Welcome to this first installment of The Fuel Desk! For the first 50-60 years in the history of the automobile, the primary fuel was leaded gasoline. While you could choose your “flavor” of petrol by selecting different grades (i.e. Octane levels), the basic chemistry was the same. Gasoline was gasoline, pure and simple.
Today, there are many variations on that theme, including fluids that bear little or no resemblance to the original explosive fuel at all. The purpose of this feature is to educate you about the fuel that you are using, and the numerous options available to motorists today that are not even gasoline!
Let’s begin this journey with a conversation about the fuel most familiar to the average American motorist – gasoline.
What it is – Gasoline is a transparent, petroleum-derived liquid that is used primarily as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives; a 42 gallon barrel of crude oil yields about 19 gallons of gasoline, when processed in an oil refinery. Ironically, gasoline was not invented but is a natural by-product of the petroleum industry with kerosene being the principal product. What was invented were the numerous processes and agents needed to improve the quality of gasoline making it a better commodity.
Material separated from crude oil via distillation, called virgin or straight-run gasoline, does not meet specifications for modern engines (particularly the octane rating), but can be pooled to the gasoline blend. Gasoline, when used in high-compression internal combustion engines, tends detonate causing damaging “engine knocking” (also called “pinging”) noise. To address this problem, tetraethyl lead (TEL) was widely adopted as an additive for gasoline in the 1920s. With the discovery of the extent of environmental and health damage caused by the lead and its incompatibility with catalytic converters, leaded gasoline was phased out beginning in 1973. This was the start of the age of unleaded gasoline in the United States.
The ‘anti-knock’ characteristic of a particular gasoline blend, when used as a fuel in internal combustion engines, is measured by its octane rating. Gasoline is produced in several grades of octane rating. While tetraethyl lead and other lead compounds are no longer used to regulate and increase octane-rating, many other additives are put into gasoline. The purpose of these additives is to improve gasoline’s chemical stability, to control corrosiveness, provide fuel system cleaning, and determine performance characteristics.
Octane and gasoline – Spark ignition engines are designed to burn gasoline in a controlled process called deflagration. In some cases, however, the unburned mixture can auto ignite by detonating from pressure and heat alone, rather than ignite from the spark plug at exactly the right time, which causes rapid pressure rise which can damage the engine. This phenomenon is often referred to as engine knocking. One way to reduce knock in spark ignition engines is to increase the gasoline’s resistance to auto ignition, which is expressed by its octane rating.
The octane rating is measured relative to a mixture of 2,2,4-timethylpentane (an isomer of octane) and n-heptane. There are different conventions for expressing octane ratings, so the same physical fuel may have several different octane ratings based on the measure used.
Octane Number Calculations – One of the best known is the research octane number (RON). RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane. Another important, but lesser known measurement of octane is called the motor octane number (MON). MON is determined at 900 rpm engine speed instead of the 600 rpm for RON. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, higher engine speed, and variable ignition timing to further stress the fuel’s knock resistance. Depending on the composition of the fuel, the MON of a modern pump gasoline will be about 8 to 12 octane lower than the RON, but there is no direct link between RON and MON.
Motorists usually see a number at the pumps which is actually the result of a formula that combines both the RON and the MON. It is called the Anti-Knock Index (AKI). You may not have realized it, but the formula used by the AKI is listed on every gasoline pump. Sometimes also referred to as the Posted Octane Number the formula is R+M/2. Basically it’s the combination of the RON plus the MON, divided by 2.
In the United States, octane ratings in unleaded fuels can vary between 85 and 87 AKI (91-92 RON) for regular, through 89-90 AKI (94-95 RON) for mid-grade, and up to 90-94 AKI (95-99 RON) for premium.