What's the difference between diesel and petrol?

Differences in refinery processing

Petroleum products, like petrol and diesel, are derived from crude oil and are produced at varying temperatures during a refining process called Fractional Distillation. Crude oil is made up of a mixture of hydrocarbons, and it is the separation of these hydrocarbons that produces different products.

The initial process involves heating crude oil in a still, or distillation column. At various temperatures, different hydrocarbons vaporise and can be recovered. For instance, petrol has a boiling range of 40 to 205 degrees, while diesel has a boiling range of 250 to 250 degrees.

Once separated, the next step is to blend these by-products with other elements to produce commercially acceptable petrol and diesel. For example, different mixtures can create fuels with varying octane ratings.

Diesel and petrol engines

Diesel engines can be up to 40% more efficient than a spark-ignited petrol engine with the same power output. They tend to last longer and are less prone to break down.

Diesel engines also produce much more torque at low rpm as compared to a petrol engine of the same displacement. For these and many other reasons, diesel is the fuel of choice for industry, transport, agriculture, mining and power generation.

When it comes to domestic vehicles, there are more cars available with petrol engines than diesel. Petrol engines run at higher rpm and have lower Co2 emissions that diesel. They also produce more power for a given engine size, they’re cheaper to make and for this reason are often cheaper to buy.

Why are petrol and diesel different prices?

Petrol and diesel prices are driven by supply and demand. As petrol is primarily used in the domestic sector and diesel used in the industrial, agricultural and transport sectors, demand for these fuels differ.

For example, during the resources boom of the late 1990s to late 2000s, the demand for diesel grew and pushed up international prices above that of petrol. However, as a result of the recent global economic downturn, demand has dropped relative to petrol, to the extent that retail prices for diesel are now below petrol.

Retail petrol and diesel prices also vary in regional and remote areas due to higher freight and distribution costs, as well as lower service station sales volumes.

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