Anoverhead valve engine, abbreviated (OHV) and sometimes called apushrod engine, is apiston engine whose valves are located in thecylinder head above thecombustion chamber. This contrasts withflathead (or "sidevalve") engines, where the valves were located below the combustion chamber in theengine block.
Although anoverhead camshaft (OHC) engine also has overhead valves, the common usage of the term "overhead valve engine" is limited to engines where the camshaft is located in the engine block. In these traditional OHV engines, the motion of the camshaft is transferred usingpushrods (hence the term "pushrod engine") androcker arms to operate the valves at the top of the engine. However, some designs have the camshaft in the cylinder head but still sit below or alongside the valves (theFord CVH andOpel CIH are good examples), so they can essentially be considered overhead valve designs.
Some earlyintake-over-exhaust engines used a hybrid design combining elements of both side-valves and overhead valves.[1]
The first internal combustion engines were based on steam engines and therefore usedslide valves.[2] This was the case for the firstOtto engine, which was first successfully run in 1876. As internal combustion engines began to develop separately to steam engines,poppet valves became increasingly common.
Beginning with the 1885Daimler Reitwagen, several cars and motorcycles used inlet valve(s) located in the cylinder head, however these valves were vacuum-actuated ("atmospheric") rather than driven by a camshaft as with typical OHV engines. The exhaust valve(s) were driven by a camshaft, but were located in the engine block as with side-valve engines.
The 1894prototype Diesel engine used overhead poppet valves actuated by acamshaft,pushrods androcker arms,[3][4] therefore becoming the first OHV engines. In 1896, U.S. patent 563,140 was taken out by William F. Davis for an OHV engine with liquid coolant used to cool the cylinder head,[5][6] but no working model was built.
In 1898, bicycle manufacturerWalter Lorenzo Marr in the United States built a motorised tricycle powered by a single-cylinder OHV engine.[7] Marr was hired byBuick (then namedBuick Auto-Vim and Power Company) from 1899–1902, where the overhead valve engine design was further refined.[8] This engine employed pushrod-actuated rocker arms, which in turn opened poppet valves parallel to the pistons. Marr returned to Buick in 1904 (having built a small quantity of theMarr Auto-Car, with one of the first known engines to use an overhead camshaft design), the same year that Buick received a patent for an overhead valve engine design. In 1904, the world's first production OHV engine was released in theBuick Model B. The engine was aflat-twin design with two valves per cylinder. The engine was very successful for Buick, with the company selling 750 such cars in 1905, and the OHV engine has powered almost all Buick automobiles since then.
Several other manufacturers began to produce OHV engines, such as the 1906–1912Wright BrothersVertical 4-Cylinder Engine.[9][10] In 1911,Chevrolet joined Buick in almost exclusive use of OHV engines.[11] However,flathead "side-valve" engines remained commonplace in the U.S. until the mid-to-late 1950s,[12] when they began to be phased out for OHV engines.
The first overhead camshaft (OHC) engine dates back to 1902, in theMarr;[13] however, use of this design was mostly limited to high-performance cars for many decades. OHC engines slowly became more common from the 1950s to the 1990s, and by the start of the 21st century, the majority of automotive engines (except for some North American V8 engines) used an OHC design.[14]
At the1994 Indianapolis 500 motor race, Team Penske entered a car powered by the custom-builtMercedes-Benz 500I pushrod engine. Due to a loophole in the rules, the pushrod engine was allowed to use a larger displacement and higher boost pressure, significantly increasing its power output compared to the OHC engines used by other teams. Team Penske qualified in pole position and won the race by a large margin.
In the early 21st century, several pushrod V8 engines from General Motors and Chrysler usedcylinder deactivation to reduce fuel consumption and exhaust emissions. In 2008, the first production pushrod engine to usevariable valve timing was introduced in theDodge Viper (fourth generation).[15]
OHV engines have several advantages compared to OHC engines:
Compared to OHC engines, OHV engines have the following disadvantages:
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