Alternative fuels

Automobile Engineering (B.E./B.Tech) · Engines and emissions · free study notes for exams, placements and interviews

In short

Alternative fuels are energy carriers other than conventional petrol and diesel used to run vehicles and engines — alcohols (ethanol, methanol), biodiesel, gaseous fuels (CNG, LPG, LNG, CBG, hydrogen), producer gas, and electricity or hydrogen through fuel cells. They matter to India because about 85% of crude oil is imported, urban air quality is poor, and the country has committed to net zero by 2070 with a large renewable push. The subject appears in university papers under "Alternative fuels / fuels and combustion", in GATE (ME) under IC engines and fuels, and in ESE and PSU interviews. Employers — Maruti Suzuki, Tata Motors, Mahindra, Bajaj, TVS, Ashok Leyland, Cummins, IOCL, BPCL, HPCL, GAIL, Praj, Reliance — hire for blending, retrofit, calibration and CBG plant roles. Expect numerical questions on blend properties and energy equivalence, plus policy questions on E20, the Ethanol Blended Petrol Programme and the National Policy on Biofuels.

Explained

Why alternative fuels, and how they are classified

Drivers: import dependence and the oil import bill, local pollutants (CO, HC, NOx, PM), greenhouse gases, and farm-income/waste-to-energy opportunity.
By origin: fossil-derived (CNG, LNG, LPG, grey methanol, grey hydrogen) versus renewable (ethanol, biodiesel, CBG/biogas, producer gas, green hydrogen, green methanol).
By physical state: liquid (ethanol, methanol, biodiesel, HVO), gaseous (CNG, LPG, hydrogen, CBG), solid-derived gas (producer gas from biomass gasification), and electricity (battery or fuel cell).
By generation of biofuel: 1G from food crops (sugarcane molasses, corn); 2G from lignocellulose (rice straw, bagasse, bamboo); 3G from algae; 4G with carbon capture.
Drop-in versus blend-in: drop-in fuels (HVO, biodiesel in small fractions, synthetic diesel) need no engine change; others need blending limits or engine modification.
Well-to-wheel view: a fuel is only clean if production, compression/liquefaction and distribution are counted, not just tailpipe. Battery and hydrogen vehicles are zero at the tailpipe, not necessarily well-to-wheel on a coal-heavy grid.

Properties that decide whether a fuel can be used

Calorific value (CV): ethanol about 26.8 MJ/kg, methanol 19.9, petrol 44, diesel 42-43, biodiesel ~37-38, CNG ~48-50 MJ/kg, hydrogen 120 MJ/kg (lower heating values; sources vary a little). Low CV means more fuel mass per unit work.
Energy density by volume matters for range: hydrogen has the best CV per kg but the worst per litre, hence 350-700 bar storage or cryogenic liquid.
Stoichiometric air-fuel ratio: petrol 14.7, diesel ~14.5, ethanol 9.0, methanol 6.45, CNG ~17.2, hydrogen 34.3 (by mass). A fuelling system sized for petrol cannot deliver enough ethanol without larger injectors.
Octane number: ethanol ~108 RON, methanol ~110, CNG ~120-130 (methane number), LPG ~105 — all allow higher compression ratios and spark advance than petrol (91-95 RON in India).
Cetane number: biodiesel 48-60 (better than diesel's ~51 minimum spec), ethanol and methanol are near-zero cetane, so they cannot self-ignite in a CI engine without an ignition improver, glow plug, dual-fuel pilot or spark conversion.
Latent heat of vaporisation: ethanol ~840 kJ/kg and methanol ~1,100 kJ/kg against petrol's ~350 — strong charge cooling (good for knock and volumetric efficiency, bad for cold start).
Flame speed, flammability limits: hydrogen's wide limits (4-75% in air) and very high flame speed allow ultra-lean burn but cause backfire and pre-ignition.
Others examined: viscosity and cold filter plugging point (biodiesel), oxygen content (ethanol 34.7% by mass, biodiesel ~11%), water affinity/hygroscopicity, material compatibility and lubricity.

Alcohol fuels: ethanol and methanol

Ethanol (C₂H₅OH) in India comes mainly from sugarcane molasses (C-heavy and B-heavy), sugarcane juice, and increasingly surplus/damaged grain and maize; 2G plants use rice straw and bagasse.
Blends: E5/E10 need no engine change. E20 (20% ethanol in petrol) is the nationwide norm — the Ethanol Blended Petrol Programme reached the 20% average blending target in 2025, well ahead of the original 2030 schedule, and E20-compliant vehicles have been sold since April 2023. Higher blends (E85, E100) need flex-fuel engines.
Effects of ethanol blending: higher octane and knock resistance; leaner operation unless fuelling is corrected (oxygen in the fuel shifts lambda); slightly lower CO and HC; NOx can rise or fall; volumetric fuel consumption rises roughly 1-2% at E10 and commonly 2-6% at E20 depending on calibration; water pickup and corrosion of aluminium, zinc, and some rubbers; vapour-lock and hot-restart issues.
Flex-fuel vehicles (FFV): a lambda/ethanol sensor lets the ECU adapt injection and timing anywhere between E0 and E85/E100; materials and fuel pump are upgraded. BS-VI flex-fuel norms exist and FFV models have been showcased in India (Toyota, Maruti, Honda two-wheelers).
Methanol (CH₃OH): made from coal, natural gas, biomass or CO₂. NITI Aayog's methanol economy roadmap targets M15 petrol blending, methanol-diesel blending (MD15), DME for CI engines, and methanol in cooking and marine use. Toxic, attacks metals, invisible flame, very low cetane.
Alcohol in CI engines: three routes — dual fuel/fumigation into the intake, blending with surfactant/emulsifier, or spark-ignition conversion with increased compression.

Biodiesel and vegetable-oil routes

Definition: biodiesel is a mixture of fatty acid methyl esters (FAME) made by transesterification of vegetable oil or animal fat with methanol and an alkali catalyst (KOH/NaOH), producing glycerol as co-product.
Why raw vegetable oil is not used directly: viscosity is 10-20 times that of diesel, causing poor atomisation, injector coking, carbon deposits, lube-oil dilution and ring sticking. Remedies are transesterification (standard), blending, pyrolysis, or micro-emulsion.
Indian feedstocks: used cooking oil (UCO), non-edible oils — jatropha, karanja (pongamia), mahua, neem — plus animal tallow and rice-bran oil. The policy is non-edible-first because edible oil is itself imported.
Blends: B5-B20 run in standard engines; B100 needs fuel-line and seal changes. India's Biodiesel Blended Diesel programme has run far below its 5% target — actual blending has stayed under about 1%, limited by feedstock. Treat current percentage figures as moving.
Pros: high cetane, sulphur-free, intrinsic lubricity, biodegradable, 40-60% lower PM and visible smoke, near-closed carbon loop.
Cons: ~8-12% lower CV, slightly higher NOx ("biodiesel NOx penalty"), poor cold-flow behaviour, oxidative instability on storage, solvent action that loosens old tank deposits and clogs filters, and higher cost.
Standards: biodiesel (B100) to IS 15607; blended diesel grades are covered by the automotive diesel specification IS 1460; ethanol-blended petrol by IS 2796 and anhydrous ethanol for blending by IS 15464. Confirm the current edition before quoting in a report.
HVO/renewable diesel is hydrotreated vegetable oil — a true drop-in paraffinic diesel, chemically unlike FAME, with cetane above 70.

Gaseous fuels: CNG, LNG, LPG, CBG, hydrogen

CNG: mostly methane, stored at 200-250 bar in type-I to type-IV cylinders; delivered through a pressure regulator and gas injectors. Bi-fuel petrol+CNG is the common Indian retrofit; heavy vehicles use dedicated spark-ignition gas engines or dual-fuel diesel.
CNG effects: very high knock resistance, no cold-start enrichment, almost no PM, 20-25% lower CO₂ per unit energy, cheaper per km. Against that: ~10-15% power loss in a retrofit (gaseous fuel displaces air, cutting volumetric efficiency), heavy cylinders, limited boot space, slower refuelling and methane slip in HC measurement.
LNG: methane liquefied at about −162 °C, ~600 times denser than gas, used for long-haul trucks; needs cryogenic tanks and tolerates boil-off losses.
LPG (Auto LPG): propane-butane mix at ~8-10 bar liquid; approved retrofit kits only, under CMVR rules; cleaner than petrol, lower power, cylinder in the boot.
CBG (compressed biogas): biogas from press mud, cattle dung, agricultural residue and MSW, upgraded to >90% methane and compressed. The SATAT initiative (Sustainable Alternative Towards Affordable Transportation, 2018) promotes CBG plants with assured offtake by oil-marketing companies; CBG blending in CNG is being phased in under a CBG Blending Obligation.
Hydrogen: colour codes — green (electrolysis on renewable power), blue (reformed gas with CCS), grey (reformed gas without CCS). The National Green Hydrogen Mission (2023) targets 5 MMT per year of green hydrogen production capacity by 2030.
Hydrogen in IC engines (H₂-ICE): works well on lean mixtures, zero carbon pollutants, only NOx and trace oil-derived HC; problems are pre-ignition and backfire (low ignition energy ~0.02 mJ), power loss with port injection due to hydrogen's large volume fraction, hydrogen embrittlement, and NOx near stoichiometric. Direct injection and EGR are the fixes.
Fuel cell (PEMFC): electrochemical, not combustion — hydrogen + oxygen → water + electricity at 50-60% efficiency, roughly twice an IC engine. Indian demonstrations include hydrogen fuel-cell buses and Indian Railways' hydrogen train programme.
Producer gas / biomass gasification: partial oxidation of biomass gives CO + H₂ + CH₄ + N₂ with a very low CV of about 4-6 MJ/m³; used in stationary dual-fuel gensets, with tar and dust cleaning the main problem.

Electric and hybrid pathways, and the Indian policy frame

BEV: battery-electric, tank-to-wheel zero emission; efficiency 70-90% from battery to wheel against 25-40% for an IC engine; limitations are charging time, battery cost, grid carbon intensity and recycling.
Hybrids: series, parallel and series-parallel; strong hybrids recover braking energy; plug-in hybrids add grid charging. Hybrid + ethanol flex-fuel is an Indian-specific combination being promoted.
Policy and law to quote: National Policy on Biofuels, 2018 (amended 2022, which advanced the 20% ethanol target to 2025-26 and allowed more feedstocks); Ethanol Blended Petrol Programme; SATAT; National Green Hydrogen Mission; FAME/PM E-DRIVE support for electric vehicles; PM-JI-VAN Yojana for 2G ethanol; Energy Conservation (Amendment) Act, 2022, which created a carbon-credit trading scheme and allowed mandates on non-fossil energy use.
Regulatory machinery: the Central Motor Vehicles Rules, 1989 under the Motor Vehicles Act, 1988 govern retrofits and type approval; CNG/LPG kits must be approved and fitted by authorised retrofitters with an RC endorsement; AIS standards (for example AIS-024 for CNG retrofit, AIS-025 for LPG) are issued by ARAI; testing agencies are ARAI Pune, ICAT Manesar, GARC Chennai and NATRAX. Emission limits remain BS-VI (Phase 2 from April 2023) with RDE and OBD-II; CAFE norms cap fleet CO₂ separately. GST on vehicles and fuels changed with the 5%/18%/40% slab structure of 22 September 2025 — check current rates before quoting.

Key points

Alternative fuel - any transport energy carrier other than conventional petrol or diesel, fossil or renewable.
Oxygenated fuel - fuel carrying oxygen in its molecule (ethanol 34.7%, biodiesel ~11%), which cuts CO and soot but lowers calorific value.
E20 - 20% ethanol in petrol, India's nationwide blend, target met in 2025; E20-compliant vehicles sold since April 2023.
Flex-fuel vehicle - ECU plus materials that tolerate any blend from E0 to E85/E100 using an ethanol or wide-range lambda sensor.
Transesterification - oil + methanol + KOH → FAME (biodiesel) + glycerol; the standard route to usable biodiesel.
Why not raw vegetable oil - viscosity 10-20× diesel causes poor spray, injector coking and lube-oil dilution.
Biodiesel NOx penalty - PM and smoke fall but NOx often rises slightly with FAME blends.
Cetane vs octane - alcohols are high-octane, near-zero cetane (SI-friendly, CI-hostile); biodiesel is high-cetane (CI-friendly).
CNG at 200-250 bar - cheapest clean retrofit; costs 10-15% power through reduced volumetric efficiency.
LNG - cryogenic methane at −162 °C, ~600× denser than gas, for long-haul trucking.
CBG and SATAT - upgraded biogas (>90% CH₄) for transport, with assured offtake and a blending obligation into CNG.
Hydrogen - 120 MJ/kg but terrible volumetric density; backfire and pre-ignition in port-injected H₂-ICE; green hydrogen from renewable electrolysis.
Fuel cell - electrochemical conversion at 50-60% efficiency, no combustion, water the only product.
Producer gas - biomass gasification gas, only 4-6 MJ/m³, stationary dual-fuel use.
Energy equivalence - comparisons must be made per unit energy (MJ or kWh), never per litre.
Well-to-wheel - the only honest basis for comparing fuels; tailpipe-only figures flatter electricity and hydrogen.
Standards - B100 to IS 15607, automotive diesel IS 1460, ethanol-blended petrol IS 2796, anhydrous ethanol IS 15464; retrofit under CMVR with AIS kit approval.
Policy stack - National Policy on Biofuels 2018 (amended 2022), EBP Programme, SATAT, National Green Hydrogen Mission 2023, PM-JI-VAN, Energy Conservation (Amendment) Act 2022.

Formulas and examples

Blend calorific value (mass basis): CV_blend = x·CV_a + (1 − x)·CV_b — x = mass fraction of fuel a; CV in MJ/kg
Blend CV (volume basis): CV_v = v·ρ_a·CV_a + (1 − v)·ρ_b·CV_b — v = volume fraction, ρ in kg/litre, result in MJ/litre
Stoichiometric A/F from CₓH_yO_z: A/F = [ (x + y/4 − z/2) × 137.9 ] / (12x + y + 16z) — kg air per kg fuel (137.9 ≈ mass of air per mole of O₂)
Brake thermal efficiency: η_bth = BP / (ṁ_f × CV) — BP in kW, ṁ_f in kg/s, CV in kJ/kg
BSFC = ṁ_f / BP (g/kWh); brake specific energy consumption BSEC = BSFC × CV (BSFC in kg/kWh, CV in MJ/kg, giving MJ/kWh) — compare different fuels on BSEC, not BSFC
Equivalent fuel quantity for the same energy: m₂ = m₁ × CV₁ / CV₂
Running cost per km = (fuel used per km) × (price per unit)
Energy of a gaseous fuel in a cylinder: E = V_water × (p/p₀) × ρ_gas,STP × CV (ideal-gas approximation; real methane needs a compressibility factor Z ≈ 0.8 at 200 bar)

Example 1 — Calorific value of E20 by volume. Densities: petrol 0.745 kg/L, ethanol 0.789 kg/L. CV: petrol 44.0 MJ/kg, ethanol 26.8 MJ/kg.

Petrol per litre of blend: 0.80 × 0.745 × 44.0 = 26.22 MJ
Ethanol: 0.20 × 0.789 × 26.8 = 4.23 MJ
CV of E20 ≈ 30.45 MJ/L; pure petrol = 0.745 × 44.0 = 32.78 MJ/L
Energy drop = (32.78 − 30.45)/32.78 = 7.1%. If the engine's efficiency is unchanged, volumetric consumption rises by 32.78/30.45 − 1 = 7.6%; in practice a recalibrated engine recovers part of this through higher spark advance, so measured penalties are usually 2-6%.

Example 2 — Stoichiometric air-fuel ratio of ethanol. Ethanol C₂H₆O: x = 2, y = 6, z = 1. Molar mass = 24 + 6 + 16 = 46 g/mol.

O₂ required = x + y/4 − z/2 = 2 + 1.5 − 0.5 = 3.0 mol
Air mass = 3.0 × 137.9 = 413.7 g
A/F = 413.7 / 46 = 9.0 kg air per kg ethanol (against 14.7 for petrol) — so for the same air mass the injector must deliver 14.7/9.0 = 1.63 times the fuel mass.

Example 3 — Diesel versus B20 on a tractor engine. Diesel CV 42.5 MJ/kg; biodiesel (FAME) CV 37.5 MJ/kg. B20 = 20% biodiesel by mass.

CV_B20 = 0.20 × 37.5 + 0.80 × 42.5 = 7.5 + 34.0 = 41.5 MJ/kg (2.35% below diesel)
The engine gives BP = 30 kW on diesel at BSFC 250 g/kWh → ṁ_f = 30 × 0.250 = 7.5 kg/h; fuel power = 7.5 × 42.5/3.6 = 88.5 kW → η_bth = 33.9%
On B20 at the same efficiency and power, fuel power must stay 88.5 kW → ṁ_f = 88.5 × 3.6/41.5 = 7.68 kg/h → BSFC = 256 g/kWh, a 2.4% rise in mass consumption with identical BSEC of 10.62 MJ/kWh.

Example 4 — CNG versus petrol running cost (illustrative prices; check current rates). A car does 15 km/L on petrol at ₹105/L and 22 km/kg on CNG at ₹78/kg.

Petrol: 105/15 = ₹7.00 per km
CNG: 78/22 = ₹3.55 per km → saving ₹3.45/km, i.e. 49%
Over 15,000 km a year the saving is 15,000 × 3.45 = ₹51,750; a ₹90,000 factory CNG premium pays back in 90,000/51,750 = 1.74 years.

Example 5 — Hydrogen storage and range. A 700-bar tank of 120 litres water volume holds hydrogen at about 40 kg/m³ (real-gas density at 700 bar, 15 °C).

Mass stored = 0.120 m³ × 40 = 4.8 kg; energy = 4.8 × 120 = 576 MJ = 160 kWh
A fuel-cell car using 0.9 kWh/km of hydrogen energy at 55% stack efficiency delivers 160 × 0.55 = 88 kWh at the wheels; at 0.5 kWh/km wheel demand the range is about 176 km from this 120-litre tank volume — the volumetric penalty that drives large, heavy composite tanks.

Fuel	CV (MJ/kg)	Stoich. A/F	Octane/Cetane	Engine type	Main limitation
Petrol	44	14.7	91-95 RON	SI	Fossil, knock-limited
Diesel	42.5	14.5	~51 CN	CI	PM and NOx
Ethanol	26.8	9.0	~108 RON	SI, flex	Low CV, water pickup, cold start
Methanol	19.9	6.45	~110 RON	SI, dual-fuel CI	Toxic, corrosive, very low CV
Biodiesel (FAME)	37.5	~12.5	48-60 CN	CI	Cold flow, storage stability, cost
CNG / CBG	48-50	17.2	~130 MN	SI, dual-fuel	Storage bulk, power loss
LPG	46	15.5	~105 RON	SI	Range, boot space
Hydrogen	120	34.3	very high	SI (H₂-ICE), fuel cell	Volumetric density, backfire, cost
Producer gas	4-6 MJ/m³	~1.2 (volume)	low	Stationary dual-fuel	Very low CV, tar

Where it is used

Oil marketing and refining: IOCL, BPCL, HPCL blending terminals; ethanol dosing, blend quality to IS 2796, CBG offtake under SATAT; 2G ethanol refineries at Panipat and Numaligarh.
Vehicle makers and calibration teams: Maruti Suzuki (CNG and E20 portfolio), Tata Motors and Ashok Leyland (CNG/LNG and hydrogen buses), Bajaj and TVS (CNG/ethanol two- and three-wheelers), Cummins and Greaves (gas and dual-fuel engines) — roles in fuel-system design, ECU calibration and emission validation.
Gas distribution and infrastructure: GAIL, IGL, Mahanagar Gas, Adani Total — CNG station design, compressor and dispenser engineering, LNG corridors for freight.
Sugar and agri-industry: distilleries attached to sugar mills producing ethanol from B-heavy and C-heavy molasses and grain; press-mud CBG plants; Praj Industries as a technology supplier.
Testing and certification: ARAI, ICAT, GARC and NATRAX — kit approval under AIS, BS-VI/RDE testing, chassis dynamometer and emission bench work.
Retrofit, fleet and stationary use: authorised CNG/LPG retrofitters under CMVR, biodiesel in Indian Railways and state transport fleets, biomass gasifier gensets in rural industry.

Interview questions

Q: Why can't ethanol be used directly in a diesel engine? A: Its cetane number is near zero and its latent heat is very high, so it will not self-ignite reliably under compression; it needs an ignition improver, dual-fuel pilot injection or conversion to spark ignition.
Q: Why does fuel consumption in litres rise with E20 even though the engine runs fine? A: Ethanol has about 60% of petrol's calorific value per kg, so E20 carries roughly 7% less energy per litre; more volume is needed for the same work.
Q: What is transesterification and why is it necessary? A: Reacting vegetable oil with methanol and an alkali catalyst to produce fatty acid methyl esters and glycerol; it cuts viscosity from 10-20× diesel to near-diesel levels so the fuel atomises properly.
Q: A CNG retrofit car feels sluggish. Why? A: CNG is gaseous, so it displaces intake air and reduces volumetric efficiency, typically costing 10-15% power; the fixed retrofit calibration and late gas injection add to it.
Q: Why does biodiesel reduce particulates but sometimes increase NOx? A: The fuel-bound oxygen and absence of aromatics and sulphur cut soot formation, while advanced injection timing and higher bulk modulus raise peak combustion temperature slightly, which raises NOx.
Q: What causes backfire in a port-injected hydrogen engine? A: Hydrogen's very low minimum ignition energy and wide flammability range let hot spots or residual gas ignite the mixture in the intake during valve overlap; direct injection avoids it.
Q: Why is a flex-fuel engine not simply a petrol engine with different software? A: It also needs larger injectors and pump, ethanol-tolerant seals, fuel lines and tank coating, a cold-start aid and an ethanol or wide-range lambda sensor.
Q: Are electric vehicles zero-emission? A: Zero at the tailpipe, but not well-to-wheel — the emissions depend on grid carbon intensity plus battery manufacturing and recycling.
Q: What is the difference between CNG and CBG? A: Chemically both are mostly methane for the same engine; CNG is fossil natural gas while CBG is upgraded biogas from waste, promoted under SATAT with a blending obligation into CNG.
Q: Why compare fuels on BSEC rather than BSFC? A: BSFC is grams of fuel per kWh, so a low-calorific fuel always looks worse; BSEC multiplies by calorific value and compares actual energy used per kWh of work.
Q: Which Indian standard covers B100 biodiesel? A: IS 15607; blended automotive diesel falls under the automotive diesel specification IS 1460 — confirm the current edition, since these specifications are revised periodically.
Q: Why is methanol attractive in India despite being toxic? A: It can be made from domestic coal, biomass or CO₂ rather than imported crude, has high octane for SI blending, and supports DME production for CI engines — the basis of NITI Aayog's methanol economy roadmap.

Common mistakes

Comparing fuels per litre or per kg instead of per unit energy, then concluding ethanol or CNG is "inefficient".
Using 14.7 as the stoichiometric ratio for an alcohol blend; it falls to about 14.1 for E10, about 13.5 for E20 and 9.0 for pure ethanol.
Treating biodiesel and raw vegetable oil as the same thing, or calling straight vegetable oil "biodiesel".
Saying E20 needs no engine change at all — it needs material compatibility and recalibration, which is exactly why E20-compliant vehicles were a separate milestone.
Forgetting that hydrogen's 120 MJ/kg is per kilogram; by volume it is the worst common fuel, which is the whole storage problem.
Quoting stale blending percentages or policy targets as fixed facts — ethanol and CBG figures move every year and biodiesel has stayed near 1% against a 5% target.
Calling a fuel cell an IC engine, or claiming a battery or hydrogen vehicle is zero-emission well-to-wheel.