The main purpose of this design was to obtain higher flame speed and to reduce the knocking tendency. The entire combustion space was concentrated over the inlet and exhaust valve. This also provided sufficient turbulence during the compression stroke and increased flame velocity. The ratio of area to volume of the space above the piston was large and it has created a quench area which is essential to avoid knocking. The different F-head arrangements are shown in Figure. This arrangement is a compromise between L-head and I-head combustion chamber. In the present F-head design, inlet valve is located in the head and exhaust valve in the block. This was used by Rover company for a long time. It consists of crown shaped piston and correspondingly sloping cylinder head. The inlet and exhaust valves are inclined. The plug is located in the flat roof which allows use of inlet valve bigger than exhaust valve. This provides the shortest flame travel and therefore the possibility of knock is very rare. This arrangement provides all the desired requirements of good combustion chamber as high volumetric efficiency, maximum compression for the given fuel resistance to knock and high thermal efficiency. It can also operate on learn A : F ratio without misfiring. Another type of F-head is shown in figure. It is used in willeys jeep. It also used wedged-shaped combustion chamber but is longer and narrower than Rover type. It provides Squish when piston reaches to TDC and creates sufficient turbulence. Inlet valve area is also larger than exhaust valve area.
The hemispherical head with inclined valves was also used in Norton motor cycle engines. This design is more preferred for supercharged engines as symmetry of design is very important because such engines fail because of high thermal stresses.
Providing a cavity in the piston cylinder creates sufficient turbulence which provides rapid flame travel and avoids knocking. A compression ratio of 9 can be used with arrangement without any danger of knocking.
