Automobiles, petrol and octane ratings

By Lionel Palliyaguru
lionelpg@gmail.com

Petrol is a colourless liquid fuel derived from crude oil, with a density lower than water. Petrol is the primary fuel used in automobiles and various other machines with internal combustion engines. It is not a single compound but a mixture of about 150 hydrocarbons compounds, mainly isooctane, butane and ethyl toluene. As petrol is highly flammable and volatile, secure storage is required. Ideally, it should be stored in an airtight container to avoid contamination with moisture which could lower the performance and cause damage to the engine. When the mixture of petrol and air is ignited inside the cylinders, heat energy is released with the formation of carbon dioxide and water.

The first crude oil well was dug in Pennsylvania in 1859 by an American businessman, Edwin Laurentide Drake. At that time only kerosene was used for lighting purposes while gasoline and other petroleum byproducts in the distillation process were discarded until the automobile was invented in the year 1892. Since then gasoline-powered automobiles were in demand all over the world and reached the nine million mark by the year 1920. Since then numerous improvements have been made to all systems of the automobile. At present, most of the functions are controlled by an inbuilt microprocessor control system.

The main purpose of fuel is to deliver power to the automobile for its mobility with minimum damage to the engine parts and environment. In internal combustion or spark-ignition engine, the required power is produced by the controlled burning of the petrol-air mixture inside the cylinders. The fuel used in automobiles is called petrol, also known as gasoline or gas. For maximum power and pollution-free operation, additives in small quantities are added to petrol.Normally, there are three grades of petrol sold at retail filling stations, Regular, Mid-grade and Premium. Some companies have different names for these grades, such as unleaded, super and super premium. They all indicate the octane rating of the fuel which reflects the antiknock properties of the fuel. Higher octane ratings are higher in prices.

In a spark ignition engine, injected petrol fumes (a mixture of petrol and air) gets compressed while the piston moves upwards during the compression stroke. Due to the compression of the petrol fumes temperature and pressure rise and reach the ignition point of the mixture causing spontaneous combustion before being ignited by the spark plug. This untimely explosion causes an unusual sound which is called knocking or pinging. Knocking could lead to engine damage if not prevented.

There are several methods of how engine knocking can be prevented. Modern vehicles are equipped with electronic systems to detect and prevent knocking by ignition timing. When petrol fumes are compressed excessively, the tendency of spontaneous combustion increases and is directly related to the compression ratio of the engine. Engines with higher compression ratios generate more power and are susceptible to knocking. As such, knocking-resistant fuels were developed by mixing suitable additives with petrol.

Currently, high-octane fuels are used to prevent engine knocking. This is achieved by adding various additives to petrol, making it less susceptible to spontaneous combustion. In other words, treated petrol can withstand higher compression. Lead (Tetraethyl lead) was used as an anti-knocking agent during the early 1920s and discontinued in the 1970s due to strict emission laws to protect the environment. Subsequently, Aromatics (benzene, toluene, and xylene) were used and found not acceptable due to health hazards. Currently, ethyl alcohol (ethanol) is added to petrol to boost the octane rating which is a biofuel produced from renewable sources.

Octane ratings are a measure of the anti-knocking capability of petrol used in automobiles. In this regard, two standards were developed. They are named, RON (Research Octane Number) and MON (Motor Octane Number). RON uses an engine speed of 600 rpm and an intake temperature of 52°C while MON uses an engine speed of 900 rpm with an intake temperature of 149°C. MON standard was developed to simulate normal driving conditions. In addition to the two main standards, the third standard was defined by taking the simple average value of RON and MON. It is named AKI (Anti Knocking Number) which is displayed as AKI (RON+MON/2) in filling stations. Octane rating is totally dependent on the standard by which it is measured. Therefore, displaying an octane rating without identifying the standard is meaningless. The value of the octane rating obtained using the RON method is different from the MON and AKI methods. It is to be noted that octane 91 based on the RON standard is equivalent to octane 87 based on the AKI standard. The approximate conversion value of the AKI octane rating could be obtained by multiplying the RON rating by a factor of 21/22. Similarly, the RON rating could be obtained by multiplying the AKI rating by a factor of 22/21.

In order to ascertain the octane rating of fuel, a test engine with a variable compression ratio cylinder is used. While running the engine with the fuel under test, the compression ratio of the cylinder is increased until the engine begins to knock at a given intensity. This compression ratio is then compared with reference fuel, consisting of iso-octane and n-heptane. The percentage of iso-octane by volume of the reference fuel mixture is taken as the octane number of the fuel under testing. Iso-octane is considered an additive with 100 percent anti-knocking properties, whereas n-heptane is considered a self-igniting additive with zero percent anti-knocking properties.

In Sri Lanka, octane ratings are based on the RON standard whereas the US and Canada use AKI ratings. All automobile manufacturers indicate the octane rating for the vehicle and users could select the appropriate octane rating depending on the country.It is advisable to use the octane rating recommended by the manufacturer as the use of a lower octane rating can reduce performance and incur additional maintenance costs in the long run. Similarly, use of a higher octane rating would be unwanted spending.