The piston’s role in combustion efficiency

We all understand the role of the piston in terms of the general operation of the engine. It is an important part of the pumping mechanism of the engine, drawing in and expelling the working fluid. It is also an important part of the combustion chamber, and it is this function of the piston that we will concentrate on here.

The piston crown shape affects the flow of the fuel-air mixture into and around the chamber during the intake stroke. Its shape is particularly important where fuel is injected directly into the chamber, as with diesels and gasoline direct-injection engines. Beyond this, the piston crown shape affects the overall efficiency of the engine. Some of us may recall that to minimise heat rejection, the ratio of combustion chamber surface area to volume should be minimised. In terms of simple geometric shapes, the ideal combustion chamber would be a sphere.

However, in the practical world of engineering, we are limited to something approaching a cylinder, where the ends are complex surfaces defined by the piston crown and combustion chamber. A ‘clean’ flat-topped piston crown with few features such as valve cut-outs or intruders will have the minimum practical surface area. In many cases, our requirement to run high compression ratios means we will add an intruder to the top of the piston, and the valve timing and lift necessary for optimum performance will often give us deep valve pockets. This is far from ideal if we want to minimise combustion chamber surface area.

There are two other disadvantages to piston crowns that have lots of such features. First, sharp external corners will tend to run hot and can be a source of detonation, which can limit performance, owing to the optimum valve timing being within a range that causes detonation. Second, sharp internal corners, as we might find in the base of valve pockets, can tend to act to as flame quench areas, with the flame not burning right into the corner.

If we look further than the crown and examine the top land of the piston skirt profile, there are opportunities here to improve efficiency. There is very thin ring of combustion chamber volume which is bounded by the piston, liner and top ring, and which is open to the rest of the chamber. This acts as a quench volume and plays no real part in the combustion event. However, while its form is not conductive to combustion, there is nothing to prevent the fuel-air mixture entering this ‘crevice volume’. Anyone chasing maximum efficiency will keep this crevice volume to a minimum by maintaining the minimum working clearance between the piston top land and cylinder bore. There is also an incentive to have the top ring as close to the piston crown as is practical.

There are other effects of the crown design on combustion. The squish effect is caused by the piston expelling the working fluids from the periphery of the chamber as the piston approaches top dead centre, and the reverse happens as the piston descends. The squish effect is potent in terms of improving combustion, and the amount of squish clearance is also thought to be important for reducing detonation damage.

Written by Wayne Ward