Types of Fuel and Octane Ratings
Petrol's octane rating is a measurement of the fuel's ability to resist engine knocking. Knock occurs when the fuel-air mix in the cylinder explodes instead of burning in a controlled way. This shockwave moves within the combustion chamber, and creates a metallic 'pinging' sound.
An octane rating is often referred to as an 'anti-knock index'. If fuel has a high octane number, it will have a higher resistance to engine knocking.
Octane Numbers
Usually, there are three different octane numbers associated with all petrols. Petrol's Research Octane Number (RON) is measured under simple test conditions. Petrol's Motor Octane Number (MON) is measured under tougher test conditions and at higher engine speed and temperature.
The average of these two values is what becomes related closer to actual driving conditions. This value is known as the Road Octane Number, and is what should be used in filling stations.
Occasionally, some filling stations will confuse these different octane numbers in a bid to embellish on their octane rating claims, and advertise their fuel's Research Octane Number, which is higher than the Road Octane Number. In many European countries, the Research Octane Number is advertised on pumps, so a much higher octane value is common when travelling in certain countries.
Diesel automatically ignites and burns when it is compressed to a very high pressure. The released energy is contained by the engine and powers the vehicle. The key difference between diesel and petrol engines is auto-ignition. A spark plug ignites the fuel in a petrol engine whereas a diesel engine auto-ignites. Diesel has a Cetane Index and Number rather than the Research Octane Number (RON) that petrol has.
Diesel drivers may find this
diesel drivers forum useful for useful information, advice and discussions.
Diesel used to be the cheaper option for motorists, but recently the cost of diesel has risen above petrol. This article discusses
why diesel costs more than petrol (even though it is easier to make).
Bio-diesel is a more environmentally friendly fuel slowly becoming available across the UK. It is produced from renewable energy sources such as sugar beet, rape seed and sunflowers and is a biological substitute for regular diesel. Bio-diesel fueled vehicles are more environmentally friendly than conventional cars which run on petrol and diesel because the fuel is not as toxic and does not produce as many damaging exhaust emissions. Read our
biodiesel page for more information on this type of fuel or to add your company to our directory if you are a biodeisel supplier.
Leaded Four Star petrol was removed from sale on British forecourts on 1st January 2000. However, Leaded Four Star is now sold in a small amount of licensed stations in the UK.
In the UK, the most common petrol types are:
Ordinary unleaded - 95 RON
Super unleaded - 98 RON
Leaded Four Star - 98 RON
Super Fuels
Many of the large petrol companies have launched 'super fuels' - petrols and diesels that have a higher research octane level. These fuels are said to increase power in many vehicles, deliver less pollution than regular fuels and help to maintain a cleaner engine. Amongst these 'super fuels' are Tesco's Super Unleaded 99 Octane petrol, supplied by
Greenergy, now sold at many stations across the UK and also BP's Ultimate 102 Unleaded which is currently the most advanced, high-performance petrol you can buy on UK forecourts.
Using Super Fuels
One of our users commented that using higher octane fuel than your engine requires actually gives no benefit and may be a waste of money. This is because virtually NO engines require 98 RON over 95, and the market for 'super' fuels seems to be based on people's misunderstanding of octane ratings and the placebo effect of filling up with 'more powerful' fuel - making motorists think their engine is running better in some way.
What do the fuel companies say then, to justify the "increased power" claims for the super grade fuels? Some companies say that while all fuels contain cleaning additives, 'super' fuels contain more or better detergents to keep the injectors cleaner than standard fuel. Others say the fuel is a few percent denser which gives slightly more power per litre. These benefits may be marginal though in comparison to the extra cost involved so it is worth ensuring that your engine will actually benefit before filling up.
Some engines actually do need higher octane fuel, such as race engines with very high compression and some turbocharged engines, such as the import version of the Nissan Skyline. Also, a few vehicles, such as the new BMW K1200R motorbike, can sense knock and adjust their engine tuning to take advantage of higher grade fuels. Another user commented that the 2004 BMW 330 also does this, according to the driver's handbook it makes 231 BHP on 98 octane and 221 BHP on 95. This ability is apparently widespread amongst German performance cars using Bosch / Siemens electronic engine controls.
