Thermochemistry & Fuels Lecture 4 1
Outline In this lecture we will discuss the properties and characteristics of diesel fuels: Cetane number and index Viscosity and cold behaviour Flash point Sulphur We will also briefly introduce a number of fuel additives and alternative fuels. 2
Diesel Fuel Diesel fuels are mixtures of a wide range of hydrocarbons with boiling points in the range of 150 to 380 C. Diesel fuels are usually divided into two categories: heavy diesel and light diesel. Light diesels have a M.W. in the range of 170, an equivalent chemical formula of C 12.3 H 22.2 and are used for automotive applications. Heavy diesels have a M.W. in the range of 200, an equivalent chemical formula of C 14.6 H 24.8 and are used for heating purposes. 3
Cetane Number The cetane number represents the readiness of the fuel to self-ignite when injected into the combustion chamber. Since C.I. engines are based on the principle of self-ignition, the cetane number of the diesel fuel is its most important characteristic. The higher the cetane number, the shorter is the ignition delay and the easier it is for the fuel to ignite. 4
Cetane Number As in the octane number for gasoline fuels, the cetane number is measured experimentally through a standard test procedure on a standard engine. During the test, ignition delay of the tested fuel is measured and is compared with that of two reference fuels: n-cetane (n-hexadecane) C 16 H 34, which is assigned a cetane number of 100. Heptamethyl nonane C 12 H 34, which is assigned a cetane number of 15. 5
Cetane Number Alkanes in general have high cetane numbers, while the opposite is true for the cyclic and aromatic compounds. One can notice a strong inverse relationship between the cetane number of a fuel and its octane number. 6
Cetane Number Accordingly, the cetane number of a fuel is determined depending on the percentages of the two standard hydrocarbons in the mixture that resembles its ID, according to the following equation: CN = % cetane + 0.15 (% HMN) The European standard EN 590 (2000) for diesel fuels determines the minimum cetane number at 51. 7
Cetane Index The Cetane Index is a calculated value that was introduced to empirically estimate the cetane number of a fuel from its density and boiling curve. The cetane index is usually slightly lower than the cetane number. The European standard EN 590 (2000) for diesel fuels determines the minimum cetane index at 46. 8
Viscosity The viscosity of the diesel fuel is usually regulated by standards since it is important for the operation of fuel injection equipment. If the viscosity is too high, this could reduce the flow of the fuel and can lead to injector-pump damage. On the other hand, if it is too low, this can significantly increase the leakage from pumping elements. The range set by EN 590 for the viscosity of diesel fuel is 2-4.5 mm 2 /s. 9
Cold Behaviour During cold weather, crystallisation of heavy alkanes contained in the diesel fuel can occur. This can lead in turn to fuel-filter blockage, and ultimately to the interruption of the fuel flow. A Cold Filter Plugging Point (CFPP) is often defined by fuel standards in the range of 0 to - 20 C 10
Flash Point The flash point is the temperature at which a combustible liquid gives off just enough vapour to produce a vapour/air mixture that will ignite in the existence of a flame. Typical flash points for diesel fuels are in the range of 55 C. This makes diesel fuels generally easy and safe to handle and store. The flash point has no effect on the engine performance or the cetane number. 11
Sulphur Diesel fuels contain chemically bonded sulphur with quantities depending on the quality of the crude oil and the cracked components that are added at the refinery. Sulphur can be reduced with hydrogen treatment. Sulphur leads to the production of SO x and sulphates which are environmental and health hazards and impair the operation of catalytic converters. 12
Sulphur As a result, the sulphur content in the fuel is usually limited by law. EN 590 currently limits the sulphur content to 350 ppm. This will be reduced to 50 ppm by 2005 and to 10 ppm by 2008. 13
Fuel Additives Along with the structure of hydrocarbons, automotive fuels contain many additives to improve the quality of the fuel in different ways: Octane or cetane enhancers Flow improvers (for winter fuel) Anti-ageing agents Intake system anti-contamination detergents Anti-corrosion agents 14
Alternative Fuels In addition to gasoline and diesel fuels, a number of other fuels are being introduced, researched and tested. Despite the possible advantages of introducing such fuels, their application still faces practical obstacles and so it is limited to controlled sectors. Liquefied Petroleum Gas (LPG), Compressed Natural Gas (CNG), Alcohols, hydrogen and biofuel are just examples of alternative fuels. 15