“Fingers” knock during acceleration: the main reasons. Troubleshooting Rules

Any extraneous noise in the car’s engine often causes owners to be wary. And even if these sounds do not affect driving performance, their very appearance makes the driver think about diagnostics. On many cars, “fingers” knock during acceleration, but this problem is often ignored. A sound appears when the car picks up speed. If you do not pay attention to it in time, much more serious problems arise. At the same time, motorists cannot independently determine the causes of knocking, as well as fix this problem. Let's look at the causes of these unpleasant sounds to the driver’s ear, and also learn how to fix these problems with the motor.

Finger knock

From an engine that is running under load, loud metallic sounds can be heard. They disappear at a certain speed. Old-school mechanics will say that “fingers” knock on acceleration. However, the driver will be surprised, and will be completely right: the sound has nothing to do with the “fingers” installed in the pistons.

The nature of this knock is of a different nature. It is caused by detonation. Sometimes, for certain reasons, fuel can burn incorrectly. The blast wave in the combustion chamber is reflected from the piston and from the cylinder walls. At the same time, those same sonorous metal knocks are created in which specialists hear the sound of “fingers”.

Why fingers?

The process of burning a combustible mixture in a fully functional engine is sequential. Near the spark plug, a flame flares up, and gradually it fills the entire cylinder. But there is another burning option - detonation. The explosion of the fuel mixture in the combustion chamber occurs abruptly. This increases the pressure and temperature. This very explosion is called detonation. That's why the driver hears a knock - it comes from a blast wave. Proper combustion implies a speed of fire propagation up to 30 m / s. The gas pressure rises gradually. With such combustion, the flame fills the cylinder gradually. Gases press the piston gently. There are no gas knocks on the walls of the combustion chambers, since there is no explosion. If the burning rate is greater, then this is the prerequisite for detonation. By the way, this phenomenon is very harmful for the engine.

Knock - what is it?

If the “fingers” knock during acceleration, this indicates detonation in the engine. Such is called the instant and very destructive in its force explosion of any flammable substances after the impact or operation of the detonator. This definition is based on Ushakov’s dictionary. Detonation of combustible substances for car engines is a quick burnout of a mixture of gasoline and air. It occurs when the engine runs under load at low speeds and low-quality fuel. This process is accompanied by knocks, vibration, temperature increase. As a result, “fingers” knock during acceleration (including VAZ-2112).

Why does knocking occur?

The octane number of fuel is an indicator that characterizes the coefficient of resistance of a combustible liquid to ignition during compression. In other words, it is detonation resistance.

Naturally, motors, where the compression ratio is quite high, need fuel with a high octane rating. Any modern engine has a high compression ratio. If low-octane gasoline is poured into it , this significantly increases the risk of detonation. Glow ignition is an independent combustion of the fuel mixture in the cylinders. One of the reasons for this phenomenon is unburned soot or high temperatures in the combustion chamber. Another reason why the fingers “knock” in the engine during acceleration is the poor fuel mixture. If you increase the amount of air in relation to the volume of fuel, this causes detonation. A mixture that is too lean at the moment it hits the cylinder will cause detonation with a higher probability than normal. Also, a similar effect occurs at high loads. “Fingers” knock on acceleration precisely because of power unit overloads. If you start moving in third gear instead of first, not only a ringing may appear, but also a characteristic metal clang.

More about the reasons for knocking

When the car picks up speed, this is a stressful situation for the engine. Especially if you need to disperse a car suddenly. When the driver squeezes the accelerator pedal to the floor for a sharp set of revolutions, for example from one to six thousand, then the driver will hear the “fingers” knock during acceleration (“Priora” is no exception).

It's quite normal. To quickly pick up speed, the electronics delivers more fuel with the same amount of air to the cylinders, which will definitely lead to detonation. But the situation is also possible at smooth starts. The driver will hear a characteristic knock. This phenomenon is not normal with a smooth set of speed. In these cases, it is necessary to quickly identify and eliminate the cause. This will help to avoid trouble.

Typical causes of ringing "fingers" during normal operation of the internal combustion engine

If the “fingers” knock during acceleration in the Kalina, the DMRV may have failed. If it does not work correctly, then the computer will receive incorrect information and issue incorrect commands. Another reason is the incorrectly set ignition timing. For this reason, the point at which the fuel will burn as much as possible is near TDC. This leads to increased pressure in the combustion chamber. If the “fingers” knock during acceleration at the Ford Focus, then the knock damping sensor may have failed. You should definitely check this item. If it stops working, it should be replaced.

Poor fuel is the cause of all the troubles that happen to cars. This has already been described in detail above. It should be noted that finger knocking is not always a problem that has formed in the process of using the car. There are situations when the machine comes from the factory with incorrectly connected sensors. In the end, this leads to knocking and knocking. This problem is especially dangerous, because the engine is running in and detonation is especially harmful to it. It should be excluded.

Effects

Knocking can cause permanent damage to the engine. This burnout and other valve damage, breakdown of the piston rings.

At this moment, the engine experiences enormous thermal and mechanical stresses. The piston edges melt, the jumpers between the rings break. Also goes to the connecting rod bearings.

How to avoid detonation?

Without exception, all modern cars are equipped with a special sensor and unit that react and suppress this destructive phenomenon.

As soon as detonation occurs, the sensor membrane detects stresses, the magnitude of which depends on the frequency and the amplitude of the blast wave in the cylinder. The executive system receives a signal from the sensor and changes the algorithm of the ignition system. If you have “knocking” fingers during acceleration (“Tavria”), the causes of this phenomenon may be in gasoline, carburetor adjustment, and ignition angle. The easiest way to avoid detonation is to prevent premature ignition. You can also increase engine speed. When driving, it is recommended to increase the speed smoothly.If you need a sharp start, then experts recommend untwisting the motor before starting, and then start moving. Detonation can be reduced by selecting the glowing number of candles. In this case, it is recommended to use hotter candles. They will burn the entire fuel mixture without residue, and there will be no turbulence.

Conclusion

Detonation always occurs unexpectedly and can greatly scare car owners. When your fingers knock, you should use these recommendations, the reasons are described in detail. If the problem cannot be solved on your own, then you should seek help from the service station.