion

Thermal stabilisation is a fundamental consideration when conducting meaningful RECD emission testing. An engine reaching operating temperature does not automatically mean that the complete emission-control system has stabilised.

By monitoring RECD outlet temperature, exhaust conditions, back-pressure and load—and by applying a clearly documented stability criterion—testing teams can reduce the risk of premature sampling and improve the comparability of results.

Why Thermal Stabilisation Matters Before Conducting an RECD Emission Test

When evaluating a Retrofit Emission Control Device (RECD) on a diesel generator set, emission testing is often treated as a straightforward process.