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.

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.

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.

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 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, though they were the basis of many early racing engines, including famous names such as Ardun (Zora 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.