- September 2, 2026
- Posted by: Aceget
- Category: GRAP & Regulatory Compliance
If you have been quoted a generator, browsed a dealer’s spec sheet, or tried to work out whether your existing DG set is still compliant, you have probably run into all three names in the same sentence: CPCB II, CPCB III, CPCB IV+. The natural assumption is that these are three sequential steps, each one a modest tightening of the last, the way BS III led to BS IV led to BS VI for vehicles.
For generators, that assumption is wrong in one important way. CPCB III barely exists as an enforced, widely-adopted standard for diesel gensets. This article explains what CPCB II and CPCB IV+ actually require, why CPCB III occupies such a strange, mostly theoretical space in between, and which tier actually determines what you need to buy or retrofit today.
The short answer, if you need it right now
If you are buying a new diesel genset in India today, it will be built to CPCB IV+, because CPCB II-spec new units have not been legally sellable since 1 July 2024. If you already own a CPCB II unit purchased before that cutoff, it remains legal to operate, subject to whatever renewal, capacity-band and retrofit requirements apply in your state, but it does not meet the current manufacturing standard. CPCB III, in the form most people expect (a distinct, numbered, widely-enforced genset tier), is not something you need to look for, because it was never broadly notified and enforced as a standalone requirement for standard diesel gensets in the way CPCB II and CPCB IV+ were.
The rest of this article unpacks why, and what each real tier actually requires.
CPCB II: the standard most of India’s existing DG fleet was built to
CPCB II became the effective manufacturing standard for diesel gensets from around 2014, following a CPCB notification that tightened the original 2004/2005 baseline (now commonly called CPCB I).
Technical profile. For the power band that covers most commercial and light-industrial installations (roughly 75 to 800 kW), CPCB II set a combined NOx-plus-hydrocarbon limit of approximately 4.0 g/kWh, a particulate matter limit of approximately 0.2 g/kWh, and a carbon monoxide limit of approximately 3.5 g/kWh. Smaller sets under 19 kW faced a looser combined NOx+HC ceiling of around 7.5 g/kWh, reflecting their smaller absolute emissions footprint.
Technology required. CPCB II is generally achievable through improved mechanical or basic electronic fuel injection and better combustion chamber design, without necessarily requiring the full after-treatment stack (particulate filters, selective catalytic reduction) that later became mandatory.
Where it stands today. CPCB II is not illegal to operate. It is the standard a very large share of India’s currently running genset fleet was built to, since it held the manufacturing baseline for roughly a decade, from 2014 until CPCB IV+ took over in 2023. What CPCB II units cannot do is satisfy requirements that specifically demand CPCB IV+ performance, most notably CAQM’s exemption from certain GRAP time restrictions in the NCR, which is reserved for CPCB IV+-certified new diesel units. A CPCB II unit operating in the NCR typically needs a certified retrofit, an RECD or a dual-fuel conversion, to bring its real-world emissions performance closer to current expectations, since the engine itself cannot be upgraded to CPCB IV+ without replacement.
CPCB III: the tier that exists on paper more than in practice
This is where most of the confusion in “CPCB II vs III vs IV+” comes from, and it deserves a direct answer rather than a vague one.
Unlike the automotive Bharat Stage system, where BS III was a genuine, widely-enforced, numbered milestone that every vehicle manufacturer had to meet before BS IV arrived, the genset emission framework did not follow the same clean sequential numbering in practice. Industry documentation, CPCB’s own published notifications, and independent technical references such as DieselNet’s India standards summary describe the genset timeline as moving from the 2004/2005 baseline, to the 2014 tightening (CPCB II), directly to the 2023 CPCB IV+ standard, without a distinct, separately numbered, broadly enforced “CPCB III” tier sitting between them for the standard diesel genset category most businesses buy.
Why the name still circulates. A few things keep “CPCB III” alive in conversation even though it is not the standard most buyers will encounter:
- The nomenclature exists conceptually in India’s broader off-road and non-road engine emission framework, which covers categories like construction equipment, and readers sometimes carry that terminology across into genset conversations where it does not directly apply in the same way.
- Some intermediate proposals and draft notifications referencing a “CPCB III” style tightening did circulate during the 2010s as CPCB worked toward what eventually became the CPCB IV+ jump, so the name has some basis in the regulatory process even though it was not the tier that was ultimately notified and enforced for standard gensets.
- Sellers and marketing material occasionally use “CPCB III” loosely to describe any generator positioned as more advanced than CPCB II but not yet fully CPCB IV+ compliant, which is imprecise but common enough to keep the term in circulation.
The practical takeaway. If a dealer, spec sheet or compliance officer mentions “CPCB III” in the context of a standard diesel genset purchase, it is worth asking specifically which CPCB notification and which numeric limits they mean, because for most standard applications, the real decision is between an existing CPCB II unit and new CPCB IV+ stock, not a three-way choice.
CPCB IV+: the current standard, and a genuinely different engine
CPCB IV+ became mandatory for new gensets from 1 July 2023, with a one-year window allowing sale of remaining CPCB II-spec stock until 1 July 2024, after which only CPCB IV+ units could legally be sold new.
Technical profile. This is not an incremental tightening of CPCB II, it is a categorical jump. Depending on the specific power band, particulate matter limits fall to roughly 0.02 to 0.03 g/kWh, down from CPCB II’s approximately 0.2 g/kWh in the comparable mid-range band, a reduction of around 90 percent. NOx is now regulated as its own separate figure, roughly 0.40 to 0.67 g/kWh depending on band, rather than combined with hydrocarbons the way CPCB II measured it. CPCB IV+ applies specifically to diesel engines between roughly 19 kW and 800 kW.
Technology required. Meeting these limits generally requires electronic fuel injection, exhaust gas recirculation, diesel particulate filters, and selective catalytic reduction, essentially the same after-treatment architecture used in international Tier 4 Final and Stage V compliant engines. This is a genuinely different engine, not a tuned version of a CPCB II block, which is why manufacturers needed the full year of transition time to shift production and why used or reconditioned CPCB II equipment cannot simply be upgraded to CPCB IV+ status through a software or minor hardware change.
A structural change worth noting. CPCB IV+ also collapsed the earlier distinction some tiers allowed between prime-duty and standby-duty gensets into a single unified emission standard covering both use cases. Older tiers sometimes permitted different limits depending on whether a genset was the primary power source or backup power, a distinction CPCB IV+ largely does away with.
The comparison, side by side
| Factor | CPCB II | CPCB III (as commonly asked about) | CPCB IV+ |
|---|---|---|---|
| Status for standard diesel gensets | Legacy standard, still legally operable, no longer sellable new since July 2024 | Not a distinct, broadly enforced standalone tier for standard gensets | Current mandatory standard for all new gensets since July 2023 |
| Effective from | Around 2014 | Not separately notified in this form for gensets | 1 July 2023 |
| PM limit (mid power band) | Approx. 0.2 g/kWh | Not applicable in the standard genset framework | Approx. 0.02 to 0.03 g/kWh |
| NOx-related limit | Combined NOx+HC, approx. 4.0 g/kWh | Not applicable in the standard genset framework | NOx alone, approx. 0.40 to 0.67 g/kWh |
| Core technology | Improved fuel injection, better combustion control | N/A | Electronic fuel injection, DPF, SCR, EGR |
| Prime vs standby distinction | Sometimes differentiated | N/A | Unified single standard for both |
| Path to compliance if you already own one | Retrofit (RECD or dual-fuel) for capacity-band-mandated locations | N/A | Already compliant as manufactured |
How to check which tier your own generator actually is
Before deciding what to do about any of this, confirm what you actually own, since assumptions here are a common source of wasted effort.
Check the engine data plate. Most manufacturers stamp the CPCB compliance tier directly on the engine’s identification plate, alongside the rated power, serial number and year of manufacture. This is the fastest and most reliable check.
Check the original purchase or commissioning documentation. The CTO application and any test certificates filed at the time of installation will typically state the compliance tier explicitly, since it is a required field for consent approval.
Cross-check against the purchase date. As a rough sanity check, a genset purchased new before mid-2023 is almost certainly CPCB II or an earlier tier, since CPCB IV+ was not yet mandatory. A genset purchased new after July 2024 is CPCB IV+, since CPCB II stock could no longer be legally sold from that date. Anything commissioned in the narrow window between July 2023 and July 2024 could technically be either, since that was the sell-through period for remaining CPCB II inventory, which makes the data plate check the only reliable method for units purchased in that specific window.
When in doubt, ask the manufacturer directly. Serial number lookups through the original equipment manufacturer will confirm the certified tier definitively if the data plate is unclear or has become illegible over time.
What the after-treatment technology actually does
The jump from CPCB II to CPCB IV+ is easier to understand once you know what each piece of added technology is actually doing, rather than treating it as an abstract compliance requirement.
Diesel Particulate Filter (DPF). Physically traps soot and particulate matter from the exhaust stream before it leaves the engine, periodically burning off the trapped material in a regeneration cycle. This is the component most directly responsible for the roughly tenfold drop in PM limits between CPCB II and CPCB IV+.
Selective Catalytic Reduction (SCR). Injects a urea-based fluid (commonly known as diesel exhaust fluid) into the exhaust stream, triggering a chemical reaction that converts NOx into harmless nitrogen and water vapour before it exits the tailpipe. This is the primary technology behind CPCB IV+’s much lower, separately tracked NOx limit.
Exhaust Gas Recirculation (EGR). Redirects a portion of exhaust gas back into the engine’s intake, lowering peak combustion temperatures and reducing NOx formation at the source, working alongside SCR rather than replacing it.
Electronic Fuel Injection (EFI). Replaces mechanically timed fuel delivery with electronically controlled injection, allowing far more precise fuel metering across varying load conditions, which improves combustion efficiency and reduces both particulate and NOx formation before after-treatment even comes into play.
None of these four exist in a typical CPCB II engine in the combination CPCB IV+ requires, which is the core technical reason a CPCB II unit cannot simply be software-updated or lightly modified into CPCB IV+ compliance. A certified retrofit device approximates some of this benefit, particularly on the particulate side, without replicating the full factory-integrated system.
Retrofit versus replacement: a rough cost comparison
Numbers vary significantly by capacity, manufacturer and installer, but the general shape of the decision holds across most mid-range commercial installations:
| Consideration | Certified retrofit (RECD or dual-fuel) on existing CPCB II unit | New CPCB IV+ replacement |
|---|---|---|
| Upfront cost | Meaningfully lower, a fraction of new equipment cost | Full equipment cost |
| Downtime during transition | Typically days | Can involve longer lead times for new equipment delivery and commissioning |
| Emissions performance achieved | Brings particulate performance much closer to current expectations, does not fully replicate CPCB IV+ NOx control | Full CPCB IV+ performance as manufactured |
| Eligibility for GRAP time-restriction exemptions | Generally not eligible, exemption is reserved for CPCB IV+ certified units | Eligible where applicable |
| Best suited to | Mechanically sound units with meaningful remaining service life | Units nearing end of life, or facilities needing GRAP exemption eligibility specifically |
For a large share of businesses, particularly those operating outside the NCR’s most tightly restricted zones, the retrofit path resolves the practical compliance question at a fraction of replacement cost. The calculation shifts meaningfully for any facility that specifically needs CPCB IV+’s GRAP exemption eligibility, since no retrofit currently confers that specific benefit.
Myths worth clearing up
“CPCB III must be stricter than CPCB II and less strict than CPCB IV+, so I should look for it.” As covered above, there is no broadly enforced standalone CPCB III tier for standard diesel gensets to look for. Searching for CPCB III-labelled equipment specifically is likely to waste time that would be better spent confirming whether you need CPCB II-plus-retrofit or full CPCB IV+.
“CPCB IV+ and BS VI are the same thing.” They are conceptually similar, both represent a major after-treatment-driven tightening, but they are separate regulatory frameworks. BS VI governs on-road vehicles, CPCB IV+ governs stationary and portable gensets, and equipment certified under one is not automatically certified under the other.
“Once retrofitted, my genset is officially reclassified as CPCB IV+.” It is not. A retrofitted CPCB II unit remains a CPCB II engine with a certified add-on device, which satisfies specific capacity-band and retrofit mandates but does not change the engine’s underlying certified tier or unlock CPCB IV+-specific exemptions like GRAP time-restriction relief.
What this means if you already own a CPCB II generator
You have three realistic paths, and the right one depends on your location, your capacity band, and how long you plan to keep using the unit.
Path one: retrofit. For most businesses with a working, otherwise sound CPCB II genset, fitting a certified RECD or converting to dual fuel is the most cost-effective way to bring real-world emissions performance closer to current expectations without replacing functioning equipment. This is also, in many NCR capacity bands, the legally required path rather than an optional upgrade. Our explainer on why RECDs are mandatory in India covers the legal basis for this requirement in detail, and our comparison of RECD versus DPF technology explains the technical difference between the retrofit approach and factory-fitted particulate filtration.
Path two: continue operating as-is, where permitted. Outside the NCR’s specific capacity-band mandates, and outside any state-specific retrofit requirement, a CPCB II unit can often continue operating legally provided its consent, electrical and other certifications remain current. Our guide on how often to renew a DG set’s pollution certificate covers exactly what that ongoing compliance looks like.
Path three: replace with CPCB IV+. For businesses planning a longer-term capacity upgrade, relocating to a more tightly regulated area, or simply reaching the natural end of an existing unit’s service life, replacing with new CPCB IV+ stock resolves the compliance question at the engine level entirely, at the cost of a full equipment purchase rather than a retrofit.
There is no single right answer across all three paths. A facility running a five-year-old CPCB II unit in good mechanical condition in a state without an aggressive retrofit mandate has a very different calculation than one running a similarly aged unit inside the NCR’s 19 to 125 kW dual-fuel-mandated band. If you are unsure which path applies to your specific installation, our certifications page outlines the standard documentation involved in each route, and our team can help assess your specific capacity band and location through our contact page.
Common questions buyers actually ask
Can I retrofit a CPCB II genset to make it officially CPCB IV+? No. A retrofit device brings real-world emissions performance closer to current expectations and satisfies specific capacity-band mandates like CAQM’s Direction No. 76, but it does not change the engine’s underlying certified emission tier. The unit remains a CPCB II engine with a certified retrofit attached, not a reclassified CPCB IV+ unit.
Is it worth buying used CPCB II equipment in 2026? This depends heavily on price, remaining mechanical life, and your specific location’s capacity-band requirements. A used CPCB II unit priced well below new CPCB IV+ equipment can still make financial sense if a retrofit brings it into compliance for your specific use case, but it is worth running that comparison explicitly rather than assuming the lower purchase price alone makes it the better deal once retrofit and ongoing compliance costs are factored in.
Does CPCB IV+ cost meaningfully more than CPCB II equipment did? Generally yes, because the after-treatment technology required (DPF, SCR, EGR, electronic fuel injection) adds real manufacturing cost compared to the simpler CPCB II architecture. This is part of why the retrofit path remains commercially attractive for existing CPCB II owners rather than blanket replacement.
Will CPCB IV+ itself be superseded soon? Nothing has been notified as of 2026 to suggest an imminent successor tier, and given how large a technical and manufacturing shift the 2023 transition already represented, most industry observers expect the current standard to hold as the baseline for at least the next several years rather than facing an imminent further tightening. Our timeline of CPCB emission norms for generators traces how long each previous tier held before being replaced, which gives useful context for how durable CPCB IV+ is likely to be as the current baseline.
Does my capacity band affect which tier I actually need, beyond just what’s legally sellable? Yes, particularly in the NCR. Our post on GRAP rules for DG sets in Delhi NCR explains how CAQM’s capacity-band framework interacts with the underlying CPCB tier, since a CPCB IV+ unit can receive operational exemptions during GRAP restrictions that an older-tier unit, retrofitted or not, does not receive. For the fuller picture of how this fits into ongoing compliance obligations, our environmental compliance checklist for generator owners walks through the complete lifecycle.