Alcohols and biodiesel as engine fuels

How ethanol, methanol and biodiesel differ from petrol and diesel in calorific value, stoichiometric air-fuel ratio, octane, cetane and latent heat, how biodiesel is made by transesterification, and what blending does to power, fuel consumption, materials and emissions.

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Why it matters

India's ethanol blending programme has moved pump petrol from E5 and E10 towards E20, and biodiesel blends are permitted in diesel, so every engine and fuel-system engineer must know how these fuels change mixture preparation, power, fuel consumption, materials and emissions. The same property comparisons — calorific value, stoichiometric air–fuel ratio, octane and cetane, latent heat — appear repeatedly in university exams and interviews.

Key ideas

Alcohols (methanol CH₃OH, ethanol C₂H₅OH).

  • Sources: ethanol from fermentation of sugarcane molasses, cane juice, grains or (second generation) cellulosic biomass; methanol mostly from natural gas or coal via synthesis gas, or from biomass.
  • Oxygenated fuels: ethanol is about 35% oxygen by mass and methanol about 50%. The bound oxygen means less air is needed per kg of fuel and the calorific value is lower: lower heating value roughly 27 MJ/kg for ethanol and 20 MJ/kg for methanol, against about 43–44 MJ/kg for petrol. Per litre, ethanol carries roughly two-thirds of the energy of petrol.
  • Stoichiometric air–fuel ratio: about 9.0 for ethanol and 6.5 for methanol, against about 14.7 for petrol. So the injector must deliver far more fuel mass, but the energy released per kg of air is almost the same as petrol — which is why alcohol engines lose little power despite the low calorific value.
  • High octane: research octane numbers above 100 (about 108–109). Alcohols are single small molecules with high autoignition resistance, burn fast, and have a high latent heat of vaporisation (about 0.9 MJ/kg for ethanol, about 1.1 MJ/kg for methanol, against about 0.35 MJ/kg for petrol). The evaporative cooling of the charge increases volumetric efficiency and further suppresses knock, allowing higher compression ratio or more spark advance.
  • Low cetane: alcohols are poor compression-ignition fuels; using them in a CI engine needs dual-fuel operation, ignition improvers or a pilot diesel injection.
  • Drawbacks: cold starting is difficult with neat alcohols (high latent heat, single boiling point, low vapour pressure for neat ethanol); alcohols absorb water and can phase-separate from petrol; they corrode aluminium, zinc and some elastomers and plastics, so fuel-system materials must be compatible; low-ethanol blends raise vapour pressure; aldehyde emissions (formaldehyde from methanol, acetaldehyde from ethanol) increase; methanol is toxic. Fuel consumption by volume rises roughly in proportion to the drop in volumetric energy content.
  • Blends: E10 means 10% ethanol by volume in petrol, E20 is 20%, and E85/E100 need flex-fuel vehicles with sensors that measure ethanol content and adapt fuelling and ignition.

Biodiesel (fatty acid methyl esters, FAME).

  • Production by transesterification: a triglyceride (vegetable oil — jatropha, karanja, used cooking oil — or animal fat) reacts with an alcohol, normally methanol, in the presence of a base catalyst such as NaOH or KOH at about 60 °C: triglyceride + 3 CH₃OH → 3 methyl esters + glycerol. Excess methanol pushes the equilibrium forward; high free-fatty-acid feedstock needs acid pre-esterification to avoid soap formation. Transesterification reduces the viscosity of the oil roughly ten-fold, close to that of diesel; straight vegetable oil is too viscous and causes poor atomisation, injector coking and ring sticking.
  • Properties relative to diesel: lower heating value about 37–38 MJ/kg (roughly 8–12% less than diesel by mass, a little less by volume because biodiesel is denser); cetane number usually equal to or higher than diesel (depends on feedstock — saturated esters give higher cetane); about 11% oxygen; practically no sulphur or aromatics; high flash point; good lubricity; higher viscosity and cloud point.
  • Emissions: CO, HC and PM generally fall because of the bound oxygen and absence of aromatics; NOx often rises slightly (effects of earlier injection due to higher bulk modulus, and higher oxygen availability). Life-cycle CO₂ is lower because the carbon was recently absorbed by the plant, although the exact saving depends on how the feedstock is grown and processed.
  • Drawbacks: poor cold-flow behaviour (gelling, filter plugging), oxidation instability during storage, solvent action that loosens old deposits and clogs filters, attack on some elastomers, and dilution of engine oil during DPF regeneration post-injection.
  • Blends: Bxx means xx% biodiesel by volume (B5, B20, B100). Low blends need no engine change; high blends need compatible seals, filters and calibration.

Connections. Octane and knock link to the SI combustion topic; cetane and delay period to the CI combustion topic; the emission effects connect to the emission-formation and after-treatment topics.

Formulas

AFR_st = [(x + y/4 − z/2) × 4.76 × 28.97] / M_fuel

  • For a fuel CₓHᵧO_z: AFR_st = stoichiometric air–fuel ratio (kg air/kg fuel); 4.76 = mol of air per mol of O₂; 28.97 g/mol = molar mass of air; M_fuel = molar mass of fuel (g/mol). Complete combustion to CO₂ and H₂O.

LHV_blend = Σ vᵢ × LHVᵢ (volumetric basis, MJ/L) or Σ wᵢ × LHVᵢ (mass basis, MJ/kg)

  • vᵢ = volume fraction, wᵢ = mass fraction, LHVᵢ = lower heating value of each component. Use volume fractions with MJ/L and mass fractions with MJ/kg — never mix.

E_air = LHV / AFR_st

  • E_air = energy released per kg of air at stoichiometric mixture (MJ/kg air). A close indicator of the power potential of a fuel in a given engine.

CN_blend ≈ Σ vᵢ × CNᵢ

  • Linear mixing estimate of cetane number for diesel–biodiesel blends (approximate; real blends may deviate).

Property values (LHV, density, octane, cetane) vary with feedstock and specification; take them from your data book or the problem statement.

Worked examples

Example 1 (standard): fuel consumption on E20. Given: petrol LHV = 32.0 MJ/L, ethanol LHV = 21.2 MJ/L; a car gives 15.0 km/L on petrol. Assume the same engine efficiency on E20. Find the E20 energy content and the expected km/L.

  1. LHV_E20 = 0.8 × 32.0 + 0.2 × 21.2 = 25.6 + 4.24 = 29.84 MJ/L.
  2. Ratio to petrol: 29.84 / 32.0 = 0.9325.
  3. Same efficiency means distance ∝ energy per litre: 15.0 × 0.9325 = 13.99 km/L.
  4. Volume increase for the same energy: 1 / 0.9325 = 1.072, i.e. 7.2% more litres. Answer: 29.8 MJ/L; about 14.0 km/L.

Example 2 (GATE level): why ethanol does not lose power. Given: ethanol C₂H₆O (M = 46.07 g/mol, LHV = 26.8 MJ/kg); iso-octane C₈H₁₈ as petrol (M = 114.23 g/mol, LHV = 44.0 MJ/kg); air 28.97 g/mol. Find each stoichiometric AFR and the energy per kg of air.

  1. Ethanol: x + y/4 − z/2 = 2 + 1.5 − 0.5 = 3 mol O₂. AFR = 3 × 4.76 × 28.97 / 46.07 = 8.98.
  2. Iso-octane: 8 + 4.5 = 12.5 mol O₂. AFR = 12.5 × 4.76 × 28.97 / 114.23 = 15.09.
  3. Energy per kg air, ethanol: 26.8 / 8.98 = 2.98 MJ/kg air.
  4. Energy per kg air, iso-octane: 44.0 / 15.09 = 2.92 MJ/kg air.
  5. Fuel mass needed for the same air: 15.09 / 8.98 = 1.68 times more ethanol. Answer: AFR 8.98 (ethanol) vs 15.09; energy per kg air 2.98 vs 2.92 MJ/kg — about 2% more for ethanol, so power is maintained even though 68% more fuel mass is injected.

Example 3: B20 properties. Given: diesel CN = 51, LHV = 35.8 MJ/L; biodiesel CN = 55, LHV = 33.0 MJ/L.

  1. CN_B20 ≈ 0.2 × 55 + 0.8 × 51 = 51.8.
  2. LHV_B20 = 0.2 × 33.0 + 0.8 × 35.8 = 35.24 MJ/L (98.4% of diesel).
  3. Biodiesel needed to replace the energy of 10 L of diesel: 10 × 35.8 / 33.0 = 10.85 L. Answers: CN ≈ 51.8; 35.2 MJ/L; 10.85 L of B100.

Common mistakes

  • Comparing fuels on calorific value alone and concluding alcohols give much less power; compare energy per kg of air.
  • Mixing mass and volume bases when blending (E10 and B20 are volume percentages).
  • Using the petrol AFR of 14.7 for blends; E20's stoichiometric AFR is lower and the ECU must add fuel.
  • Saying ethanol has a high cetane number — it has a high octane number and a very low cetane number.
  • Claiming biodiesel reduces every pollutant; NOx often increases slightly.
  • Confusing transesterification (oil + alcohol → ester + glycerol) with fermentation (sugar → ethanol + CO₂).
  • Forgetting that alcohols' high latent heat both helps (charge cooling) and hurts (cold start).

For GATE ME

Expect conceptual questions on which property makes a fuel suitable for SI or CI engines (octane vs cetane), on oxygenated fuels and their emission effects, and on transesterification; and numericals on stoichiometric AFR of CₓHᵧO_z fuels, blend calorific value, energy per kg of air and fuel-consumption changes. Practise the combustion-equation balance with fuel-bound oxygen.

Quick check

  1. What is the stoichiometric AFR of methanol (CH₃OH, M = 32.04 g/mol)?
  2. Why does ethanol's high latent heat improve volumetric efficiency?
  3. What are the products of transesterification of a triglyceride with methanol?
  4. Is E20's volumetric energy content higher or lower than petrol's?
  5. Which emission usually increases slightly with biodiesel?

Answers: 1. 1.5 × 4.76 × 28.97 / 32.04 = 6.46. 2. Evaporation cools the charge, raising its density. 3. Fatty acid methyl esters (biodiesel) and glycerol. 4. Lower. 5. NOx.

Interview questions

All Automotive Engines, Emissions and Alternate Fuels interview questions

Try answering each one aloud before you open it.

  1. 1.What are alcohols and biodiesel, and how are they used as engine fuels?Concept

    Alcohols, such as ethanol and methanol, are organic compounds with hydroxyl groups. They can be used as alternative fuels in internal combustion engines due to their high octane numbers and clean-burning properties. Biodiesel is a renewable fuel made from vegetable oils or animal fats through a process called transesterification. It can be used in diesel engines with little or no modification, offering reduced emissions compared to conventional diesel.

  2. 2.Explain the process of transesterification in the production of biodiesel.Concept

    Transesterification is a chemical process that converts triglycerides in oils or fats into biodiesel and glycerol. It involves reacting the oil or fat with an alcohol, usually methanol or ethanol, in the presence of a catalyst such as sodium hydroxide or potassium hydroxide. The reaction breaks the triglycerides into methyl or ethyl esters (biodiesel) and glycerol as a byproduct.

  3. 3.Why is ethanol considered a cleaner fuel compared to gasoline?Application

    Ethanol is about 35% oxygen by mass, contains no sulphur or aromatics and is a single small molecule, so it burns more completely and tends to lower CO, HC and particulate emissions, especially in blends with older or rich-running engines. Its high octane number allows higher compression ratio and therefore better efficiency. Its CO₂ advantage is a life-cycle one: the carbon released was recently absorbed by the crop, so the net saving depends on how the feedstock is grown and processed. It does increase aldehyde (acetaldehyde) emissions and, in low blends, evaporative emissions through higher vapour pressure.

  4. 4.What are the advantages and disadvantages of using biodiesel in diesel engines?Application

    Advantages: biodiesel is renewable and biodegradable, has a cetane number similar to or higher than diesel, contains almost no sulphur or aromatics, has good lubricity and a high flash point, and generally lowers CO, HC and particulate emissions while giving a life-cycle CO₂ saving. Disadvantages: its heating value is roughly 8–12% lower, so power and fuel economy drop slightly at high blends; NOx often rises a little; it has poor cold-flow properties and oxidation stability; it can attack some elastomers, loosen deposits that clog filters and dilute engine oil. Low blends such as B5–B20 work in most engines without modification.

  5. 5.How does the use of methanol as a fuel affect engine performance and emissions?Application

    Methanol has a high octane number, which can improve engine performance by allowing higher compression ratios and more efficient combustion. It also burns cleaner than gasoline, reducing emissions of carbon monoxide and hydrocarbons. However, methanol has a lower energy content than gasoline, which can result in reduced fuel economy. It is also corrosive and requires modifications to the fuel system to prevent damage.

  6. 6.What modifications are typically required for an engine to run on ethanol-blended fuels?Application

    Low blends such as E10 need no change on modern fuel-injected cars. For E20 and above, fuel-system materials (seals, hoses, pump and tank coatings) must tolerate ethanol, and the ECU calibration must deliver more fuel because the blend's stoichiometric air–fuel ratio is lower; the lambda sensor corrects small errors in closed loop. Ignition timing can be advanced to exploit the higher octane. Flex-fuel vehicles for E85–E100 add an ethanol-content sensor, larger injectors and cold-start aids, because neat ethanol vaporises poorly when cold.

  7. 7.A diesel engine uses 10 L of diesel (35.8 MJ/L) on a trip. How many litres of B100 biodiesel (33.0 MJ/L) would supply the same energy?Numerical

    Energy needed = 10 L × 35.8 MJ/L = 358 MJ. Volume of biodiesel = 358 MJ ÷ 33.0 MJ/L = 10.85 L. So about 8.5% more fuel by volume is needed, assuming the same engine efficiency, because biodiesel has a lower heating value than diesel.

  8. 8.What happens if biodiesel is used in a non-compatible engine without any modifications?Application

    Using biodiesel in a non-compatible engine without modifications can lead to several issues. Biodiesel can cause fuel filter clogging due to its solvent properties, which may clean out deposits in the fuel system. It can also degrade rubber components and seals not designed for biodiesel, leading to leaks. Additionally, there may be issues with cold weather performance, as biodiesel can gel at lower temperatures.

  9. 9.Explain why alcohols like ethanol and methanol have higher octane numbers compared to gasoline.Concept

    Octane number measures resistance to autoignition of the end gas. Methanol and ethanol are small molecules whose oxidation chemistry needs high temperatures to reach the low-temperature chain-branching that causes knock, whereas straight-chain hydrocarbons in petrol autoignite easily; so alcohols have research octane numbers of about 108–109. They also have a high latent heat of vaporisation, which cools the charge and further lowers end-gas temperature, and they burn fast, leaving less time for the end gas to autoignite. The same properties give them very low cetane numbers, so they are poor fuels for compression ignition.

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