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Flathead engine

Flathead engine or Sidevalve engine (sometimes called a flatty) refers to an internal combustion engine with 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.

Advantages

Generally the flathead uses a small chamber on one side of the cylinder to carry the valves. This has a number of advantages, primarily making the cylinder head much simpler. It also means a valve can be operated by pushing directly up on it, as opposed to needing some sort of mechanical arrangement to push it down or to drive overhead cams, as on a "valve-in-head" engine. It may also lead to slightly easier cooling, as valve and pushrods 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.


Disadvantages

On the downside, the flathead also requires the airflow to make at least a 90-degree turn to enter the cylinder, which makes it less efficient, colloquially called poorer "breathing". Breathing was not greatly important for early production cars because engines could not run long and reliably at high speed, and all engines had poor combustion anyway, so this was a minor concern given the benefits in simplicity. The maximum compression ratio is also low at only about 7:1, further reducing efficiency (although it means the engine can run on low-octane fuel).

A more serious concern is exhaust, which often follows a more complicated path to leave the engine. This virtually guarantees the engine will overheat under sustained 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.

History and applications

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, though they were the basis of many early racing engines, including famous names such as Ardun (Zora Arkus-Duntov) and Frontenac (Louis Chevrolet). 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.

Harley-Davidson motorcycle flathead engines

The flathead engine saw service in Harley-Davidson motorcycles beginning with the "sport" model opposed twin produced from 1919-1923, and continuing in 1924 with single cylinder export 350cc and 500cc singles and continued in the Servicars until the 1970s. In the domestic U.S. market, the DL model, 1929-36, started Harley's side valve tradition in the 45 cubic inch displacement. The DLs featured a total loss oiling system (oil's always clean!), and were succeeded in 1937 by the WL, which had recirculating oil. The WL went on to serve in WWII as the U.S. and Canadian Army's primary two wheeled mount and subsequently as a civilian middleweight through 1952. The engine continued virtually unchanged with the GA designation in the three wheeled Servicar until production ceased in 1976.

In 1952, the W series was supplanted by the K series flatheads, designed to compete with British sporting motorcycles of the time, as the American motorcycle Association allowed the 750cc sidevalvers to compete against 500cc overhead-valve bikes. The K models featured unit construction engine and transmission cases, right side foot shift and left side foot brake, and evolved from 45 cubic inches to 55 cubic inches over the short life of the retail market run. The K series was replaced by the overhead valve Sportster series in the retail market in 1957. However, racing versions of the K model continued to be produced in very limited numbers for some time after, winning both roadraces and dirt track through 1969, when the American Motorcycle Association decided to change the rules and make the venerable flatheads uncompetitive. The K racers were replaced first by the iron XR 750cc overhead valve engine, and two years later by the alloy head XR, which continues in service in flat track racing to this day.

In 1930, the 74 cubic inch VL flathead replaced the JD, which featured intake over exhaust (IoE) valve configuration. The VL had a single downtube frame and total loss oiling, culminating in an 80 cubic inch version in 1936. In 1937, the engine was given the U designation and went into the same frame and running gear configuration as the overhead valve Knucklehead, which originated in 1936. The U continued to be produced, in varying configurations as both a 74 cubic inch and 80 cubic inch motor through 1948. By that time, the first year of the Panhead, it had been thoroughly superseded and outsold in the marketplace by the superior performance of the overhead valve model big twins.