10.10.2006, 23:00
The thing that allowed the 1951 Chrysler HEMI to produce so much more power than other engines of the day was the efficiency of the combustion chamber.
In a HEMI engine, the top of the combustion chamber is hemispherical, as seen in the image above. The combustion area in the head is shaped like half of a sphere. An engine like this is said to have "hemispherical heads." In a HEMI head, the spark plug is normally located at the top of the combustion chamber, and the valves open on opposite sides of the combustion chamber.
Most cars prior to the 1950s used what was known as a flat head, and many lawn mower engines still use the flathead design today because it is less expensive to manufacture. In a flathead engine, the valves are in the block, rather than in the head, and they open in a chamber beside the piston.
The head in a flathead engine is extremely simple -- it is a solid metal casting with a hole drilled in it to accept the spark plug. The camshaft in the block pushes directly on the valve stems to open the valves, eliminating the need for pushrods and rocker arms. Everything is simpler in the flathead design. The problem with a flathead engine is its thermal efficiency, which we'll discuss next.
Advantages
There are many different parts of an engine's design that control the amount of power you can extract from each combustion stroke. For example:
You want to burn all of the gas in the cylinder. If the design leaves any of the gas unburned, that is untapped energy.
You want the maximum cylinder pressure to occur when the crankshaft is at the right angle, so that you extract all of the energy from the pressure.
You want to waste as little of the engine's energy as possible sucking air and fuel into the combustion chamber and pushing exhaust out.
You want to lose as little heat as possible to the heads and the cylinder walls. Heat is one of the things creating pressure in the cylinder, so lost heat means lower peak pressures.
The last item in the list is one of the key advantages of the HEMI head versus the flathead engine. Surface area causes heat loss. Fuel that is near the head walls may be so cool that it does not burn efficiently. With a flat head, the amount of surface area relative to volume of the combustion chamber is large. In a HEMI engine, the surface area is much smaller than in a flat head, so less heat escapes and peak pressure can be higher.
Another factor with a HEMI head is the size of the valves. Since the valves are on opposite sides of the head, there is more room for valves. The engine design that preceded the HEMI was a wedge-shaped combustion chamber with the valves in line with each other. The inline arrangement limited valve size. In a HEMI engine, valves can be large so the airflow through the engine is improved
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In a HEMI engine, the top of the combustion chamber is hemispherical, as seen in the image above. The combustion area in the head is shaped like half of a sphere. An engine like this is said to have "hemispherical heads." In a HEMI head, the spark plug is normally located at the top of the combustion chamber, and the valves open on opposite sides of the combustion chamber.
Most cars prior to the 1950s used what was known as a flat head, and many lawn mower engines still use the flathead design today because it is less expensive to manufacture. In a flathead engine, the valves are in the block, rather than in the head, and they open in a chamber beside the piston.
The head in a flathead engine is extremely simple -- it is a solid metal casting with a hole drilled in it to accept the spark plug. The camshaft in the block pushes directly on the valve stems to open the valves, eliminating the need for pushrods and rocker arms. Everything is simpler in the flathead design. The problem with a flathead engine is its thermal efficiency, which we'll discuss next.
Advantages
There are many different parts of an engine's design that control the amount of power you can extract from each combustion stroke. For example:
You want to burn all of the gas in the cylinder. If the design leaves any of the gas unburned, that is untapped energy.
You want the maximum cylinder pressure to occur when the crankshaft is at the right angle, so that you extract all of the energy from the pressure.
You want to waste as little of the engine's energy as possible sucking air and fuel into the combustion chamber and pushing exhaust out.
You want to lose as little heat as possible to the heads and the cylinder walls. Heat is one of the things creating pressure in the cylinder, so lost heat means lower peak pressures.
The last item in the list is one of the key advantages of the HEMI head versus the flathead engine. Surface area causes heat loss. Fuel that is near the head walls may be so cool that it does not burn efficiently. With a flat head, the amount of surface area relative to volume of the combustion chamber is large. In a HEMI engine, the surface area is much smaller than in a flat head, so less heat escapes and peak pressure can be higher.
Another factor with a HEMI head is the size of the valves. Since the valves are on opposite sides of the head, there is more room for valves. The engine design that preceded the HEMI was a wedge-shaped combustion chamber with the valves in line with each other. The inline arrangement limited valve size. In a HEMI engine, valves can be large so the airflow through the engine is improved
2000 dodge ram 5.9 cummings,quadcab sport svart, Edge juice w/additude,FASS-HPFP-150, K&N cold air intake kit, MBRP 4" mere kommer