(17) Fuel Oils.
Crude oil, a natural product, is the base of the fuels most commonly used in internal combustion engines, especially in the smaller sizes. From this compound the following derivatives are obtained by the process of distillation, a separation possible because of the different boiling points of the various oils. As each derivative or DISTILLATE has a different boiling point, the temperature of the crude oil is maintained at the boiling point of that product that is desired, and the resulting vapor is condensed. The following list is not anywhere near complete for there are several hundred distinctly different distillates, but it contains those that are of the most interest to the engine man.
- 1. Crude Oil.
- 2. Gasoline.
- 3. Naphtha.
- 4. Solar Oil.
- 5. Kerosene.
The specific gravity of the crude oil as obtained in the field will range from 12° to 56° Beaumé scale. The crude from Pennsylvania will average 40° Beaumé while that from Texas will average 20°. The accompanying table will give the calorific values and general properties of the principle liquid fuels. It should be noted that the weight or density of the liquids is given in terms of specific gravity or Beaumé scale, in which the SPECIFIC GRAVITY of the fuel is the ratio of its weight per unit volume to the weight of an equivalent volume of water. The specific gravity of a liquid is generally determined by an instrument known as a HYDROMETER which consists of a glass tube sealed at both ends carrying a graduated scale on the upper portion of the stem, and a ballast weight of shot or mercury at the bottom.
The hydrometer is floated in the liquid to be tested, and the lower the specific gravity, the lower the hydrometer sinks, and vice versa. The specific gravity of the liquid is read directly from the graduation on the stem that are on a level with the surface of the liquid under test. As in the case of thermometers, hydrometers are all graduated in two different scales, the specific gravity scale and the Beaumé scale. The specific gravity scale reads at 1.00 when floated on distilled water, and the Beaumé at 10.00 when floated on the same liquid.
A difference in temperature affects the density of a liquid, hence all hydrometers are graduated for a standard temperature of 60°F unless otherwise specified. For a difference of 10°F there is a variation of one degree gravity in the Beaumé scale, and for a difference of 20°F in temperature there is a change of one degree on the specific gravity scale. If the temperature differs from 60°F, the corresponding correction should be made in the reading.
To convert the Beaumé reading (B) to terms of the specific gravity scale (S) use the following formula:
| 140 | ||
| S = | = specific gravity. | |
| 130 + B |
| 140 | ||
| B = | = Beaumé scale. | |
| S |
| PROPERTIES OF OILS | |||||
|---|---|---|---|---|---|
| Degrees Beaumé | Specific Gravity | Weight per gal. | B.T.U.’S per lb. | B.T.U.’S per gal. | |
| Gasoline | 67.2 | .7125 | 5.932 | 21120 | 125,284 |
| Heavy naphtha | 64.6 | .7216 | 6.011 | 20527 | 123,388 |
| Kerosene | 48.8 | .7848 | 6.538 | 20018 | 130,877 |
| W. Virginia crude | 40.0 | .8251 | 6.874 | 19766 | 135,871 |
| Penn. fuel oil | 31.9 | .8660 | 7.215 | 19656 | 141,818 |
| Kansas crude | 29.0 | .8816 | 7.345 | 19435 | 142,750 |
| Fuel oil | 22.7 | .9176 | 7.645 | 19103 | 146,042 |
| California crude | 22.5 | .9248 | 7.710 | 18779 | 144,786 |
| California crude | 15.2 | .9646 | 8.036 | 18589 | 149,381 |
| Alcohol, 95% | 41.9 | .816 | 6.798 | 10500 | 71,380 |
It will be noted that the petroleum products contain an enormous amount of heat energy, nearly 25% more than that of the same weight of pure carbon. It will also be noted that the lighter products such as gasoline, kerosene, etc., have more heat per pound but less per gallon than the heavier oils. This is rather confusing at first, but as will be seen after deliberation that the heavier fuel is the most economical since the least is used per horse-power, and is bought by the gallon. The calorific values given in the table are obtained by a calorimeter, and are burnt in the open air, and consequently have a different heating value when under compression in the cylinder of the engine.
In all cases the liquids are vaporized before being introduced in the cylinder, the more volatile liquids such as gasoline being converted into vapor at atmospheric temperature, and the heavier non-volatiles by being sprayed into a heated vessel or preheated air. The percentage of liquid fuel contained in a cubic foot of air vapor mixture depends on the temperature, the boiling point of the liquid and upon the pressure and humidity.
Gasoline consists principally of compounds of the methane series, the one representative of gasoline being Hexane (C6H14). It requires 15.5 pounds of air for combustion theoretically and about 10 per cent. more in practice. The formation of gasoline vapor produces a drop in temperature of 50°F, and should be heated 100°F above the atmosphere for the best results. The volume of air required for the combustion is about 192 cubic feet. With alcohol at 20 cents per gallon and gasoline at 12½ cents the number of B.T.U.’s for one cent in the case of alcohol is 3594 and 9265 in the case of gasoline. In the engine the difference is not so great owing to the difference in compression pressures.