Pistons are one of the hardest working components of an internal combustion engine, where temperatures inside cylinders can reach over 1,800 degrees F. Early pistons were of cast iron, but there were obvious benefits for engine balancing if a lighter alloy could be used. To produce pistons that could survive engine combustion temperatures, it was necessary to develop new alloys such as Y alloy and hiduminium, specifically for use as pistons. A few early gas engines had double-acting cylinders, but otherwise effectively all internal combustion engine pistons are single-acting.
In general, a piston is a lubricated sliding shaft that is contained by a cylinder and is made gas-tight by piston rings. Its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. The following diagram shows the components of a typical piston:
In further detail, an internal combustion engine is acted upon by the pressure of the expanding combustion gases in the combustion chamber space at the top of the cylinder. This force then acts downwards through the connecting rod and onto the crankshaft. The connecting rod is attached to the piston by a swiveling wrist pin. This pin is mounted within the piston.
The pin itself is of hardened steel and is fixed in the piston, but free to move in the connecting rod. A few designs use a “fully floating” design that is loose in both components. All pins must be prevented from moving sideways and the ends of the pin digging into the cylinder wall, usually by circlips.
Gas sealing is achieved by the use of piston rings. These are a number of narrow iron rings, fitted loosely into grooves in the piston, just below the crown. The rings are split at a point in the rim, allowing them to press against the cylinder with a light spring pressure. Two types of ring are used: the upper rings have solid faces and provide gas sealing; lower rings have narrow edges and a U-shaped profile, to act as oil scrapers. There are many proprietary and detail design features associated with piston rings res associated with piston rings.
Pistons are cast from aluminum alloys. For better strength and fatigue life, some racing pistons may be forged instead. Billet pistons are also used in racing engines because they do not rely on the size and architecture of available forgings, allowing for last-minute design changes. Although not commonly visible to the naked eye, pistons themselves are designed with a certain level of ovality and profile taper, meaning they are not perfectly round, and their diameter is larger near the bottom of the skirt than at the crown.
A compression ratio is exactly what it sounds like- a ratio where you’re compressing the maximum cylinder volume into the minimum cylinder volume. That’s the volume of the cylinder when a piston is all the way down compared to all the way up to the top. It’s written out and said as a ratio. As compression ratio increases, the piston moves higher in the bore at top dead center, hence there is additional force for the expansion stroke (additional force for the same amount of fuel equals higher efficiency). The basic point of this all is that a higher compression ratio means that the engine is getting more work out of the same amount of fuel. That’s good for power and also miles per gallon.
In production gasoline engines from the past 20 years, compression ratios are typically between 8∶1 and 12∶1. There is a limit of how high you can go with compression. Anything over 14.5:1 compression ratio would run very hot and risk auto-ignition. This could shoot out a rod or spin a bearing. This is what’s casually referred to as “blowing up.” Metal simply cannot withstand such high levels of stress. Several production engines have used higher compression ratios, including:
- Cars built from 1955–1972 which were designed for high-octane leaded gasoline, which allowed compression ratios up to 13∶1.
- Some Mazda SkyActiv engines released since 2012 have compression ratios up to 14.0∶1. The SkyActiv engine achieves this compression ratio with ordinary unleaded gasoline through improved scavenging of exhaust gases (which ensures cylinder temperature is as low as possible before the intake stroke), in addition to direct injection.
- The 2014 Ferrari 458 Speciale also has a compression ratio of 14.0∶1.
Low compression prevents detonation in boosted engines. Basically it's all about the pressure in the cylinder. An engine without forced induction (supercharger or turbocharger) has to generate more cylinder pressure by using the piston to compress the charge after the intake valve closes. The forced induction engine forces more air in while the intake valve is still open, so the piston does not have to compress the charge as much. The measured compression ratio is all about the pressure created by the piston motion. Too much cylinder pressure results in issues with street octane fuel since it can create knock, which are really rapid, high magnitude pressure pulses in the cylinder that can destroy pistons, bearings, etc.
When a turbocharger or supercharger is used (e.g., 6.2 Hellcat, Redeye or Demon), the compression ratio is often lower than naturally aspirated engines (e.g., 3.6, 5.7 or 6.4). This is due to the turbocharger/supercharger already having compressed the air before it enters the cylinders. Engines using port fuel-injection typically run lower boost pressures and/or compression ratios than direct injected engines because port fuel injection causes the air/fuel mixture to be heated together, leading to detonation. Conversely, directly injected engines can run higher boost because heated air will not detonate without a fuel being present.
Compression ratios can be increased or decreased by the shape of the piston head. A domed piston will result in greater compression. Here are two examples of a flat head and a domed piston:
Flat Head Low Compression Piston
Domed High Compression Piston
Here are the compression ratios for the modern-day Challenger engines:
3.5L= 9.9:1 (87 octane)
3.6L= 10.1:1 (87 octane)
5.7L= 10.5:1 (89 octane)
6.1L= 10.3:1 (91-93 octane)
6.2L= 9.5:1 (87 octane), supercharged
6.4L= 10.9:1 (91-93 octane)