RECD Selection for DG Sets with Frequent Short Start-Stop Cycles

Some generators live a quiet life. They wake up for a monthly test, carry the building through the odd long outage and go back to sleep. Others never get a moment’s rest. The grid flickers, the AMF panel calls for the DG, the engine fires, the supply comes back four minutes later and the set winds down. By evening it has done it eight more times.

Owners of the second kind of set often ask a fair question when an RECD becomes necessary: will a particulate filter cope with this? The short answer is yes, provided the device is chosen with the start-stop pattern in mind and the site removes the starts it does not need. The longer answer is the subject of this guide.

We will follow a single short run from the moment of cranking to the moment the exhaust goes cold, look at what that run leaves behind in the filter, work out how many such runs a typical site really has, and then turn all of that into selection criteria, questions for the vendor and operating changes.

What a Seven-Minute Run Looks Like from Inside the Exhaust

Imagine a 160 kVA set at a shopping complex in Ghaziabad. It is a cool January morning. The grid drops at 10:12, the AMF panel starts the engine and the supply returns at 10:19. Here is what happens in the exhaust over those seven minutes.

Time from start	What the engine is doing	What the exhaust and RECD experience
0 to 10 seconds	Cranking and firing on a cold block	Incomplete combustion; a visible puff of smoke, unburned fuel and water vapour
10 to 30 seconds	Speed settles at 1500 rpm, voltage builds	Exhaust still cool; moisture condenses on cold pipe walls and on the device inlet face
30 to 60 seconds	Changeover closes, building load lands on the set	A sharp load step; brief extra smoke as the governor adds fuel
1 to 4 minutes	Running at a steady but modest load	Exhaust temperature climbing; pipe, silencer and device housing soaking up heat
4 to 7 minutes	Still running; device inlet temperature approaching but not yet at working level	Catalyst only partly active; most soot captured but little burned
7 minutes	Grid returns, changeover opens, engine runs a short cool-down and stops	Exhaust flow ends; device begins to cool with a fresh layer of soot and moisture on board

The important line is the fourth one. Every metre of steel between the engine and the filter has to warm up before the exhaust reaching the device is hot enough to do useful work. On a cold engine, with an RECD a few metres downstream, that warm-up can easily take longer than the run itself.

Why Short Runs Are Hard on a Particulate Filter

An RECD of the kind used on Indian DG sets usually combines a catalyst with a wall-flow particulate filter. The filter traps soot. The catalyst helps that soot burn away at temperatures the exhaust can reach during normal running. That self-cleaning, called passive regeneration, is described in our explainer on what a diesel particulate filter is and how it helps and in the general Wikipedia article on diesel particulate filters.

Short start-stop duty attacks that process in four ways.

1. The soot arrives before the heat does

The first seconds after a cold start are among the dirtiest of the whole run. The cylinders are cold, fuel does not vaporise well and the engine produces more particulate per minute than it will once warm. That soot reaches the filter while the filter is at its coldest.

2. The run ends before regeneration starts

If the device never reaches the temperature at which its catalyst becomes effective, the soot from that run simply stays. One such run does no harm. Ten a day, every day, add up.

3. Water joins the soot

Combustion produces water vapour. On a cold pipe it condenses, mixes with soot and fuel residue and forms a damp, sticky layer. Sulphur in the fuel can make that condensate mildly acidic. Damp deposits are harder to burn off than dry soot, and repeated wetting and drying is not kind to gaskets, clamps and sensor fittings. Using low-sulphur fuel helps; our article on how ultra low sulfur diesel improves emission control explains why.

4. The engine itself gets dirtier

Engines that rarely reach operating temperature suffer from fuel dilution of the lubricating oil, carbon build-up around injectors and, over time, glazing of the cylinder walls. All of these raise smoke output and oil consumption, which means more soot and more incombustible ash heading for the filter. The engine-side condition is often called wet stacking.

Research on city buses, another stop-start duty, points the same way. A study from the University of Pretoria on DPF loading and regeneration issues in city bus operation looked at how low-speed, stop-and-go service affects filter loading and the need for regeneration support. A DG set at a site with an unstable grid behaves in a similar spirit, only with longer gaps between runs.

The Heat-Up Clock: How Long Is “Too Short”?

There is no single answer, because warm-up time depends on the engine, the load, the ambient temperature and the layout. But you can reason about it.

Three things set the length of the heat-up clock:

• Load during the run. Higher load means hotter exhaust and a faster warm-up. A set carrying 15% of its rating may take a very long time to bring the device to working temperature; at 50% it happens much sooner.
• Thermal mass between engine and filter. A long uninsulated pipe, a heavy silencer upstream of the device and a thick-walled RECD housing all absorb heat before any of it reaches the catalyst.
• Starting temperature. A January start in Haryana begins from a much colder block and pipe than a May start.

As a practical working assumption for planning, many engineers treat runs under about 15 minutes as unlikely to contribute meaningfully to regeneration on a typical installation, and runs of 30 minutes or more at reasonable load as useful. Those figures are not taken from a standard; they are a starting point. If you want a firmer number for your own set, a temporary thermocouple at the device inlet logged over a few runs will show you exactly how long the exhaust takes to reach working temperature.

Where Frequent Starts Come From

Before choosing a device, find out why the set starts so often. The cause decides which starts can be removed.

Cause of frequent starts	Typical sign in the log	Can it be reduced?
Momentary grid dips and flickers	Many runs of 2 to 6 minutes, clustered in bad-weather periods	Often yes, with an AMF start delay and UPS cover for sensitive loads
Very short outages from local feeder switching	Runs of a few minutes at predictable times	Sometimes; talk to the distribution company and review timers
AMF settings too sensitive	Starts on voltage sag even when supply never fully fails	Yes, by setting proper under-voltage thresholds and time delays
Synchronised multi-set plants with load-demand start/stop	Second or third set starting and stopping repeatedly as load moves around a threshold	Yes, by widening the start/stop hysteresis and adding minimum run timers
Peak shaving or tariff-based running	Short runs at the start and end of tariff windows	Sometimes; schedule longer blocks instead of many short ones
Frequent short no-load tests	Regular 5 to 10 minute runs at zero load	Yes, by switching to fewer, longer, loaded tests
Operator habit	Irregular manual starts, often brief	Yes, with a written procedure

Our guide on understanding generator load factor and duty cycle explains how the pattern of running affects an engine. For sites under GRAP restrictions in NCR, the season can also change how often the DG runs; how GRAP restrictions affect businesses using DG sets covers that side.

Measuring Your Start-Stop Pattern

Pull at least a month of run records from the DG controller event log, the AMF panel or the manual logbook. Then calculate four things:

1. Starts per day, averaged over the period and in the worst week
2. Median run length, the run duration that half of all runs fall below
3. Share of runs under 15 minutes
4. Longest run in each week, which tells you how often the filter gets a real chance to regenerate

A simple classification helps when you brief a vendor:

Class	Starts per day	Median run	Longest weekly run	RECD outlook
Light cycling	Fewer than 2	Over 30 minutes	Several hours	Few special measures needed
Moderate cycling	2 to 5	15 to 30 minutes	At least one hour	Workable with attention to device placement and periodic loaded runs
Heavy cycling	5 to 10	Under 15 minutes	Under one hour in some weeks	Needs a device chosen for cycling duty plus operating changes
Severe cycling	More than 10	Under 10 minutes	Rarely over 30 minutes	Fix the starts first; device choice alone will struggle

These bands are illustrative. They are meant to start a conversation, not to replace an engineer’s review.

Selection Criteria for Start-Stop Duty

Once you know your class, these are the device and installation features that matter most.

Catalyst light-off behaviour

Ask the vendor at what inlet temperature the catalyst becomes effective for soot and for carbon monoxide and hydrocarbons, and how quickly the device reaches that point from cold at your typical load. Catalyst formulations differ. A formulation that is active at lower temperatures gives short runs a better chance of contributing something.

Thermal mass and warm-up speed

A device that heats quickly helps. Housing design, substrate size and any internal mass all affect warm-up. A device that is oversized “to be safe” can work against you on a cycling set, because more mass takes longer to heat. The right answer is a device sized for your flow and backpressure budget, not the biggest one available.

Position in the exhaust line

Every metre closer to the engine means hotter gas at the device. Where the layout and the engine maker’s guidance allow, placing the RECD close to the turbocharger or exhaust outlet, ahead of long pipe runs, gives the catalyst a head start. Our article on whether an RECD can replace a generator’s exhaust silencer is relevant when the existing silencer sits in the best spot.

Insulation

Insulating the pipe between engine and device, and in some cases the device housing itself, holds heat in. This is one of the cheapest and most effective measures for cycling duty. Insulation must be suitable for exhaust temperatures and must leave sensors and inspection points accessible.

Soot holding capacity

Between regeneration opportunities, the filter has to store whatever soot the short runs deposit. A filter with more storage capacity tolerates longer stretches of cycling before backpressure becomes a concern. That has to be balanced against warm-up speed, which is why a vendor should design the two together.

Condensate handling

Ask how the device and pipework deal with water. A low point with a drain upstream of the device, correct pipe slopes and a rain cap that keeps water out of a cold stack all reduce damp deposits.

Regeneration support options

Some installations on difficult duty cycles add an aid to regeneration, such as an arrangement that raises exhaust temperature during a planned loaded run, or an off-line cleaning service at set intervals. Ask the vendor which options apply to the device they are proposing and what each costs to run. Do not assume an option exists for every product.

Monitoring and alarms

A cycling set needs backpressure and temperature monitoring more than most, because the trend between regeneration events is what tells you whether the filter is keeping up. Ask for a display or IoT system that records backpressure and inlet temperature for every run, with alarms set at sensible levels. The RECD maintenance guide explains what to watch.

Backpressure margin

A filter that carries more soot between regenerations will sit at higher backpressure for longer. Make sure the design leaves margin within the engine maker’s limit at the soot load you expect, not only at a clean filter. Our explainer on how exhaust backpressure affects a DG set shows why this matters.

Questions to Put to the Vendor

Send these with your run records:

1. How long does the proposed device take to reach effective catalyst temperature from a cold start at our typical load?
2. What proportion of our runs, based on our log, do you expect to contribute to regeneration?
3. How much soot can the filter hold before backpressure reaches your recommended service point?
4. Where do you propose to mount the device, and how far is that from the engine outlet?
5. What insulation do you recommend, and is it included?
6. How will condensate be drained or prevented?
7. What regeneration support or cleaning service do you recommend for our pattern, and how often?
8. What does your monitoring record per run, and what alarm thresholds will you set?
9. Is the proposed model covered by a valid CPCB approval for our engine’s power?

The last question matters for every RECD. CPCB’s System and Procedure for Emission Compliance Testing of RECDs sets out how devices are tested and approved. Our guide on how to read an RECD test certificate helps you check what you are being offered, and choosing CPCB approved RECD vendors suggests what else to ask.

Cutting the Starts You Do Not Need

The cheapest way to make an RECD comfortable on a cycling set is to give it fewer, longer runs. These changes should be carried out by a qualified electrical contractor or the DG service provider, with attention to safety and critical loads.

Add a sensible start delay

Many AMF panels allow a delay between detecting grid failure and starting the engine. A delay of a few seconds to a minute, chosen with care, can ride through momentary dips without starting the DG at all. Sensitive loads such as IT equipment, fire systems and emergency lighting must stay protected by UPS or battery systems during that window, so check their autonomy first.

Set a minimum run time

Once the engine has started, a minimum run timer keeps it running for a set period, perhaps 20 to 30 minutes, even if the grid returns sooner. The building transfers back to grid when the timer expires. This turns many four-minute runs into fewer longer ones and gives the exhaust a chance to heat up. It does use some extra fuel, which has to be weighed against fewer cold starts and a healthier filter and engine.

Use a return-to-grid delay

A delay before transferring back to the grid avoids the situation where the supply flickers back for a few seconds, the set stops and then restarts moments later.

Widen load-demand thresholds on multi-set plants

Where two or more synchronised sets share load, a second set that starts at 80% and stops at 70% may cycle repeatedly when the load hovers around those numbers. Widening the gap between start and stop thresholds, adding time delays and setting minimum run times stops that hunting.

Replace short tests with fewer loaded runs

A ten-minute no-load test every few days does nothing for the filter and little for the engine. A loaded run of 30 to 60 minutes once a week or fortnight is better for both. If transferring building load is not practical, a load bank can supply it.

Ask the distribution company

Repeated short outages at predictable times sometimes come from feeder switching or protection settings upstream. A formal complaint or a conversation with the local utility office occasionally removes a whole category of starts.

A Worked Example: The Ghaziabad Shopping Complex

Here is the 160 kVA set from the opening, using an illustrative month of data.

Before changes

• Average starts per day: 7.4
• Median run length: 6 minutes
• Runs under 15 minutes: 81%
• Longest run in a typical week: 48 minutes
• Weekly test: 10 minutes at no load, twice a week
• Device position under consideration: on a frame beside the canopy, about 6 metres of uninsulated pipe from the engine outlet

This is heavy cycling bordering on severe. An RECD fitted to this set as it stands would have very few runs hot and long enough to regenerate.

Changes agreed

1. AMF start delay set to cover momentary dips, after confirming UPS autonomy for billing counters, CCTV and the fire panel
2. Minimum run timer of 25 minutes
3. Twice-weekly no-load tests replaced by one 45-minute loaded test each week, carrying the food court and common area load
4. Device relocated to the canopy roof in place of the existing silencer, reducing the pipe length to about 1.5 metres, with insulation on the remaining pipe

After changes, illustrative

• Average starts per day: 2.6
• Median run length: 25 minutes
• Runs under 15 minutes: 12%
• Longest run in a typical week: 2 hours or more, including the loaded test

The set moved from heavy to moderate cycling. Diesel use rose slightly because of the minimum run timer, but the shopping complex now had a duty pattern an RECD could live with. Malls face a particular coordination challenge with tenants and common areas; our article on RECDs for shopping malls looks at that.

Sites Where Short Cycling Is Common

Certain kinds of property see short cycling more than others:

• Residential societies in areas with patchy evening supply, where the DG starts for every flicker. See RECDs for housing societies.
• Retail and malls, where AMF settings are often tuned to avoid any visible interruption to customers.
• Small factories at the end of rural feeders, where voltage sags are frequent.
• Telecom and small server rooms, though many of these rely on batteries for short outages and start the DG only for longer ones.
• Multi-set plants using automatic load-demand control without enough hysteresis.

If your site is one of these, start collecting run data before you request an RECD quote. Our post on whether an RECD suits a DG set that runs only occasionally deals with the related case of sets that run rarely but for reasonable periods.

Looking After a Cycling Set After Fitment

A set with heavy start-stop duty needs a slightly tighter routine than a set with long runs.

Watch backpressure at a fixed load

Note the device backpressure at a known load during each loaded test. A slow, steady rise over several weeks means soot is accumulating faster than it is burning off. A sudden rise suggests an engine problem.

Plan regeneration runs

If the monitoring shows backpressure creeping up, schedule a longer loaded run before it reaches the service point. Many sites find that a planned run every week or two keeps things in balance.

Keep the engine healthy

Check injectors, air filter, oil level and oil condition more often. Fuel dilution of the oil is a common consequence of short runs. Our monthly maintenance checklist for diesel generators is a good base, and how often to change generator engine oil covers oil intervals.

Inspect for moisture

Check drains, joints and the inlet area of the device for signs of condensate and corrosion, particularly after the monsoon and in winter.

Keep records

Run logs, backpressure readings and service records together show an inspector that the device is installed and working as intended. See what happens during a pollution control board inspection.

Misconceptions About Start-Stop Duty and RECDs

“Short runs are too brief to make much soot, so they do not matter.”

Each run is brief, but the cold-start portion is the dirtiest part of any run, and short runs are almost entirely cold-start. Many short runs can load a filter faster than fewer long ones with the same total hours.

“A bigger filter fixes everything.”

More storage capacity helps tolerate gaps between regenerations, but a larger device also takes longer to warm up. Without fewer starts or longer runs, a bigger filter delays the problem rather than solving it.

“The minimum run timer wastes fuel.”

It does use some extra fuel. Against that, it avoids repeated cold starts, reduces engine wear and fuel dilution, and lets the RECD regenerate. For most heavy-cycling sites, the trade is worth examining with real numbers.

“Our set is new, so cycling will not hurt it.”

New engines produce less soot than old ones, which helps. But warm-up physics is the same for every engine. A new set with a short-cycling pattern will still deposit soot in a cold filter.

Frequently Asked Questions

Can an RECD be fitted to a DG set that starts many times a day?

Yes. The device should be selected with the start-stop pattern in mind, placed close to the engine and insulated where possible, and the site should remove unnecessary starts with start delays, minimum run timers and better test routines.

How short is a “short run” for an RECD?

It depends on load, layout and weather, but runs under about 15 minutes often end before the device reaches working temperature. Logging inlet temperature for a few runs will show you the real figure for your installation.

Why does my DG start for a few minutes several times a day?

Common causes are momentary grid dips, AMF settings that are too sensitive, short feeder outages and load-demand control on multi-set plants. The controller or AMF event log will usually show the pattern.

Will a minimum run timer help the RECD?

Usually yes. It turns many brief runs into fewer longer ones, giving the exhaust time to heat up and the filter a chance to regenerate.

Do short no-load tests harm an RECD?

They add soot and moisture without helping regeneration. Replacing them with fewer loaded tests of 30 to 60 minutes is better for the filter and the engine.

What should I send an RECD vendor if my set cycles often?

At least a month of run records with start times, durations and loads, the engine details, photos of the exhaust route, and your AMF and changeover settings if available.

Fewer Starts, Better Results

A DG set that starts and stops all day is not disqualified from an RECD. It simply needs more thought at the selection stage and some housekeeping on the electrical side. Measure the pattern, remove the starts that serve no purpose, give the device every chance to heat up through good placement and insulation, and choose a design whose catalyst, capacity and monitoring suit the duty that remains.

Aceget designs RECD installations around how each set actually runs, with CPCB type-approved devices for engines from 25 kVA to 2500 kVA and monitoring that records every run. For the size used in this article’s example, see the 160 kVA DG set RECD. Share your run log with the Aceget team for a review, or browse the complete retrofit emission control device range.