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A flathead engine (aka sidevalve engine (SV), flathead, or flatty) is 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. As the cylinder cross-section has the shape of an upside-down L, other names such as L-block or L-head are also common.
The sidevalve's poppet valves are usually sited on one side of the cylinder(s). A recess in the cylinder head creates a corridor connecting the valves and the combustion chamber. The valve gear comprises a camshaft which operates the valves via simple tappets, without any further valvetrain paraphernalia (such as pushrods, rocker arms, overhead valves or overhead camshafts).
The sidevalve arrangement was once the most common across all motor industries (automotive, agricultural, marine, aviation, and others), but it has since fallen from favor in most multicylinder applications, such as automotive and aviation, having been displaced by overhead-valve designs. Flathead designs are still commonly built new for many small engine applications of one and two cylinders, such as lawnmowers, rotary tillers, and two-wheel tractors.
The main advantages of a sidevalve engine are simplicity, reliability, cheapness, compactness, responsive low-speed power, and insensitivity to low-octane fuel. The absence of a complicated ohv valvetrain allows a compact engine that is cheaper to manufacture, as the cylinder head may be little more than a simple metal casting. These advantages may explain why ohv designs were first adopted only in high-performance applications such as aircraft, luxury cars, and sports cars, while economy cars, trucks, and agricultural engines continued to use flathead engines for some time. Even today, flatheads are common in lawnmowers and basic farm machinery.
An important benefit of the sidevalve design is that, if a valve should drop, only limited damage would occur, and an engine can often operate safely with a broken valve. By comparison, an ohv or ohc engine can suffer piston damage and catastrophic engine failure when a valve drops. The flathead design is clearly not an interference engine design, and is virtually immune from damage at high rpm.
Because intake and exhaust gases must pass through the same corridor between the block and head, and because the light valve gear allows the valves to open and close quickly, flatheads can be designed with little or no valve overlap, so that no fresh fuel/air mixture can escape into the exhaust port. Also, at tdc, the piston gets very close to the flat cylinder head, and the resultant squish turbulence gives excellent fuel/air mixing. These two factors together give better low rpm performance than with ohv or ohc engines which use valve overlap to gain high rpm. However, 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.
The typical flathead engine has its valves adjacent, but the "T-head" variant acts as a "cross-flow" head. Although it is possible to have some valve overlap with a "T-head", it is virtually pointless, as the compression ratio would be even lower than the standard flathead.
The main disadvantages of a sidevalve engine are poor gas flow, poor combustion chamber shape, and low compression ratio, all of which result in a low power output.
The sidevalve configuration makes intake and exhaust gases follow a circuitous route, with low volumetric efficiency, or "poor breathing". High volumetric efficiency was less important for early cars because their engines rarely sustained extended high speeds; but designers seeking higher outputs had to abandon the sidevalve. A compromise used by Willys (Jeep), Rover and Rolls-Royce in the 1950s was the F-head (aka intake-over-exhaust configuration), in which there is one side valve and one overhead valve per cylinder.
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 Ricardo head moved the exhaust valve farther from the center of the cylinder than the intake valve (whereas they had previously been equidistant). Ricardo observed how the form of the inlet and exhaust tracts affected gas flow and turbulence both in the inlet stream and within the combustion chamber.
Because the exhaust follows a complicated path to leave the engine, there is a tendency for the engine to overheat. In a T-head engine, a sidevalve engine has a crossflow configuration, so exhaust gases leave on the opposite side of the cylinder from the intake valve. American LaFrance powered their fire engines with T-head engines from the 1920s to the 1950s. The Cleveland Motorcycle Manufacturing Company produced a T-head four-cylinder in-line motorcycle engine in the 1920s; and early Stutz engines were T heads.
History and applications
Flathead inline 4- and 6-cylinder engines were frequently used for automobiles, tractors, and other products from the beginning of the automotive industry until the 1960s, when overhead valve designs began to outnumber flathead designs in production numbers. Flathead designs are still commonly built new for applications such as lawnmowers, rotary tillers, two-wheel tractors, and other one- and two-cylinder equipment.
Due to cooling and efficiency problems, flathead engines fell out of favor in "high power" applications, such as aircraft engines, prior to World War I, with rare execeptions like the American Aeronca E-107 opposed twin of 1930. 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 (except in some rodding and customizing circles), that most often use restored and reproduction examples of the classic 1932-1953 "model years" Ford flathead V8 engine, with the flathead valve configuration 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.
A new arrival is the Belgian D-Motor LF26, a compact 2.7-litre flat-four aero-engine that has direct drive to a propeller, rather than a reduction gearbox. As the engine was designed to produce peak power of 90 bhp at only 2800 rpm, this very low engine speed meant the designers could dispense with the complexity of an ohv valvetrain. The resulting simplicity, lightness, compactness and reliability are ideal in an aero-engine.  D-Motor is developing a 125 bhp 6-cylinder version.
Harley-Davidson motorcycle flathead engines
Model W, singles, and 45s (DL, RL, and WL)
The flathead engine saw service in Harley-Davidson motorcycles beginning with the Model W's flat-twin, produced from 1919 to 1923, and continuing in 1924 with single-cylinder export-model 21-cubic-inch (340 cc) and 30.5-cubic-inch (500 cc) singles and continued in Servi-Cars until 1973. In the domestic U.S. market, the 45-cubic-inch (740 cc) DL model (1929 to 1931) and its technical descendant, the RL model (1932 to 1936), started Harley's side-valve tradition in the 45-cubic-inch displacement class. The DL and RL models featured a total-loss oiling system and were succeeded in 1937 by the WL 45, which had recirculating oil lubrication. The WL went on to serve in World War II 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 various G-based designations in the three-wheeled "Servi-Car" until production ceased in 1973.
K-series and developments
In 1952, the K series flatheads was introduced, selling in parallel with the W series (which was discontinued after 1952), designed to compete with British sporting motorcycles of the time, as the American motorcycle Association allowed the 750 cc sidevalves to compete against 500 cc overhead-valve bikes. The K models featured a unit construction engine and transmission case, right side foot shift and left side foot brake, and evolved from 45 cubic inches (1952 to 1953) to 55 cubic inches by a 0.75-inch (19 mm) increase in stroke length (1954 to 1956) over its five year 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 750 cc K model, designated KR, continued to be produced in very limited numbers for some time after, winning both roadraces and dirt track events against overhead valve bikes limited to 500 cc through 1969, when the American Motorcycle Association finally decided to change the rules and make the venerable flatheads uncompetitive. The K racers were replaced first by the iron-head XR 750 cc overhead valve engine, and two years later by the alloy-head XR, which continues in service in flat track racing to this day.
Flathead big twins
In 1930, the 74-cubic-inch (1,210 cc) VL flathead replaced the JD Big Twin, which had featured intake-over-exhaust (IoE) valve configuration. The VL had a single downtube frame and total loss oiling, culminating in an 80-cubic-inch (1,300 cc) version (VLH) in 1935. In 1937, that engine was redesigned to include a recirculating lubrication system, and designated the model U, and it went into the same frame and running gear configuration as the model E Knucklehead, which had originated in 1936. The U continued to be produced in varying configurations as a 74 cubic inch U & UL (1937 to 1948), and 80 cubic inch UH & ULH engine (1937 to 1941). By that time, the first year of the aluminum-head Panhead, it had been thoroughly superseded and outsold in the marketplace by the superior performance of the overhead valve model Big Twins.
- American Rodder, 6/94, pp.45 & 93.
- "valve overlap" = where both intake and exhaust valves are simultaneously open
- Road and Track, some time in the 1960s
- "Street Rodder, 1/85, p.72.
- "Kapelstraat 198 8540 Deerlijk - Recent information". D-motor.eu. Retrieved 2011-12-06.
- "Kapelstraat 198 8540 Deerlijk - Product / Produit". D-motor1.vpweb.be. Retrieved 2011-12-06.
- Wagner, Herbert; Mitchell, Mark (May 1999). "Chapter 6: The Gang's All Here". Classic Harley-Davidson, 1903-1941. Enthusiast Color. Motorbooks International. pp. 46–47. ISBN 0-7603-1805-0. Retrieved 2012-08-05. "The Sport Model engine was Harley's first side-valve."