Piston Design Secrets

Piston Ring Sealing

At the centre of any internal combustion engine lies it’s beating heart. Pistons travel up and down the bore at immense speeds, sucking and expelling combustion gases in and out of the combustion chamber. The piston has to withstand the immense loads from inertia and gas pressure. But they also have to provide a stable housing for the sealing rings to function correctly. Read on for some of the piston design secrets that we can reveal…

The sealing rings have to stop the combustion gases from escaping from the combustion chamber and into the crankcase. They also must prevent oil from travelling in the opposite direction, from the crankcase into the combustion chamber. So its vital that the piston is designed to keep the rings in place at all times.

Good ring groove design is of course important, but there are a couple of other design tricks that piston manufacturers can employ. These features can use the high gas pressures around the rings to their advantage. Take a look at our Cosworth YB piston for example, and you’ll see a series of shallow grooves above and between the compression ring grooves. What are they, and why are they there?

Cosworth PA2062 Piston Design Features
Piston Accumulator Groove

Accumulator groove

Look closely between the top and second compression ring grooves and you’ll notice a very small groove. This groove is called an accumulator groove, and its job is to help to control the movement of the top ring.

It is inevitable that some combustion gases will escape past the top compression ring. This might be due to oversize ring gaps. Another reason is the unsettling of the top ring when the piston rapidly changes direction at TDC at high engine speeds. To combat this, the small additional volume in the accumulator groove reduces the pressure on the underside of the top ring.

This reduction in pressure is an effective method to reduce or eliminate unwanted ring flutter. This is a phenomenon that can have an adverse effect on engine performance and can potentially lead to high levels of blow-by. (Blow-by is the escape of combustion gases past the ring).

The reduction in pressure in the accumulator groove also has a beneficial reduction in pressure above the second compression ring. Stabilising both compression rings will improve the sealing performance of the rings.

Anti-detonation bands

Another design feature that can help to control the rings are what we call anti-detonation bands (sometimes they are also referred to as contact reduction grooves). The bands are a series of very shallow rectangular grooves above the top ring groove. The volume within these grooves, although small, allows a space for the additional atomisation of the fuel and air mixture.

As the name suggest, they are there to reduce the possibility of detonation around the outer edges of the piston. In doing this, they help to protect the top ring from the disruption of any high-pressure pulse waves from detonation.

These bands can also prevent the build-up of carbon above the compression ring which would otherwise cause the ring to stick in the ring groove. They also have the added benefit of reducing the amount of contact between the top band of the piston and the cylinder bore wall, and so are very beneficial at high engine speeds and piston temperatures.

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Piston Anti-detonation Bands