How Fuel and Engine Condition Can Confound RECD Performance Tests

A facility in Faridabad fitted a Retrofit Emission Control Device to an ageing 320 kVA generator. The acceptance test showed an 83% particulate reduction, well clear of the 70% requirement. Everyone was pleased until a year later, when a periodic check on the same device came in at 71%.

The RECD hadn’t deteriorated much. What had changed was the engine. During the RECD installation, the service team had also replaced two leaking injectors and a badly clogged air filter. The “before” test was run on a smoky, poorly maintained engine. The “after” test was run on a freshly serviced one. The 83% included the effect of the service visit. The device on its own was closer to 72%.

In research terms, fuel and engine condition are confounders. They affect both the measurement and the thing you think you’re measuring, and if they change between tests, they become tangled up with the device’s effect. This article catalogues the most common fuel and engine confounders in RECD testing, explains which way each pushes the result, and shows how to control them.

Why Confounders Matter in an RECD Comparison

An RECD comparison calculates:

Reduction = (PM before − PM after) ÷ PM before

That formula assumes the only meaningful difference between “before” and “after” is the RECD. Anything else that changes emissions between the two measurements is credited to, or blamed on, the device:

• If something raises emissions in the baseline test only, the reduction looks bigger (flattered).
• If something raises emissions in the post-RECD test only, the reduction looks smaller (penalised).
• If the engine is so dirty that the device is overloaded, the result describes an unusual condition, not normal operation.

Same-day upstream/downstream tests are protected against most of these, because the engine and fuel are identical for both readings. Sequential tests, with the baseline taken days or weeks before the RECD test, are much more exposed. If you’re choosing a test design, see planning an RECD emissions test before and after installation.

Part 1: Fuel Confounders

Sulphur content

What it does: Sulphur in fuel forms sulfur dioxide during combustion. Some of it oxidises further, especially over a hot catalyst, to form sulfate particles. These count as particulate mass on the filter.

How it confounds: If the post-RECD test uses fuel with higher sulphur than the baseline, sulfate formation on the RECD’s catalyst can add particulate mass downstream and make the device look worse. The effect is strongest at high load and during regeneration, when temperatures are highest.

What to control: Diesel sold through retail outlets in India meets the BS-VI specification with a very low sulphur limit, which keeps this effect small. Fuel bought in bulk from other sources, stored for a long time, or mixed in a shared tank should be checked. Keep the supplier certificate for each delivery used in testing. Background: how ultra-low sulphur diesel improves emission control.

Fuel density, cetane quality and distillation

What they do: Heavier fuels and fuels with poor ignition quality (low cetane) tend to burn less cleanly, producing more soot and unburnt hydrocarbons, particularly at light load and during cold running.

How it confounds: A change of fuel supplier or batch between tests can raise or lower engine-out particulate independent of the RECD.

What to control: Use fuel from the same batch for both tests where possible. If that isn’t possible, record the supplier, delivery date and certificate for each, and ask for density and cetane data if available. The Indian diesel specification is published by the Bureau of Indian Standards.

Biodiesel blends

What they do: Biodiesel blends often reduce soot but can increase the soluble organic fraction of particulate, and they change how quickly soot burns in a filter.

How it confounds: A switch from conventional diesel to a blend, or the reverse, between tests changes both the baseline emissions and the device’s behaviour.

What to control: Don’t change blend ratio between tests. If the site uses blends routinely, test on the blend actually used. See using biodiesel blends in an RECD-equipped generator.

Water and contamination

What they do: Water, microbial growth and sediment in stored fuel affect injection and combustion. They can also clog filters, starving the engine of fuel at higher loads.

How it confounds: Contaminated fuel tends to raise smoke and particulate, often unevenly from one test to the next.

What to control: Drain water from tank sumps and fuel-water separators before testing, check the fuel filter, and avoid testing just after a tank has been topped up from an unknown source. See the generator fuel filter replacement guide.

Dual-fuel operation

If the generator has been converted to dual-fuel operation, the share of gas versus diesel changes emissions sharply. Testing at a different substitution rate before and after is a major confounder. See RECD after diesel-to-dual-fuel conversion.

Part 2: Engine Condition Confounders

Fuel injectors

What they do: Injectors atomise fuel into a fine spray. Worn, dirty or leaking injectors produce poor spray patterns and dribble fuel, which increases soot and unburnt hydrocarbons.

How it confounds: This is the classic baseline inflation problem. A baseline taken with bad injectors is far dirtier than normal. If the injectors are serviced during RECD installation, the “after” measurement benefits from both the service and the device.

What to check: smoke behaviour under load, injector service history, and cylinder exhaust temperatures if available (uneven temperatures across cylinders suggest injector problems). See why is my generator emitting black smoke.

Air filter and intake restriction

What it does: A clogged air filter restricts air into the engine. With less air for the same fuel, combustion gets richer and soot rises, especially at higher loads.

How it confounds: A filter changed between tests lowers engine-out particulate. A filter that clogs progressively during a long test campaign raises it.

What to check: intake restriction against the engine maker’s limit, and the filter’s condition and change date. See the generator air filter maintenance guide.

Turbocharger

What it does: Supplies compressed air to the engine. A worn turbocharger with damaged blades, worn bearings or oil leaks delivers less air and can leak oil into the intake or exhaust.

How it confounds: Reduced boost raises soot. Oil leaking past turbo seals adds oil-derived particulate and ash, which also loads the RECD.

What to check: boost pressure if measurable, oil traces at the turbo outlet, and any whistling or unusual noise.

Lube oil consumption: piston rings, liners and valve guides

What it does: Oil that gets into the combustion chamber, past worn rings or valve stem seals, partly burns and partly passes through as organic particulate and ash.

How it confounds: Oil-derived particulate has a large organic share. Some of it passes through a filter as vapour and condenses downstream, so the device can appear less effective. Ash builds up permanently in the filter and raises back-pressure. Blue smoke is the typical sign: see why is my generator emitting blue smoke.

What to check: oil top-up records over recent operating hours, and any blue smoke. See engine oil consumption and RECD suitability.

Injection timing and fuel pump settings

What they do: Timing and pump calibration set when and how much fuel is injected. Incorrect or tampered settings can raise soot or NOx considerably.

How it confounds: Any adjustment between tests, even well-intentioned, changes engine-out emissions. Mechanically governed engines are more prone to drift than electronically controlled ones. See RECD for mechanically governed vs electronic DG engines.

What to check: service records for any pump or timing work, and confirm nothing will be adjusted between tests.

Cooling system and operating temperature

What it does: An engine running too cool, for example with a stuck-open thermostat, burns less completely, producing more hydrocarbons and organic particulate. One running too hot may derate or behave erratically.

How it confounds: Coolant temperature affects combustion and therefore engine-out emissions. See common causes of generator overheating.

What to check: coolant temperature at each load point, logged alongside emissions.

Exhaust leaks upstream of the sampling point

Leaks at the manifold or turbo flange can draw in air or lose exhaust, depending on local pressure. Either way the sample no longer represents the engine. This is a measurement confounder as much as an engine one. See how dilution can mislead RECD before-and-after measurements.

Back-pressure added by the RECD itself

The RECD adds back-pressure, which slightly changes how the engine breathes. If total back-pressure stays within the engine maker’s limit, the effect on engine-out emissions is small. If it exceeds the limit, the engine can smoke more, and part of the RECD’s measured performance is lost to a problem the installation created. Record back-pressure in both tests. See how exhaust back-pressure affects a DG set.

Part 3: Ambient Conditions, the Confounder Nobody Owns

Fuel and engine condition have obvious owners: the fuel supplier and the maintenance team. Ambient conditions don’t, so they often go unrecorded. They still affect engine-out emissions.

Intake air temperature

Hot intake air is less dense, so the engine receives less oxygen for the same fuel. In north Indian summers, DG rooms and canopies can be far hotter than outside air, especially with poor ventilation. Less oxygen means richer combustion and more soot, particularly at high load. A baseline taken in a cool January and a post-RECD test in a hot May are not on equal terms. See assessing an RECD for very hot ambient conditions and summer maintenance tips to prevent generator overheating.

Humidity

Higher humidity slightly changes combustion temperatures and tends to lower NOx. Its effect on particulate is smaller, but it changes the moisture content of the exhaust, which matters when results are converted between wet and dry bases.

Altitude and barometric pressure

At higher altitude there is less air per cylinder-full, with an effect similar to hot intake air. This mostly matters for hill sites, and for sequential tests where weather systems bring large pressure changes. See RECD selection for diesel generators at high altitude.

What to record

• Ambient temperature outside and inside the DG room or canopy
• Intake air temperature, if a sensor is fitted
• Relative humidity
• Barometric pressure

If the baseline and post-RECD tests are separated by a season, these records explain part of the difference. Better still, a same-day upstream/downstream test removes ambient effects from the comparison altogether.

The Confounder Map: Direction of Bias

Confounder	Change between tests	Effect on apparent reduction
Injectors serviced after baseline	Cleaner “after” engine	Flattered
Air filter replaced after baseline	Cleaner “after” engine	Flattered
Engine deteriorates between tests	Dirtier “after” engine	Penalised
Higher-sulphur fuel in “after” test	More sulfate downstream	Penalised
Fuel batch change (density/cetane)	Either direction	Unpredictable
Biodiesel blend change	Either direction	Unpredictable
Rising oil consumption	More organic PM and ash	Usually penalised
Pump/timing adjusted between tests	Either direction	Unpredictable
Excess back-pressure after RECD	Dirtier engine-out with RECD	Penalised

Notice how many confounders are unpredictable. You can’t correct for them after the fact. You can only prevent them by controlling conditions before testing.

Worked Example: Baseline Inflation

The figures are illustrative and follow the Faridabad case at a matched 60% load.

What actually happened:

Condition	PM (mg/Nm³, corrected)
Baseline: worn injectors, clogged air filter, no RECD	140
After: serviced engine, with RECD	24
Reported reduction	(140 − 24) ÷ 140 = 82.9%

What would have been measured with the engine serviced before the baseline:

Condition	PM (mg/Nm³, corrected)
Baseline: serviced engine, no RECD	90
After: serviced engine, with RECD	24
True device reduction	(90 − 24) ÷ 90 = 73.3%

The service visit accounted for nearly 10 percentage points of the “reduction.” The device is still compliant, but much closer to the line than the acceptance report suggested. When the periodic check came in at 71%, it looked like a large drop when it was really a small one.

There is a second risk. A very dirty baseline engine sends far more soot into a new filter, which can affect early regeneration and back-pressure. The device may be loading at an unusual rate during the acceptance test.

Control Strategy: Fix the Engine First, Then Freeze It

The most reliable approach is simple:

1. Bring the engine to a healthy, documented condition before the baseline. Fix leaking injectors, replace clogged filters and resolve any obvious oil consumption problem. Our guide on whether to complete an engine overhaul before fitting an RECD explains when deeper work is justified.
2. Freeze the engine between tests. No injector, pump, timing, turbo or filter changes between the baseline and the post-RECD test, unless the change is documented and both parties agree it is unavoidable.
3. Freeze the fuel. Use the same batch for both tests, or document each batch with a certificate.
4. Record engine health indicators at each test so any change can be seen, even if it wasn’t planned.
5. Prefer a same-day upstream/downstream design wherever the exhaust layout allows it. It removes nearly all fuel and engine confounders in one step.

Pre-Test Engine and Fuel Checklist

Use this before both the baseline and post-RECD tests, and attach the completed sheets to the test report.

Engine records

• Running hours and date of last service
• Injector service history; any recent injector work
• Air filter change date and intake restriction reading
• Lube oil consumption over the last 100 running hours (or similar period)
• Any pump, timing or governor adjustments since the last test
• Turbocharger condition notes

On-the-day engine readings (at each load point)

• Coolant and lube oil temperature
• Exhaust temperature before the RECD (per cylinder if available)
• Engine back-pressure
• Load in kW and power factor
• Visible smoke observations

Fuel

• Supplier, delivery date and batch for fuel in the day tank
• Supplier certificate (including sulphur) on file
• Water drained from sump and separator; fuel filter condition
• Biodiesel or other blend ratio, if any
• For dual-fuel engines: substitution rate

Our monthly maintenance checklist for diesel generators covers much of the engine side and can serve as the base for these records.

What If the Engine Is Already in Poor Condition?

Sometimes there isn’t time or budget to fix the engine before testing. In that case:

• Use a same-day upstream/downstream test. The engine is still dirty, but it is equally dirty for both readings, so the comparison is fair for that condition.
• Report the engine condition openly in the test report, so nobody mistakes a dirty-engine result for normal performance.
• Be aware the device may be stressed. A very dirty engine loads a filter faster, which affects back-pressure and regeneration. The result may not represent the device’s performance on a healthy engine.
• Plan the engine work. An RECD is not a substitute for basic engine maintenance. See signs your generator needs professional servicing.

Fuel and Engine Condition After Acceptance

The same confounders affect every later test. If a periodic check shows a lower reduction than at acceptance, check engine and fuel records before assuming the device has deteriorated. Common causes include:

• The acceptance baseline was inflated (as in the worked example)
• The engine has since worn, raising oil consumption or soot
• A new fuel supplier was introduced
• Maintenance has lapsed, for example on air filters

Keeping engine and fuel records with each test, as part of your RECD maintenance routine, makes these questions quick to answer.

Related Reading on RECD Testing

• Which instrument calibration records should accompany RECD tests
• How to compare RECD test results from different laboratories
• How to set up a witnessed RECD performance demonstration
• Estimating kilograms of PM avoided after an RECD retrofit
• Why generator load must be matched in RECD comparison tests

Frequently Asked Questions

Should I service the engine before or after the baseline test?

Before. Service the engine, let it settle with some running hours, then take the baseline. If the service happens between the baseline and the post-RECD test, its effect gets credited to the device.

Can I test with fuel from a different supplier than usual?

You can, but use the same fuel for both tests and keep the supplier certificate. Changing fuel between tests introduces an effect that can’t be separated from the device’s.

Does an RECD hide engine problems?

It reduces particulate from whatever the engine produces, but it doesn’t fix the engine. A worn engine still burns more fuel, may produce more oil-derived particulate and ash, and can load the device faster. Test results should be read alongside engine condition.

My acceptance result was 85% and the latest check is 72%. Is the RECD failing?

Not necessarily. Check whether the acceptance baseline was taken on a poorly maintained engine that was serviced during installation, whether load and fuel matched, and whether engine condition has changed since. A controlled retest will show the device’s current performance.

Is an upstream/downstream test always better?

For controlling fuel and engine confounders, yes, because both readings come from the same engine at the same moment. It needs suitable ports and access on both sides of the device. See how exhaust sampling location affects RECD test results.

Test the Device, Not the Service Visit

A clean, honest RECD result starts with a healthy engine and stable fuel. Aceget supplies CPCB type-approved Retrofit Emission Control Devices for DG sets from 25 kVA to 2500 kVA. We help clients assess engine condition before installation, so the acceptance test measures the device itself. Send us your DG nameplate photos and service history, or contact our team to plan a test.