Over roughly the last two decades, Russia's largest carmaker, the Volga Automobile Plant, widely used so-called "non-interference" pistons in its engines. At first, this informal name became attached to the pistons themselves, and later to the engines as a whole. The reason was the distinctive design of the piston crown: special recesses were made in it. Because of these cutouts, the parts looked quite impressive and gave the impression of a complex high-tech design.
However, it now appears that the company is gradually abandoning this solution. At the very least, the new VAZ-11184 engine installed in the Niva Travel already uses different pistons, without the familiar recesses. Car enthusiasts usually call such pistons "interference" pistons: their crown is completely smooth.
The 11184 engine itself, apparently, is regarded by the plant as a promising development. Its displacement is 1.8 liters, a parameter well suited for passenger cars. Therefore, it is assumed that this engine will later be modernized and used in future LADA models.
How the idea of the "non-interference" piston appeared
The solution with grooves on the piston crown itself was initially hard to call successful. More than twenty years ago, this design first appeared on Lada Priora engines. At that time, it was viewed more as a temporary measure, since Priora itself in that period was accompanied by numerous technical problems.
The quality of engine assembly left much to be desired. In addition, breaks or stretching of the timing belt drive occurred fairly often. In such cases, the pistons at the top of their stroke collided with the valves, leading to deformation of the guides and serious damage to the parts themselves.
It was precisely then, apparently, that the slang expression meaning "to hit" came into use to describe such a collision. Pistons of the previous design began to be called "interference" pistons. The solution to the problem turned out to be fairly obvious: increase the thickness of the piston crown and make special recesses on its surface, valve reliefs, matched exactly to the valve diameter.
Later, when the design was deemed suitable for serial production, the recesses began to be formed directly during part manufacturing. They were made by hot stamping together with the grooves for the piston rings. Thanks to this, when the belt broke, the valves no longer collided with the piston: the valve head simply entered the corresponding relief.
The drawbacks of the "non-interference" solution
Since then, "non-interference" pistons have been installed in most AvtoVAZ engines. However, instead of solving the problem of timing belt quality, the engineers effectively chose a workaround. The piston design itself turned out to be a compromise and ultimately seriously limited the development of new engines.
Theoretically, the problem could have been solved differently, for example by modernizing belt production. Replacing equipment and using higher-quality materials would have made it possible to increase belt life, which at the time was only about 25,000 to 35,000 kilometers, and it was impossible to predict when it would break. An alternative could have been the purchase of modern production equipment for them.
The "non-interference" pistons developed several serious disadvantages. First, they were significantly more expensive to manufacture. It is impossible to form valve reliefs accurately by conventional casting, so the pistons were produced by hot stamping from expensive powder alloys based on nickel, aluminum, and silicon. As a result, the average cost of such a piston, for example for the Lada Priora, was about three times higher than the price of a part obtained by conventional casting into a steel mold.
Second, the combustion chamber geometry was far from optimal. Any reduction in piston height automatically reduced the compression ratio of the fuel-air mixture. Because of this, a new engine lost up to about 7% of its power, and after more than 30,000 kilometers of use, losses could reach 12 to 15%. The cause again was those same valve reliefs.
Coke gradually accumulated on the inner surface of the recesses and continued to smolder even after a fresh mixture was supplied. This led to pre-ignition and detonation. In addition, the recesses themselves, although only a few millimeters deep, noticeably worsened the movement of airflows and exhaust gases.
This geometry complicated the management of intake and exhaust timing, which directly affected fuel consumption and the engine's overall efficiency coefficient.
Why this layout began to be abandoned
Interestingly, a similar period of enthusiasm for "non-interference" solutions also existed among European and Japanese manufacturers. This took place before the mass transition to high-octane gasoline and before the widespread introduction of catalytic converters. However, over time it became clear that such a design was economically and technically disadvantageous.
Later, with the spread of turbocharging, some companies again tried to return to the idea of pistons with recesses. But even then, it turned out that other methods were more effective: using long connecting rods, increasing piston stroke, and employing crankshafts with complex geometry.
It is precisely this approach that is implemented, for example, in the design of the VAZ-11184 engine, where the former "non-interference" scheme has already been abandoned.