overhead valve engine

File:Experimental Diesel Engine.jpg ]]

File:Valve-In-Head 1904 patent.jpg

The first internal combustion engines were based on steam engines and therefore used slide valves.{{cite web |title=Part I: V-engines |url=https://www.topspeed.com/cars/engine-s-history-ar11139.html |archive-url=https://archive.today/20200127174925/https://www.topspeed.com/cars/engine-s-history-ar11139.html |url-status=dead |archive-date=January 27, 2020 |website=TopSpeed.com |date=29 July 2006 |access-date=13 December 2019 }} This was the case for the first Otto 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 1885 Daimler 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 1894 prototype Diesel engine used overhead poppet valves actuated by a camshaft, pushrods and rocker arms,{{cite book |last1=Diesel |first1=Rudolf |title=Die Entstehung des Dieselmotors. |date=1913 |publisher=Springer |location=Berlin |isbn=978-3-642-64940-0 |page=17}}{{cite book |last1=Diesel |first1=Rudolf |title=Theorie und Konstruktion eines rationellen Wärmemotors zum Ersatz der Dampfmaschinen und der heute bekannten Verbrennungsmotoren |date=1893 |publisher=Springer Berlin Heidelberg |location=Berlin |isbn=978-3-642-64949-3 |pages=5,62}} 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,{{cite web |url=http://patentimages.storage.googleapis.com/pages/US563140-0.png |title= U.S. patent 563140 |access-date=2017-06-07 |url-status=live |archive-url=https://web.archive.org/web/20170828104245/http://patentimages.storage.googleapis.com/pages/US563140-0.png |archive-date=2017-08-28 }}{{cite web|url=http://pdfpiw.uspto.gov/.piw?Docid=00563140&homeurl=http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1%2526Sect2=HITOFF%2526d=PALL%2526p=1%2526u=%25252Fnetahtml%25252FPTO%25252Fsrchnum.htm%2526r=1%2526f=G%2526l=50%2526s1=0563,140.PN.%2526OS=PN/0563,140%2526RS=PN/0563,140&PageNum=&Rtype=&SectionNum=&idkey=NONE&Input=View+first+page|title=Patent Images|website=pdfpiw.uspto.gov|access-date=8 May 2018|url-status=dead|archive-url=https://web.archive.org/web/20170828110158/http://pdfpiw.uspto.gov/.piw?Docid=00563140&homeurl=http%3A%2F%2Fpatft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO1%2526Sect2%3DHITOFF%2526d%3DPALL%2526p%3D1%2526u%3D%25252Fnetahtml%25252FPTO%25252Fsrchnum.htm%2526r%3D1%2526f%3DG%2526l%3D50%2526s1%3D0563,140.PN.%2526OS%3DPN%2F0563,140%2526RS%3DPN%2F0563,140&PageNum=&Rtype=&SectionNum=&idkey=NONE&Input=View+first+page|archive-date=28 August 2017}} but no working model was built.

In 1898, bicycle manufacturer Walter Lorenzo Marr in the United States built a motorised tricycle powered by a single-cylinder OHV engine.{{cite book |title=Walter L. Marr, Buick's Amazing Engineer |first1=Beverly Rae |last1=Kimes |author-link1=Beverly Rae Kimes |first2=James H. |last2=Cox |publisher=Racemaker Press |location=Boston |year=2007 |page=14}} Marr was hired by Buick (then named Buick Auto-Vim and Power Company) from 1899–1902, where the overhead valve engine design was further refined."The Buick, A Complete History," third ed., 1987, Terry P. Dunham and Lawrence Gustin. 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 engine was a flat-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 1906–1912 Wright Brothers Vertical 4-Cylinder Engine.Hobbs, Leonard S. The Wright Brothers' Engines and Their design. Washington, D.C.: Smithsonian Institution Press, 1971, p 61, 63.{{Cite web|url=http://www.wright-brothers.org/Information_Desk/Just_the_Facts/Engines_&_Props/Wright%20Engines.htm|archive-url=https://web.archive.org/web/20160428085141/http://www.wright-brothers.org/Information_Desk/Just_the_Facts/Engines_%26_Props/Wright%20Engines.htm|url-status=dead|title=Wright Engines|archive-date=April 28, 2016}} In 1911, Chevrolet joined Buick in almost exclusive use of OHV engines.{{cite web | url=http://www.carnut.com/specs/gen/chv30.html | title=Chevrolet 1930s General Specs }} However, flathead "side-valve" engines remained commonplace in the U.S. until the mid-to-late 1950s,{{cite web | url=https://www.autoweek.com/car-life/a1713546/what-was-final-year-new-flathead-powered-american-car/ | title=What Was the Final Year for a New Flathead-Powered American Car? | date=20 December 2018 }} when they began to be phased out for OHV engines.

{{main|Overhead camshaft engine}}

The first overhead camshaft (OHC) engine dates back to 1902, in the Marr;{{cite encyclopedia|title= Maudslay|last= Georgano|first= G. N.|author-link= G. N. Georgano|page= [https://archive.org/details/newencyclopediao0000unse_v2r4/page/407 407]|encyclopedia= The New Encyclopedia of Motorcars 1885 to the Present|editor-last= Georgano|editor-first= G. N.|year= 1982|orig-year= 1968|publisher= E. P. Dutton|location= New York|edition= Third|isbn= 0-525-93254-2|lccn= 81-71857|ref= none|url= https://archive.org/details/newencyclopediao0000unse_v2r4/page/407}} 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.Now, manufacturers such as Honda even use OHC motors for lawnmowers!

At the 1994 Indianapolis 500 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 to reduce fuel consumption and exhaust emissions. In 2008, the first production pushrod engine to use variable valve timing was introduced in the Dodge Viper (fourth generation).{{cite web |url=http://www.sae.org/automag/technewsletter/070402Powertrain/04.htm |title=Automotive Engineering International Online: Powertrain Technology Newsletter |publisher=Sae.org |access-date=2011-09-07 |url-status=dead |archive-url=https://web.archive.org/web/20110805085957/http://www.sae.org/automag/technewsletter/070402Powertrain/04.htm |archive-date=2011-08-05 }}

File:pushrod engine.jpg (with intake manifold removed), showing the camshaft, pushrods, and rockers]]

OHV engines have several advantages compared with OHC engines:

• Smaller overall packaging: The cam-in-block 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.{{Cite web |url= http://www.caranddriver.com/features/04q2/the_pushrod_engine_finally_gets_its_due-column|title= The Pushrod Engine Finally Gets its Due|first= Larry|last= Webster|date= May 2004|website= Car and Driver|access-date= 31 December 2014|url-status= live|archive-url= https://web.archive.org/web/20140826193302/http://www.caranddriver.com/columns/the-pushrod-engine-finally-gets-its-due|archive-date= 26 August 2014}}
• Simpler camshaft drive system: OHV engines have a less complex drive system for the camshaft than OHC engines. Most OHC engines drive the camshaft or camshafts using a timing belt, a chain, 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, 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 (i.e., the lifters, pushrods, and rockers). Inertia from these valvetrain parts makes OHV engines more susceptible to valve float 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.{{cite web |title=What is the difference between OHV, OHC, SOHC and DOHC engines? |url=https://www.samarins.com/glossary/dohc.html |website=www.samarins.com |access-date=19 December 2019}}

{{reflist}}

{{Internal combustion engine}}

Category:Cam-in-block valvetrain configurations

Category:Engine valvetrain configurations

Category:Motorcycle engines

Category:Scottish inventions