CHAPTER V
PETROLEUM ENGINES
There is a great deal of difference, from an economic point of view, between spirit or carburetted air engines which we have just described and motors using ordinary commercial petroleum oil. As a rule such oil is much less dangerous, having a lower flashing-point, while at the same time it is cheaper than the more artificial products such as benzoline, naphtha, etc. We shall presently give a table comparing the cost of power produced by various systems of motors, and which will explain why the petroleum engine is gradually ousting the carburetted air motor from its place.
The first motor using ordinary petroleum was brought out twenty-six years ago by Brayton in 1872, and may be said to be the father of the large family of motors which consume oils varying from naphtha to others which have boiling-points as high as 150° C.
Ready motor (Brayton).—This motor belongs to the third group of the table on page 21 (combustion and compression). There are two cylinders, one motive the other auxiliary, and used for compressing air. The compressed air passes through a space filled with some absorbent material such as felt, and kept saturated with petroleum injected into it by a special pump. The carburation of the air proceeds as follows: the high-pressure jet of air causes the petroleum to froth up, and in passing through it draws along a quantity of oil in the shape of finely-divided particles. This petroleum mist passes on to the cylinder. The advantage of this method of obtaining an explosive mixture is that it matters not what oil is used, in fact, the heavier oils are even to be preferred, as their partial condensation in the cylinder helps to lubricate it. One volume of petroleum will by this process produce 24,000 of explosive mixture.
The motor is constructed in both the horizontal and vertical form, the auxiliary being superposed upon the motor cylinder. The admission of the pulverized petroleum lasts for one-third of the forward stroke, and the return stroke expels the burnt gases. The engine is double-acting, using both sides of the piston.
A reservoir of compressed air is used for starting the engine, and saves the trouble of turning the heavy fly-wheel round by hand to give it the necessary impetus.
The Brayton motors are constructed by an American firm from 1 to 10 horse-power. The cost of maintenance and fuel is moderate, rendering them exceedingly practicable where coal gas or the lighter petroleum are unobtainable. A later type than the one we have just described appeared in 1890 which is slightly more economic, single-acting, and uses the Otto cycle. The process for obtaining a spray of oil is also much improved. Both the auxiliary and the motor cylinders are connected by a beam and connecting rods to the shaft. A red-hot platinum tube ignites the explosive mixture.
Sécurité petroleum motor.—This engine was patented in 1887 by MM. Belmont Chabout and Diedrichs, and appeared at the Paris Exhibition of 1889. An Otto cycle is used, and ignition is obtained by a platinum capsule heated to incandescence by a jet of carburetted air which passes through a spiral coil warmed by the waste heat of the cylinder. This ignition tube is situated at the back of the cylinder, and is placed in communication with it by a port and valve, which determines the exact instant at which the explosion takes place. The petroleum is vaporized by passing through pipes coiled up in a vessel traversed by the heated products of combustion. The vapour thus formed has a high enough pressure to draw air along with it as it passes across an air space between two nozzles; an explosive mixture is therefore formed. The motor is started by carburating a small quantity of air by means of a reservoir containing a light oil, and this supplies sufficient energy to work the motor until the spiral coils are hot enough to vaporize the petroleum. The whole apparatus is rather complicated, especially as there are two separate vaporizers using different oils. This disadvantage, combined with only a moderate efficiency and a high prime cost, has severely handicapped the engine.
Priestman oil engine.—The first patent of this interesting engine was taken out in 1886 by Messrs. Dent and Priestman. The engine as a whole represents probably the highest point of perfection which it is possible to attain to in this class of motor, and the general arrangement is somewhat similar to that of Dr. Otto’s engines, but in addition there is a very ingenious apparatus for working with heavy petroleum oils. A single-stroke pump, driven by an eccentric keyed to an auxiliary shaft turning half as fast as the main shaft, compresses the air to a standard pressure (depending in value, on the size of the engine) in a reservoir supplied with a governing valve and placed near the front of the engine. This reservoir contains petroleum, and the pressure of the air forces it up into a spray-maker, where it is pulverized. From thence the petroleum spray passes to a vaporizer heated by the waste gases, and becomes mixed with a certain volume of air forming an explosive mixture, which is admitted to the cylinder by an automatic valve. Compression and ignition take place in the cylinder, and the waste gases escape through a valve worked by the same eccentric which drives the air-pump. The number of parts of the mechanism is by this means reduced to a minimum, and it is found that this peculiar vaporizing process completely prevents the cylinder from being fouled, as is usual when petroleum is heated to such a high temperature, often decomposing it. The Priestman engine gave a fresh lease to the life of oil motors, which had at one time been almost abandoned by engineers. It is well adapted for all kinds of industrial operations requiring powers of from 1 to 50 horse-power, and large vertical engines have been constructed for marine purposes up to 100 horse-power, and have always given complete satisfaction to persons using them. A few minutes are required for the apparatus to become sufficiently heated to be able to start, after which the consumption of fuel is about 380 to 500 grammes of petroleum per horse-power hour, or about ·85 lb. The six-horse size weighs about 1½ tons. Ignition can be had either electric or by a flame, according to the wishes of the purchaser.