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5.1.8. Compression

The compression shows a condition of the engine. At ignition of fuel-air mix in the combustion chamber there is very high pressure. It leads to big loads of pistons, rings, walls of cylinders, and also on the valve and their saddle. Damage of laying of a head of the block leads to increase in a consumption of oil and fuel, bad production of the fulfilled gases and, respectively, smoking, falling of power, difficulties at start of the cold engine.
   Nominal size of a compression
The size of a compression is various for different types of engines. Usually at the heated-up motor it makes from 26 to 32 bars, a minimum – 18 bars.
The difference should not exceed 3 bars. If the size of a compression differs more, than on 3 bars, at different cylinders, then the reason of it should be looked for in the following:
1. Wear of pistons or piston rings.
2. Sticking of rings in pistons.
3. Wear of the cylinder (often consequence of sticking of piston rings).
4. A deposit on valves and saddles.
5. The damaged valve.
6. The burned-through valve (at too small gap).
   Measurement of a compression
You can independently measure a compression in the presence of a kompressometr. You need the assistant who will scroll the engine while you do measurements. For exact measurement of a compression the valvate mechanism has to be adjusted. On STOA there is a device of measurement of losses of pressure. With its help it is possible to find a detail which is responsible for loss of pressure.

1. Warm up the engine up to the working temperature.
2. Lift a cowl.

Fig. 4.22. Arrangement of a cover of nozzles: 1 – highway; 2 – cover of nozzles

3. Remove a branch pipe of the soaking-up highway (fig. 4.22).
4. Uncover nozzles.
5. Turn out nozzles.
6. Put the car on the emergency brake, and the gear shifting lever — in neutral situation.
7. Do not turn the engine the ignition key, otherwise in the engine fuel will move that it can lead to damage of the measuring device. Use recording компрессометр, the having contact switch to which the engine connects electrically.

Fig. 4.23. Measurement of a compression: 1 – nozzles; 2 – компрессометр; 3 – pipeline of high pressure

8. Screw компрессометр in the corresponding carving part forkamer (fig. 4.23).
9. Uncover the control unit, disconnect contact "50" and connect компрессометр.
10. Uncover with "+" on the left internal arch of a motor compartment.
11. Connect a connecting cable of a kompressometr to "+" (contact "30").
12. Ask the assistant to press an accelerator pedal against the stop and to hold it pressed throughout all measurement.
13. Turn the engine a starter, pressing the contact switch of a kompressometr (about 5 c).
14. Connect компрессометр to the following cylinder.
15. Check one by one all cylinders and compare the written-down values to nominal sizes.

Detection of malfunctions
1. Dig a small amount of oil in an opening of a nozzle and then once again measure a compression.
2. If the received values too low, it can be the certificate that valves, saddles of valves, valvate channels, a head of cylinders or laying of a head of the block of cylinders are damaged.
3. If you received the best results, then the reason should be looked for in piston rings or in cylinders. Oil for a short time improves consolidation of pistons and walls of cylinders and therefore compressed air leaves with a big resistance (in comparison with the previous option).

Leak of compressed air during the operation of the engine
If during the operation of the motor leak of compressed air around a motor compartment is observed, then the reasons can be the following:
1. Inlet pipeline or damper: the inlet valve is faulty.
2. When opening a radiator or a broad tank: laying of a head of cylinders, a crack in a head of cylinders is faulty.
3. When opening a mouth of filling of oil or at the taken-out oil probe: walls of cylinders, pistons or piston rings are worn-out.
4. "Shots" in the muffler: the final valve is faulty.

   Test for loss of pressure
Along with a compression often check loss of pressure for STOA. For carrying out measurements the engine has to be heated-up, all nozzles — are turned out, and the casing of the soaking-up highway, a cover of a maslozalivny mouth and a cover of a radiator — are open. For measurement the piston of the checked cylinder has to be in the top dead point. The tester is connected to a nest of a nozzle of the corresponding cylinder and the compressor. The scale of the manometer is calibrated before air is given to the combustion chamber. If in the combustion chamber there is a loss of pressure, then it is shown as a percentage on a device scale. The general loss of pressure should not exceed 25%, and on valves and consolidation of a head of cylinders loss should not be less than 10%, and on piston rings – 20%. It is possible to establish by listening whether there is an air through the soaking-up highway (the delivery pipeline), a final collector, a cover of the gulf of oil, the cooling system or via the camera of the next cylinder.