The Piston as Part of the Combustion Chamber
It is often said that the combustion chamber is the heart of any internal combustion engine, although the lungs might be an anatomically more accurate analogy. What is in no doubt though is that it has a fundamental bearing on the efficient operation of the engine; it is where we convert our chemical reagents into other substances and release energy in the process. The piston is an important part of this in any race engine. In a diesel engine, the combustion chamber is mostly formed in the piston crown. However, diesels are but a small niche application in racing; spark-ignition engines are dominant.
Even in a spark-ignition engine, the piston crown has a big impact on combustion, affecting it right through the engine cycle. If we look at the role of the piston during inlet, the shape of the crown and any features protruding from or formed in it will affect the motion of the mixture and the distribution of fuel within the chamber. As the piston rises on the compression stroke, the valves shut, we ignite the mixture, and the flame spreads from the spark plug outwards and downwards.
In order to force the mixture at the margins of the chamber towards the advancing flame front, engines tend to take advantage of the squish effect. The term ’squish’ refers to the thin part of the combustion chamber at the outside of the piston where the piston most closely approaches the head. The mixture is forced inwards at a velocity affected by various factors including the squish area and shape, engine stroke and engine speed. The reverse happens as the piston descends, with the enflamed volume being drawn out towards the margins of the chamber.
It is inevitable that the piston will absorb some heat from the hot combustion process, and this is also affected by piston crown geometry as well as piston temperature, thermal conductivity and other factors. The rate at which the surface heats up is affected by the ratio of surface area to volume of different areas of the piston - sharp-pointed sections will absorb heat at a greater rate than concave corners. Such hot-spots can provoke uncontrolled combustion, which can damage the piston and cylinder head.
We have mentioned in passing that the piston can affect the distribution of fuel and motion of the mixture. This is true in general for all types of engine, but especially for one class of race gasoline engine, the direct injection spark-ignition engine. Here, the fuel is injected directly into the cylinder, where it impinges on the piston crown. The form of the piston needs to be carefully considered in order to ensure that the fuel is where we want it to be (and in the right proportion compared to the available oxygen) at the start of ignition. If the mixture in the vicinity of the spark plug is either too rich or too weak in terms of air-to-fuel ratio, combustion will not be possible or may be inefficient. The piston crowns used in gasoline direct-injection engines often have a smooth area where the fuel spray interacts with the piston crown.
Written by Wayne Ward