In short. A diesel generator engine produces oxides of nitrogen, particulate matter, hydrocarbons and carbon monoxide, and reducing one tends to increase another. Engines built to CPCB IV+ norms meet the limits with electronic common-rail injection plus exhaust aftertreatment, typically an oxidation catalyst, a particulate filter and selective catalytic reduction with urea. These systems work well on loaded engines and poorly on standby sets that idle for years, so the operating regime matters as much as the hardware.
Where each pollutant comes from
Oxides of nitrogen form when combustion is hot and oxygen is plentiful, exactly the condition that gives a diesel its efficiency. Particulate matter, mostly soot with adsorbed hydrocarbons, forms where fuel is rich and combustion incomplete, typically in the core of the spray. Measures that cool combustion to cut NOx, such as retarded timing or exhaust gas recirculation, raise soot and fuel consumption; measures that burn soot cleanly raise NOx. For two decades designers traded one against the other inside the cylinder. Current limits are too tight for that alone, so the exhaust is treated after it leaves the engine.
Carbon monoxide and unburnt hydrocarbons are a smaller share on a healthy diesel but climb sharply at light load and during cold running, the normal state of a standby set exercised at no load every week. White smoke is unburnt fuel, blue smoke is oil, and black smoke is soot from overfuelling, a choked air filter or a tired turbocharger.
The Indian regulatory picture
The Central Pollution Control Board sets emission limits for new diesel generating sets under the Environment (Protection) Rules. The CPCB II limits that applied from 2014 were met largely with in-cylinder measures. The CPCB IV+ limits, in force for new sets up to 800 kW from mid-2023, cut NOx and particulate matter to a small fraction of the earlier values and in practice require electronic injection and aftertreatment; sets above 800 kW fall under separate limits. Existing sets are not retrospectively required to meet the new limits, but state boards and regional authorities can restrict their operation, as the Delhi NCR regime already does, which is why retrofit emission control devices exist. The notification, the engine’s type approval and the state board’s consent conditions should be read together for any site; our India genset emissions standards page tracks the current position.
What each aftertreatment device does
| Device | Target pollutant | How it works | What it needs to function |
|---|---|---|---|
| Diesel oxidation catalyst (DOC) | CO, hydrocarbons, soluble fraction of PM | Platinum-group catalyst oxidises CO and HC to CO2 and water, and converts NO to NO2 for downstream use | Exhaust temperature above its light-off point, low-sulphur fuel |
| Diesel particulate filter (DPF) | Particulate matter | Wall-flow ceramic traps soot; soot is burnt off by NO2 passively or by raised exhaust temperature actively | Regular periods of exhaust heat, low-ash engine oil, periodic ash cleaning |
| Selective catalytic reduction (SCR) | NOx | Urea solution is injected, decomposes to ammonia, and reduces NOx to nitrogen over a catalyst | Exhaust above roughly 200 degrees for dosing, urea solution to ISO 22241, working NOx sensors |
| Exhaust gas recirculation (EGR) | NOx | Cooled exhaust is fed back to the intake to lower peak combustion temperature | Clean EGR cooler and valve, tolerant lubricant |
| Retrofit emission control device (RECD) | Mainly PM on legacy sets | Add-on filter or catalyst package certified for a stated engine family | Correct match to engine, exhaust backpressure within limits, maintenance regime |
A CPCB IV+ set in the mid and upper kW range commonly carries a DOC, a DPF and SCR in one canister, with urea in a separate tank, all managed by the engine control module whose sensors are described under engine ECM and sensors.
Why standby duty is the hard case
Aftertreatment is designed around a loaded engine with a hot exhaust. A standby set exercised for fifteen minutes a week on no load never reaches the temperature at which the DOC lights off, the DPF regenerates or the SCR can dose. Soot accumulates, backpressure rises, and the control module eventually forces an active regeneration by injecting extra fuel late in the cycle. If the set is shut down before regeneration completes, as happens when the mains returns, the engine logs an aftertreatment fault. Over months this becomes a derate, and the set bought for emergencies is limited to a fraction of its rating.
The remedy is operational. Exercise runs should be long enough and loaded enough to bring the exhaust to temperature, and where site load is small, a resistive load bank should be connected for the run. Our load bank testing service is used for exactly this on CPCB IV+ installations. Urea solution should be bought in sealed containers, kept out of sunlight and used within its shelf life, because contaminated or crystallised urea blocks dosing nozzles. Oil must be the low-ash grade the manufacturer specifies, since ash does not burn off and shortens the interval between DPF cleanings.
Exhaust design and backpressure
Every device in the exhaust adds restriction, and the engine manufacturer states a maximum backpressure at the turbocharger outlet. Long pipe runs, bends, an undersized silencer or a soot-loaded filter all consume that allowance, and excess backpressure raises exhaust temperature, fuel consumption and turbocharger wear. Where a retrofit device is added, the backpressure calculation must be redone with the device in place and the exhaust route checked for condition and leaks first. Connecting several engines to one common header is not acceptable unless engineered for it, because exhaust from a running set flows back into an idle one and wets its filter and turbocharger. The method is set out under exhaust backpressure.
Retrofit, replace or restrict: the legacy decision
For a facility with CPCB II or older sets, three paths exist. A certified retrofit device brings particulate emissions down to a level the state board accepts, at the cost of added maintenance. Dual-fuel conversion to gas reduces diesel consumption and some pollutants but does not make an old engine compliant with new-set limits. Replacement with a CPCB IV+ set gives a clean regulatory position at the highest capital cost. The right answer depends on engine condition, running hours, gas availability and whether local restrictions are likely to tighten. The framework is laid out in RECD versus dual fuel versus replacement, and the device-level detail in retrofit emission control devices for DG sets.
Related photographs and videos
Common mistakes
- Exercising a CPCB IV+ standby set on no load every week; the particulate filter never regenerates and the engine derates when it is finally needed.
- Assuming that an old set with a new silencer or a new exhaust route is compliant; the pipework proves nothing about emissions.
- Fitting a retrofit device certified for a different engine family; the backpressure, the temperature window and the certificate are all wrong.
- Topping up the urea tank with water or agricultural urea; the dosing system blocks and the engine enters inducement derate.
- Deleting or bypassing aftertreatment to cure a fault; the set loses its certified status, and the underlying fault, usually low exhaust temperature, remains.
What to send us
Send photographs of the genset and engine nameplates, the emission compliance plate or type approval reference if the set carries one, the exhaust arrangement from manifold to outlet, and any aftertreatment or emission codes on the controller. Tell us the typical load, the weekly exercise routine and whether the site is subject to any state board or regional restriction. Send a generator sales or compliance enquiry and we will advise whether retrofit, operating changes or replacement is the sensible route.
Questions engineers ask
Does a CPCB IV+ generator cost more to run than a CPCB II set?
Fuel consumption per kWh is similar or slightly better because of common-rail injection. The additional running costs are urea solution, low-ash oil, periodic DPF ash cleaning and the discipline of loaded exercise runs. For a set that runs a few hundred hours a year, these are modest beside the capital cost difference.
Can I still buy a non-CPCB IV+ generator?
New sets up to 800 kW sold in India must meet the current norms, so a new non-compliant set in that range is not a legal purchase. Used older sets change hands, and their operation depends on the state board’s consent conditions for the site.
What exhaust temperature does SCR need?
Dosing is normally inhibited below roughly 200 degrees at the catalyst inlet, and conversion efficiency is best well above that. The exact thresholds are in the engine manufacturer’s data. A set at 20 percent load in a cool room may never reach them, which is why loading matters.
Will a retrofit device make my old set equivalent to CPCB IV+?
No. A certified retrofit device reduces particulate matter, and some designs reduce other pollutants, to a level the regulator accepts for continued operation of an existing set. It does not re-certify the engine to new-set limits, and its certificate applies to a stated engine family only.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.