It is common to talk about auto start y detonation in the combustion chambers of engines, but not everyone is totally clear about the differences between these two terms. We are going to clear up those doubts and we will also talk about other concepts, such as pitted connecting rod or ignition advance.
The first thing we must review is that in gasoline engines the burning of the air and fuel mixture must originate from a spark jump. This spark is created by the plug several degrees before the piston reaches Top Dead Center (TDC) on the stroke. compression. But of course, not everything happens instantly in practice.
The combustion of the mixture is not instantaneous, but it is carried out as if by layers from the spark plug to the rest of the chamber, it is what is known as flame front. That is why there is a ignition advance several degrees before the piston reaches TDC. The pressure increases considerably and just after reaching TDC, it pushes the piston towards Bottom Dead Center (BDC) on the expansion stroke. In this expansion phase the pressure in the cylinder drops.
For its part, in diesel engines the method is very similar to that of gasoline engines; however, there is no spark plug to create that electric arc that burns the mixture. In these propellants the explosion is created due to high temperatures and compression. That is why they are called compression ignition or self-ignition engines. The compression ratio It is much higher than diesel.
How do modern diesels control the exact moment for fuel combustion? With the high pressure fuel injection. When the piston is close to TDC and all the air is at a very high pressure, the injection system sprays the fuel into the combustion chamber at very high pressure. It immediately comes into combustion and "throws" the piston towards its BDC.
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Auto start
Once we have a basic idea of how "work" originates in gasoline and diesel engines, we can now talk about these phenomena that can occur. As you have seen, the flame is controlled in the engines, be they gasoline or diesel. But there are times when failures occur. We start with self-ignition, which can also be known as surface ignition in some countries.
El auto start occurs when any area, point in the combustion chamber or particle is glowing. It can also be due to some chip, carbon or even the electrode of the spark plug itself being at a very high temperature. At a certain moment, this "hot spot" in the combustion chamber causes the air-fuel mixture to garment before spark jump, starting combustion at an inappropriate time.
Logically can cause breakdown, because the flame and its propagation are not controlled. The sooner this self-ignition occurs with respect to the moment in which the spark should jump under normal conditions, the more damaging it can be for the engine. Not only will performance drop, but it will produce vibrations and pressures that can be critical for the mechanics.
In old carburettor cars it may be the case that we disconnect the ignition key and the engine does not stop due to self-ignition. This is because, in some cases, this type of engine does not cut off the fuel supply when we remove the ignition key, but rather the ignition system. Having a hot spot in the cylinder continues to cause explosions and the engine continues to rotate, thus drawing air and fuel from the carburetor. feeds back, so to speak.
Detonation
Detonation is different from self-ignition, although it is no less true that they are often confused. In this case, the air-fuel mixture does not burn through a hot spot (as in self-ignition), but to reach a higher pressure than it should. As a general rule, when this happens, it happens with very few moments of difference with the spark jump caused by the spark plug.
Thus, in the combustion chamber we have two flame fronts and two shock waves that collide with each other. Fuel burning is uncontrolled and the flame is not expanding as the manufacturer intended. Remember that the entire design of the combustion chamber, including the piston head, is painstakingly created. Likewise, major breakdowns can occur and, of course, the performance of the engine is significantly reduced.
pitted connecting rod
El pitted connecting rod It is the phenomenon that occurs when the ignition of the mixture occurs too soon. If the spark plug produces the spark jump when the engine is not turning at high revolutions and the piston is still far from TDC, the flame front will push the piston back to BDC before it has completed the intake stroke.
Logically, this is very dangerous and can cause the motor to break, since the force of the flame front tries to reverse the rotation of the mechanics and all its inertia. In extreme cases, some connecting rod has been seen bent because it is the one who has absorbed those shocking forces.
To do this, for years all engines have had a connecting rod knock sensor. This sensor is there to detect the frequencies caused by knocking and, when this is the case, it sends the order to the electronic engine management system. Instantaneously varies the ignition advance and fuel injection, avoiding such a costly failure.
In very old cars this sensor does not exist. That is why in these vehicles you can notice the typical cranking sound, which usually appears when driving in a gear that is too high, at low revolutions and accelerating hard. It is a fairly characteristic metallic sound that must be avoided at all costs.
How to avoid self-ignition, detonation and connecting rod knock?
Logically current cars, thanks to electronics, are able to control these situations by themselves. However, not all the cars we have on our streets are modern and, on the other hand, it is always better to do it "naturally", that is, without the electronics having to act. It will always be healthier.
These three phenomena are mainly caused by driving with the engine flooded, by high temperatures and pressures inside the cylinders, in addition to the use of low octane fuel.
Circulating in very high gears and accelerating fully (or almost fully) is not good for the mechanics. It costs nothing to drop a gear or two so that the engine runs at the appropriate revolutions and is in an optimal operating zone. The response will be much better and the mechanics will not suffer, creating healthier explosions in the cylinders. You have to know how to use the gears correctly.
Very important is also the use of proper octane. In a normal vehicle with standard mechanics, using more octane than the manufacturer recommends will not offer great advantages; but it won't cause any problems either. However, we should never refuel with fuel with a lower octane rating than recommended.
The octane number (95 rum, 98 rum, etc.) is the ability of the fuel to ignite on its own. The higher the octane number, the more difficult it is for the mixture to explode without the spark jump caused by the spark plug. Therefore, if we use a fuel with a lower octane number than that recommended by the brand, it is easy for these detonations, self-ignition and pitting of the connecting rod to occur. In the case of electronics correct it, it is likely that we will have a loss of performance.
