What are the differences between Petrol, Diesel and LPG fuels?

When it comes to powering vehicles through fossil fuels, there are three main fuel types that motorists use: Petrol, Diesel and Liquified Petroleum Gas (LPG). You could argue that electric cars are also powered via fossil fuels through the burning of coal in power stations, however we’ll stick to these three for now!

One process used at an oil refinery is called fractional distillation and uses heat to split the crude oil into different components. LPG comes out at around 20°C, base petrol is split at around 150°C and diesel is split at around 300°C. There are heavier fuels in there as well and these are “cracked” to split into even more fuel sources. This is a very simplified description as there are many processes undertaken to clean and refine the end products. I was actually born and raised near many major oil refineries in South Wales and can remember when the cracking plants were first installed to maximise the oil coming in from the ships.

LPG is more commonly known as Butane or Propane and is a fuel that burns cleanly and is often known as Autogas. LPG can be used in the same types of engines as petrol however it produces less power for the same volume of fuel so therefore can be less efficient. It’s main advantage is cost – it is typically much cheaper to buy than petrol or diesel!

Petrol, used as a fuel for the internal combustion engine, is created by adding benzene and other oil based refined chemicals. This mixture improves the Octane Number but also makes it more volatile than the original distilled liquid.

Diesel from crude oil, is about 75% saturated hydrocarbons (i.e. paraffin) and 25% aromatic hydrocarbons (naphthalene and alkylbenzene). Diesel can also be developed using other oils such as vegetable oil and many new bio-diesels are being developed to re-use vegetation waste and even algae!

Each fuel uses a different type of engine
Petrol and LPG engines use a mix of air and fuel that is ignited by a spark. There are typically 3 types:
1. The 2-Stroke – two strokes to suck in fuel, compress, ignite and exhaust the gases. The 2-stroke was developed by Dugald Clark around 1880 and has a lower fuel intake than a 4-Stroke. It also combines the exhaust with the input so the compression starts when the piston rises after releasing the exhaust and filling with new fuel.
2. The 4-Stroke – four strokes to provide a better-balanced engine, known as the Otto Cycle. It was developed by a number of different people in the 1860s but named after Nicolaus Otto. It uses 1 stroke for each function: suck, compress, ignite & exhaust.
3. Finally there is the Wankel engine – a rotary style engine developed by Felix Wankel in the 1950s and today is most commonly used by Mazda in their RX series.
I will write a post on the different types of 4-Stroke engines as well in the near future.

Diesel engines compress air and at the maximum compression, the fuel is injected that causes combustion. This is known as the Diesel Cycle and was invented by Rudolph Diesel in the 1890s. Interestingly, he developed it originally for vegetable oils to help local farmers! They say things go in a full circle and this appears to be the case now with vegetable oils coming back into vogue as bio-diesels.

Usage
Each fuel has a place in the market and specific uses – taxis for example use more LPG because they cover higher distances and therefore need a cheaper, less powerful fuel. Many cars can be converted to be hybrid petrol and LPG to allow a longer time between refuelling or longer distances travelled, however this will impact on useable cargo space due to having two tanks. Diesel vehicles used to get lower consumption and less power but consume more oil (hence the nickname oil-burners) however today, a diesel turbo engine is highly efficient and with great consumption figures.

For many buyers, the fuel source is a factor along with size, configuration and brand of the vehicle when making a purchase decision and Governments can reward or penalise the use of any fuel through the management of taxes or rebates, which doesn’t help the manufacturers in their forward planning!