Fuels, motor

Although distillation and elemental analysis of the fractions provide a good evaluation of the qualities of a crude oil, they are nevertheless insufficient. Indeed, the numerous uses of petroleum demand a detailed molecular analysis. This is true for all distillation fractions, certain crude oils being valued essentially for their light fractions used in motor fuels, others because they make quality lubricating oils and still others because they make excellent base stocks for paving asphalt. 

Knowledge of their qu nt ty tjieir distribution by number of carbon atoms is Indispensable for the evaluation of low temperature behavior of diesel motor fuels as well as the production and transport characteristics of paraffinic crudes. 

Analysis of Aromatics in Diesel Motor Fuels by Liquid Chromatography... 

In this chapter, we will discuss petroleum products used for energy purposes, that is, motor fuels and heating fuels. Chapter 6 will be devoted to other products such as special gasolines, lubricants, petrochemical bases, and asphalts. 

First of all, a technical clarification is necessary in the wider sense, motor fuels are chemical compounds, liquid or gas, which are burned in the presence of air to enable thermal engines to run gasoline, diesel fuel, jet fuels. The term heating fuel is reserved for the production of heat energy in boilers, furnaces, power plants, etc. 

Thus, according to the definitions, diesel fuel (or gas oil) is not a heating fuel but a motor fuel. Incidentally, heavy fuel can be considered a heating fuel or a motor fuel depending on its application in a burner or in a marine diesel engine. 

It is useful to specify at the start the principal quality criteria for each type of product (motor fuels, heating fuels), imposed by the requirements for the different kinds of energy converters motors, turbines, burners. 

With respect to fuels utilized as heating fuels for industrial furnaces, or as motor fuels for large diesel engines such as those in ships or power generation sets, the characteristics of primary importance are viscosity, sulfur content and the content of extremely heavy materials (asphaltenes) whose combustion can cause high emissions of particulates which are incompatible with antipollution legislation. 

There also exist relatively simple correlations between the heating value of motor fuels and certain characteristics such as density and composition by chemical family supplied by FIA analysis. Refer to Chapter 3. 

The magnitude of the NHV has economic importance because the consumption and cost of motor fuels are frequently expressed in liters/100 km and in Francs/liter in France. From the technical viewpoint, the NHV, establishes the maximum range for a transport system with a given load. This is a decisive criterion for applications like aviation. 

Weight and volume net heating values of commercial motor fuels (average values). 

In the expression for heating value, it is useful to define the physical state of the motor fuel for conventional motor fuels such as gasoline, diesei fuel, and jet fuels, the liquid state is chosen most often as the reference. Nevertheless, if the material is already in its vapor state before entering the combustion system because of mechanical action like atomization or thermal effects such as preheating by exhaust gases, an increase of usefui energy resufts that is not previously taken into consideration. 

For hydrocarbons, this gain remains moderate since their mass enthalpy of vaporization, between 300 and 500 kJ/kg, usually represents only 0.8% to 1% of the NHV. It is thus not extremely useful to know in this case whether the motor fuel is introduced as a vapor or a iiquid. The same is not true for... 

In the applications where the compactness of the energy conversion system is the determining factor as in the case of engines, it is important to know the quantity of energy contained in a given volume of the fuel-air mixture to be burned. This information is used to establish the ultimate relations between the nature of the motor fuel and the power developed by the motor it is of prime consideration in the development of fuels for racing cars. 

If one imagine.s that the fuel is used in the liquid state in the form of droplets —as in the case of fuel injection— the specific energy of the motor fuel (SE) is expressed in kilojoules per kilogram of air utilized, under predetermined conditions of equivalence ratio (stoichiometry for example). The SE is none other than the NHY /r quotient where r represents the previously defined stoichiometric ratio. 

The density and the volatility, expressed by the distillation curve and the vapor pressure, constitute the most important physical characteristics of motor fuels for obtaining satisfactory operation of a vehicle in all circumstances. 

Several parameters come into the relation between density and equivalence ratio. Generally, the variations act in the following sense a too-dense motor fuel results in too lean a mixture causing a potential unstable operation a motor fuel that is too light causes a rich mixture that generates greater pollution from unburned material. These problems are usually minimized by the widespread use of closed loop fuel-air ratio control systems installed on new vehicles with catalytic converters. 

In practice, the user prefers the densest possible motor fuel that is compatible with the specifications, for it gives him the best volume NHV and highest fuel economy. It is estimated that an increase in density of 4 to 5% brings a reduction in consumption of 3 to 5%. Finally for the refiner, a margin of 50 thousandths accorded for the density of each type of gasoline, makes an acceptable compromise, while a tightening of the specification would be too constraining. 

The criterion retained up to now in the specifications is not the true vapor pressure, but an associated value called the Reid vapor pressure, RVP. The procedure is to measure the relative pressure developed by the vapors from a sample of motor fuel put in a metallic cylinder at a temperature of 37.8°C. The variations characteristic of the standard method are around 15 millibar in repeatability and 25 millibar in reproducibility. 

In the standard method, the metal enclosure (called the air chamber) used to hold the hydrocarbon vapors is immersed in water before the test, then drained but not dried. This mode of operation, often designated as the wet bomb" is stipulated for all materials that are exclusively petroleum. But if the fuels contain alcohols or other organic products soluble in water, the apparatus must be dried in order that the vapors are not absorbed by the water on the walls. This technique is called the dry bomb" it results in RVP values higher by about 100 mbar for some oxygenated motor fuels. When examining the numerical results, it is thus important to know the technique employed. In any case, the dry bomb method is preferred. 

In order to characterize the behavior of motor fuels or their components with regard to knocking resistance but without involving chemical composition criteria which are complex and not easy to quantify, the traditional method that has been universally employed for more than 50 years consists of introducing the concept of octane number. 

The motor fuel under test is compared to two pure hydrocarbons chosen as references. The first is 2, 2, 4-trimethylpentane or isooctane which is very resistant to auto-ignition and to which is arbitrarily given the number 100 /... 

A motor fuel has an octane number X if it behaves under tightly defined experimental conditions the same as a mixture of X volume % of isooctane and (100 - X)% of n-heptane. The isooctane-heptane binary mixtures are called primary reference fuels. Octane numbers higher than 100 can also be defined the reference material is isooctane with small quantities of tetraethyl lead added the way in which this additive acts will be discussed later. 

Measurement of octane numbers is carried out using a reference motor called CFR (Cooperative Fuel Research), referring to a series of studies conducted in 1928 in the United States in order to standardize the methods for characterizing motor fuels. 

The measurement error for conventional motor fuels is around 0.3 points and 0.7 points for the RON and the MON respectively. The RON is the characteristic more often used and more widespread than the MON moreover, when the octane number is used without reference either procedure, it is taken to be the RON. 

Specifications for octane numbers of motor fuels in France. 

In the United States, the different categories of motor fuels are not defined as they are in Europe by minimum RON and MON values, but by a combination... 

The A/ 100 corresponds to the difference between the RON of the motor fuel and that of the fraction distilled at 100°C. The determination is carried out in three stages ... 

The base products, TEL and TML, are liquids having boiling points of 205° and 110° respectively. The contents of additives used are usually expressed in grams of lead per liter of fuel in the past they have reached 0.85 g Pb/1. These concentrations are still found in some of the countries of Africa. Elsewhere, when part or all of the motor fuel pool contains lead, the concentrations are much smaller. Thus in Western Europe they no longer exceed 0.15 g Pb/1. 

---