The differences between petrol and diesel fuelled cars

For many motorists, filling up at the garage with petrol or diesel is something they do with little thought – as long as they select the correct fuel for their vehicle! However, although both alternatives are produced from mineral oil, each has their own advantages.

Taking the time to understand each fuel can help you to choose a more cost-effective vehicle and an efficient solution for your requirements.

Petrol v diesel

Diesel contains more energy per litre than petrol and therefore produces a more effective energy combustion process, offering better fuel efficiency and lower CO2 emissions. This means a diesel engine can be up to 40% more efficient than a petrol engine with the equivalent power.

How is fuel produced?

Crude oil contains different types of hydrocarbons and impurities, so to produce engine fuel, the hydrocarbons must be separated to refine the oil. This is done using fractional distillation. The crude oil is heated and the different hydrocarbons are extracted as vapour. They each have different vaporisation temperatures which makes this possible. They are then re-condensed.

Petrol is made of a mix of cycloalkanes and alkanes which boil between 40°C and 205°C, while diesel is made up of alkanes containing 12 or more carbon atoms with a boiling point of between 250°C and 350°C.

How do the engines work?

Diesel and petrol engines are theoretically quite similar; both being internal combustion engines that convert the fuel’s chemical energy into mechanical energy to move the pistons up and down. In both engines, the pistons are connected to the crankshaft and create the rotary motion to turn the vehicles’ wheels.

Both types of engine convert the fuel through a series of small combustions or explosions. In a petrol engine, the fuel mixes with air and is compressed by the pistons, to be ignited by sparks from the spark plugs. In a diesel engine, the air heats up when it’s compressed and this causes the fuel to ignite.

Pros and cons

Diesel vehicles are often costlier to buy but can have a longer life-span than their petrol counterparts. Diesel can also be more expensive to buy at the pump but because diesel vehicles tend to be more economical with fuel, they can be cheaper in the long run.

If you plan to keep your car for a few years and drive a lot of miles, diesel can be more cost-effective in the long term but if you’re selling it quite quickly and are a low-mileage driver, petrol may be the better option.

Road tax is calculated based on C02 emissions, so diesel vehicles often win because they emit less C02 than petrol cars – so they can be cheaper to tax.


Sales of diesel and petrol vehicles were similar in 2015 but figures for February 2017 showed that 50.7% of cars were petrol, 45.1% diesel and the remaining market share of 4.2% went to alternative fuel vehicles, including hybrids. Overall vehicle sales in the UK in January 2017 were at their highest January level in 12 years, with a total of 174,564 cars being sold.

Experts in vehicle leasing, ALD Automotive specialises in affordable vehicle funding solutions. Whether it’s a fleet of company cars on contract hire or a bespoke vehicle solution you’re after, ALD has a wide range of petrol and diesel-powered vehicles to suit your requirements.

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