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Overhead valve (OHV) engines can typically be identified by the location of the camshaft and valves: the camshaft is located within the block and the valves are above the block, inside the cylinder head. Pushrods typically run from the camshaft into the head and are moved up and down by the eccentric lobes of the cam. As the pushrods move, they can activate rocker arms that in turn push the valves open. The first overhead valve engines offered some benefits over previous side valve engines in the matter of efficiency, while later overhead cam (OHC) designs can often operate at higher revolutions per minute (RPMs).
Early internal combustion engines used a variety of designs, such as sleeve or side valves. These designs located the valve train inside the cylinder block with the pistons. The first overhead valve engine was designed around 1902, and offered benefits such as better top end performance and greater efficiency. In 1928, the first US patent was issued for an OHV engine design, which included plans for converting older side valve engines to the newer specification. As later OHC valvetrain configurations technically use overhead valves, nearly all modern automobiles with internal combustion engines use some type of OHV design.
These engines can offer benefits over both older side valve and newer OHC configurations. The main benefits offered over side valve engines are related to efficiency and performance, with an OHV engine taking in a mixture of air and fuel and then expelling exhaust gases more efficiently, and also rotating more quickly in some cases. When compared to an OHC design, this engine tends to be simpler, with a less complex mechanism for driving the camshaft. An overhead valve engine can also typically be smaller in size than an OHC design of similar displacement.
Despite the advantages it can offer, the design has been largely replaced by overhead cams. Overhead cam engines can typically operate better at higher engine speeds and may also work more efficiently. This may be partially due to the greater number of components involved in the valvetrain of an OHV design, particularly the pushrods. OHV engines are typically limited to around 10,000 RPMs even in racing applications, but OHC designs may achieve nearly double that. Overhead cam engines may also use multiple exhaust and intake valves per cylinder, which can further increase their efficiency.
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