Overhead-valve engine

An overhead valve engine (OHV engine) is an engine in which the valves are placed in the cylinder head. This was an improvement over the older flathead engine, where the valves were placed in the block next to the piston. Overhead camshaft (OHC) engines, while still overhead valve by definition, are usually categorized apart from other OHV engines. The type of valve typically used are Poppet valves.

In a piston engine configuration where the valves are overhead but the camshaft is not, informally called pushrod engine or I-head engine, the camshaft is placed within the cylinder block (usually beside and slightly above the crankshaft in a straight engine or directly above the crankshaft in the V of a V engine), and uses pushrods or rods to actuate rocker arms above the cylinder head to actuate the valves. Lifters or tappets are located in the engine block between the camshaft and pushrods. By contrast, overhead camshaft design avoids the use of pushrods by putting the camshaft directly above the valves in the cylinder head, thus simplifying the valvetrain.

The overhead valve internal combustion engine was known by at least 1896 when U.S. patent 563,140, awarded to William F. Davis, illustrated an internal combustion engine with valves in the head. The concept of push rods and rocker arms was being widely used in steam engines before then. Henry Ford's Quadricycle of 1896 had OHV for exhaust valves only. In 1898, Detroit bicycle manufacturer Walter Lorenzo Marr built a motor-trike with a one-cylinder OHV engine with a bore and stroke of 3 inches each. In 1900, David Buick hired Marr as chief engineer at the Buick Auto-Vim and Power Company in Detroit, where he worked until 1902. Marr's engine employed pushrod-actuated rocker arms, which in turn pushed valves parallel to the pistons, and this is still in use today. This contrasts with previous designs which made use of side valves and sleeve valves. Marr left Buick briefly to start his own automobile company in 1902, the Marr Auto-Car, and made a handful of cars with overhead valve engines, before coming back to Buick in 1904. The OHV engine was patented in 1902 (awarded 1904) by Buick's second chief engineer Eugene Richard, at the Buick Manufacturing Company, precursor to the Buick Motor Company. The world's first production overhead valve internal combustion engine was put into the first production Buick automobile, the 1904 Model B, which used a 2-cylinder Flat twin engine, with 2 valves in each head. The engine was designed by Marr and David Buick.

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