The intake/inlet over exhaust (IOE) engine, also known as F-head and pocket valve, is a valvetrain configuration used in early four-stroke internal combustion engines. The configuration consists of intake valves located in the cylinder head and exhaust valves located in the cylinder or engine block.
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In a F-head engine, the intake manifold and its valves are located atop the cylinders, in the cylinder head, and are operated by rocker arms which reverse the motion of the pushrods so that the intake valves open downward into the combustion chamber. The exhaust manifold and its valves are located beside or as part of the cylinders, in the block. The exhaust valves are either roughly or exactly parallel with the pistons; their faces point upwards and they are not operated by pushrods, but by contact with a camshaft through the tappet or valve lifter.
The earliest IOE layouts used atmospheric inlet valves which were held closed with a weak spring and were opened by the suction created when the piston went down on the inlet stroke.[1] When the limits of this system were reached, the design was improved without substantial changes to the head casting by adding a mechanical system to open the inlet valves and stronger springs to close them.[2] In both cases, the exhaust valves were in the block and were opened by contact with a camshaft through a tappet or valve lifter and closed by springs.[1]
The IOE design allowed the use of much larger valves than a side valve or overhead valve engine. Its advantages over the flathead also included better intake mixture flow and greater reliability from its cooling of the exhaust valve and its spring.
Rover used a more advanced form of IOE engine. It was designed by Jack Swaine in the mid-late 1940s and was in production from 1948 to the early 1980s. Unlike the F-head, this had an efficient combustion chamber designed for good combustion, rather than simple manufacture.[3] The top surface of the block was machined at an angle, with the piston crowns angled in a "pitched roof" to match. At TDC, the piston almost touched the angled inlet valve and provided good 'squish' to the combustion chamber itself, offset to the side by half a cylinder diameter. The resultant combustion chamber shape was a near-ideal hemisphere, although inverted and tilted from the usual "hemi-head" design. The spark plug was centrally mounted and this, together with the turbulence generated by the squish, provided a short flame path. The thinness of the gas layer between piston and inlet valve was so confined as to reduce the risk of detonation on poor fuel, one factor that kept it in service with Land Rover for so long. During the late 1940s and early 1950s when the only petrol available was low octane 'pool' petrol it also allowed Rover to run higher compression ratios than many competitors with the more usual side valve (flat head) or overhead valve designs.
The unusual combustion chamber arrangement with its angled valves also lead to an unusual valve train. The block-mounted camshaft operates small wedge shaped rockers, one for each valve. In early models the camshaft acts on a simple pad on the rocker, but for later models this pad was replaced by a roller follower. The exhaust rockers act directly on the valves, whilst the inlet rockers act on pushrods running up to a second set of longer flat rockers operating the inlet valves. The Rover engine, like many 1940s designs, was a small bore, long stroke engine to keep the RAC HP rating as low as possible, thus keeping the road tax as low as possible. The IOE layout enabled Rover to use much larger valves than would normally be possible in a small bore engine, allowing much better breathing and better performance.
The shape of the combustion chamber as an "inverted hemi-head", along with the angled cylinder head joint and pitched-roof piston crowns, had earlier been used in the 1930 Van Ranst-designed Packard V12 engine, although in this case the valves were both in the block as side valves and the spark plug was poorly placed at the extremity of the combustion chamber.[4]
The Rover IOE engine family encompassed straight-4 (1.6- and 2.0-litres) and straight-6 (2.1-, 2.2-, 2.3-, 2.4-, 2.6- and 3.0-litres) engines and powered much of the company's post-war range in the form of the P3, P4 and P5 models. Adapted versions of the 1.6 and 2.0 IOE engines were used in early version of the Land Rover as well. Power outputs ranged from 50bhp (Land Rover 1.6) to 134bhp (P5 3 litre MkII & III). The 2.6 6-cylinder IOE engine had a particularly long career. After being used in Rover P4 saloon cars it was added to long-wheelbase Land Rover models from 1963 in the 2A Forward Control models, then in 1967 in the bonneted 109",[5] and remained an optional fitment until 1980 when it was replaced by the Rover V8.
The IOE valvetrain layout was used extensively in American motorcycles. Harley-Davidson used IOE engines with atmospheric inlet valves until 1912,[6] and with mechanically driven inlet valves from 1911 to the late 1920s. Indian used IOE valvetrains on all of their four cylinder bikes except those built in 1936 and 1937.[7][8] Other American motorcycle manufacturers that use IOE engines include Excelsior, Henderson, and Ace.
Hudson used an IOE straight-six engine from 1927 to 1929. Willys, and its successor Kaiser-Jeep, used variants of the Willys Hurricane engine from 1950 to 1971. Rolls-Royce used variants of their B60 IOE straight-six engine in their Wraith and Silver Wraith cars.[9] A short-stroke oversquare version of the B60 engine was also used in the Princess 4-litre-R saloon car, displacing 3909cc and producing around 175 hp (manufacturers figures). This was known as the FB60 and was substantially upgraded from the common-or-garden B60 - it had hydraulic lifters, different carburation, hotter cam and other improvements. These changes liberated a 55 hp increase from the B60 engines' 120 hp output. Over 6000 of these cars were made.
In 1936 and 1937, the Indian Four had the valve positions reversed, with the exhaust valve in the head and the inlet valve in the block. In theory, this would improve fuel vaporization, and the engine was actually more powerful. However, the new system made the cylinder head very hot. The exhaust valve linkage required frequent adjustment. The design was returned to the original IOE configuration in 1938.[7][8]