Engine pistons vary in many ways, and the right piston type depends on several factors. Usually, one piston type will have superior qualities over the other but mostly in certain applications. We compiled a description of different automotive piston design, material, and manufacturing method. Use the information to help you make a wise decision, especially if you are considering an engine rebuild.
First, about the working of a piston in a car engine.
An engine piston reciprocates in a cylinder and transfers the movements via a connecting rod to the crankshaft. The crankshaft rotates and turns the wheels of the vehicle. To facilitate a smooth back and forth movement, the piston in car engines comprises various parts.
Piston parts include the piston head or crown, piston skirt, connecting rod, and smaller components, such as bolts, bearings, and piston rings. These parts have a role to play in the functioning of the piston and the larger engine.
The piston head has an influence on the combustion process and, therefore, overall engine performance. It forms the combustion chamber floor, which means several things. The piston head will determine the quality of intake air swirl or turbulence, flame dispersion, and fuel distribution.
Automotive piston manufacturers make these auto parts in different head designs. Each design has its best attributes. The downsides, too. To give you an idea of these variations, here are the piston type names based on the design of the piston head.
Piston Type According to the Head Design
As the name suggests, this piston type features a flat top. The design enables the piston to achieve one of the most efficient combustion processes. With the head flat, the flame spreads evenly, and fuel burns more effectively.
Because of the reduced surface area and uniform combustion, the flat top piston creates a high amount of reciprocating force. These piston types are easy to make. It reduces the piston price but also the amount to rebuild or purchase an engine.
This piston type is also known as bowl piston. It features a plate-like shape with raised outer edges. Because of the increased combustion chamber volume, dish pistons achieve a lower compression ratio. Although a drawback, this characteristic is an advantage in some situations where high reciprocating forces are not necessary.
Dish pistons are often used in engines with turbo charged or supercharged combustion. They help to prevent knock or detonation that would be caused by the boosted compression. In some engines, pistons help to contain the fuel spray, helping to enhance the combustion process. In older engines, the process will be influenced by several factors: piston type, carburetor design, and the volume of the cylinder bore.
In terms of design, this piston type is the opposite of the dish piston. As its name implies, the piston assumes the shape of a raised center. This increases the surface area of the head significantly. The combustion gases have to travel further. As a result, dome pistons are known to create a poorly performing combustion chambers with insufficient burning. This lowers the compression ratio.
The reduced compression may be a disadvantage of the dome piston. However, it is a requirement in some engines. It limits the amount of force that the pistons can produce and protects the engine. In other words, the engine only attains the amount of power it can sustain. In a modern engine, using the dome piston type, metering device action, and other car systems can help improve fuel economy.
Piston Type By Material
Classification can also be based on piston material. When selecting a piston, the type of material dictates many of its performance characteristics. The qualities include thermal conductivity and expansion rates, wear resistance, ability to withstand heat and stress, durability, and more.
Piston manufacturers use different metals are used to make connecting rods and pistons. The main ones cast iron and aluminum alloy. Although cast iron pistons are still in production today, aluminum alloy dominates the automotive piston market. Let us see a description of each piston type according to material.
Cast Iron Pistons
Cast iron is a type of piston material that contains 2% or more carbon. Variants of this material include white cast iron, gray cast iron, ductile cast iron, and ductile cast iron. A piston made of iron expands less, usually at the rate of the cylinder inside which it reciprocates.
The low expansion rate helps avoid piston slap. It also prevents loss of compression that would result from changes in the piston clearance. A downside of iron pistons is their higher weight for both piston and crankshaft, which increases the inertia of the reciprocating mass. For this reason, cast iron pistons are only ideal for low RPM engines.
Aluminum Alloy Pistons
The aluminum alloy used by piston manufacturers contains several elements (copper, zinc, manganese, and others) and aluminum as the main material. Aluminum is a lightweight metal. When used to make pistons, it helps to reduce inertial forces. This makes the material suitable for high RPM engines.
In addition to the weight savings, the aluminum alloy piston type has a higher thermal conductivity than iron. The material offers efficient conduction of heat between piston and cylinder. This is an advantage given the high heat levels of the combustion chamber that leads to the formation of carbon. Aluminum pistons also experience less temperature variation within the assembly, especially between the head of the piston and ring area.
Piston Type By Manufacturing Method
Automotive piston manufacturers use different methods to make these auto parts. They include casting, forging, and the hypereutectic process. The methods produce pistons of different qualities to suit different applications.
Forged pistons are made by using presses to hammer a chunk of metal into the shape of a piston. Although labor-intensive, forging produces stronger pistons than the other methods. This is due to the resulting grain structure.
But forged pistons expand and contract at a higher rate, which makes them require more piston and cylinder bore clearance. The pistons are often used in heavy-duty applications where there is a lot of load to be borne by the piston assembly.
These piston types are made by pouring liquid metal into molds. Cast pistons are not as strong as the forged types, but can still hold when the engine application is mild. They have better wear and thermal characteristics than the forged types. As a result tighter piston-cylinder clearances are possible. It is for this reason that iron is used to make both piston and piston ring parts.
Cast pistons have their drawbacks. They are only recommended for low-power engines. It is not advisable to use them on boosted engines such as those that incorporate turbo charger or supercharger. Besides, modern science has produced aluminum alloys with excellent characteristics. Older cars may still as cast iron pistons since they are usually mild engine vehicles.
Made using the casting method, hypereutectic pistons have silicon added during manufacture. The addition of silicon material improves wear resistance, thermal resistance, and other qualities. As a result, this piston type is stronger than the usual cast piston, and more durable.
Hypereutectic pistons are not as sturdy as the forged type, though. If subjected to extreme loads and forces, they will exhibit poor ductility and break. The pistons are suitable for engines that are mostly street use, and occasionally high speed and rough situations.
Automotive pistons can take many forms from different materials, designs, to methods used to produce them. The piston type used produces different characteristics, and to some degree, influences the engine output. When rebuilding an engine, you would need to choose the right piston type, or you will not get the best results. This article should help you decide from an informed viewpoint.