A piston accumulator groove is one way that the sealing of the piston rings can be improved. But why is ring sealing so important?
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.
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 is another design trick that piston manufacturers can employ is with the usage of what is termed an accumulator groove – you’ll see one in our Cosworth YB piston for example.
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.