In 2021, the Government of India had said it will move to 20% ethanol blending in petrol by 2025 with the two-pronged goal of cutting carbon emissions and reducing the country’s dependence on foreign oil. While vehicles modified to be compatible with the new composition started rolling out in April 2025, the government’s push for 20% ethanol-blended petrol (E20) has left vehicle-owners worried about the impact on their older vehicles and about a surge in maintenance costs.
Ethanol, or ethyl alcohol, is a biofuel: it is made from plant waste called biomass. Regular petrol is a hydrocarbon made from the fossilised remains of organic matter buried for millions of years.
When mixed with a fossil fuel like petrol, ethanol acts as an oxygenate that helps the petrol burn better.
Making ethanol
Under India’s ethanol-blending programme, the government procures ethanol either from sugarcane-based raw materials like C-heavy molasses, B-heavy molasses, sugarcane juice, sugar or sugar syrup, or damaged food grains like broken rice, maize or cellulosic and lignocellulosic materials.
Molasses is a byproduct of sugarcane production. It is a thick, dark syrup about 40% rich in sugars that can be fermented but which can’t be extracted further.
C-heavy molasses are the final byproduct of the sugar production process, with molasses content around 28-32%. B-heavy molasses are an intermediary byproduct of the same process and have a higher molasses content, around 48-52%.
Ethanol is made from molasses by fermentation — using yeast enzymes to catalyse the breakdown of sugar molecules in the presence of water. An everyday example of fermentation is ginger soda. If you put ginger, sugar, and water in an airtight container for a few days, it becomes fizzy. This is because the microbes in ginger feed on the sugar, releasing carbon dioxide, which makes carbonic acid with water.
In the first step of ethanol production from molasses, sucrose molecules in the syrup are diluted with water. Then they break down into glucose molecules in the presence of invertase.
These glucose molecules further react in the presence of zymase to form ethanol and release carbon dioxide.
Producing ethanol from food grains and lignocellulosic materials also involves other processes that break them down to fermentable sugars first. Lignocellulosic biomass is plant matter with a high carbohydrate content and is usually composed of parts not used for food or feed. This biomass is rich in cellulose, hemicellulose, and lignin.
Chemical nature, energy efficiency
Two factors are key to understanding ethanol’s energy efficiency: the calorific value and the octane number.
The calorific value of a fuel denotes its yield: a higher calorific value means more energy. The calorific value of ethanol is significantly lower than that of petrol, so the fuel’s overall burning efficiency should theoretically decrease. However, the government has maintained that the drop in fuel performance is not significant and that it is governed by a mix of other factors, including “driving habits, maintenance practices such as oil changes and air filter cleanliness, tyre pressure and alignment, and even air conditioning load”.
The octane number is a measure of a fuel’s resistance to engine knocking or burning prematurely. Ethanol has a higher octane number than petrol. Thus it has the potential to reduce the knock resistance significantly. However, because of its lower energy content, the amount of energy the engine can extract per litre of blended fuel decreases with increasing ethanol content.
This said, the drop in mileage drivers have raised concerns about won’t be significant, Sudheer Kumar Kuppili, a research fellow at London School of Hygiene and Tropical Medicine, said.
“The compensation for having lower energy would be quite low when you are moving from E10 to E20,” Kuppili said. “It would be significant only when we shift to 100% ethanol. Most of the uproar these days about the reduction in vehicular mileage is fuelled by many factors, which are difficult to assess.”
In fact, experts said the thing to focus on here is ethanol’s hygroscopic nature. That is, ethanol has a considerable tendency to attract and attach water molecules to itself. This in turn affects the vehicle’s components and fuel performance in new ways.
Independent expert Noble Varghese said the main concern is the increased possibility of corrosion.
“Ethanol affects the rubber components of a fuel system, which is mainly the piping, the fuel tank, the injectors, the filters, and the combustion chambers. The combustion chamber and the engine block itself are not as affected by ethanol, but what is affected is the fuel tank, the rubber piping, and the injector,” Varghese said.
“Ethanol tends to attract water, so if the vehicle is not used everyday, water tends to collect in the fuel tank, which is corrosive for the tank. This causes another problem too: rust particles mix with fuel and go into the fuel line and clog it,” he added.
“That in itself will reduce the mileage. This is not primarily because of ethanol’s thermodynamic properties but more because of the components which are not suited to E20 fuel.”
The government has however maintained that replacing older rubber parts and gaskets designed for the non-blended fuel is “inexpensive and can be easily managed during routine servicing.”
A statement released by the Ministry of Petroleum and Natural Gas (MoPNG) on August 12 also said this change would be required only “once in the lifetime of the vehicle” and at “any authorised workshop”.
Kuppili also said moisture won’t be a significant problem.
“This might be a problem for older vehicles, which are probably already worn out. This should not be a problem for BS-IV or BS-VI vehicles. The fuel’s hygroscopic nature might have a noticeable effect when you are in colder parts of the country. However, in those regions, fuels are usually mixed with additives to avoid freezing,” he explained.
“Also, when you are using a vehicle regularly, any moisture that accumulates will be evaporated by the high engine temperatures (>400º C),” he said.
Kuppili wished to clarify that he hasn’t worked on this particular issue of the fuels.
Comparisons with Brazil
Comparing the Indian ethanol-blending story with that of Brazil may not be justified as the South American country currently uses E27 petrol as well as paced this upgrade out over decades.
Brazil first decided to lower its dependence on imported petrol in 1975 with its Proálcool programme during the oil crisis in the 1970s. Over the next few years, it focused on building capacity by working with researchers, expanding the production of ethanol from sugarcane with the specific purpose of mixing it with petrol, providing subsidies, incentives, and tax breaks to reduce the costs of production, and increasing the competitiveness of ethanol.
The country also has a large market for vehicles fit with flex-fuel engines that allow vehicles to readily adapt to petrol with different ethanol fractions.
This isn’t the case with Indian vehicles. According to Varghese, the electronic control unit in an engine is hypersensitive to fuel content, which it detects automatically.
“If you put more ethanol in fuel, its chemical properties change and therefore the engine management system has to be able to adapt, and older cars might not be able to do that,” he said.
By way of solution, he said mechanics could quickly connect the unit to a computer and update the fuel composition setting.
However, one senior industry executive told The Hindu that vehicles that don’t have electronic control units and injectors can’t be made compatible with E20 as fuel. The executive said “about 95%” of all vehicles until the rollout of the BS-VI emission standards (in April 2020) were mechanically carbureted, meaning they didn’t utilise any electronic units with sensors to regulate the delivery and treatment of fuel.
“When the blending happens, it alters the stoichiometric ratio, i.e. air-to-fuel ratio, which in turn affects the speed of combustion and heat release,” Kuppili said. “Modern vehicles have sensors called electronic control units, which monitor the oxygen content and alter the amount of fuel and air needed.”
With ethanol-blended petrol, he added, “you would need less air than usual, as ethanol has an oxygen atom. So this reduction can actually reduce pollutant emissions such as NOX, PM, and CO.”
“The ethanol-blending policy itself is excellent. It has been successful in other parts of the world,” Varghese said. “The ideas of energy security and saving import duty are excellent and true. It just needs to be paced better.”
The principle of recalibration
The executive said the sensor’s main purpose is to manage the spark timing — which determines the release pattern of the fuel and combustion inside the cylinder. The electronic control unit uses the spark plug to create the spark that ignites the fuel-air blend in the combustion chamber. Older vehicles, i.e. those lacking the unit, have been tuned to a different fuel and are thus hard-coded for a different spark timing.
Since E20 brings more oxygen to the fuel-air mix, the executive explained, engines may have to be recalibrated by, among other measures, advancing the pressure for combustion and its eventual timing by two to three degrees. In other words, the engine will have to make sure combustion happens earlier so that there are no issues with starting the vehicle and with maintaining an adequate air-fill ratio across all load conditions and speeds.
This customisation isn’t possible with mechanically carbureted vehicles.
The executive also affirmed that the adjustment would increase cost.
“You have to pay the calibration engineer and the vehicle component engineers,” he said. The latter “charge good money.”
(With inputs from Saptaparno Ghosh)
