Heron cylinder head
A Heron cylinder head, or simply Heron head, is a design for the combustion chambers of the cylinder head on an internal combustion piston engine. The head is machined flat, with recesses only for inlet and exhaust valves, spark plugs, injectors and so on. The combustion chamber itself is contained within a dished depression in the top of the piston. The Heron head is suitable for petrol and diesel engines, for ohv and ohc valve-gear, and for small and large engine displacement capacities. While a flat cylinder head could be combined with simple flat-top pistons , that option ignores the reasons for having a depression in the top of each piston, namely: (i) it provides a compact space for combustion to begin, allowing an optimal flame front; and (ii) it creates significant "squish" as the piston reaches TDC. This causes turbulence, which is desirable because it promotes more extensive mixing of the fuel/air mixture: cf: cf1, cf2, cf3. Having no space available at the top of piston travel to maintain compression ratio would also mean that the air-fuel mixture would be compressed to zero volume (or close), which is far too high a compression  ratio for any internal combustion engine to run on (due to detonation before the piston even reached top dead center) unless the stroke was carefully designed to leave a small gap between the top of the cylinder and the head when the piston is at TDC; in a normal engine the size of the combustion chamber dictates the compression ratio of the engine (i.e. the volume of the cylinder is compressed into the space of the chamber at TDC; a flat piston and flat head would leave no space for the air-fuel mixture, unless designed so the piston does n't actually reach the top of the cylinder bore)