Chapter 3
The Gas Engine
A petroleum motor car consists of three parts - first, the carriage, consisting of the underframe, steerage, and wheels; second, the engine, with possibly its water jacket and electrical firing gear, and sundry accessories for starting and stopping; and third, the transmission gear for communicating motion to the driving wheels and altering the speed.
In a steam engine the piston is pushed backwards and forwards by the force of steam, a comparatively light flywheel giving sufficient impetus to the crank to carry it over the dead point, i.e., the two points at each end of the stroke when the connecting rod and crank are in one straight line.
In the gas engine it is different. It may be mentioned that a petroleum engine or light oil engine is simply a gas engine, and would work equally well, or nearly so, with coal gas.
In the gas engine the pressure of the explosion is on one side of the piston only, the other side being open to atmospheric pressure, but as the gas or petroleum vapour is not under pressure as steam is, it must be by some means introduced into the cylinder. Many attempts were made in past years to pump or force the gas into the cylinder. All these attempts more or less failed; at least they are superseded by what is known as the Otto cycle, so called after Dr. Otto, of Deutz, who was the first to make a really practical and economical gas engine, in the Otto engine the piston and cylinder act alternately as a pump and a motor. In the first out stroke gas and air are sucked in; in the next stroke or in-stroke they are compressed. At the end of the stroke the mixture is fired either by a hot tube or by an electric spark. The explosion drives out the piston and does the work. The next stroke, an in-stroke, the products of combustion are exhausted to the atmosphere.
This will be better understood by the diagram fig. 5, which represents the four different strokes. Two valves will be noticed, the upper one the suction for gas and air, the lower one the exhaust, to allow the burnt gases to escape.
To open the air and gas valves and the exhaust valve at the proper time, a smaller shaft gears with the crankshaft, making one revolution to two of the crankshaft. On this second shaft there are projections technically called cams, which open the valves at the proper moment. In some engines, however, the air and gas valves open by atmospheric pressure only, but in all cases the exhaust valve must be opened and closed mechanically at the proper time.
This is the principle of the Otto gas engine. Thousands of these are working in England, and they are to be found all over the world.
If the reader will take the trouble to spend a quarter of an hour watching a gas engine at work, and will question the attendant as to the use of various parts, he will soon understand its working, and get a better practical knowledge of it than by simply reading a printed description of the motor.
Of course, to carry coal gas to work the engine would be impracticable, even if it were compressed in cylinders, such as are used for producing the oxyhydrogen light in the optical lantern. It is necessary to find some substitute for gas. Explosives were suggested, and it is quite possible that in the future lyddite may be used for motor cars, instead of being fired at ones enemies. But the light hydrocarbons make an excellent substitute for gas. Gasoline is one of the lightest products of petroleum distillation, and this has been largely used for carburetting air for house lighting, both in America and England, but in the latter country the cost as compared with coal gas is very much greater, and the so-called air gas machines have for this and sundry other reasons (chiefly the introduction of electric light) almost gone out of use. Gasoline which has a specific gravity of .66 (water being 1) is used for motor car work in America, but in Europe a slightly heavier product termed in the trade “petrol,” or “motor car spirit,” is used. This has a specific gravity of .68. Benzoline, which can also be used when petrol is not obtainable, has a specific gravity of .72, while lamp oil, such as paraffin, or kerosene, has a specific gravity of .8, and is only used in heavy oil engines, and at present there are only one or two cars on the market that use this heavy oil, the chief objections to its use being the smell and smoke and the necessary preliminary heating up of the vaporiser. With the lighter oils, gasoline and petrol, which give off a vapour at ordinary temperatures, the motor can be started at once - a great advantage in everyday work.
To make the vapour for the engine (for it must be understood that petrol vapour is not a gas, but air charged with vapour), the petrol is introduced into a carburetter, where the air is “carburetted”, or literally charged with carbon. This air is then explosive, and is thus sucked into the cylinder.
Small petrol or gas engines must, in order to obtain a fair proportion of power for their size and weight, run at a high speed; 600 to 1,000 revolutions per minute are common speeds, but as the speed of the road or driving wheels of a car is very much less than this, there must be some means of gearing down the road wheels, just the reverse of the action of a man’s legs with a bicycle, a man’s action on the cycle being most effective at a slow speed (sixty to eighty per minute), so to get a fair speed the cycle is geared up.
A three-foot wheel, a diameter often used on light cars, makes ninety-three revolutions a minute at ten miles an hour, the engine at the same time running 600 or 800 revolutions. This reduction of speed is done by gearing, by cog wheels, or belts, or chains, but the gas and petrol engines lose power and efficiency at low speed, so that when a hill is encountered and more power required, the speed of the engine must be increased by altering the ratio of gearing between the engine and the road wheel. There are many methods of doing this. In the Benz the two speeds are obtained by using two belts on pulleys of different diameters, one belt giving the fast speed of twelve or fourteen miles per hour, and the other the slow speed of something like half this speed (four and a half or five miles an hour) for hill-climbing and starting.
In the Daimler, Decauville, and other cars the change of speed is obtained by gearing, cogwheels of different diameter being thrown into gear by levers and clutches; both methods have their advantages and disadvantages.
There are several varieties of carburetters, but these will be described later.