PISTON
A piston of an internal combustion engine is in the form of an inverted bucket shape and it is free to slide in the cylinder barrel. The gas tightness is secured by means of flexible piston rings, which are in the grooves of the piston. These grooves are cut in the upper part of the piston. 
A piston of an internal combustion engine serves three functions:
1. It forms a moveable wall of the combustion chamber.
2. It transmits turning force to the crankshaft via the connecting rod.
3. It functions like a crosshead and transmits side thrust, which is due to the angularity of the connecting rods, to the cylinder walls.
The piston must possess the following qualities:
1. It must be strong enough to withstand high pressure caused due to the combustion of fuel.
2. It must be very light in weight to have minimum primary and secondary forces, which are caused due to the inertia forces of the reciprocating masses. A light piston permits higher speed of the crank.
3. The piston material must be a good conductor of heat so that detonation is suppressed, and higher compression ratio is possible to get fuel economy.
It is interesting to know that an engine having a piston and cylinder head of aluminium alloy can be used at a compression ratio of 6.3 and it gives more power and fuel economy than similar engine having a cast iron piston and cylinder head at a compression ratio of 5.3 as shown in Fig. 3.6. This is due to the improved thermal efficiency, which is due to the thermal conductivity of aluminium alloy. Apart from the qualities mentioned previously, the piston must meet the following requirements:
1. The piston operation must not be noisy.
2. The piston must be of less coefficient of expansion.
It has been found by experiments that the maximum temperature produced is in the centre of the piston head, and the temperature decreases towards the edge of the piston head, and also decreases rapidly down side of the piston. Most of the heat is passed into the cylinder block at the ring belt, and some temperature drop takes place from the skirt.
In early years, cast iron pistons were used because of their strength and excellent wearing qualities. However, cast iron has lesser heat conductivity than aluminium alloy and consequently cast iron pistons run hotter than aluminium alloy pistons. Figure 3.7 shows the construction of a piston and the experimental results of piston temperature have been compared between cast iron pistons and aluminium alloy pistons.
In Fig. 3.7, an aluminium alloy piston with a T slot skirt has been shown. 'A' is the head or crown of the piston. In this type of piston, the head grooves are cut to fit the piston rings B'. The piston skirt 'C' functions like a bearing and guiding surface in contact with the cylinder walls. In modern pistons, the length of the skirt is 0.75 to 0.8 times the piston diameter, and the overall length of the piston is from 1.0 to 1.1 times the piston diameter. It is seen that longer skirts do not reduce the rate of cylinder wear. However, piston slap is reduced in pistons with longer skirts because of the effective bearing surface provided. In Fig 3.7, 'D' indicates the bosses inside the skirt. These bosses are for fitting the gudgeon pin 'E' which is across the diameter of the skirt. There is an oil scraper ring 'F which prevents excess oil from reaching the combustion chamber. In some modern pistons the oil ring is provided below the gudgeon pin or the piston pin.
