Petrol, Diesel and Kerosene

Petrol, diesel, and kerosene are liquid hydrocarbon fuels derived from the fractional distillation of crude petroleum. They are complex mixtures of alkanes (paraffins), cycloalkanes (naphthenes), and aromatic hydrocarbons, categorized based on their boiling point ranges and carbon chain lengths.

Carbon Chain and Distillation Matrix

Petrol (Gasoline)

Petrol is a highly volatile, flammable liquid mixture. Its efficiency is governed by how smoothly it burns under compression inside a spark-ignition engine.

The Phenomenon of Engine Knocking

In a standard petrol engine, the fuel-air mixture is compressed by a piston and then ignited at a precise microsecond by a spark plug. If the fuel is unstable, the intense heat and pressure cause the unburnt mixture to auto-ignite prematurely in pockets across the cylinder. These competing shockwaves collide, creating a sharp metallic noise known as knocking. Knocking drastically reduces engine efficiency and causes severe mechanical damage over time.

Octane Rating System

The anti-knock capability of petrol is quantified by its Octane Number. This scale is calibrated using two reference hydrocarbons:

• Iso-octane (2,2,4-trimethylpentane): High branched-chain stability; burns smoothly without knocking. It is assigned an Octane Number of 100.
• n-heptane: Straight-chain configuration; highly prone to premature auto-ignition and severe knocking. It is assigned an Octane Number of 0.

The Octane Number of a commercial petrol sample equals the percentage volume of iso-octane in a reference mixture that matches the knocking behavior of the fuel. High-octane fuel allows higher engine compression ratios, yielding greater power.

Octane Boosting Additives

To elevate the octane rating of straight-run petrol fractions, refineries historically added Tetraethyl Lead (TEL). Due to toxic lead emissions that cause neurological damage and poison catalytic converters, TEL has been globally phased out. Modern refineries utilize non-leaded alternatives:

• MTBE (Methyl Tertiary Butyl Ether): An oxygenated hydrocarbon that provides clean combustion.
• Ethanol Blending: Adding anhydrous ethyl alcohol increases the oxygen content, lowering carbon monoxide emissions while raising the octane index.

Diesel Oil

Diesel is a dense, oily petroleum fraction used primarily in heavy-duty commercial transport. Its operational chemistry is completely different from petrol.

Compression-Ignition Mechanics

Diesel engines do not use spark plugs. Instead, air is compressed inside the cylinder until it reaches a very high temperature (above 500°C). Diesel fuel is then injected into this superheated air as a fine mist, where it auto-ignites spontaneously.

Cetane Rating System

Unlike petrol, where auto-ignition must be prevented, diesel fuel must auto-ignite as quickly as possible once injected. The time delay between fuel injection and the start of combustion is called the ignition delay. The shorter the delay, the smoother the diesel engine runs. This ignition quality is measured by the Cetane Number:

• Hexadecane (Cetane, C₁₆H₃₄): A straight-chain alkane that ignites rapidly under compression. It is assigned a Cetane Number of 100.
• Alpha-methylnaphthalene: A cyclic aromatic compound with a long ignition delay. It is assigned a Cetane Number of 0.

A higher Cetane Number signifies a premium diesel fuel that guarantees easy cold starts, quiet idling, and minimal soot formation.

Kerosene (Paraffin Oil)

Kerosene is a intermediate distillate with a volatility profile positioned between petrol and diesel.

Properties and Domestic Utility

• Physical Nature: It is a clear, low-viscosity liquid with high chemical stability. It is less volatile than petrol, reducing accidental fire hazards during storage.
• Domestic Applications: Historically used as an illuminant in wicker lamps and a cooking fuel in pressurized stoves. It undergoes complete combustion when fed through a clean wick, burning with a blue, soot-free flame.

Aviation Turbine Fuel (ATF)

The most critical modern industrial application of high-grade kerosene is as Jet Fuel (Jet A or Jet A-1) for commercial aircraft. Kerosene is selected for aviation due to specific chemical properties:

• Low Freezing Point: High-altitude flight zones experience extreme cold (down to -50°C). ATF is highly refined to maintain a freezing point below -47°C, ensuring the fuel lines never plug with wax crystals.
• High Flash Point: It possesses a high flash point (typically >38°C), providing an indispensable safety margin against accidental fires during aircraft refueling and operations.
• High Energy Density: It delivers a high volumetric energy content, maximizing flight range per unit volume of wing-tank space.

Environmental Chemistry: Sulfur and Emissions

The combustion of fossil distillates releases pollutants like Carbon Monoxide (CO), Nitrogen Oxides (NO_x), Particulate Matter (PM_2.5 and PM₁₀), and Sulfur Dioxide (SO₂).

Hydrodesulfurization

Crude petroleum contains organic sulfur compounds. If left untreated, burning these fuels generates SO₂, which reacts with atmospheric moisture to cause acid rain. Refineries pass the fuel fractions through a Hydrodesulfurization (HDS) unit, reacting them with hydrogen gas over a cobalt-molybdenum catalyst to strip out sulfur as hydrogen sulfide gas (H₂S).

Evolution of Fuel Standards (Bharat Stage Context)

To curb urban air pollution, countries implement strict fuel quality mandates. In India, the transition from BS-IV directly to BS-VI (Bharat Stage 6) standards implemented a drastic reduction in permissible sulfur levels. Both petrol and diesel were mandated to lower their sulfur content from 50 parts per million (ppm) down to an ultra-low threshold of 10 ppm. While lowering sulfur protects human health and prevents the degradation of advanced exhaust after-treatment systems (like Diesel Particulate Filters), it reduces the natural lubricating properties of diesel fuel. To counter this, minute quantities of synthetic lubricity-enhancing additives are blended into modern low-sulfur diesel to protect fuel injection pumps from friction and wear.