THE PISTON

TKART Staff

04 July 2016

It is one of the engines most highly stressed components, an essential part that permits the transformation of pressure in the combustion chamber into movement of the crank mechanism

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

The piston is an (almost perfectly) cylindrical component that, in the case of 2T racing kart engines, is made of aluminium alloy. The upper part, known as the crown, usually has a slightly convex shape, but can vary in some cases.
Just below the crown is a key part which increases the strength of the piston’s seal: the piston ring or segment. Shaped like a ring that is open on one side, it is preformed so that the diameter is greater than the piston and can adapt to a smaller diameter, thereby pressing on the cylinder wall.
The section of the piston ring can be rectangular or L-shaped. The latter tends to regulate the opening and closing of the ports more effectively and create a better seal, which is why it is used more often on non-shifter engines such as the OK and the KF, which require high levels of thrust even at low rpm. On the other hand, the rectangular section

is more prevalent on KZ engines, since it reduces friction and is more reliable.
Theoretically, the piston, which acts as a plunger in the cylinder, performs several functions. Firstly, it seals the gasses which, as a consequence of the mixture’s combustion, reach very high pressures in the combustion chamber. Furthermore, in two-stroke engines, it replaces the intake and exhaust valves and its alternating movement regulates the opening and closing of the transfer and exhaust ports.
Other functions include “discharging” much of the heat from the combustion chamber onto the cylinder walls, as well as sliding along the cylinder liner with the least possible friction.
It is evident that the stresses that this component is subjected to are considerable: the design and structure of the piston are therefore essential both in terms of performance and reliability.

THE PISTON PERFORMS VARIOUS FUNCTIONS: AS A SEAL; TO CONTROL PORTS; TO TRANSMIT HEAT AND SLIDE UP AND DOWN IN THE CYLINDER

The law that returns the position of the piston based on the ratio of the crank radius to the connecting rod length, and the degrees of crankshaft rotation

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

PISTON

PISTON RING

PISTON PIN

1 di 3

The piston is not a perfect cylinder, but has a slightly truncated cone shape, with the narrowest part at the top. In fact, this is the area that reaches the highest temperature during operation and, therefore, expands the most. It is the same reason why the piston is also slightly elliptical, with a minor axis along the piston rod where, inside the piston, there is more material and, therefore, greater expansion when the temperature rises.
To ensure consistent performance that does not vary excessively with the rise in temperature, which in the combustion chamber, depending on the engine rpm, timing advance, load on the engine and external temperature, varies from 350° C to 400° C, the piston is made of aluminium alloy with high silicon content (18%, usually) which reduces expansion.

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

It is produced casting or hot pressing, with the latter being more expensive, but offering better results and used on racing karts. The piston surface area is made by turning and has minimum tolerances.
Another key component is the elastic ring, which provides a greater seal for flue gasses, but also increases friction. In high performance and high rpm engines, there is typically only one ring; in other cases there are two. The ring also helps to dissipate heat: it is estimated that its firm contact with the cylinder wall transfers 40% to 70% of the heat inside the piston.
Cast iron is used to manufacture such a mechanically resistant component with good heat transmission properties. Nowadays, however, alloy steel rings are under development.

Other elements of the piston include: the piston pin, which is inserted in two side holes in the piston, connecting it to the connecting rod, and the two elastic rings that fasten it in place. The ends of the rings may be folded 90 degrees to create a tab that facilitates assembly. However, this is not the case for two-stroke competition engines, where there is no tab to avoid it being subjected to stresses, breaking and damaging the engine.
A roller bearing is mounted on the piston pin and, just like the internal surface of the big end, its external surface is polished and hardened by surface tempering treatments and the use of nitrided or carburised steel. The lubrication of the cage is optimised thanks to holes on the small end.

DIFFERENT PISTONS

Engines by: Tecniengines, Iame, TM, Modena Engines

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