Overhead valve engine

Components of a traditional OHV enginevalvetrain

Anoverhead valve engine,abbreviated (OHV) and sometimes called apushrod engine,is apiston enginewhose valves are located in thecylinder headabove 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 armsto 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 CVHandOpel CIHare good examples), so they can essentially be considered overhead valve designs.

Some earlyintake-over-exhaustengines used a hybrid design combining elements of both side-valves and overhead valves.

History

1894 prototype overhead valveDiesel engine

1904 patent for Buick overhead valve engine

Predecessors

The first internal combustion engines were based on steam engines and therefore usedslide valves.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 valvesbecame 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 engineused overhead poppet valves actuated by acamshaft,pushrodsandrocker arms,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 manufacturerWalter Lorenzo Marrin the United States built a motorised tricycle powered by a single-cylinder OHV engine.Marr was hired byBuick(then namedBuick 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 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-twindesign 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.In 1911,Chevroletjoined 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

Main article:Overhead camshaft engine

The first overhead camshaft (OHC) engine dates back to 1902, in theMarr;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 the1994 Indianapolis 500motor race, Team Penske entered a car powered by the custom-builtMercedes-Benz 500Ipushrod 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 deactivationto reduce fuel consumption and exhaust emissions. In 2008, the first production pushrod engine to usevariable valve timingwas introduced in theDodge Viper (fourth generation).

Design

V8 engine(with intake manifold removed), showing the camshaft, pushrods, and rockers

OHV engines have several advantages compared with OHC engines:

• Smaller overall packaging:Thecam-in-blockdesign 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.
• Using the same cylinder head casting for both cylinder banks:A V-type engine design allows both cylinder heads to be mirror images of each other at the front of each cylinder bank. In a V-type OHV engine design, it is possible to use the same cylinder head casting for both banks, by simply flipping it around for the second bank. TheGeneral Motors LS-based small-block enginefamily is a popular example of these kinds of OHV V-type engines.
• Simpler camshaft drive system:OHV engines have a less complex drive system for the camshaft thanOHCengines.Most OHC engines drive thecamshaftorcamshaftsusing atiming belt,achain,or multiple chains. These systems require the use oftensioners,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,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 thevalvetrain(i.e., the lifters, pushrods, and rockers). Inertia from these valvetrain parts makes OHV engines more susceptible tovalve floatat 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.