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An overhead valve engine, abbreviated (OHV) and sometimes called a pushrod engine, is a
piston engine A reciprocating engine, more often known as a piston engine, is a heat engine that uses one or more Reciprocating motion, reciprocating pistons to convert high temperature and high pressure into a Circular motion, rotating motion. This article ...
whose valves are located in the
cylinder head In a piston engine, the cylinder head sits above the cylinders, forming the roof of the combustion chamber. In sidevalve engines the head is a simple plate of metal containing the spark plugs and possibly heat dissipation fins. In more modern ...
above the
combustion chamber A combustion chamber is part of an internal combustion engine in which the air–fuel ratio, fuel/air mix is burned. For steam engines, the term has also been used for an extension of the Firebox (steam engine), firebox which is used to allow a mo ...
. This contrasts with flathead (or "sidevalve") engines, where the valves were located below the combustion chamber in the
engine block In an internal combustion engine, the engine block is the structure that contains the cylinders and other components. The engine block in an early automotive engine consisted of just the cylinder block, to which a separate crankcase was attach ...
. Although an
overhead camshaft An overhead camshaft (OHC) engine is a piston engine in which the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustio ...
(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 using
pushrod A valvetrain is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture (or air alone for direct-injected engines) into the combu ...
s (hence the term "pushrod engine") and
rocker arm A rocker arm is a valvetrain component that typically transfers the motion of a pushrod in an overhead valve engine, overhead valve internal combustion engine to the corresponding intake/exhaust poppet valve, valve. Rocker arms in automobiles are ...
s 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 (the
Ford CVH The Ford CVH engine is a Straight-four engine, straight-four automobile engine produced by the Ford Motor Company. The engine's name is an acronym for either ''Compound Valve-angle Hemispherical'' or ''Canted Valve Hemispherical'', where "Hemisph ...
and Opel CIH are good examples), so they can essentially be considered overhead valve designs. Some early intake-over-exhaust engines used a hybrid design combining elements of both side-valves and overhead valves.


History


Predecessors

The first internal combustion engines were based on steam engines and therefore used slide valves. This was the case for the first
Otto engine The Otto engine is a large stationary single-cylinder internal combustion engine, internal combustion four-stroke engine, designed by the German Nicolaus Otto. It was a low-RPM machine, and only fired every other stroke due to the Otto cycle, a ...
, which was first successfully run in 1876. As internal combustion engines began to develop separately to steam engines,
poppet valve A poppet valve (also sometimes called mushroom valve) is a valve typically used to control the timing and quantity of petrol (gas) or vapour flow into or out of an engine, but with many other applications. It consists of a hole or open-ended ch ...
s became increasingly common. Beginning with the 1885
Daimler Reitwagen The Daimler ''Reitwagen'' ("riding car") or ''Einspur'' ("single track") was a motor vehicle made by Gottlieb Daimler and Wilhelm Maybach in 1885. It is widely recognized as the first motorcycle. Daimler is often called "the father of the motorc ...
, 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 1894 prototype Diesel engine used overhead poppet valves actuated by a
camshaft A camshaft is a shaft that contains a row of pointed cams in order to convert rotational motion to reciprocating motion. Camshafts are used in piston engines (to operate the intake and exhaust valves), mechanically controlled ignition syst ...
,
pushrod A valvetrain is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture (or air alone for direct-injected engines) into the combu ...
s and
rocker arm A rocker arm is a valvetrain component that typically transfers the motion of a pushrod in an overhead valve engine, overhead valve internal combustion engine to the corresponding intake/exhaust poppet valve, valve. Rocker arms in automobiles are ...
s, 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, but no working model was built.


Production OHV engines

In 1898, bicycle manufacturer Walter Lorenzo Marr in the United States built a motorised tricycle powered by a single-cylinder OHV engine. Marr was hired by
Buick Buick () is a division (business), division of the Automotive industry in the United States, American automobile manufacturer General Motors (GM). Started by automotive pioneer David Dunbar Buick in 1899, it was among the first American automobil ...
(then named ''Buick Auto-Vim and Power Company'') from 1899–1902, where the overhead valve engine design was further refined. 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 the Marr 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 the
Buick Model B The Buick Model B was Buick's first model as an independent company, later becoming part of General Motors in 1908. It was built in Jackson, Michigan. A model B was exhibited in 1905 at the New York Auto Show as a promotion of the model C which ...
. The engine was a
flat-twin A flat-twin engine is a two-cylinder internal combustion engine with the cylinders on opposite sides of the crankshaft. The most common type of flat-twin engine is the boxer-twin engine, where both pistons move inwards and outwards at the same ti ...
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 1906–1912
Wright Brothers The Wright brothers, Orville Wright (August 19, 1871 – January 30, 1948) and Wilbur Wright (April 16, 1867 – May 30, 1912), were American aviation List of aviation pioneers, pioneers generally credited with inventing, building, and flyin ...
''Vertical 4-Cylinder Engine''. In 1911,
Chevrolet Chevrolet ( ) is an American automobile division of the manufacturer General Motors (GM). In North America, Chevrolet produces and sells a wide range of vehicles, from subcompact automobiles to medium-duty commercial trucks. Due to the promi ...
joined Buick in almost exclusive use of OHV engines. However, flathead "side-valve" engines remained commonplace in the U.S. until the mid-to-late 1950s, when they began to be phased out for OHV engines.


Overhead camshaft engines

The first overhead camshaft (OHC) engine dates back to 1902, in the Marr; 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. At the
1994 Indianapolis 500 The 78th Indianapolis 500 was held at the Indianapolis Motor Speedway in Speedway, Indiana on Sunday, May 29, 1994. The race was sanctioned by United States Auto Club (USAC), and was included as race number 4 of 16 of the 1994 PPG IndyCar World ...
motor race, Team Penske entered a car powered by the custom-built Mercedes-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 used
cylinder deactivation Variable displacement is an automobile engine technology that allows the engine displacement to change, usually by deactivating cylinders, for improved fuel economy. The technology is primarily used in large multi-cylinder engines. Many automobil ...
to reduce fuel consumption and exhaust emissions. In 2008, the first production pushrod engine to use
variable valve timing Variable valve timing (VVT) is the process of altering the timing of a Poppet valve, valve lift event in an internal combustion engine, and is often used to improve performance, fuel economy or emissions. It is increasingly being used in combina ...
was introduced in the Dodge Viper (fourth generation).


Design

OHV engines have several advantages compared with OHC engines: * Smaller overall packaging: The
cam-in-block A cam-in-block engine is where the camshaft is located in the engine block In an internal combustion engine, the engine block is the structure that contains the cylinders and other components. The engine block in an early automotive engine con ...
design of an OHV engine results in a smaller overall size, compared with an equivalent OHC engine, which can have some advantages in center of gravity and hood height in V-engine designs. * Simpler camshaft drive system: OHV engines have a less complex drive system for the camshaft than
OHC An overhead camshaft (OHC) engine is a piston engine in which the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustio ...
engines. Most OHC engines drive the
camshaft A camshaft is a shaft that contains a row of pointed cams in order to convert rotational motion to reciprocating motion. Camshafts are used in piston engines (to operate the intake and exhaust valves), mechanically controlled ignition syst ...
or
camshaft A camshaft is a shaft that contains a row of pointed cams in order to convert rotational motion to reciprocating motion. Camshafts are used in piston engines (to operate the intake and exhaust valves), mechanically controlled ignition syst ...
s using a
timing belt A toothed belt, timing belt, cogged belt, cog belt, or synchronous belt is a flexible belt with teeth moulded onto its inner surface. Toothed belts are usually designed to run over matching toothed pulleys or sprockets. Toothed belts are used in ...
, a
chain A chain is a serial assembly of connected pieces, called links, typically made of metal, with an overall character similar to that of a rope in that it is flexible and curved in compression but linear, rigid, and load-bearing in tension. A ...
, or multiple chains. These systems require the use of tensioners, which add complexity. In contrast, an OHV engine has the camshaft positioned close to the crankshaft, which may be driven by a much shorter chain or even direct gear connection. However, this is somewhat negated by a more complex valvetrain requiring pushrods. * Simpler lubrication system: The lubrication requirements for OHV cylinder heads are much lower, as they typically have a single camshaft within the block, where an OHC engine can have two per bank of cylinders, requiring oil passages through the cylinder head. OHV heads only need lubrication for the rocker arms at the pushrod end,
trunnion A trunnion () is a cylinder, cylindrical Boss (engineering), protrusion used as a mounting or pivoting point. First associated with cannons, they are an important military development. In mechanical engineering (see the Trunnion#Trunnion bearin ...
, and rocker tip. This lubrication is typically provided through the hollow pushrods themselves rather than a dedicated lubrication system in the head. The reduced lubrication requirements can also mean that a smaller, lower-capacity oil pump is used. Compared with OHC engines, OHV engines have the following disadvantages: * Limited engine speeds: Although OHV engines have simpler drive systems for the camshaft, there are a greater number of moving parts in the
valvetrain A valvetrain is a mechanical system that controls the operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture (or air alone for direct-injected engines) into the combu ...
(i.e., the lifters, pushrods, and rockers). Inertia from these valvetrain parts makes OHV engines more susceptible to
valve float Valve float is an adverse condition which can occur at high engine speeds when the poppet valves in an internal combustion engine valvetrain do not properly follow the closure phase of the cam lobe profile. This reduces engine efficiency and perfo ...
at high engine speeds (RPM). * Constraints on valve quantity and location: In OHV engines, the size and shape of the intake ports as well as the position of the valves are limited by the pushrods and the need to accommodate them in the head casting. As a result, it is rare for an OHV engine to have more than two valves per cylinder, while OHC engines often have four, allowing for higher rates of combustion and exhaust.


References

{{Internal combustion engine Cam-in-block valvetrain configurations Engine valvetrain configurations Motorcycle engines Scottish inventions