TOPICS ON MECHANICS

GAS ENGINES

A gas engine is an internal combustion engine which uses blast furnace gas, producer gas, natural gas and others as fuel.

As it was a German engineer, Dr. Otto who, in 1876, built the first gas engine, his name is used to call a special cycle of operations. '-Thus the Otto cycle, or four cycle, is a four-stroke cycle for internal combustion engines that consists of induction stroke, compression stroke, power stroke and exhaust stroke, two revolutions of the crankshaft being necessary to complete the cycle of events.

Modern gas motors are of single, twin and multicylinder type.

The cylinder is equipped with two valves which are alternately opened at the right moment by a cam operated by a camshaft.

In the suction stroke, the piston starts its downward movement in the cylinder and by the action of a cam, the inlet valve opens slightly before top dead centre. The mixture is drawn into the cylinder from the carburettor and the inlet valve closes after the piston has completed its downward stroke.

In the compression stroke, the piston ascends the cylinder and as the inlet and outlet valves are closed, the mixture is compressed within the cylinder. As the piston gets to the end of the upstroke and the gases are wholly compressed, the spark is produced in the cylinder igniting the charge.

The resulting explosion from this charge makes the gases expand very quickly, the pressure rises, and the gases act with great force on the top of the piston driving it down. This is the power stroke.

In the last stroke - exhaust stroke - the piston travels up the cylinder and the burnt gases are forced out through the open exhaust valve into the exhaust system. When the piston reaches the end of its upstroke, the exhaust valve closes and the cycle is complete to start again.

The first practical gas engine was built in 1860 by a Frenchman named Lenoir, but as it was largely improved by Dr. Otto, his name was given to its cycle of operations.

He made his first gas engine in the year 1876. It consisted of a single cylinder, a small combustion chamber and two ports to let the gas in and expel the products of combustion. A water jacket surrounded the cylinder keeping it cool. A piston, moving within the cylinder was connected to a simple crank and shaft by a connectingrod. The forward motion thus imparted to the piston was turned into a rotary movement by the crank. A heavy flywheel kept the movement going and compressed the charge. The latter was ignited by a hot tube. After the explosion took place the exhaust port served as outlet for the burnt gases.

DIESEL ENGINES

A Diesel engine, or compression-ignition engine, is an internal combustion engine which requires no electric spark to ignite the charge in the cylinder. It has neither carburettor nor ignition system and the fuel is injected in the form of a very fine spray, by means of a nozzle, into the combustion chamber where it is ignited by the heat of compression of the air with which the chamber is already charged.

A four-stroke Diesel engine resembles a gasoline engine as it works on the four-stroke cycle, that is: admission, compression, power and exhaust.

In the moment the piston gets down on the admission stroke, the depression created conveys a charge of air into the cylinder through the inlet valve which opens just before top dead centre.

Once the piston has passed the bottom dead centre and is beginning to ascend, the admission valve closes and the movement of the piston compresses the air charge in the cylinder causing a quick rise of temperature.

Before the second stroke is over, the charge of fuel oil is gradually injected into the cylinder by an injector.

The burning of the air-fuel charge makes the gases expand; they push the piston downwards and produce the power stroke.

Before the piston has got to bottom dead centre, the exhaust valve opens and, as the piston goes up again, the burnt gases are forced out through the exhaust pipes and silencer.

Just before top dead centre the inlet valve opens and the cycle begins again.

On account of the high compression required to ignite the charge, the fuel must be injected at a very high pressure to penetrate the dense volume of air, and therefore a special type of injection pump is used.

A small amount of fuel is poured through a nozzle inserted in the combustion chamber. The fuel is raised from the principal tank by means of a pump and is taken by a pipe-line passing through some filters to the injection system.

The injector consists of a main body with a nozzle at one end. A spring needle keeps the nozzle closed. When the pressure from the injection pump is enough, the needle valve is lifted off its place and through very small holes or through an annular space, the fuel injection takes place.

Diesel engines are widely used nowadays in heavy industry, electric generating plants, marine and traction services, motor-trucks, airplanes and so on.

THE MOTOR-CAR

The motor-car, which marks the beginning of the speedy age in which we live, is possibly one of the most complicated machines that are mass produced.

Before a car begins to take shape, the different parts must be collected ready for assembly, each one being built by a specialist in his own type of work.

The main parts of a motor-car are: the engine, chassis frame, radiator, bumper, mudguards, head-lamps, petrol pump, steeringwheel, gear-box, clutch and brake pedals, accelerator pedals, gear lever, springs, petrol-tank, propeller shaft, hand brake, and so on.

In mass-production factories, a mechanical conveyor system is used for carrying the parts from one department to another.

Various metals are used in the foundry ranging from aluminum alloys for gear-boxes and other cases to fine steels for crankshafts. Samples of the metals are subject to test before the castings are made as a metal may be able to resist stretching or crushing but not sudden blows, that is, it may be strong but brittle..

Complete engine units that are either of a new design or contain new parts , are run under-all kinds of abnormal conditions on experimental test benches equipped with numberless special devices which record various data such as the horse-power developed, the number of revolutions of the engine per minute and so on.

The work of the engine begins in the foundry where such parts as crankcases, cylinder blocks, cylinder heads, pistons, connecting-rods, axle casings and the shells of gear boxes are made.

The exterior accessories such as the ignition coil and air cleaner are added during assembly work. Valves are among the small but important parts that must be made before the engine assembly can be completed.

The assembly begins with the crankshaft by which the reciprocating or up-and-down motion of the piston is turned into a rotary or round and round motion. The crankshaft revolves in main bearings which are in two semicircular pieces, each being carried in each half of the crankcase.

If the engine is fitted with side valves the camshaft will fit into the crankcase. The rotation of the camshaft causes the cams in their correct order to lift the tappets by which the valves are opened for a given time according to the design of the cams.

After fitting certain small items, the lower part of the crankcase can be attached. Then follow the timing pinions and their case, the next step being to bolt the cylinder head into position. There are still some important outer items - dynamo, carburetor and sparking-plugs - to be fixed.

The clutch mechanism is keyed to the rear end of the crankshaft and then, encased in its aluminium alloy casing. At the rear of the clutch mechanism, the gear-box completes the engine unit. The controls such as the gear lever and clutch pedal may be carried on the engine unit or fitted to other parts of the chassis and linked with control arms on the engine unit.

Once the engine is completed, a number of adjustment operations is required before the engine can receive its running test.

The earliest predecessor of the motor-car is believed to have been a steampropelled vehicle built about 1770 by Nicholas Cugnot but modern motor-cars did not come into being until after the patenting of Daimler's internal combustion engines in 1887. In 1893, Henry Ford finished his first engine and some ten years after his work was in the required conditions to be offered to the public. He became the king of the motor-car industry and held the ideal that everybody ought to have a car so as to satisfy the necessity of overcoming time and distance.

Henry Ford, the promoter of the motor-car industry, was born near Detroit on the 30th of July of 1863. He was the son of a farmer and his natural liking for mechanics led him to think of new methods for making the work on the farm lighter.

In 1893, when he was thirty years old, he finished his first engine but devoted his next ten years in perfecting it. By 1980 some 20.000.000 cars had come out from the Ford plant.

<< Previous - Next >>

Home

<< Previous - Next >>