What is Engine Detonation and How to Fix It

All about engine detonation: what is dangerous and how to avoid it.

Of all the possible symptoms of malfunctions, detonation is one of the most dangerous phenomena in the engine. What it is and how to protect the main unit of the car.

What is Engine Detonation

Engine detonation is a process in which the fuel and air mixture in the cylinder begins to spontaneously ignite uncontrollably. Detonation can occur both during the piston compression process (when it is just going up) and at its top position, almost simultaneously with the moment the spark plug ignites the mixture.

As a result of these microexplosions (actually, detonations), powerful shock waves arise in the cylinder, which have a detrimental effect on the parts, especially pistons and connecting rods.

Note that this article will examine issues of detonation in a gasoline engine. In diesel, combustion processes proceed according to a slightly different algorithm, and detonation in the classic “gasoline” sense almost never occurs.

How Does Detonation Differ From the Normal Ignition of a Mixture?

For a clearer understanding, it is worth giving some explanations of how detonation differs from the normal combustion of a mixture of gasoline and air.

• Moment of occurrence. Detonation almost always either precedes or occurs in parallel with the start of combustion from the spark plug. If detonation is early, it appears on the compression stroke. The resulting microexplosions, due to the expansion of gases, actually prevent the piston from moving upward. If detonation is late (the most common option), it occurs closer to the highest point of the piston movement. In this case, the detonation wave goes towards the normal combustion front, which moves from the spark plug. This interferes with the calculated ignition process and creates powerful shock and thermal loads in the combustion chamber. Figuratively, but very clearly, this option is shown in the picture below.

• Place of origin. The point of normal combustion of the air-fuel mixture is always the same: in the area of ​​the spark on the spark plug. From this point, the flame front consistently spreads across the chamber, and then the working gases relatively smoothly push the piston down. During detonation, local explosions can occur almost anywhere in the cylinder, and most often this phenomenon is formed at the end of the compression stroke, at the point farthest from the spark plug.

• Speed ​​of propagation. The combustion process of the fuel-air mixture (the so-called combustion front) calculated by the designers is relatively slow and occurs at a speed below the speed of sound. Whereas the waves from detonation microexplosions propagate at supersonic speeds. With such a spontaneous process, the piston simply does not keep up with the sharp expansion of gases – an effect of impact on it and, at the same time, on the walls of the cylinder chamber with all other parts occurs. This is also accompanied by a sharp increase in temperature relative to the calculated one.

Causes of Engine Detonation

Without going into the depths of theory, there are only two main causes of detonation: the wrong brand of gasoline and a malfunction of the engine itself. Let’s look at them in more detail.

• Too low octane number of gasoline. What this characteristic is and how important it is for normal engine operation, we discussed in a separate article. Now we will simply remind you that the number after the designation “AI” at the gas station directly affects the probability of detonation. The higher the value, the higher the anti-knock resistance of gasoline and the lower the chance of “catch detonation”.

• Engine malfunction. Among the classic causes of detonation, we can note such malfunctions as incorrect ignition timing, incorrect glow plug number and fuel system malfunction. Also, the risk of detonation is affected by the presence of significant deposits in the combustion chamber or inefficient operation of the cooling system. In the first case, the compression ratio increases (which the engine is not designed for), and in the second – obviously, the temperature increases, which also contributes to spontaneous combustion of the mixture.

• The ignition advance angle (IAA) is the moment the spark plug discharges. It changes depending on the load and speed. The glow number of the spark plug is a characteristic that determines the spark plug’s resistance to overheating.

The sum of the factors resulting from tuning. This can be either incorrectly performed chip tuning in your own garage, or “hardware” changes for which the correct control program was not selected: installation of a non-standard piston, replacement of a turbocharger, camshafts, etc.

How to Identify Engine Detonation

The phenomenon described is almost always audible and is characterized by sharp knocking sounds. However, very often real detonation (which occurs quite rarely on serviceable cars with correct settings) is confused with similar symptoms of other malfunctions.

To the point that sometimes the knock of hydraulic compensators, piston pins and even the characteristic clatter of the exhaust tract in some operating modes are taken for detonation. For an accurate determination, the following methods are used.

• Computer diagnostics. Any modern engine can accurately determine the presence and moment of detonation. Moreover, it is not at all necessary that the control unit will immediately issue a “check engine“. If you look into the log (event archive) stored in memory, it turns out that over the past year, any car, one way or another, has almost certainly encountered detonation. It’s just that these moments are rare and very short-lived in the vast majority of cases. Then the electronic “brain” records the failure in memory, but if the incident does not repeat itself within a few minutes (or hours), it does not raise the alarm.

• Acoustic diagnostics. An “old-fashioned” method, the accuracy of which depends entirely on the qualifications of the examiner and, in general, leaves much to be desired. A car stethoscope is used for determination, which, by analogy with a medical device, is applied to the engine. However, given that many modern “masters” confuse the chirping of injectors with the operation of the adsorber, and the noise of bearings is taken for chain wear, you should not rely on such a check. Moreover, detonation almost never occurs at idle speed, and in motion, for obvious reasons, it is extremely difficult to perform an acoustic check.

How do Electronics Combat Detonation?

As mentioned above, one-off periodic incidents are not something out of the ordinary – all other things being equal, “detonation” is successfully corrected by the electronic control unit. When the ECU detects detonation (this happens in a split second), the classic algorithm is to increase the fuel supply, set an earlier ignition timing (the so-called angle rollback) and reduce the turbocharging pressure if there is one.

Here it is important to focus on the fuel supply – many people mistakenly believe that when detonating, it is necessary to inject less fuel so that “it does not burn so much.” This is not true. The fact is that one of the catalysts of the process under discussion is precisely a lean mixture – that is, a combination where there is a lot of air and little gasoline. In this case, the temperature in the combustion chamber increases, as does the risk of detonation.

If the mixture is rich (little air, a lot of gasoline), then the “extra” gasoline, evaporating in the combustion chamber, cools both the mixture and the walls of the chamber. Thereby reducing the likelihood of chaotic spontaneous combustion.

Consequences

Detonation is one of the most destructive modes of operation of the internal combustion engine. Only an operation with severe oil starvation or critical overheating can “compete” with it. In all three cases, the time during which the engine can become unusable is measured in minutes.

If for some reason it is not possible to prevent detonation using standard means, and it continues for several minutes, the consequences are very sad. First of all, the connecting rod and piston group suffers. Strong shock loads in the cylinder (you can look at the illustration with the fist again) at high speeds can simply bend the connecting rod. At the same time, the extremely high temperature contributes to the melting of the piston, valves and spark plug.

The cylinder head gasket and even the walls between the cylinders can also burn out. Of course, not all of this will necessarily happen at once, but even one consequence will be enough to disassemble the engine. In general, everything usually ends with a major overhaul, and sometimes even that is impossible.

How to Eliminate Engine Detonation

In short: it is not detonation that needs to be fought, but its causes. First of all, make sure that the gasoline being filled not only formally corresponds to the required grade, but is also of the proper quality. Detonation often stops with a banal change of gas station from the “closest” to a normal one.

Secondly, if the engine is loaded with tuning, you need to be sure that it is done correctly, with a thorough check in all modes and specifically for your engine. That is, carried out with the so-called “rollback”. Unfortunately, not all tuning studios go this way.

Finally, if both of the above options do not raise any questions, all that remains is a thorough diagnosis. It is always important to remember that detonation is not a mystical phenomenon that occurs on its own, but only a consequence, the causes of which must be eliminated as soon as possible.