Knocking in the engine: causes and consequences

Every motorist occasionally has to hear knocking while the engine is running. What causes these sounds and can you determine by hearing which part is out of order?

Common causes

As a rule, a knock on the engine with normal lubrication and cooling occurs due to the fact that the gap between the parts in the joint increases. Usually, this sound is clearly audible when the gap becomes approximately three times the nominal size. As the clearance increases, the wear rate of the engine parts increases, and the knocking becomes louder.

Sometimes there is a situation where a knock in the engine occurs and with a normal gap size. In this case, it can be caused by such reasons as large loads on the engine, skewing and jamming one of the parts, reducing the viscosity of the oil due to overheating or dilution with another liquid (for example, when fuel enters the oil).

Determine by sound

We will make a reservation at once - to define knocking in the engine on hearing at times very uneasy even to the motorist with the experience. But the general laws still exist.

If the knock appears regularly, it is worthwhile to test the engine in different modes. As is known, the parameters of the engine operating mode are the crankshaft speed and load, which increases as the throttle valve is opened . If, with an increase in the engine speed, the sound amplifies, then, most likely, problems should be sought in the piston group and the gas distribution mechanism.

As the load increases, the pressure in the cylinders increases and, accordingly, the load on the moving parts of the engine increases, primarily the piston group. Therefore, in the presence of defects in the crank mechanism, the knock in the engine increases as the load increases.

Now let's try to determine the "knocking" detail according to the key of the knock. If, while the engine is running at idle, you hear a thud, which then, as the number of turns increases, becomes louder and more often, it crushes the crankshaft in which the crankshaft bearings or crankshaft bearings have worn out. The reason may be an untimely change of oil or oil filter.

A dead knock in the engine "on the cold", that is, when starting a cold engine, can indicate the wear of the camshaft bearings. As a rule, such a knock lasts not more than a couple of seconds, until oil falls on the bearings. If the knock then disappears, then with such a camshaft you can still drive up to 50,000 km. Later, the knock becomes more audible even when a warm engine is started, and neglect to the engine leads to increased fuel consumption, deterioration of the starting qualities of the power unit, and deposits on the valve seat. The reason for the wear of the camshaft bearings may be poor oil quality, bearing operation in the absence of oil or its absence, oil or impurities entering the oil.

A slight metallic knock in the engine, which occurs when the revolutions increase, can indicate detonation. This is the name of the phenomenon in which fuel burns hundreds of times faster than under normal conditions. When detonation occurs shock wave, which during the movement of the piston down tens of times is reflected from the piston and cylinder head, while causing the same knock. In addition to knocking, detonation can be indicated by a drop in engine power, shaking of the unit during operation, overheating of the cylinder head. The reason for the detonation is the use of low-octane fuel, the formation of carbon deposits in the combustion chamber, the failure of the exhaust gas recirculation valve. The result of detonation is the damage to the pistons and piston rings.