HOW IS A KART ENGINE?

Engine is mainly divided into 2 parts: the first one is the head (divided in other 2 parts, the high part which is a kind of coverage and in which the sparking plug is put, and the low one, where there's the cylinder); the second part is the carter, where you can find the crankshaft. These two parts work in a kind of related way one from each other. Let's start analyzing all parts and how they are made.

Sparking plug and the high part: the sparking plug is the element which, thanks to the power provided by the starter coil, causes some sparks responsible for the combustion - the motor fuel is burnt. The high part is made by aluminium (as the whole head) and it's composed by a surface with some small wings (fins) put at a certain distance: these fins increase the surface which has contact with fresh air and help the engine cooling.

The middle part contains the cylinder in which the piston runs. Also this part is covered by fins for the same reasons we said before. The cylinder can be defined as a cast iron pipe with some holes (called lights) used for the exhaust and the transferport; the cylinder is made by cast iron because is very slightly affected by heat, since it has very small thermal expansion: it decreases the risk of seizing; in fact the cause of seizing is not found in the cylinder but in the piston. The piston is made by aluminium which is affected by high thermal expansion (high in comparison with other metals). So why is the piston made by aluminium? The reason is simple, in order of going fast, you need a light piston, able to go up and down with little effort. With a piston made by cast iron probably you couldn't move. Also the piston is kind of pipe, with one side open and the other one closed with a spherical end. Actually it has internally some reinforcement, because it has to be both light and strong. It has also 2 side hole, used to insert the segher/piston pin (blocked by two small rings in both sides) and then is linked to the end of the connecting rod and the connecting rod is linked do the crankshaft from the front part.

Carter contains shaft, oil seal, bearings and ignition. Let's start talking about shaft, which is an axis with cams. At the end of one side the pinion is sticked, and at the end of the other one there's the magneto. This magneto is linked to the coil which brings current to the sparking plug (20.000 V); this is the ignition mechanism. Bearings have the function described in the theoretical card and they are placed where the shaft goes through the carter. Oil seal have the function not to let the petroil going out the carter and at the same time they have to keep a low friction with the spinning shaft, for this reason you can have an oil layer between shaft and the internal part of oil seal.

HOW IT WORKS

Two-stroke kart engine means all phases of the cycle occur in just 2 runs of the piston and there's a combustion every time the piston goes up. Since it's a cycle, there is not a beginning and an end, so we can start wherever we want in order of describing it. Let's define one thing: BDC is the lowest point the piston can reach; TBC is the highest point the piston can reach. Now we can start describing the cycle, considering the piston goes down from the TBC, after combustion. The distance between PMI and PMS is called RUN.

When the piston goes down, it opens exhaust lights (lights are holes put on the cylinder), so combustion waste-products can be expelled (discharged phase) and in the meanwhile from other lights new petroil mixed with fresh air can enter through some holes around the cylinder (decanting phase); at this point there's the washing phase, it means replacing old combusted gases with new petroil. In this phase it's very important new petroil doesn't go to the exhaust lights, for this reason during years several exhaust setting ups have been studied and the research goes on. With two-stroke kart engine there are some wastes and that's a disadvantage of this kind of engine. In the meanwhile the piston started its run to the top point and going up, it closes both exhaust and decanting lights, then it's at the top point and the new petroil is in a suction/intake phase; during its run to the top we are already in this situation, where the depression caused by cylinder and plate swallows petroil from carburetor. Two parts of the engine are well divided by piston belt and for this reason this belt is very important: in case of breakage, the engine would start running very bad and it could be damaged. During suction phase, after its beginning, compression phase starts (when petroil is compressed) then the combustion phase (very fast, when petroil is burnt after the sparking plug explosion) and the expansion phase follows (when exhaust gases go out) and ends when the suction is ended. It's just to underline how long the suction lasts. The cycle is closed.
Here you can find a picture with all phases shown.

Here you can see how phases work, in relation with (BDC and TBI) and rotation point; this scheme is not fixed since suction has a variable length.