Hemi engine
The abbreviation Hemi ( hemispherically-shaped combustion chambers ) denotes an engine with hemispherical (hemispherical) combustion chambers ( Hemi-Motor ); ancient Greek ἡμισφαίριον hemisphairion , German 'hemisphere' .
description
The millings of the combustion chambers within the cylinder heads are hemispherical so that an optimal, even distribution of force can take place within the combustion chambers. With a flat piston head , the large volume of the combustion chamber formed in this way would result in an undesirably low compression ratio, which is why the piston head in Hemi engines is usually arched up. In order to avoid a collision with the fully or partially open valves, so-called valve pockets are usually milled into the piston crown.
The combination of a hemispherical combustion chamber with a domed piston head at very high compression ratios creates a sickle-shaped combustion chamber, which makes it difficult for the flame front to spread from the central spark plug. For this reason, for example, Alfa Romeo's Hemi racing engines used two spark plugs that could ignite the mixture more quickly. Modern Chrysler Hemi engines also have two spark plugs to reduce fuel consumption and nitrogen oxide emissions.
In order to achieve better swirling of the sucked-in mixture, the combustion chamber hemisphere usually does not cover the entire diameter of the bore of the engine, but is slightly smaller, so that a flat edge results. With a suitably shaped piston crown, the mixture can be squeezed out of this edge and then rotates at high speed in the combustion chamber. This allows the flame front to spread quickly at the point of ignition and the mixture burns almost completely. The use of this form of combustion chamber is not limited to two-valve engines. In four-valve engines, it requires a radial arrangement of the valves, as was the case in motorcycle engines from the English brand Rudge long before the Second World War .
The inlet and outlet valves of two-valve engines are usually arranged parallel to one another in the cylinder head. This offers manufacturing advantages because almost all processing steps (drilling valve guides and bolt holes for rocker arm axis or camshaft bearings, milling valve seat, cutting thread) can be done in one operation without reclamping the workpiece. A hemispherical combustion chamber cannot be realized in this way.
Combustion chambers can also be roof-shaped or wedge-shaped. On some engines, the bottom of the cylinder head is a flat surface. The latter construction is called the Heron cylinder head, the combustion chamber is then a recess in the piston crown.
Advantages and disadvantages
The hemispherical combustion chamber offers several advantages. The gas exchange is made easier because the valves on the spherical surface are radially diagonally opposite. The movement of the air-gasoline mixture into the combustion chamber and the removal of the exhaust gases take place on a more fluid path, which also leads to better filling of the cylinder. Another advantage of the hemispherical design of the combustion chamber is that the flame front takes the same length in all directions from the centrally located spark plug to the edge of the combustion chamber and thus, at least in theory, uniform combustion. In practice, however, this advantage can hardly be realized, since a centrally arranged spark plug would restrict the possible size of the valves too much, which is why the spark plug is usually screwed into the side of the combustion chamber in favor of larger valve diameters. Alternatively, two side spark plugs are used, as in new Chrysler Hemi engines. In Hemi engines with only one spark plug, the nitrogen oxide values are particularly poor, which meant the end with the introduction of emission standards for many Hemi engines.
Another important factor for power delivery is how completely the fuel is burned in the cylinder. The temperatures on the combustion chamber wall are lower and the mixture located there is only incompletely burned. The hemisphere offers a better space-to-surface ratio in this regard. Hemispherical combustion chambers designed with a pinch edge enable good and intensive swirling of the sucked in fuel-air mixture, which promotes rapid and thus performance-enhancing combustion. In addition, in the hemi-head the spark plug can theoretically be attached between the valves - a property that otherwise only multi-valve engines (three-, four- and five-valve engines) have. In practice, this design feature is usually not implemented as it would restrict the possible valve diameter too much.
The Hemi principle enables a high degree of efficiency and high engine output even at 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, however, requires a somewhat more complex design of the valve train. This is one of the reasons for the Hemi's relatively high price even at the beginning. Inlet and outlet valves are not parallel and must be controlled by rocker arms or two camshafts. This is why Hemi engines often have a cylinder head and cover that is similar in width to multi-valve engines.
The hemispherical shape of the combustion chambers was combined with multi-valve designs as early as the 1920s. The English motorcycle manufacturer Rudge had engines with semi-radial (inlet valves parallel, outlet valves radial) and fully radial (all valves radial) valve arrangement in its range. The later designs by Ludwig Apfelbeck for BMW also became known. In comparison with conventional multi-valve engines, however, these could not achieve any decisive advantages. With their roof-shaped combustion bowl and central spark plug, these offer exactly the advantages that the Hemi head could once claim for itself.
Chrysler - "Hemi" as a brand
The term "Hemi" was established by the Chrysler Corporation and is still a brand of Chrysler today . The 426 cubic inch Hemi V8 engines (6981 cm³) offered from 1965 to 1971 were very successful in racing and became very popular in the age of muscle cars .
The 426 and other Hemi engines are still available as new parts from Mopar for racing vehicles or restoration projects. Hemi engines still dominate the drag racing field ; There, however, especially in the higher classes, not only original cast blocks, but also various replicas, for example made of aluminum, are used.
In the early 2000s, Chrysler marketed the principle as "New Hemi". Chrysler currently has four Hemi engines on offer: a 5.7 L engine, a 6.1 L engine, a 6.4 L engine and a 6.2 L supercharged engine .
other producers
- Alfa Romeo
- Aston Martin
- BMW
- Citroën
- Dodge
- Duesenberg
- ford
- jaguar
- Lancia
- lotus
- Mitsubishi
- Moskvich
- NSU
- Porsche
- Steyr-Puch
Literary sources
- Gert Hack, Fritz Indra : Multi-valve engines. Motorbuchverlag, ISBN 978-3-613-02260-7 .
Web links
- The official Chrysler Hemi page (English)
- English page with a schematic representation of a hemi-combustion chamber and information
- allpar.com: 426 Hemi: the Mopar elephant engine used by Chrysler, Dodge, and Plymouth (English)
