IOE engine

Type of combustion engines

The intake/inlet over exhaust, or "IOE" engine, known in the US as F head, is a four stroke internal combustion engine whose valvetrain comprises OHV inlet valves within the cylinder head and exhaust side valves within the engine block. IOE engines were widely used in early motorcycles, initially with the inlet valve being operated by engine suction instead of a cam activated valvetrain. When the suction operated inlet valves reached their limits as engine speeds increased, the manufacturers modified the designs by adding a mechanical valvetrain for the inlet valve. A few automobile manufacturers, including Willys, Rolls Royce and Humber also made IOE engines for both cars and military vehicles. Rover manufactured inline four and six cylinder engines with a particularly efficient version of the IOE induction system. A few designs with the reverse system, exhaust over inlet (EOI), have been manufactured, such as the Ford Quadricycle of 1896.

In a F head/IOE engine, the intake manifold and its valves are located in the cylinder head above the cylinders, 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. An F head engine combines features from both overhead valve and flathead type engines, the inlet valve operating via pushrod and rocker arm and opening downward like an overhead valve engine, while the exhaust valve is offset from the cylinder and opens upward via an integrated pushrod/valve stem directly actuated by the camshaft, much like the valves in a flathead engine.

The earliest IOE layouts used atmospheric inlet valves which were held closed with a weak spring and were opened by the pressure differential created when the piston went down on the inlet stroke. This worked well with low speed early engines and had the benefit of being very simple and cheap, but the weak spring was unable to close the valve fast enough as engine speed increased. This required stronger springs, which in turn required direct mechanical action to open, as the atmospheric pressure of 15 PSI limits the total force available from creating a pressure differential, meaning that a spring is the strongest that can be used (for practical purposes, it would have to be lighter still). 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. 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. Its advantages over the sidevalve/flathead also include a compact combustion chamber, a well located spark plug, and a cooling effect from the mixture swirl, Unlike the conventional F head IOE, this had an efficient combustion chamber designed for good combustion, rather than simple manufacture. 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 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 or overhead valve designs. and remained an optional fitment until 1980 when it was replaced by the Rover V8.

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.

Some engines have been made with the reverse configuration, having the exhaust valve located in the cylinder head and the intake valve in the block. The ABC Skootamota began production with an engine of this configuration, but this was changed to an overhead valve engine before production ended. 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 returned to the original IOE configuration in 1938.