Differences Between Petrol and Diesel: A Comprehensive Guide

Petrol and diesel, made using precise refining methods, differ in hydrocarbon length, ignition type, efficiency, and emissions. These differences all affect the engine design, performance, fuel economy, and durability. Find out more below.

Introduction to petrol and diesel engines

While diesel and petrol motors both convert fuel into mechanical energy, they differ in design, engine combustion processes, exhaust systems. Both fuels are produced from crude oil, undergoing a refining process to separate the organic compounds. However, petrol is lighter and vapourises at lower temperatures, making it suitable for spark ignition but more volatile. Diesel hydrocarbons, on the other hand, are heavier, requiring high compression to ignite.

Accordingly, petrol engines have spark plugs and a lighter structure, while diesel engines are stronger to handle higher pressures. Petrol combustion mixes air and fuel before ignition, giving smooth, fast power whereas diesel is injected directly into compressed air, producing slower, high-torque power.

Five key differences between petrol and diesel engines

1. Fuel type and combustion: petrol engines use spark plugs to ignite the air-fuel mixture. Modern diesel engines rely on compression ignition, where air is compressed to a high temperature before fuel injection. Glow plugs warm the air up so that the motor can start in cold conditions.

2. Fuel efficiency: diesel engines are generally more fuel-efficient, producing more energy by volume than spark-ignited petrol engines. They operate with higher compression ratios, extracting more energy per litre of fuel. Petrol engines consume more and are better suited to short journeys.

3. Power and torque: compression diesels produce higher torque at lower RPMs, making them ideal for heavy vehicles and towing. Petrol vehicles generate less torque but typically achieve higher maximum RPMs, providing smoother acceleration and higher top-end power output.

4. CO2 emissions: petrol vehicles usually emit more carbon dioxide (CO₂) per litre of fuel burned than diesel. However, diesel engines can emit more nitrogen oxides (NOx) and particulates, which are harmful pollutants, even though CO₂ emissions are lower. This is why manufacturers introduced the use of Ad Blue and diesel particulate filters. You may also have to pay more road tax as a result.

5. Engine construction and maintenance: diesel motors are built to withstand higher pressure levels, making them heavier and often more expensive. Petrol motors are lighter and generally cheaper to maintain.

How to tell the difference between petrol and diesel cars

Telling petrol and diesel cars apart involves looking at performance characteristics and specifications. Diesel engines are generally heavier and built with stronger components to handle higher compression, while petrol engines are lighter. Diesel cars produce more torque at low RPMs, making acceleration feel steady rather than quick bursts typical of petrol engines.

The engine sound differs: diesels have a distinct, deeper clatter, whereas petrol models usually run more quietly and smoothly. Exhaust characteristics can also help as diesel fuel sometimes emits darker smoke due to incomplete combustion. Checking the exact composition of the fuel system or consulting the vehicle manual provides a definitive identification.

Colour difference between petrol and diesel fuel

Firstly, standard and premium petrol are typically transparent and light-coloured, often being described as slightly yellow or greenish in colour. The fluid looks quite thin and watery compared to diesel, which is oilier in texture. In the UK, most petrol pumps are green or have green stickers whereas diesel pumps are usually black or dark grey. Fresh diesel is usually yellow or straw-coloured although some regions may add dye (red/blue/green) to distinguish between different types.

Mileage differences between petrol and diesel vehicles

On average, petrol cars typically last around 200,000 kilometres, while diesel cars often reach 300,000 kilometres or more, thanks to sturdier engine construction. This lifespan difference contributes to long-term value for diesel owners. In terms of fuel consumption, diesel vehicles generally provide 20–30% better mileage than petrol cars.

The main reason for this is the difference in energy density: diesel fuel contains more energy per liter than petrol, enabling diesel engines to extract greater efficiency. Additionally, the engines operate at higher compression ratios, enhancing thermal efficiency and reducing wasted energy. Petrol cars, however, are lighter, quieter, and often more responsive, making them ideal for city driving.

Differences in clutch and transmission systems of petrol and diesel cars

Diesel engines produce more torque at lower RPMs, so they require strong clutches and transmissions. The cars typically have reinforced parts and special gear ratios for improved performance and durability. Petrol engines make less torque, so their clutches and transmissions can be lighter. These differences may affect drivetrain options, with European cars often offering more diesel models with manual gearboxes.

Differences between petrol and diesel fuel

First, we need to talk about fuel composition. Petrol has shorter hydrocarbon chain lengths – around C4 to C12, whereas diesel hydrocarbons are typically between C10 and C20. Meanwhile petrol has a high octane rating, usually 91–98, preventing engine knocking in spark-ignition engines. In contrast, diesel is measured by cetane number, reflecting ignition ease. Petrol offers smoother response, while diesel ensures greater efficiency and durability.

Produced through fractional distillation, petrol is considered cleaner as it has less impurities and ensures more complete combustion. This results in lower levels of emissions of soot and small particles.

FAQ

Which type of engine is better for long-distance driving?

Diesel engines are usually preferred for long-distance travel due to higher mileage and durability, while petrol engines are better suited for shorter, frequent trips.

Are petrol cars cheaper to insure than diesel cars?

A: Yes, petrol cars are often cheaper to insure since they usually cost less to repair and replace compared to diesel vehicles.

Do petrol and diesel cars require different driving styles?

A: Yes. Diesel cars perform best with steady, lower RPM driving for efficiency, whereas petrol cars are more responsive to higher RPMs and quick acceleration.