In short. A dual-fuel generator is a diesel engine that draws natural gas, biogas or LPG vapour into its intake air and ignites it with a reduced pilot injection of diesel. Approved systems typically replace 40 to 70 percent of the diesel energy at mid to high load, less at light load, and fall back to full diesel if gas fails. Substitution is an operating envelope across load, not one guaranteed number.
Define the architecture first
The words dual fuel are used loosely in the market, so the first question on any proposal is which two fuels are involved and how the engine actually uses them. In the strict sense used for generators in India, dual fuel means compression ignition is retained: diesel is still injected and still ignites the charge, while gas premixed with the intake air supplies part of the energy. This differs from a bi-fuel spark-ignition engine that runs on either petrol-type gas or another gas, and from a purpose-built gas engine that runs on gas alone. CPCB’s emission procedures recognise dual fuel as a distinct engine and fuel category for this reason. A buyer comparing offers should insist that each supplier describes the architecture, the gas admission method and the control strategy in writing.
How a dual-fuel engine behaves
At any load the governor demands a total fuel energy. The dual-fuel controller decides how much of that comes from gas and trims the diesel injection to make up the rest, within limits set by knock, exhaust temperature, misfire and the minimum pilot quantity needed for reliable ignition. At light load the premixed gas charge is too lean to burn completely, so methane slips through unburnt and the controller keeps substitution low or switches gas off entirely. At mid to high load the charge is rich enough to burn well and substitution rises. Near full load the knock margin narrows, especially with warm intake air in a Hyderabad summer, and substitution is capped again. The result is a curve, not a constant.
| Generator load | Typical substitution behaviour | Main limiting factor |
|---|---|---|
| Below about 30 percent | Gas off or low, often under 20 percent | Lean charge, incomplete combustion, methane slip |
| About 40 to 80 percent | Highest substitution, commonly 40 to 70 percent for approved kits | Gas quality, exhaust temperature |
| Above about 85 percent | Reduced substitution | Knock margin, intake temperature |
| Large load step | Gas momentarily reduced, diesel carries the transient | Governor response, controller logic |
These ranges are indicative of the technology in general. The figures for a specific engine and kit come from the supplier’s test data and should state the load, ambient and gas at which they were measured.
Substitution: what the percentage means
S (%) = (Ed,0 − Ed,DF) / Ed,0 × 100
S is the gas substitution rate on an energy basis, Ed,0 is the diesel energy the engine uses on diesel alone at a given electrical output, and Ed,DF is the diesel energy it uses in dual-fuel mode at the same output. Worked example (illustrative): a 400 kW set carrying 240 kW, which is 60 percent load, burns 64 litres per hour on diesel alone. In dual-fuel mode at the same 240 kW it burns 26 litres per hour plus gas. The substitution is (64 − 26) / 64 = 59 percent. Note that the gas energy supplied is usually slightly more than the diesel energy it replaces, because thermal efficiency in dual-fuel mode is often a little lower; the saving must be calculated from both fuels’ prices and quantities, not from the substitution rate alone. A quoted percentage that does not state the load, the basis and the measurement method cannot be compared with any other.
Retrofit conversion versus manufacturer-engineered dual fuel
Dual fuel reaches a site in two ways. A manufacturer may offer a dual-fuel genset that has been designed, calibrated and type approved in that configuration, with the warranty and emission certification covering both modes. Alternatively a third-party conversion kit may be fitted to an existing diesel engine, adding a gas train, a mixer or port injection, sensors and a controller that trims the original fuel system. Conversion keeps a working engine in service and can be economic, but it is not automatically equivalent: the engine maker’s warranty, the original emission certification, and the long-term reliability under the new combustion regime all need to be addressed. The detailed assessment for an existing set is set out in dual-fuel compatibility and conversion assessment, and the substitution testing method in dual-fuel performance and gas substitution.
Gas supply and the three operating states
Gas availability is the word that decides whether dual fuel pays. The project should model three states: gas fully available, gas partially available at reduced pressure or flow, and gas unavailable. In each state the questions are what happens to output, operating cost and emissions, and whether the transition between states is smooth. A well-designed system closes the gas valves on low pressure, gas leak, knock, high exhaust temperature or loss of a sensor, and returns to full diesel without dropping the load. That fallback must be proved during commissioning, not assumed from the brochure. The valve fail state, controller logic and alarm behaviour should be documented. Gas supply sources include pipeline natural gas, treated biogas and LPG vapour, each with its own pressure and composition checks.
Emissions, compliance and fuel cost
Substitution is a fuel measure, not an emissions measure. A 60 percent substitution does not mean 60 percent lower emissions. Particulate matter usually falls substantially because less diesel is burnt, but methane slip and carbon monoxide can rise, particularly at light load, and oxides of nitrogen may move either way. For CPCB compliance the relevant question is whether the configuration has been tested and approved as dual fuel, and for sites under GRAP or similar restrictions whether that approval permits operation. Fuel savings depend on the relative delivered prices of diesel and gas per unit of energy, the hours run and the load profile; consumption estimates for the diesel side can be checked with the generator fuel consumption guidance. The broader choice between dual fuel, emission control retrofits and replacement is compared in RECD versus dual fuel versus replacement.
Common mistakes
- Accepting a single substitution figure such as 70 percent as a guarantee. The site runs at 35 percent load most of the time and actual substitution is a third of the promise.
- Converting an engine without checking its condition. Worn injectors or turbochargers cause knock and high exhaust temperatures, and the controller keeps gas switched off.
- Treating fallback to diesel as automatic. A valve that fails open or a controller that trips the set on gas loss turns a fuel interruption into a power outage.
- Reporting substitution as emissions reduction. Auditors and pollution control boards ask for measured emissions, and the claim cannot be supported.
What to send us
Send the nameplate photos of the engine and alternator, the hours run and recent service history, a log or estimate of typical load across the day, and the gas source with available pressure and flow. Include photographs of the generator room or canopy and the proposed gas pipe route, and any conversion offers you have received. Use the application engineering enquiry and we will assess whether the engine is a sound candidate and what substitution its load profile can realistically achieve.
Questions engineers ask
Can any diesel generator be converted to dual fuel?
No. The engine must be in sound mechanical condition, a conversion kit must be engineered and validated for that engine family, and the knock and exhaust temperature margins must allow useful substitution. Older mechanically governed engines and very small sets are often poor candidates. Each engine is assessed individually.
Does dual fuel reduce the generator’s power output?
A properly calibrated system should deliver the rated output, but at high load the controller may reduce gas to protect against knock, so the set reaches full power with more diesel. On high-ambient days some systems limit substitution further. Load acceptance on gas may also be slightly softer, which is why the controller usually shifts transients onto diesel.
What happens if the gas supply fails while the generator is running?
In a correctly designed system the gas shut-off valves close and the governor increases diesel injection to carry the full load, with a brief frequency dip. This transition should be tested at commissioning under load. If the design trips the engine instead, the dual-fuel system has reduced reliability rather than added flexibility.
Is dual fuel compliant with CPCB norms?
CPCB procedures include a dual-fuel category, and compliance depends on the specific engine and kit configuration being tested and approved. A conversion kit fitted without that approval does not carry the original diesel engine’s certification into dual-fuel operation. Confirm the approval document before purchase.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.