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Many products from oil can be gained in the refining process that can be widely used directly and indirectly in which the quality of the fuel has been continuously developed to be up to date with the endless evolution of engines. At this point, we will look at two familiar and widely used type of fuel which is benzene and diesel.

Thailand's current trade in fuel is in accordance with the free trade system whereby oil price varies depending on market mechanism that relies on many factors such as production status. Utilizing the global situation to observe reveals that oil prices at various petrol stations of several companies shifts in the same direction in accordance with global oil prices. The distribution price of oil in the country can be categorized in two ways as follows:

1. Wholesale price in front of the refinery is composed of the price at the refinery plus tax from the revenue office and the municipality, oil fund, energy conservation promotion fund and value added tax or VAT.
2. Retail price at the petrol station comprised of the wholesale price in front of the refinery plus the market value and VAT so we buy petrol from the various stations at the retail price in which the price structure is made up of 3 important parts:

Benzene

Octane Value is a fuel's characteristic that shows its ability to resist the explosion before the appropriate time in benzene engines. In other words, this number refers to the resistance to knockout of the fuel in benzene engines due to the increase in temperature within the mixture between air and fuel that is being injected by a flame before the termination of the combustion process in the injection cylinder of the engine in which the knockout of an engine can cut its strength. However, each type of engine has a different octane value depending on the design by the engine manufacturer.

Amount of Lead Even though at present, there is no lead added in benzene to increase its octane value but lead might be mixed in crude oil or the production process in which lead is considered a pollutant in exhaust gas and dangerous to the human body so a benchmark to control its amount must be established.

Amount of Sulfur Once sulfur in fuel is burned, it can corrode and cause the engine to be worn out as well as become dust making the engine dirty and pollute the atmosphere.

Amount of Phosphorus usually originates from the adding of substance into benzene to increase its quality which could damage the exhaust filter.

Corrosion Oil that has substances mixed in it such as sulfur can cause corrosion in metallic components wearing out the engine in which the corrosion value is the index that shows the wear out status in engines.

Stability towards oxidation reaction is the index that shows the ability of the fuel to result in a reaction with oxygen generating a sticky substance having an impact on exhaust, combustion chamber and oil reserve.

Amount of sticky substance Once oil that has a component of nitrogen making reaction with oxygen in the air, a sticky substance will be created which is considered a dirty substance in the exhaust system and combustion chamber freezing the valve and ring as well as obstructing the carburetor.

Evaporation rate is the feature that shows the heavy and light weight component of the oil as well as how and how long it will be continuously burned. If there is more light weight substance, it will be hard to start but if oil slowly evaporates at a consistent rate, gradually increasing the temperature will make the combustion more continuous with the engine running more smoothly. Hence, the evaporation rate of oil can effect the start up of an engine, its acceleration and energy saving.

Exhaust pressure must not exceed the standard to prevent the problem of exhaust blocking the oil passage tube causing the flow of oil to be inconsistent in the carburetor which could make the engine different to start, jerk or turn off suddenly.

Amount of benzene substance Benzene is an aromatic substance with a high octane value but dangerous to the human respiratory system, brain in which inhaling for a long period of time could lead to cancer.

Amount of aromatics has a high octane value in which certain aromatics such as benzene, polycyclic aromatics has caused cancer in experimented animals. Apart from this, burning oil with a high amount of aromatic substance can generate a high amount of ash as well as exhaust with aromatics in the case of incomplete combustion.

Color Generally, benzene has no color but vendors usually add color in order to easily differentiate the various types of fuel and prevent counterfeits in which benzene octane 91 will have a red color while benzene octane 95 will have a yellow color.

Amount of water can rapidly damage the fuel and cause blockage at a low temperature as wall as make the angina not to run as smooth.

Oxygenate substances added in benzene to help increase oxygen such as MTBE [Tertiary Butyl Ether] can help improve combustion and reduce pollutants such as carbon monoxide. At the same time, MTBE with a high oxygen value more than 100 will help increase octane in benzene but vendors will usually control its amount to be at a suitable level because MTBE absorbs water very well.

Quality increasing substances in benzene Adding these types of substances can increase the effectiveness of its application as follows
- Cleaning substances helps to wash dirt left in the fuel system and maintain the carburetor to always be clean
- Air mixture resistance substances help to prevent benzene from mixing with oxygen in the atmosphere as well as sticky substances dangerous to the fuel system from happening
- Corrosion and rust prevention substances prevents rust from blocking the oil filter in the carburetor

Diesel fuel

Diesel is one of the products gained from refining crude oil but has a boiling temperature and concentration index higher than benzene. Diesel engines have a different operating principle from benzene engines in which the explosion of diesel engines use heat from air injection in the cylinder eventually injecting the fuel to create combustion unlike explosion using a start plug in benzene engines.

Characteristic and feature of diesel fuel

Inflammation This feature can show the ability in starting an engine at a low temperature and prevent the knockout of the engine during combustion in the cylinder, The combustion is fast with high effectiveness in which these features come out in the form of a cetane index or cetane number whereby the cetane index must be high enough to meet the cycle [rpm] of the engine making the start up of the engine easy without any knockout and helping to save energy.

Cleanliness is the important feature of diesel fuel whereby the fuel must be clean before and after combustion. For instance, there must be a minimum amount of sediment, ash, water or residues because diesel fuel requires en oil pump and fuel injection head in combustion.

Distribution in the form of minute drops The concentration or transparency will determine the distribution of diesel. The right amount of concentration will make the oil to distribute in minute drops very well in which the concentration of diesel has en impact on the oil pumping system for while the pump injects fuel into the combustion chamber the oil itself will lubricate the piston.

Evaporation rate of diesel can affect the boiling and inflammation point.

Color Normally, diesel has a light tea-like color but sometimes it could change due to the fact that in the refining process, crude oil from different locations with the same features might be used in which color is not the important factor in determining the quality of the fuel. Vendors have set up 3 standard colors similar to tea in which the color might intensify if kept for long periods but if it turns green or dark black, counterfeits might be made from furnace oil or used engine oil.

Amount of sulfur in oil is not desirable in which there are 2 types of corrosion by sulfur in oil. The first type comes from the corrosion after combustion whereby components of sulfur after combustion will generate sulfur dioxide which integrated with water will have the features of acid corrode the various parts of the engine. The second type comes from sulfur in the fuel that is oil will corrode the injection system of the diesel engine whereby the amount of sulfur in diesel fuel will be more or less depending on the type of crude oil and refining process used. Sulfur components with corrosion features come in several forms such as mercaptain, disulfide or heterocyclic such as Thiopental.

Concentration and transparency has an effect on the shape of the oil particles sprayed from the injection head. If the oil has a high concentration index, spraying in minute drops will not be good for the particles will be big and spout far instead of being sprayed in small particles causing oil not to mix well with air making the combustion incomplete and lowering the effectiveness of the engine. If the oil has a low concentration value, the spraying of oil will be delicate but it won't go as far making the combustion bad as well as there could be leakage back in the pump and injection head. Thus, diesel will generally have a concentration or transparency index at 1.8-4.1 centistokes at a temperature of 40 degrees Celsius.

Choosing the suitable type of diesel fuel

Diesel fuel or Zola is another name of high speed diesel to differentiate it from heavy diesel. Choosing diesel fuel is different from benzene because only high speed diesel is sold at petrol stations while heavy diesel is sold by oil companies directly to industries. The important point is that most oars or trucks at present use both benzene and diesel engines so the opening for filling oil in cars should have a sign that clearly states the type of fuel. If misused, engine problems could follow.

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