Flathead engine

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Ford flathead V8 engine, modified for power, depicted on cover of Hot Rod magazine. The "flat head" is evident.

Flathead engine refers to an internal combustion engine that has the valves placed in the engine block beside the piston, instead of in the cylinder head, as in an overhead valve engine. The design was common on early engine designs, but has since fallen from use.

Generally the flathead engine uses a small chamber on one side of the cylinder to carry the valves. This has a number of advantages, primarily that it makes the cylinder head much simpler. It also means that the valve can be operated by pushing directly up on it, as opposed to needing some sort of mechanical arrangement to push the valves down or to drive overhead cams, as on a "valve-in-head" engine. It may also lead to slightly easier cooling, as the valves and operating rods are out of the way of the cylinder, making a cooling jacket simpler to construct (but see below). The line of intakes along the side of the engine leads to the alternate name L-block (or L-head), due to the cylinders having the shape of an upside-down L. This configuration is also known as sidevalve.

On the downside, the flathead engine also requires the airflow to make at least a 90-degree turn to enter the cylinder, which makes it less efficient; colloquially it's said that such an engine has poorer "breathing". Breathing was not greatly important for production cars, in those days, because engines could not run long and reliably at high speed. In the early days all engines had poor combustion anyway, so this was a minor concern given the benefits in simplicity.

A more serious concern is that the exhaust often follows a more complicated path to leave the engine. This virtually guarantees that the engine will overheat under sustained steady heavy use. It is sometimes possible to arrange the engine layout so the exhaust will be taken through a second set of similar chambers moved to the other side of the cylinder, in which case the layout is referred to as a T-block.

Although flathead in-line 4 and 6 cylinder engines were frequently used for automobiles, tractors, etc., the best known flathead automotive engine is the early 20th century Ford V-8, which has both sets of valves (intake and exhaust) located on the inside of the "Vee," and which are all operated by a single camshaft located above the crankshaft.

Due to the heating and efficiency problems, flathead engines fell from "high power" uses such as aircraft engines fairly quickly, prior to World War I. However they lived on for some time in the automotive world and were used on the Jeep for instance. Flatheads are no longer in common use for automobiles, although they are still used for some small-engine applications like lawnmowers. Because of their design, the size of valves and the compression ratio are limited, which in turn reduces available power and economy.

[edit] Harley-Davidson motorcycle flathead engines

The flathead engine popular in Harley-Davidson motorcycles in the 1930s and used in the Servicars till 1970s had what was referred to as the "Ricardo Head".

[edit] Harley-Davidson engine timeline

Flathead: 1929–1936 *Big twins until 1948 commercial market as models UL and ULH, possible limited market until 1952-some literature indicating existence up to 1952 engine serial numbers displayed at motorcycle shows, old dealer catalog showed as optional engine until 1952 for limited market- sidecar use or military/ government contract completion . K models 1956 - was model prior to OHV CH /sportster. KRs made/raced even later. Servicar/'trike' noted preceding sentence to 1970s. RN*
Knucklehead: 1936–1947
Panhead: 1948–1965
Shovelhead: 1966–1985
Evolution: 1984–1999
Twin Cam 88: 1999—
Revolution: 2002—

[edit] See also

[edit] External link

• Images of each style of Harley-Davidson engine