Compression

This article deals with the design, operation and maintenance of air-cooled, four-stroke engines. These engines are available for operation on petrol or vaporizing oil and are being used for driving stationary and mobile farm machinery. We examine these engines under five head­ings: 1) compression, 2) lubrication, 3) fuel, 4) ignition, 5) cooling.

Compression. Compression is the basis of engine power, and the components used to obtain it are crankcase and crankshaft; connecting-rod, piston and rings; cylinder, cylinder head and gaskets; valves and tappets. The crankshaft is carried by (1) two main bearings mounted in the crankcase. The piston carries two or three compression rings and one oil scraper ring; pressure rings should be renewed if the gap measures (2) 0.030 in. or more

The compression ratio is relationship between the volume of the fuel and air mixture (combustible mixture) in the cylinder when the piston is at the bottom and the volume of the same combustible mixture when the piston is at the top of the stroke. If the volume of combustible mixture in a cylinder when the piston is at the bottom of the stroke is 72 cubic inches, and the mixture is com­pressed to a volume of 12 cubic inches when the piston is at the top of the stroke, the compression ratio is 6 to 1. The significance of the compression ratio of an engine can be fully understood if one remembers that the amount of pressure on the piston determines the power of the engine. Other things being equal (3), the greater the pressure at the beginning of the power stroke, the greater will be the power developed.

Compression ratio is directly related to the kind of fuel that can be used without knocking. As the compression ratio goes up, the tendency to knock increases.

NOTES:

1) is carried by — опирается на

(2) if the gap measures — если зазор составляет

(3) other things being equal — при прочих равных условиях

1. Study the following words:

compression, lubrication, fuel, ignition, cooling, crankcase crankcase, crankshaft, bearings, rings, scraper, ratio, significance, tappets, connecting-rod, gaskets, amount, pressure, mounted

2. Complete the sentences with appropriate words from the text:

a) ….. is the basis of engine power.

b) … is carried by two main bearings mounted in the crankcase.

c) The … carries two or three compression rings.

3. Define the part of speech of the following words:

Equal, ignition, cooling, crankcase, crankshaft, bearings, rings, scraper, ratio, significance, tappets, connecting-rod, gaskets, amount, pressure, mounted, relationship

4. Exercises in translation:

1. In the four-stroke "cycle" employed in most engines, pressure is created in the cylinder by the burning of a compressed charge of petrol-vapour and air once in every four strokes of the piston. 2. The valves can open and close openings in the top of the cylinder. 3. If the piston is on the down-stroke, pressure in the cylinder will drive the piston downward and assist the turning of the fly­wheel. 4. If the piston is on the up-stroke, pressure in the cylinder will oppose any rising of the piston and turning of the flywheel. 5. If these instructions are followed, the art of tightening the cylinder head will then lie in know­ing when to stop. 6. If a broken valve spring is found on an overhead valve engine, it can often be renewed without removing the cylinder head.

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