Hemi engine

The abbreviation Hemi ( hemispherically -shaped combustion chambers ) denotes an engine with hemispherical ( hemispheric ) combustion chambers ( Hemi engine ).

The hemisphere formed by the cylinder head. With a flat piston head would result by the large volume of the combustion chamber thus formed an undesirably low compression ratio, which is why the piston head with Hemi engines is carried out usually highly arched. In order to avoid the collision with the fully or partially open valves, so-called valve pockets are machined into the piston head generally.

By the combination of a hemispherical combustion chamber with a high-arched piston head at very high compression ratios results in a crescent-shaped combustion chamber, which impedes the propagation of the flame front from the central spark plug. Therefore, two spark plugs were used for example in the Hemi racing engines from Alfa Romeo, which could ignite the mixture faster.

To achieve a better swirling of the intake mixture, the combustion chamber hemisphere usually not covered the full diameter of the hole of the engine, but is slightly smaller, so that there is a flat edge. With a suitably shaped piston head, the mixture can be squeezed out of this edge, and then rotated at high speed in the combustion chamber. Thus, the flame front to the ignition timing can spread quickly and the mixture burns almost completely. The application of this combustion chamber shape is not limited to two-valve engines. In four- valve engines it requires a radial arrangement of the valves, as implemented well before the Second World War in the English brand Rudge motorcycle engines.

The intake and exhaust valves of two-valve engines are usually arranged in parallel in the cylinder head. This provides manufacturing advantages because almost all processing steps ( valve guide and bolt holes for the rocker shaft or camshaft bearing drilling, milling valve seat, thread cutting) in a single operation without re-clamping of the workpiece can be made. A hemispherical combustion chamber can be so but do not realize.

Combustion chambers can also be roof - or wedge-shaped. In some motors, the bottom of the cylinder head has a plane surface. The latter design is called Heron cylinder head, the combustion chamber is then a recess in the piston crown.

The hemispherical combustion chamber has several advantages. Thus, the gas exchange is facilitated because the valves radially inclined face on the spherical surface. The movement of the air-fuel mixture in the combustion chamber and the discharge of the exhaust gases takes place at a smoother web, which also leads to a better filling of the cylinder. Another advantage of the hemispherical combustion chamber design is the same in all directions long path of the flame front of the centrally arranged ignition plug to the edge of the combustion chamber and thus at least in theory, a uniform combustion. In practice, this benefit, however, can hardly realize, because a centrally located spark plug would limit too much the potential size of the valves, and therefore in favor of larger diameter valve, the spark plug is usually screwed laterally into the combustion chamber.

Important for the power delivery is also how completely the fuel is burned in the cylinder. At the combustion chamber wall temperatures are lower and located there is only incompletely combusted mixture. The hemisphere has, in this respect, the optimal ratio of space and surface. Constructed with hemispherical combustion chambers squish band allow a good and intensive swirling of the intake fuel -air mixture, which promotes fast and therefore performance-enhancing combustion. Moreover, in the hemi- head, the spark plug can theoretically be mounted between the valves - a property that would otherwise have only multi-valve engines ( three-, four -and five -valve ). In practice, this design feature is usually not realized, as that would restrict too much the possible valve diameter.

The Hemi principle allows a high efficiency and high power output at a relatively low compression. With the same or even lower compression a Hemi engine can do more than a two-valve engine with normal combustion chambers.

The position of the valves caused, however, a very complex design of the valve train. That's one of the reasons for those already at the beginning of times comparatively high price of the Hemi. Intake and exhaust valves are not parallel, and must be driven by rocking levers and two camshafts. Therefore Hemi engines often have a similarly wide cylinder head and cover as multi-valve engines.

Already in the 1920s, the hemispherical shape of the combustion chambers combined with multi- valve designs. The British motorcycle manufacturer Rudge had engines with semi- radial ( parallel intake valves, exhaust valves radial) and fully radial (all valves radially ) valve assembly in the program. The later constructions of Ludwig Beck apple for BMW became known. In comparison with conventional multi-valve engines but these could not achieve significant advantages. This offer with its roof combustion chamber recess and the central spark plug exactly the benefits that once could claim solely for the hemi- head.

" Hemi " as a brand

The term " Hemi " was established by the Chrysler Corporation and is still a trademark of Chrysler. The offered 1965-1971 426 cubic inch Hemi V8 engine ( 6981 cc ) were very successful in racing and thus arrived at the time of muscle cars are gaining huge popularity.

As before, the 426er and other Hemi engines are as new parts from Mopar to purchase for racing cars or restoration objects. Hemi engines still dominate the field in the area of drag racing; there are, however, especially in the higher classes, not only original cast blocks, but also a variety of replicas, such as aluminum, are used.

Literary sources

• Gert Hack, Fritz Indra: multi-valve engines. Engine book publishing house, ISBN 978-3-613-02260-7.