Basic Knowledge About Piston

1. What are the different structural forms of pistons? What are the different cooling methods?

Answer: The body of a diesel engine piston consists of a piston head and a piston skirt. The head mainly bears mechanical and thermal stress; the skirt plays different roles depending on the type of diesel engine. In cylindrical pistons, the skirt bears the lateral pressure of combustion pressure and acts as a piston guide.

Currently, the most common cylindrical pistons generally have the piston head and skirt cast as one piece. For some low-speed, high-power diesel engines, the piston head and skirt are manufactured separately and then assembled together with bolts. The head is usually made of high-quality heat-resistant material. 

The length of the piston skirt depends on the type of diesel engine. Since the guiding task of the crosshead diesel engine piston is undertaken by the slider and guide plate, the skirt length of this type of piston is generally shorter.

 In some two-stroke crosshead diesel engines, a longer skirt is retained, mainly to control the opening and closing of the cylinder liner's air ports, preventing both the intake and exhaust ports from opening when the piston reaches top dead center, thus avoiding the waste of scavenging air.

The piston crown is one of the components of the combustion chamber. Therefore, the piston top must be properly cooled.

Based on the properties of the piston coolant, piston cooling methods can be divided into two types: oil cooling and water cooling.

Based on the coolant supply mechanism, cooling methods can be divided into three types: injection cooling, sleeve cooling, and hinged cooling.

1. Injection Cooling: The most common type utilizes pressurized lubricating oil from the diesel engine's lubrication system. 

This oil passes through the main bearings and connecting rod bearings, and then through small holes at the connecting rod's center bore and small end, spraying onto the inner surface of the piston top. After absorbing some of the heat from the piston top, the oil splashes into the crankcase oil sump. This injection cooling method is commonly used in four-stroke diesel engines.

Some diesel engines with high performance parameters often employ the following methods:

(1) Installing a dedicated lubricating oil nozzle inside the crankcase to spray lubricating oil onto the inner surface of the piston for cooling;

(2) Installing an oil supply sleeve on the top of the connecting rod small end, allowing lubricating oil to flow through the connecting rod bore and then through this sleeve for spray cooling;

(3) Installing a serpentine cooling pipe channel in the high-temperature zone of the piston top, with lubricating oil flowing through the pipe for cooling.

These three methods are commonly seen in medium- and high-speed diesel engine structures. Among them, the serpentine pipe structure has only recently been adopted in high-speed, high-power diesel engines, and it has a better cooling effect.

2. Sleeve-type: This cooling method is used for piston top cooling in low-speed, high-horsepower diesel enginesWater or oil can be used as the coolant, but when using water cooling, all moving parts in the mechanism should maintain good sealing and should be isolated from the crankcase to prevent coolant leakage into the crankcase, diluting the lubricating oil and damaging the engine parts. 

The sleeve-type cooling mechanism consists of an inlet pipe and an outlet pipe as an assembly. This assembly is installed at the bottom of each piston and is supplied with circulating water by an external independent cooling water pump (or oil pump), forming a separate piston cooling water (oil) system.

Both the inlet and outlet pipe assemblies extend into the top cooling space inside the piston. Typically, a nozzle is installed at the outlet of the inlet pipe to spray water (oil) onto the inner surface of the piston top for cooling. 

The inlet of the outlet pipe should be higher than the outlet of the inlet pipe to maintain a certain water (oil) level in the cooling space.

3. Hinge Type: The hinge type cooling mechanism consists of a complete set of pipes, a hollow shaft, bearings, and hinges. The oil supplied by the cooling oil pump passes through this set of parts and enters the top cooling space inside the piston, absorbing some heat before flowing back to the oil sump through a hinge mechanism.

This cooling mechanism is more complex and requires higher precision. Besides good sealing, it also requires that each mechanism can swing flexibly.

Therefore, hinged cooling mechanisms can only use oil cooling, not water cooling. This structure is no longer used in high-speed diesel engines. 

Modern large, low-speed engines all use oil cooling, with oil pipes drilled inside the piston rod, forming oil inlet and outlet passages inside and outside the pipes.

2. What are the signs and causes of piston crown erosion? What should be considered in management?

Answer: Piston crown erosion manifests as the piston crown layer gradually thinning and peeling away, exhibiting pitting. The pitting varies in depth and size and is unevenly distributed. In severe cases, it can even burn through.

The main reason is that the piston crown surface is difficult to cool sufficiently, making it the hottest part of the combustion chamber, easily reaching the temperature that causes vanadium corrosion, resulting in high-temperature corrosion.

 Especially when carbon deposits form on the inner surface of the piston crown, or scale forms on the cooling water, or when carbon deposits form on the piston crown due to poor combustion, or when operating under prolonged overload, the piston crown will overheat, leading to severe erosion.

This erosion phenomenon is exacerbated by increasing the cylinder diameter, boost pressure, and heat load of the diesel engine. Especially when using heavy fuel oil, the corrosive effect of combustion products accelerates the development of piston crown erosion.

Avoiding piston crown erosion inevitably involves fuel quality issues, thus affecting economic matters, and cannot be easily resolved. Therefore, only by ensuring efficient combustion in the diesel engine through proper management and regularly removing carbon deposits from the piston crown and scale from the cooling space to ensure adequate cooling of the piston crown can the occurrence of the aforementioned accidents be reduced. 

In older engines, the shape of the erosion corresponds to the position of the fuel injection holes, indicating that atomization quality is the main cause of erosion. Severe erosion requires repair at a workshop.