Last reviewed 1 Oct 2026 · 10 min read
Fuels for IC engines
A fuel is a substance that burns to release heat. Farm engines use liquid petroleum fuels (petrol, diesel, kerosene), gaseous fuels (LPG, CNG, biogas), and alternative fuels (ethanol, biodiesel).
Petroleum refining (outline)
Crude oil is distilled in a fractionating column; fractions (lightest to heaviest): LPG and gases → naphtha/petrol → kerosene → diesel (gas oil) → lubricating oil, fuel oil → bitumen. Cracking (thermal or catalytic) converts heavier fractions into petrol; reforming raises the octane number.
Properties of fuels
| Property | Meaning and importance |
|---|---|
| Calorific value (CV) | Heat released by the complete combustion of unit mass of fuel; higher (gross) and lower (net) CV (the latter excludes the latent heat of the water vapour). Petrol ≈ 44–47 MJ/kg, diesel ≈ 42–45 MJ/kg, ethanol ≈ 27, biodiesel ≈ 37–40, LPG ≈ 46, natural gas ≈ 50, hydrogen ≈ 120 MJ/kg |
| Volatility | Tendency to vaporise; essential for petrol (starting, mixture distribution, vapour lock); measured by distillation curve and Reid vapour pressure |
| Density (specific gravity) | Petrol ≈ 0.72–0.77, diesel ≈ 0.82–0.86 kg/L; used for metering and volume–mass conversions |
| Viscosity | Flow resistance; for diesel it affects atomisation and pump lubrication (about 2–4.5 cSt at 40 °C); too high → poor spray; too low → wear |
| Flash point | Lowest temperature at which the vapour flashes with a flame; safety classification (diesel above about 35–66 °C; petrol below 0 °C — highly flammable) |
| Fire (burning) point and autoignition temperature | Fuel safety; diesel auto-ignites at lower temperature than petrol (about 250 °C vs 400 °C) |
| Cloud point and pour point | Cloud point — wax crystals appear; pour point — the fuel stops flowing; important for diesel in cold climates |
| Sulphur content | Causes corrosion and SO₂ emissions; BS-VI diesel has ≤ 10 ppm sulphur (BS-IV 50 ppm) |
| Carbon residue, ash, water and sediment | Cleanliness; cause deposits and injector wear |
| Octane number (petrol) | Resistance to knock |
| Cetane number (diesel) | Ease of self-ignition |
Knocking and octane number (SI engines)
Knocking (detonation) is the sudden, uncontrolled combustion of the end gas ahead of the flame front, giving metallic pinging, high pressure spikes, loss of power and engine damage.
- Octane number measures resistance to knock: it is the percentage by volume of iso-octane in a mixture with n-heptane that matches the fuel's knock behaviour in a standard CFR engine (iso-octane = 100, n-heptane = 0). Regular petrol: 87–91 (RON); premium: 95 and above.
- Factors reducing knock: higher octane fuel, lower compression ratio, retarded ignition, richer mixture, lower intake temperature, good cooling and clean combustion chamber, turbulence.
- Anti-knock additives: tetra-ethyl lead (TEL) — banned (leaded petrol discontinued in India in 2000); modern petrol uses oxygenates (ethanol, MTBE), aromatic blending components, and catalytic reforming.
- Octane rating methods: RON (research octane number — lower speed) and MON (motor octane number — higher severity).
Diesel knock and cetane number (CI engines)
Diesel knock is due to delayed ignition (a large accumulated charge burns suddenly). A short ignition delay means smooth running.
- Cetane number — the percentage by volume of cetane (n-hexadecane, CN = 100) in a mixture with alpha-methyl-naphthalene (CN = 0) that matches the fuel's ignition quality. Diesel for tractors: CN about 45–55; at least 40–51 in standards.
- Cetane improvers (alkyl nitrates) raise the number. Higher cetane gives easier starting, less knock and smoke, lower noise.
- The cetane and octane numbers behave oppositely: a good petrol (high octane) is a poor diesel (low cetane) and vice versa.
A fuel that behaves like a blend of 88 % iso-octane and 12 % n-heptane has an octane number of 88. A diesel behaving like a blend of 52 % cetane and 48 % alpha-methyl-naphthalene has a cetane number of 52.
Combustion basics
- Air–fuel ratio (A/F) (by mass): stoichiometric — petrol ≈ 14.7 : 1, diesel ≈ 14.5 : 1, ethanol ≈ 9 : 1, methane ≈ 17.2 : 1. Excess air ratio (λ) . Diesel engines run lean ( 1.2–2 at full load, much leaner at light load) and are controlled by fuel quantity; petrol engines run near and are controlled by throttling.
- Complete combustion: ; . Incomplete combustion produces CO and soot.
- Emissions: CO, HC (unburnt hydrocarbons), NOx, particulate matter (PM), SO₂, CO₂. BS-VI norms apply to vehicles; tractors follow TREM IV (from 2024 for tractors above 50 hp, check the notifications). Catalytic converters and DPF (diesel particulate filters) control emissions.