Chapter 6: Internal Combustion Engines

OIL ENGINES--ENGINES WORKED WITH PRODUCER GAS--BLAST FURNACE GAS ENGINES

If carbon and oxygen be made to combine chemically, the process is accompanied by the phenomenon called heat. If heat be applied to a liquid or gas in a confined space it causes a violent separation of its molecules, and power is developed.

In the case of a steam-engine the fuel is coal (carbon in a more or less pure form), the fluid, water. By burning the fuel under a boiler, a gas is formed which, if confined, rapidly increases the pressure on the walls of the confining vessel. If allowed to pass into a cylinder, the molecules of steam, struggling to get as far as possible from one another, will do useful work on a piston connected by rods to a revolving crank.

We here see the combustion of fuel external to the cylinder, i.e. under the boiler, and the fuel and fluid kept apart out of actual contact. In the gas or oil-vapour engine the fuel is brought into contact with the fluid which does the work, mixed with it, and burnt inside the cylinder. Therefore these engines are termed internal combustion engines.

Supposing that a little gunpowder were placed in a cylinder, of which the piston had been pushed almost as far in as it would go, and that the powder were fired by electricity. The charcoal would unite with the oxygen contained in the saltpetre and form a large volume of gas. This gas, being heated by the ignition, would instantaneously expand and drive out the piston violently.

A very similar thing happens at each explosion of an internal combustion engine. Into the cylinder is drawn a charge of gas, containing carbon, oxygen, and hydrogen, and also a proportion of air. This charge is squeezed by the inward movement of the piston; its temperature is raised by the compression, and at the proper moment it is ignited. The oxygen and carbon seize on one another and burn (or combine), the heat being increased by the combustion of the hydrogen. The air atoms are expanded by the heat, and work is done on the piston. But the explosion is much gentler than in the case of gunpowder.

During recent years the internal combustion engine has been making rapid progress, ousting steam power from many positions in which it once reigned supreme. We see it propelling vehicles along roads and rails, driving boats through the water, and doing duty in generating stations and smelting works to turn dynamos or drive air-pumps--not to mention the thousand other forms of usefulness which, were they enumerated here, would fill several pages.

A decade ago an internal combustion engine of 100 h.p. was a wonder; to-day single engines are built to develop 3,000 h.p., and in a few years even this enormous capacity will doubtless be increased.

It is interesting to note that the rival systems--gas and steam--were being experimented with at the same time by Robert Street and James Watt respectively. While Watt applied his genius to the useful development of the power latent in boiling water, Street, in 1794, took out letters patent for an engine to be worked by the explosions caused by vaporising spirits of turpentine on a hot metal surface, mixing the vapour with air in a cylinder, exploding the mixture, and using the explosion to move a piston. In his, and subsequent designs, the mixture was pumped in from a separate cylinder under slight pressure. Lenoir, in 1860, conceived the idea of making the piston suck in the charge, so abolishing the need of a separate pump; and many engines built under his patents were long in use, though, if judged by modern standards, they were very wasteful of fuel. Two years later Alphonse Beau de Rochas proposed the further improvement of utilising the cylinder, not only as a suction pump, but also as a compressor; since he saw that a compressed mixture would ignite very much more readily than one not under pressure. Rochas held the secret of success in his grasp, but failed to turn it to practical account. The “Otto cycle,” invented by Dr. Otto in 1876, is really only Rochas’s suggestion materialised. The large majority of internal combustion engines employ this “cycle” of operations, so we may state its exact meaning:--

(1) A mixture of explosive gas and air is drawn into the cylinder by the piston as it passes outwards (i.e. in the direction of the crank), through the inlet valve.

(2) The valve closes, and the returning piston compresses the mixture.

(3) The mixture is fired as the piston commences its second journey outwards, and gives the “power” stroke.

(4) The piston, returning again, ejects the exploded mixture through the outlet or exhaust valve, which began to open towards the end of the third stroke.

Briefly stated, the “cycle” is--suction, compression, explosion, expulsion; one impulse being given during each cycle, which occupies two complete revolutions of the fly-wheel. Since the first, second, and third operations all absorb energy, the wheel must be heavy enough to store sufficient momentum during the “power” stroke to carry the piston through all its three other duties.

Year by year, the compression of the mixture has been increased, and improvements have been made in the methods of governing the speed of the engine, so that it may be suitable for work in which the “load” is constantly varying. By doubling, trebling, and quadrupling the cylinders the drive is rendered more and more steady, and the elasticity of a steam-engine more nearly approached.

The internal combustion engine has “arrived” so late because in the earlier part of last century conditions were not favourable to its development. Illuminating gas had not come into general use, and such coal gas as was made was expensive. The great oil-fields of America and Russia had not been discovered. But while the proper fuels for this type of motor were absent, coal, the food of the steam-engine, lay ready to hand, and in forms which, though useless for many purposes, could be advantageously burnt under a boiler.

Now the situation has altered. Gas is abundant; and oil of the right sort costs only a few pence a gallon. Inventors and manufacturers have grasped the opportunity. To-day over 3,000,000 h.p. is developed continuously by the internal combustion engine.

Steam would not have met so formidable a rival had not that rival had some great advantages to offer. What are these? Well, first enter a factory driven by steam power, and carefully note what you see. Then visit a large gas- or oil-engine plant. You will conclude that the latter scores on many points. There are no stokers required. No boilers threaten possible explosions. The heat is less. The dust and dirt are less. The space occupied by the engines is less. There is no noisome smoke to be led away through tall and expensive chimneys. If work is stopped for an hour or a day, there are no fires to be banked or drawn--involving waste in either case.

Above all, the gas engine is more efficient, or, if you like to express the same thing in other words, more economical. If you use only one horse-power for one hour a day, it doesn’t much matter whether that horse-power-hour costs 4d. or 5d. But in a factory where a thousand horse-power is required all day long, the extra pence make a big total. If, therefore, the proprietor finds that a shilling’s-worth of gas or oil does a quarter as much work again as a shilling’s-worth of coal, and that either form of fuel is easily obtained, you may be sure that, so far as economy is concerned, he will make up his mind without difficulty as to the class of engine to be employed. A pound of coal burnt under the best type of steam-engine gives but 10 per cent. of its heating value in useful work. A good oil-engine gives 20-25 per cent., and in special types the figures are said to rise to 35-40 per cent. We may notice another point, viz. that, while a steam-engine must be kept as hot as possible to be efficient, an internal combustion engine must be cooled. In the former case no advantage, beyond increased efficiency, results. But in the latter the water passed round the cylinders to take up the surplus heat has a value for warming the building or for manufacturing processes.

Putting one thing with another, experts agree that the explosion engine is the prime mover of the future. Steam has apparently been developed almost to its limit. Its rival is but half-grown, though already a giant.

Some internal combustion engines use petroleum as their fuel, converting it into gas before it is mixed with air to form the charge; others use coal-gas drawn from the lighting mains; “poor gas” made in special plants for power purposes; or natural gas issuing from the ground. Natural gas occurs in very large quantities in the United States, where it is conveyed through pipes under pressure for hundreds of miles, and distributed among factories and houses for driving machinery, heating, and cooking. In England and Europe the petroleum engine and coal-gas engine have been most utilised; but of late the employment of smelting-furnace gases--formerly blown into the air and wasted--and of “producer” gas has come into great favour with manufacturers. The latest development is the “suction” gas engine, which makes its own gas by drawing steam and air through glowing fuel during the suction stroke.

We will consider the various types under separate headings devoted

(1) To the oil-fuel engine,

(2) The producer-gas engine and the suction-gas engine,

(3) Blast-furnace gas engines, with reference to the installations used in connection with the last two.

All explosion engines (excepting the very small types employed on motor cycles) have a water-jacket round the cylinders to absorb some of the heat of combustion, which would otherwise render the metal so hot as to make proper lubrication impossible, and also would unduly expand the incoming charge of gas and air before compression. The ideal engine would take in a full charge of cold mixture, which would receive no heat from the walls of the cylinder, and during the explosion would pass no heat through the walls. In other words, the ideal metal for the cylinders would be one absolutely non-receptive of heat. In the absence of this, engineers are obliged to make a compromise, and to keep the cylinder at such a temperature that it can be lubricated fittingly, while not becoming so cold as to absorb too much of the heat of explosion.

OIL ENGINES

These fall into two main classes:--

(a) Those using light, volatile, mineral oils--such as petrol and benzoline--and alcohol, a vegetable product.

(b) Those using heavy oils, such as paraffin oil (kerosene) and the denser constituents of rock-oil left in the stills after the kerosene has been driven off. American petroleum is rich in burning-oil and petrol; Russian in the very heavy residue, called astakti. Given the proper apparatus for vaporisation, mineral oils of any density can be used in the explosion engine.

The first class is so well known as the mover of motor vehicles and boats that we need not linger here on it. It may, however, be remarked that engines using the easily-vaporised oils are not of large powers, since the fuel is too expensive to make them valuable for installations where large units of power are needed. They have been adopted for locomotives on account of their lightness, and the ease with which they can be started. Petrol vaporises at ordinary temperatures, so that air merely passed over the spirit absorbs sufficient vapour to form an explosive mixture. The “jet” carburetter, now generally employed, makes the mixture more positive by atomising the spirit as it passes through a very fine nozzle into the mixing chamber under the suction from the cylinder. On account of their small size spirit engines work at very high speeds as compared with the large oil or gas engine. Thus, while a 2,000 h.p. Körting gas engine develops full power at eighty-five revolutions a minute, the tiny cycle motor must be driven at 2,000 to 3,000 revolutions. Speaking generally, as the size increases the speed decreases.

Of heavy oil engines there are some dozens of well-tried types. They differ in their methods of effecting the following operations.

1. The feeding of the oil fuel to the engine. 2. The conversion of the oil into vapour. 3. The ignition of the charge. 4. The governing of speed.

All these engines have a vaporiser, or chamber wherein the oil is converted into gas by the action of heat. When starting-up the engine, this chamber must be heated by a specially designed lamp, similar in principle to that used by house painters for burning old paint off wood or metal.

Let us now consider the operations enumerated above in some detail.

1. The oil supply. Fuel is transferred from the storage tank to the vaporiser either by the action of gravity through a regulating device to prevent “flooding,” or by means of a small pump, or by the suction of the piston, which lifts the liquid. In some engines the air and gas enter the cylinder through a single valve; in others through separate valves.

2. Vaporisation. As already remarked, the vaporising chamber must be heated to start the engine. When work has begun the lamp may be removed if the engine is so designed that the chamber stores up sufficient heat in its walls from each explosion to vaporise the charge for the next power stroke. The Crossley engine has a lamp continuously burning; the Hornsby-Ackroyd depends upon the storage of heat from explosions in a chamber opening into the cylinder. The best designs are fairly equally divided between the two systems.

3. Ignition of the compressed charge is effected in one of four ways: by bringing the charge, at the end of the compression stroke, into contact with a closed tube projecting from the cylinder and heated outside by a continuously burning lamp; by the heat stored in some part of the combustion chamber (i.e. that portion of the cylinder not swept by the piston); by an electric spark; or by the mere heat of compression. The second and third methods are confined to comparatively few makes; and the Diesel Oil Engine (of which more presently) has a monopoly of the fourth.