Gasoline Octane Rating
In the United States, the Octane scale is used for the evaluation of gasoline. Specificlly, the Octane scale is used as a measure of a gasoline's resistance to knocking, as compared with the standard fuel with a high resistance to knocking, called iso-octane.
Note: Technically, the name "iso-octane" is incorrect. The actually chemical name for the standard fuel is 2,2,4-Trimethyl pentane.
Engine Knock
Knocking or pinging occurs in the automobile engine when the last portion of the air/fuel mixture detonates as it is being compressed. This sudden rise in pressure can seriously damage the engine.
Physics of Gases
As a gas, or a mixture of gases, is compressed it heats up. This fact follows from two fundamental laws of physics:- Boyle's Law states that at a constant temperature, the volume of a given mass of gas
varies with the pressure. Mathematically, this can be stated as:
V1/V2 = P2/P1
Where:
- V1 = the original volume
- V2 = the new volume
- P1 = the original pressure
- P2 = the new pressure
- Gay-Lussac's (also called Charles's) Law states that at a constant pressure, the volume of a given mass of gas
varies with its absolute temperature. Mathematically, this can be stated as:
V1/V2 = T1/T2
Where:
- V1 = the original volume
- V2 = the new volume
- T1 = the original temperature
- T2 = the new temperature
These two laws can be combined into an equation that states that the volume of a given mass of gas varies directly with its absolute temperature and inversely with the pressure:
P1V1/T1 = P2V2/T2
Note: A more exact statement of this relationship, which uses the concepts of molar volume and kelvin temperature, is known as the Ideal-Gas equation.
What this means in the automobile engine is that as the piston in a cylinder compresses the gas/air mixture, the volume decreases and the pressure and temperature increase. This increase in temperature is called the heat of compression. High compression engines, in which the air/gasoline vapor mix is compressed into a small volume, have a high heat of compression.
Ignition and the Flame Front
Ignition occurs when the air/fuel mixture in the cylinder is ignited by the spark plug at the end of the compression stroke. A flame front of buring air/fuel mix spreads out from the point of ignition. The from this flame front increases the temperature and pressure in the unburned air/fuel mix ahead of it. If the temperature reaches a critical point, detonation of the remaining air/fuel mix occurs, causing knocking.
Note: Because of its high heat of compression brings the air/fuel mix closer to the critical temperature where knocking takes place, a high-compression engine has a greater tendency to knock than a low-compression engine. This fact must be compensated for in the engine design as well as in the formualation of the gasoline.
Gasoline Characteristics and Knocking
The critical temperature at which detotation of the last part of the air/fuel mix and consequently knocking occurs depends on the "anti-knock" characteristics of the gasoline. As gasolines were developed, it was found that straight chain hydrocarbons have a greater tendency to knock that branched chain hydrocarbons. The compound with the poorest anti-knock characteristics was found to be n-Heptane, and it was given a rating of 0. The compound with the best anti-knock characteristics was found to be 2,2,4-Trimethyl pentane ("iso-octane"), and it was given a rating of 100.
Octane Rating
The octance rating of a gasoline is defined as the percentages of iso-octane and n-Heptane that has the same knock characteristics as the gasoline under test. For example, if the gasoline has the same characteristics as a mixture of 13% n-Heptane and 87% iso-octane, the octane rating of the gasoline is 87.