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A flathead engine, also known as a sidevalve engine American Rodder, 6/94, pp.45 & 93.(As the cylinder cross-section has the shape of an inverted L, other names such as "L-block" or "L-head" are also used) or valve-in-block engine, is an internal combustion engine with its contained within the Cam-in-block, instead of in the cylinder head, as in an overhead valve engine.
Flatheads were widely used internationally by automobile manufacturers from the late 1890s until the mid-1960s but were replaced by more efficient overhead valve and overhead camshaft engines. They are currently experiencing a revival in low-revving aviation engine such as the D-Motor.
The sidevalve engine's combustion chamber is not above the piston (as in an OHV (overhead valve) engine) but to the side, above the valves. The spark plug may be sited over the piston (as in an OHV engine) or above the valves; but aircraft designs with Dual ignition may use either or both positions.The D-motor flathead aero-engines have both spark pugs above the valves.
"Pop-up pistons" may be used with compatible heads to increase compression ratio and improve the combustion chamber's shape to prevent Engine knocking. "Pop-up" pistons are so called because, at top dead centre, they protrude above the top of the cylinder block.
At top dead centre, the piston gets very close to the flat portion of the cylinder head above, and the resultant squish turbulence produces excellent fuel/air mixing. A feature of the sidevalve design (particularly beneficial for an aero-engine) is that if a valve should seize in its guide and remain partially open, the piston would not be damaged, and the engine would continue operating safely on its other cylinders.
Sidevalve engines can only be used for engines operating on the otto cycle. The combustion chamber shape is unsuitable for ,Anton Pischinger (author): Die Steuerung der Verbrennungskraftmaschinen, in Hans List (ed): Die Verbrennungskraftmaschine, volume 9, Springer, Wien 1948, , p. 14 which require a high compression ratio for Combustion to occur.
In a sidevalve engine, intake and exhaust gases follow a circuitous route, with low volumetric efficiency, or "poor breathing", not least because the exhaust gases interfere with the incoming charge. Because the exhaust follows a lengthy path to leave the engine, there is a tendency for the engine to Thermal shock. (Note: this is true for V-type flathead engines but less of an issue for inline engines which typically have the intake and exhaust ports on the same side of the engine block.) Although a sidevalve engine can safely operate at high speed, its volumetric efficiency swiftly deteriorates, so that high power outputs are not feasible at speed. High volumetric efficiency was less important for early cars because their engines rarely sustained extended high speeds, but designers seeking higher power outputs had to abandon the sidevalve. A compromise used by the Willys MB, Rover Company, Land Rover, and Rolls-Royce in the 1950s was the F-head engine (or "intake-over-exhaust" valving), which has one sidevalve and one overhead valve per cylinder. Road and Track, some time in the 1960s
The flathead's elongated combustion chamber is prone to Engine knocking (or "knocking") if compression ratio is increased, but improvements such as laser ignition or microwave enhanced ignition might help prevent knocking. Turbulence grooves may increase swirl inside the combustion chamber, thus increasing torque, especially at low rpm. Better mixing of the fuel/air charge improves combustion and helps to prevent knocking. The test report reveals that fuel consumption and temperatures decreased at low engine speed while torque increased.Patent Somender Singh: "Design to improve turbulence in combustion chambers"
An advance in flathead technology resulted from experimentation in the 1920s by Sir Harry Ricardo, who improved their efficiency after studying the gas-flow characteristics of sidevalve engines. The internal-combustion engine by Harry Ralph Ricardo, Blackie and Son Limited.
The difficulty in designing a high-compression-ratio flathead means that most tend to be spark-ignition designs, and flathead diesels are virtually unknown.
After WWII, flathead designs began to be superseded by OHV (overhead valve) designs. Flatheads were no longer common in automobile, but they continued in more rudimentary vehicles such as off-road Jeep. In US custom car and hot rod circles, restored examples of early Ford flathead V8s are still seen. Street Rodder, 1/85, p.72.
Flathead aero-engines
The simplicity, lightness, compactness and reliability might seem ideal for an aircraft engine, but because of their low efficiency, early flathead engines were deemed unsuitable. Two notable exceptions were the American Aeronca E-107 opposed twin aero engine of 1930 and the Continental A40 flat four of 1931, which became one of the most popular light aircraft engines of the 1930s. Two modern flatheads are the Belgian D-Motor and .Although very small and compact, the D-Motor flat-six displaces nearly 4 litres. These are extremely oversquare and compact aero-engines with direct drive to a propeller.Tacke, Willi; Marino Boric; et al: World Directory of Light Aviation 2015-16, pages 256-257. Flying Pages Europe SARL, 2015.
Flathead motorcycles
Flathead designs have been used on a number of early pre-war motorcycles, in particular US V-twins such as Harley-Davidson and Indian, some British singles, BMW and Russian copies thereof.For example, some Dnepr and Ural used flathead designs that BMW had licensed to the Soviets. The Cleveland Motorcycle Manufacturing Company produced a T-head four-cylinder in-line motorcycle engine in the 1920s.
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