The mystery of a single «Chinese»

9 октября, 2021 от arto Выкл

The automotive expert is simply obliged to be curious. At least, so that in the «ordinary» breakdown in the stream, do not miss the very «not ordinary», the occurrence of which cannot be explained by one «error in operation». Expert almost a detective. The meticulous, persistent and pedantic Sherlock Holmes in search of the present cause of this breakdown.

We offer the material of Alexander Khrulev, Forensic Expert, Candidate of Technical Sciences, Motor Center «AB-Engineering»

All defects (breakdowns) can be divided into two groups — operational and factory (manufacturing). The most common is, of course, operational.

Factory defects, on the contrary, are very rare. When the car is produced by large series on automated production, the probability of error is very low. Nevertheless, from time to time, mistakes happen. The emergence of such a defect is probabilistic in nature — as items neither make it nor check, and some of them («with a marriage») has a chance to slip into assembly. However, despite the greatest rarity, it is factory defects that put the most difficult tasks before the expert.

This will be discussed in this series of almost detective stories.

… This defect was a combination of production and structural factors. In cases where the defect is integrated, either fails to uniquely indicate a purely production cause, they speak about a constructive-manufacturing defect.

The object is a practically new car of one of the Chinese automakers, with a mileage of about 9 thousand km. Completely made, with a modern motor — a good design, almost «Toyotovskaya». In general, the complaints of the quality of manufacture do not break. But the engine failed. Outsided sounds appeared, chimney, oil consumption has increased dramatically.

The owner came to the dealer, began to explore — and found: the problem is in the skirts of the pistons. They turned out to be strong. Such cases are known — they often cause «oil starvation». And who is responsible for such a «starvation»? Of course, the owner, in time, did not follow the oil level in the engine.

Everything would be simple if not one «but». The car did not even have time to go through that on the service. Yes, and by itself a new one. And the increased consumption of oil appeared only at the very last moment, and before that — nothing like that.

The second interesting point — only the skirts of the pistons and cylinders were injured. Inserts were in good condition. So, «oil starvation» was obtained by some selective.

The cause of such a selective defect could be fuel if it was poured into the cylinders by the river and washed the oil from cylinders. But checking nozzles did not give any results: they worked normally. The car diagnosis also showed that everything is fine. And in general, the car itself came to the service — which already practically eliminates the fuel injection of the engine.

By the way, the oil in the engine was still the original, the first «factory» fill — it did not change, because the time did not come true. But which oil was flooded into the engine at the factory — it is not known, it is necessary to request.

Our Holmes sent a request. It turned out, the «All-season» oil, the viscosity of 10W-40 by SAE. Well, oil and oil — commonly used. In general, nothing special.

Then the expert went to another way: when was the car bought and when did the breakage happen? — It turned out that it was bought in November, and the fault was manifested in January. And where was the machine exploited? — In Siberia. And frosts there what were at this time? — Yes, about -35 ° C.

Holmes already guessed, what is the attenuation. In any case, close to it.

Remember: 10W-40 oil with such cold does not apply! It works fine at temperatures up to -20 ° C.

And the deduction method suggested that there should be something else. Because if the engine fails, there are usually several reasons that «converge» in one place and at one time. Each of them would not lead to disastrous consequences. But together, they make the effect of synergies — and the engine breaks down.

We began to look, from which material is the piston, and from what is the coating of his skirt. One of the main components of aluminum «piston» alloys — silicon. It not only prevents thermal expansion when heated, but also provides a low coefficient of friction pair of «cast iron-aluminum». Without silicon, the details slide «do not want», and for normal operation it should be it in alloy 11-13%. And in the alloy of «Chinese» pistons of silicon turned out to be less than 8%.

Farther. Modern pistons are coated, most often containing antifriction graphite. The coating with graphite sharply reduces friction, and the piston can work for a certain time even without oil. Graphite prevents the piston from the appearance of scaling, for example, with heavy launches. However, in the study it turned out that there is no graphite piston in the coating. The coating was similar to «solid anodizing» — in fact, on ceramics.

Ah yeah manufacturer! Low silicon aluminum piston — too soft. In order to increase the hardness, the manufacturer may have decided to cover it with ceramics. And even though she protects the piston, but does not have good antifriction properties of cast iron. If there is a lubricant, such an engine would work normally and warmly. But not at -35 ° C.

In the cold, the oil of 10W-40, naturally thickened, his feed found it difficult, and the splashing and stopped at all. The coverage of the piston is inherently antifriction. So the skirts of the pistons were shared — and the engine failed.

Here it is a randering: at one time and in one place there are four factors at once — soft piston material, it is too solid coating, thick oil and severe frosts. Remove something one thing — and the danger could be avoided. For example, if at the factory, we took into account the geography of the region, where cars are sent, but another oil would be flooded, then it would be quite possible, nothing would have happened. Pistons would work without an accident.

And in fact? The owner of the car received a new motor from the importer.

Thanks to Sherlock Holmes.

(to be continued)

Help: Specialized Motor Center «AB-Engineering»: Mechanical processing of parts, repair of engines and fault examination.