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.

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 invented in 1898, in Detroit, by bicycle manufacturer Walter Lorenzo Marr, who built a motor-trike that year with a one-cylinder OHV engine with a bore and stroke of 3 inches each. In 1900, Marr was hired as chief engineer at the Buick Auto-Vim and Power Company in Detroit, where he worked until 1902. Marr said he got the idea of overhead valves when making the small tricycle engine, because that was the only way he could get the valves to fit. 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 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 of course designed by Marr.

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