The piston is a component of the internal combustion engine. Together with the connecting rod and crankshaft, it forms the crank mechanism of the engine. The main function of the piston is to transform the pressure generated by the burning air-fuel mixture into force, acting on the crankshaft. There are also secondary functions fulfilled by the piston:
- contributes to heat dissipation generated during combustion
- ensures the sealing of the combustion chamber, preventing gas leakages from it and oil penetration into the combustion chamber
- guides the movement of the connecting rod
- ensures to the continuous change of gases in the combustion chamber
- generates the variable volume in the combustion chamber
The piston is the component of the internal combustion engine (ICE) which has to sustain the most mechanical and thermal stress. Due to the piston, the power of the ICE is limited. In case of very high thermal or mechanical stress, the piston is the first component to fail (compared to engine block, valves, cylinder head). This is because the piston must be a compromise between mass and mechanical and thermal stress resistance.
The geometry of the piston is constrained due to the cubic capacity of the ICE. Therefore, the main way to increase the mechanical and thermal resistance of the piston is by increasing its mass. This is not recommended because a piston with high mass, has high inertia which translates in high dynamic forces, especially during high engine speed. The resistance of the piston can be improved by geometry optimization but there will be always a compromise between mass and mechanical and thermal resistance.
At a first look, the piston seems a simple component, but its geometry is quite complex:
- bowl diameter
- piston crown
- combustion chamber (bowl)
- piston crown edge
- piston top land
- compression ring groove
- ring land
- groove base
- recessed ring land
- groove sides
- oil scraper ring groove
- oil return bore
- piston pin boss
- retention for groove distance
- groove for retainer ring
- piston boss distance
- piston boss distance
- stepped edge
- piston diameter 90 °C against the piston pin bore
- piston pin bore
- bowl depth
- ring zone
- piston compression height
- piston length
- oil cooler duct
- ring carrier
- bolt bush
- diameter measuring window
- crown camber
As you can see there are significant differences between diesel and gasoline pistons. Diesel pistons must withstand higher pressures and temperatures, therefore they are bigger, bulkier and heavier. They can be manufactured from aluminium alloys, steel or a combination of both. Gasoline engine pistons are lighter, designed for higher engine speeds. They are manufactured from aluminium alloys.
The piston crown comes in direct contact with the burning gases, within the combustion chamber, so it’s exposed to high thermal and mechanical stress. Depending on the type of the engine (diesel or gasoline) and the type of fuel injection (direct or indirect injection), the piston crown can be flat or can contain a bowl.
After the piston crown comes to ring zone. Most of the pistons have three ring grooves, in which piston rings are mounted. The top ring is called the compression ring, the middle on is the scraper ring and the bottom one is the oil control ring.
The piston skirt keeps the piston balanced inside the cylinder. It is usually covered with a low friction material to reduce the friction losses. The piston pin bore is hosting the piston pin, which connects the piston to the connecting rod.
The combustion process has different characteristics for diesel and gasoline ICE. In a diesel engine, the peak gas pressure, during combustion, can reach 150 – 160 bar. In a gasoline engine, the maximum pressure is below 100 bar. Because of higher pressure, diesel pistons have to withstand higher mechanical stress.
In order to work without any failure in such harsh conditions, diesel engine pistons are designed heavier, sturdier with more mass. The drawback is higher inertia, higher dynamic forces so lower maximum engine speed. One reason for which diesel engines have lower maximum speed (approx. 4500 rpm), compared to gasoline engines (approx. 6500 rpm), is heavier mechanical components (pistons, connecting rods, crankshaft, etc.).
The heat generated during combustion is partially absorbed by the piston. Most of the heat is transferred through the ring area of the piston (around 70%). The piston skirt evacuates 25% of the heat and the rest is transferred further to the piston pin, connecting rod and oil. Higher engine speed means higher piston temperature. This happens because the accumulated heat doesn’t have time to dissipate between two consecutive combustion cycles. In the same time, higher engine load means higher piston temperature, because there is more air-fuel mixture burning, which generates more heat.
The piston temperature can be lowered by circulating oil through the piston mid region. This can be accomplished by using oil jet devices mounted on the engine block, which inject engine oil, through an orifice, when the piston is close to bottom dead center (BDC).
There are several advanced piston technologies, each having the purpose of increasing the mechanical and/or thermal resistance, lowering the friction coefficient or lowering the overall mass (keeping in the same time the mechanical and thermal properties).
Below you can find examples of modern pistons, manufactured by Kolbenschmidt, each with distinctive technologies.
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