Volatility of petrol and its performance

by L.S. Ananda WEDAARACHCHI

The Sunday Observer interviewed Cyril Suduwella, Msc, Diploma in Petrochemistry, former Head of laboratory at Sapugaskanda Refinery and eminent Petro Scientist with many years experience in petroleum testing and quality of refined products. He held top posts in Sri Lankan and overseas refineries dealing with all aspects of fossil fuels and alternative fuels. He is also a visiting lecturer in Petroleum Chemistry at Post Graduate Institute at Peradeniya University.

Excerpts of the interview:

Q: What is Petrol?

A: Petrol (Automotive Gasolines)

Automotive Gasoline or Petrol are essentially blends of hydrocarbons derived from Petroleum. In addition, they may contain selected addives that impart specific features to the finished gasoline. The Hydrocarbons are derived from fractional distillation of Crude oil and from complex process that increase either the amount or quality of the gasoline. Gasolines contain hundreds of individual hydrocarbons which range from normal butane to C12 hydrocarbons such as methyl naphthalene. The properties of commercial petrol are predominantly influenced by the refinery practices and partially by the nature of the crude oils from which they are produced. Finished Petrol boils from 30 to 225 degrees Celsius.

Gasoline are blended to satisfy diverse automotive requirements. Antiknock rating (Octane Number), distillation characteristics, vapor pressure, sulfer content, oxidation stability, and anti corrosive behaviour are balanced to provide satisfactory vehicle performance. Additives may be used to provide or enhance specific performance features.

Q: What is Octane Number?

A: The anti knock rating (Octane Number) of petrol is an important characteristic. If antiknock rating is very low knock occurs. Knock is a high pitch, metallic rapping noise. Petrol with antiknock rating higher than that required for knock free operation does not improve performance. However, vehicles equipped with knock limiters may show a performance improvement as the antiknock quality of petrol is increased; conversely antiknock rating decreases may cause vehicle performance loss. The loss of power and the damage to an automotive engine due to knocking are generally not significant until the knock intensity becomes very severe. Heavy and prolonged knocking may cause power loss and damage to the engine.

The Octane Number of petrol is measured by several methods. These procedures employ single cylinder variable compression ratio single cylinder laboratory engines. By definition, the octane number of iso-octane is 100, and the octane number of n-heptane is zero. The Octane number of a given Petrol is the percentage by volume of iso-octane in a blend with n-heptane that knocks with the same intensity at the same compression ratio as the Petrol, when compared by one of the standardized engine test methods.

Q: Why does the quality of petrol depend on its volatility properties?

A: The volatility properties of petrol are of prime importance to the drivability of vehicles under all conditions encountered in normal service. Petrol that vaporizes too readily in pumps, fuel lines, and carburetors will cause decreased fuel flow to the engine, resulting in rough engine operation or stoppage (vapor lock). Conversely, petrol that do not vaporize readily enough may cause hard starting and poor worm-up and acceleration, as well as unequal distribution of fuel to the individual cylinders, which may cause knock.

Petrol that vaporises too readily also may cause, if certain atmospheric conditions exist, ice formation in the carburetor throat, resulting in rough idle and stalling.

The volatility of Petrol must be carefully "balanced" to provide the optimum compromise among performance features that depend upon the vaporisation behaviour. Superior performance in one respect may give serious trouble in another, unless correct specifications are imposed. Therefore, volatiilty characteristics of petrol must be adjusted for seasonal variations in atmospheric temperatures and geographical variation in altitude. The temperature at which 10%, 50% and 90% evaporation occurs are used to characterise that volatility of Petrol.

Q: What are the other quality determining parameters?

A: In addition to the performance and volatility characteristics, petrol must also provide for satisfactory engine cleanliness. The following properties have a direct bearing on the overall performance of Petrol;

* Workmanship and contamination

* Lead content

* Phosphorous content

* Manganese content

* Suulfur content

* Existent Gum and Oxidation stability

* Density or Specific Gravity

* Rust and Corrosion

* Hydrocarbon composition

* Additives

Q: What is water Tolerance?

A: Petrol and water are almost entirely immiscible; when mixed, they readily separate into two phases. Blends will dissolve some water but will also separate into two phases when contacted with more water than they can dissolve. This was may be absorbed from ambient air or may occur as liquid water in the bottom of tanks in the storage distribution and vehicle fuel system. Such separation may not be a problem with Petrol-Ether blends. However, it is a matter of concern with Petrol-Alcohol blends because low molecular weight alcohols are readily extracted from blends leaving an alcohol poor petrol phase and an alcohol rich aqueous phase. This aqueous phase may be highly corrosive many metals and the engine will not operate on it.

In addition, the properties of the petrol phase will differ from those of the blend, for example, octane rating, volatility, stoichiometry etc., and engine performance may suffer.

All plastics and elastomers used in current automotive fuel systems such as gaskets, 'O"-rings, diaphragms, filters, seals etc., may be affected in time by exposure to petrol. These effects include dimensional changes, embrittlement, softening, delamination, increase in permeability, loss of plasticizers, and disintegration. Petrol-Oxygenate blends may aggravate these effects.