Design features to improve top ring sealing

Monday, November 10, 2014

Tags :  pistons-rings

Sometimes, when we run a new design of race engine, or have made a big change to one, we find we cannot get a good ring seal. Poor piston ring sealing can cause loss of performance through loss of combustion pressure and poor oil consumption – neither of which are desirable in a race engine. We can detect problems when we see high levels of blow-by on the dyno using our blow-by meter, or alternatively an unusually high rate of oil use. If this is an effect which is repeated over a number of engines or rebuilds, it may indicate a problem that requires a design fix, and there are some design features that can be incorporated into the piston to improve ring sealing.

The piston ring has an inherent amount of radial pressure, which we feel as drag when we insert a piston into a bore, or via the force when we check the fit of the rings in the bore before fitting them to the piston. This is often done when confirming or adjusting ring end gaps. Indeed, the radial pressure or ‘ring tension’ is often expressed as a force needed to bring the ring into a form that will fit in the nominal bore size.

This radial pressure is augmented by gas pressure behind the ring, and it is this component of the ring force that can be increased through detailed design. The technique to improve pressure behind the ring is known as ‘gas porting’ or ‘gas jetting’, and the idea is to use the pressure in the combustion chamber to enhance the pressure behind the ring. This can be done by using a series of small drillings directly from close to the periphery of the piston crown through into the top ring groove. These are usually known as vertical gas jets, although they are not necessarily absolutely vertical.

The second type of gas porting, which is more common, is radial gas porting, also known as horizontal gas porting, side gas porting or lateral gas porting. These features are produced using a milling cutter rather than a drill, especially if they are added to existing pistons. The reason for this is that the radial type of gas jet is not a continuous hole, but is a semicircular cut with its axis intersecting with the top side of the ring groove.

Among the reasons that piston rings may not seal is insufficient side clearance between the ring and the groove – that is, across the width of the piston ring. To avoid ring flutter, small clearances are used, and this can prevent sufficient pressure reaching the back of the piston ring groove.

However, the use of low-tension rings and gas ports is often intentional, being a design feature from the start. Through the use of gas porting, the only time there is sufficient radial force on the ring to seal combustion pressure is when pressure is present – when we need it. At all other times, the ring tension alone forces the ring against the cylinder wall, and this means the average radial force is lower by using a low-tension ring and gas porting than would be achieved using a piston ring of higher tension and no gas porting.

Written by Wayne Ward

Leave your comment