Generally speaking, engines have pistons. The piston moves up and down inside a cylinder, where a ring (or group of rings) around the piston guide the movement inside the cylinder. Connected to a rod, which is in turn connected to a crankshaft, a piston's job is to translate the force of combustion into rotation.
Pistons can be found in everything from a 1800's steam locomotive to a modern air compressor, but we're going to focus on pistons found in the modern Internal Combustion Engine (ICE).
Basic Piston Function
In order to harness the energy inside gasoline or diesel fuel, a small amount of this fuel is mixed with air anddetonated inside an engine cylinder.
This explosion occurs in the small space between the top of the piston (the crown) and the top of the cylinder (the cylinder head). This small space is known as the combustion chamber.
Intake and exhaust valves control the amount of air and fuel that enter the chamber, as well as the exhaust gases to exit the chamber once combustion is completed. As the piston up and down in the cylinder, it works in concert with the valves to either a) draw in air and fuel or b) expel exhaust. We refer to the modern engine as a four stroke engine, as it takes four strokes of the piston (two up and two down) to expel exhaust, draw in air and fuel, compress the air and fuel mixture, and then harness the explosive power.
In order to make sure that engines are as efficient as possible, many engine designs have the valves going far into the combustion chamber - so far that the top of the piston must have special indentations to make sure the piston doesn't hit the valves when it reaches the very top of the cylinder. These indentations are called valve reliefs, and they can vary from small indentations to large pockets. There are also flat-top pistons that have no reliefs or special shape, but these are not very common in modern vehicles.
NOTE: Some engine designs have the valves coming so far into the combustion chamber that the piston will definitely hit a valve unless the timing between the valves and the piston is perfect. This is known as a 'zero clearance' or 'interference' engine design. If you have an interference engine, you likely have a very specific maintenance requirement in your owner's manual regarding replacing your vehicle's timing belt. This strict maintenance requirement is due to the fact that a timing problem in your engine will result in valve and/or piston damage.
All pistons also have groves around the barrel of the piston that hold rings, and the rings serve to seal the combustion chamber, help guide the piston in the cylinder, and also lubricate the piston as it moves up and down. The exact position of these ring grooves impacts performance ? if the grooves are closer to the piston crown, the engine will have a higher compression ratio. Of course, placing rings closer to the crown also means they'll be subjected to more heat, which means that the ring groves must be larger to allow for greater thermal expansion, which in turn can effect ring durability.
Position of the ring groves relative to one another is important too. Most pistons have three separate grooves, and the specific spacing of these groves can determine how the piston holds up to wear and tear, how well it is lubricated, and more.
The shape of the piston crown can also effect compression. If the crown is shaped to provide as little clearance as possible, compression will increase. However, too much compression is a bad thing, as it can lead to detonation (pre-ignition of the air-fuel mixture), and detonation can lead to damage to the piston, rings, valves, and more.
Additionally, many engines benefit when the piston crown is dish-shaped, which increases the size of the combustion chamber. While this will drop compression ratio (and therefore reduce thermodynamic efficiency), the reduction in compression ratio can be compensated for by improved efficiency in combustion. Or not ? too much dish hurts just as much as too little dish.
Piston skirt length effects performance too. Deeper skirts provide better cooling and quiet operation, but also can increase weight and friction. Shorter skirts, on the other hand, are generally louder at cold start and also increase operating temperatures, which can increase ring wear and increase the risk of detonation.
Finally, the wristpin ? which connects the piston to the rod and bears the brunt of the load that's placed upon the piston during the power stroke ? is an important component as well. If the piston is allowed to float on the wristpin, performance is generally improved, but the cost of this type of piston is typically higher than interference fit wristpins, which don't allow the piston to move as freely.
As you can see, the design of a piston can be changed in a variety of ways that all effect performance.
1. Piston weight helps to determine how quickly an engine can accelerate. Lightweight pistons boost acceleration rates and generally generate less friction, but they also can reduce low-end torque. Heavier pistons don't accelerate as rapidly, but they can provide more power at lower RPMs, so piston weight is at least partially determined by intended vehicle use.
2. Piston materials can vary pretty dramatically too. While a piston can be made from any strong metal, most modern pistons are made from aluminum. However, some pistons use aluminum that is mixed with a high percentage of silicon, while others use pistons that have almost no silicon.
3. Silicon content effects expansion and contraction as well as durability. Pistons with a high percentage of silicon don't expand or contract nearly as much as pistons that don't contain silicon, which means they operate quietly at start up and don't need to warm-up to reach peak efficiency. However, pistons that contain almost no silicon are stronger and perform better at higher temperatures, so racing engine designers almost always go with pistons that contain almost no silicon and live with the loud clicking noise (sometimes called piston slap) which is heard at start-up because the pistons haven't expanded enough to completely fill the cylinder.
Speaking of heating and cooling, pistons are not technically round ? they are very slightly elliptical. This slightly out of round shape is intentional, as a piston is expected to expand at least a little bit during operation. By using an elliptical shape, the natural expansion of the piston can be harnessed to put the piston into round.
A Note About Piston Slap: 'Piston slap' refers to the sound of a piston slapping into the side of a cylinder wall, and generally speaking piston slap is bad because it can lead to cylinder wall scuffing, ring wear, increased oil use, etc.
Many brand-new vehicles use short-skirt piston designs in concert with very high engine tolerances, and the designers are trying to minimize friction loses while maximizing efficiency. However, a consequence of these design choices is that some new engines make quite a bit of noise during cold start-up. This noise is caused by a very small discrepancy between cylinder bore diameter and piston-ring diameter. Commonly (and incorrectly) referred to as piston slap, this noise at cold start is in fact just a normal consequence of using a short-skirt piston in a cylinder with a very small margin for error.
Forged vs Cast Pistons
Finally, no discussion of piston design would be complete without discussing the difference between cast and forged pistons.
Cast pistons are manufactured using a standard casting process, where molten aluminum alloy is poured into a mold and allowed to cool. Because this cooling process can sometimes be uneven, and because uneven cooling can lead to weakness in the aluminum, cast pistons must be poured a little thicker than necessary to ensure strength.
Forged pistons, on the other hand, are manufactured by stamping heated aluminum in a die to create a blank, and then machining that blank to create a complete piston. Since forged pistons are stamped, they tend to be stronger and more uniform than cast aluminum. This means that a forged piston can be made a little thinner than a cast piston without compromising strength.
However, having said this, the operational difference between a quality cast aluminum piston (which is the standard for almost all new vehicles) and a quality forged aluminum piston isn't dramatic. Forged pistons are advantageous in racing applications where weight is more important, but generally don't offer a huge improvement over cast pistons in most typical applications. In fact, cast pistons tend to be more carefully engineered and manufactured than forged pistons, as the casting process requires a very high level of precision.
If you found this article interesting, be sure to read our article about Eutectic, Hypoeutectic, and Hypereutectic Pistons too.