PISTON ACCUMULATOR GROOVE TECH

A piston accumulator groove is one way that the sealing of the piston rings can be improved. But why is ring sealing so important?

Piston Accumulator Groove

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.

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.

Cosworth PA2062 Piston Design Features

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.


Want to know more about the range of  Cosworth pistons that we supply? Then get in touch via our Contact Us page.

HEAD GASKET TECH

Cosworth YB1429 Head Gasket

Multi layer steel (MLS) head gaskets, like our popular Cosworth YB1429 head gasket pictured here, comprise of a number of layers of thin sheets of spring or carbon steel that sandwich the sealing material.

Cosworth YB1429 Head Gasket

The upper and lower steel sheets contain a pressed beading. This beading runs around the perimeter of the bores and passageways to increase the local sealing capability. When the head fasteners are tightened, the beading deforms to create extra sealing pressure. This pressure ensures that the combustion gases, lubricating oil and coolant that transfer between head and block all stay away from each other and remain inside the engine. Ideally the deformation will be mostly elastic, so that the beading can adapt to changes in temperature and load. Most MLS gaskets including ours also contain an inner layer called a stopper that prevents plastic deformation of the beading.

Further, the top and bottom layers have an elastomer coating to reduce friction. This can help to prevent microscopic movement of the gasket. Without this coating, there could be damage to the gasket and the firefaces on the head and block.

Our YB1429 MLS head gasket was designed by the engineers at Cosworth and Victor Reinz. They wanted to create a superior gasket. They wanted one that would provide excellent sealing between the block and head for all of the high performance applications that the YB engine ends up in.

If you’d like to know more or wish to order one then take a look in our on-line shop.