Automotive engines use different types of pistons. These offer both advantages and disadvantages depending on the application. When choosing one for your vehicle, several factors come into play such as the ability to withstand wear, heat, and impact. That’s what I will be covering in this chapter: the different piston types and their characteristics. For those looking to know how pistons are mode, there’s a section for that at the end of the chapter.
Common Engine Piston Types
The types of pistons today are many and varied. Their variations mostly concern design differences, material, and manufacturing method. Based on these considerations, we have these types of engine pistons and categories.
- Based on head design- Flathead pistons, Dome Pistons, Dish Pistons
- Based on manufacturing method– forged pistons, cast pistons, hypereutectic pistons
- Based on Material- aluminum pistons, cast iron pistons, steel pistons
Piston Types by Head Design
Engine configurations and power levels vary. That means the need for different compression levels. Consequently, piston heads are designed to control combustion and, therefore, the compression ratio. That gives the different types of pistons based on head design as listed below.
The head is a flat shape. As a result, the flame spreads uniformly. The reduced surface area also enhances the combustion process. That, in turn, means a more efficient reaction of the air fuel mixture and higher compression ratio. Flathead pistons are, therefore, known to produce higher reciprocating forces.
The main advantages of flathead piston types include the simple design which makes them less costly to produce. As a result, they’re cheaper to use in an engine. Another good thing about the pistons is their ability to enhance the combustion process to produce more force. They are mostly used in gasoline engines.
These are named so for their raised middle that bubbles to form the shape of a dome. This piston design causes combustion gases to travel a longer distance. At the same time, the large surface area alters the combustion process to make it less efficient.
Dome pistons are needed in engines that do not need to produce a lot of power. In these applications, the reduced compression also helps to protect the engine from damage. Examples of such uses include the two-stroke engines of motorcycles.
As the name suggests, these types of pistons are a dish shape. The center of the head or crown is recessed and the edges curled up. This design helps to increase the volume of the combustion chamber, thereby reducing the compression.
Just like the dome types, dish pistons commonly find application where compression and reciprocating force have to remain low. These include those that use turbo charger or super charger systems. In these boosted engines, higher compression levels can be destructive.
Piston Types by Manufacturing Method
Engine pistons are also classified according to the way they are made or, in other words, the manufacturing process. The methods include forging, casting, and the addition of silicon to harden iron. The types of engine pistons in this category are described below.
Forged pistons are among the most common types. To make them, a piece of metal is pressed into the required shape or design. The resulting grain structure makes the piston possess higher characteristics in terms of structural integrity.
The strength of forged pistons fits applications where the part has to bear higher loads, such as high-performance or heavy duty vehicles. However, these pistons expand more and require larger clearances to prevent seizing at higher temperatures.
These are an alternative to the forged types. Their manufacture involves using molten metal to produce the piston shape. These types of pistons can be aluminum or iron, sometimes even steel depending on the required properties.
Engine pistons made using the casting method are not as robust as those made by forging metal pieces. However, they are still suitable for light engines. In these applications, the mechanical and thermal stress is usually low enough to cause early damage.
Hypereutectic pistons are a type of cast pistons. The only difference is that silicone is added during manufacture. This is normally done in order to make them more robust and wear resistant, which makes them suit conditions that are somewhat harsher.
Hypereutectic pistons may not be as strong as those made using the forging process. However, they possess better qualities than the standard cast types. Their advantages over cast pistons include the ability to stand up to thermal and mechanical stress better.
Piston Types by Material
Piston material is another way to classify the engine part. In that regard, the types of pistons in this category includes aluminum, iron, and steel. The advantages and disadvantages of each material when used are explained below.
These are made from aluminum alloy that comprises copper, manganese, zinc, and other elements. Aluminum alloy pistons are usually either forged or cast from molten metal, with each type offering different properties in terms of wear and sturdiness.
Aluminum pistons offer the advantage of being low weight to fit high-speed engines. They are also best suited for light applications where the piston does not suffer a lot of abuse. As such, these types of pistons are mostly used in gasoline engines.
When compared to the other types, aluminum pistons do not stand up to wear well. They are also less sturdy and the reason why they do not fit heavy duty engines or diesels. That said, modern manufacturing methods are producing alloys that make aluminum pistons better at withstanding wear and mechanical or thermal stress.
Cast Iron Pistons
As the name implies, these types of pistons are made using the casting process. Cast iron pistons possess both good and bad qualities, which makes the fit some engines and not others.
Starting with the good qualities, the pistons show better resistance to wear than the forged types. They also have better thermal characteristics and can operate in high heat environments without risk of damage.
The downsides of cast iron pistons include their weak structure which makes them susceptible to damage by impacts or mechanical stress. They are, therefore, mostly suited for lighter engines than the modern turbo charger or supercharged engine.
How are Pistons Made?
Piston manufacture involves either a forging or casting process. The material can also be or aluminum alloy, among other materials. The forging process is the most used process today, and aluminum the most popular material. Here is how pistons are made using aluminum alloy and the forging method.
- The piston manufacturing process begins with large rods of aluminum. The rods typically measure up to 9 feet long.
- A saw cuts the aluminum rods into small pieces in an automated machine.
- The cut pieces, now called slugs, move through an oven and are heated.
- A punching press is also heated in readiness to receive the slugs. This is the forging machine.
- The slugs are moved from the oven to the press, which then applies a pressure of up to 2 000 tons. This shapes the slug into the shape of the required piston.
- The super-hot pistons are moved out of the press and air cooled for about an hour.
- After cooling, the pistons are once again placed in an oven, this time to strengthen them.
- Next, the pistons are transferred to a lower temperature oven. The low heat serves to stabilize them.
- The pistons are then taken into a lathe machine and excess parts removed. Tiny oil holes are also drilled into the piston assembly at this stage.
- The lathe machine removes material to produce the piston grooves. A large hole is also drilled through the piston. This is the hole that will hold the piston pin.
- The piston is taken to a milling machine which removes some material from the skirt to reduce weight.
- A lathe removes some material from the edges of the crown. This is done to allow for piston expansion without binding.
- Another machine engraves the piston number and other data.
- The piston is finally smoothed out by hand and the wrist hole taken through a lathe to machine it to specifications.
- The last step in the piston manufacturing process involves spraying it with hot and deionized water. This is done to remove oil that may have accumulated in the different products processes. After that the piston is dried in readiness for final touches and packing.
Different types of pistons offer levels of performance and durability. When selecting the type to use, these properties are important. The piston design and material options discussed here should help you make the right decisions. Regardless of the type, though, pistons are prone to damage by the harsh conditions of the combustion chamber. Find out these affect engine pistons next.