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"OHV" redirects here. OHV may also refer to Off highway vehicle.
Components of a pushrod valve actuation system
Picture of a V8 engine block (with intake manifold removed), showing the camshaft, pushrods, and rockers.

An overhead valve (OHV) engine is just what it says, the valves are placed over the head of the pistons. This was an improvement over the old flat head engine, where the valves were placed in the block next to the piston. OHC, SOHC and DOHC engines place the camshaft overhead but they are still OHV engines.

This type of valvetrain installation, also informally called pushrod engine or I-head engine, is a type of piston engine that places the camshaft within the cylinder block (usually beside and slightly above the crankshaft in a straight engine or directly above the crankshaft in the V of a V engine), and uses pushrods or rods to actuate rocker arms above the cylinder head to actuate the valves. Lifters or tappets are located in the engine block between the camshaft and pushrods.[1] The more modern overhead camshaft (OHC) design (still literally overhead valve) avoids the use of pushrods by putting the camshaft in the cylinder head.

In 1949, Oldsmobile introduced the Rocket V8. It was the first high-compression I-head design, and is the archetype for most modern pushrod engines. General Motors is the world's largest pushrod engine producer,[citation needed] producing both V6 and V8 pushrod engines.

Few pushrod type engines remain in production outside of the United States market. This is in part a result of some countries passing laws to tax engines based on displacement, because displacement is somewhat related to the emissions and fuel efficiency of an automobile. This has given OHC engines a regulatory advantage in those countries, which resulted in few manufacturers wanting to design both OHV and OHC engines.

However, in 2002, Chrysler introduced a new pushrod engine: a 5.7 litre Hemi engine. The new Chrysler Hemi engine presents advanced features such as variable displacement technology and has been a popular option with buyers. The Hemi was on the Ward's 10 Best Engines list for 2003 through 2007. Chrysler also produced the world's first production variable valve OHV engine with independent intake and exhaust phasing. The system is called CamInCam,[2] and was first used in the 600 horsepower (447 kW) SRT-10 engine for the 2008 Dodge Viper.

History[edit]

In automotive engineering, an overhead valve internal combustion engine is one in which the intake and exhaust valves and ports are contained within the cylinder head.

The original overhead valve or OHV piston engine was developed by the Scottish-American David Dunbar Buick. It employs pushrod-actuated valves parallel to the pistons, and this is still in use today. This contrasts with previous designs which made use of side valves and sleeve valves.

Arthur Chevrolet was awarded US Patent #1,744,526 for an Overhead Valve Engine design. This patent included an adapter that could be applied to an existing engine, thus transforming it into an Overhead Valve Engine.

Nowadays, automotive use of side-valves has virtually disappeared, and valves are almost all "overhead". However most are now driven more directly by the overhead camshaft system, and these are designated OHC instead—either single overhead camshaft (SOHC) or double overhead camshaft (DOHC).

Advantages[edit]

Overhead valve (OHV) engines have specific advantages:

  • Smaller overall packaging: because of the camshaft's location inside the engine block, OHV engines are more compact than an overhead cam engine of comparable displacement. For example, Ford's 4.6 L OHC modular V8 is larger than the 5.0 L I-head Windsor V8 it replaced. GM's 4.6 L OHC Northstar V8 is slightly taller and wider than GM's larger displacement 5.7 to 7.0 L I-head LS V8. The Ford Ka uses the venerable Kent Crossflow OHV engine to fit under its low bonnet line.
  • Less complex drive system: OHV engines have a less complex drive system to time the camshaft when compared with 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 some complexity to the engine. In contrast an OHV engine has the camshaft positioned just above crankshaft and can be run with a much smaller chain or even direct gear connection.

Limitations[edit]

Some specific problems that remain with overhead valve (OHV) engines:

  • Limited engine speeds or RPM: OHV engines have more valvetrain moving parts, thus more valvetrain inertia and mass, as a result they suffer more easily from valve "float", and may exhibit a tendency for the pushrods, if improperly designed, to flex or snap at high engine speeds. Therefore, OHV engine designs cannot revolve ("rev") at engine speeds as high as OHC[1] Modern OHV engines are usually limited to about 6,000 to 8,000 revolutions per minute (rpm) in production cars, and 9,000 rpm to 10,500 rpm in racing applications. In contrast, many modern DOHC engines may have rev limits from 6,000 rpm to 9,000 rpm in road car engines, and in excess of 20,000 rpm (though now limited to 18,000 rpm) in current Formula One race engines using pneumatic valve springs. High-revving pushrod engines are normally solid (mechanical) lifter designs, flat and roller. In 1969, Chevrolet offered a Corvette and a Camaro model with a solid lifter cam pushrod V8 (the ZL-1) that could rev to 8,000 rpm. The Volvo B18 and B20 engines can rev to more than 7,000 rpm with their solid lifter camshaft. However, the LS7 of the C6 Corvette Z06 is the first production hydraulic roller cam pushrod engine to have a redline of 7,100 rpm. The Honda CX500 motorcycle engine has a 9750rpm redline, well above the usual limits for auto engines, due to the lighter weight of components.
  • Limited cylinder head design flexibility: overhead camshaft (OHC) engines benefit substantially from the ability to use multiple valves per cylinder, as well as much greater freedom of component placement, and intake and exhaust port geometry. Most modern OHV engines have two valves per cylinder, while many OHC engines can have three, four or even five valves per cylinder to achieve greater power. Though multi-valve OHV engines exist, their use is somewhat limited due to their complexity and is mostly restricted to low- and medium-speed diesel engines, with a few notable exceptions such as the four valve per cylinder Honda CX500 motorcycle. 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.
  • Noise and Refinement: OHV engines are generally noisier than their OHC counterparts owing to the increased complexity of the valvetrain and the adoption of chain or gear based camshaft drive.
  • Maintenance: The location of the camshaft in the cylinder block often necessitates removal of the engine whenever camshaft work is required. This is particularly true for front wheel drive applications with a transversely mounted engine. Longitudinally mounted OHV engines suffer less from this problem as the camshaft can be withdrawn from the front of the engine after removal of the radiator.

1994 Mercedes/Ilmor Indianapolis 500 engine[edit]

See also: Penske PC-23

The Indy 500 race in Indianapolis each year bears some vestige of its original purpose as a proving ground for automobile manufacturers, in that it once gave an advantage in engine displacement to engines based on stock production engines, as distinct from out-and-out racing engines designed from scratch. One factor in identifying production engines from racing engines was the use of pushrods, rather than the overhead camshafts used on most modern racing engines; Mercedes-Benz realized before the 1994 race that they could very carefully tailor a purpose-built racing engine using pushrods to meet the requirements of the Indy rules and take advantage of the 'production based' loophole, but still design it to be a state of the art racing engine in all other ways, without any of the drawbacks of a real production-based engine. They entered this engine in 1994, and because of the higher boost pressure and larger displacement that the "loophole" allowed pushrod engines, dominated the race. After the race, the rules were changed in order to reduce the amount of boost pressure supplied by the turbocharger. This amount was still 13% higher than what was allowed for the OHC engines, the engine was also allowed to retain its considerable displacement advantage. The inability of the engine to produce competitive power output after this change caused it to become obsolete after just the one race. Mercedes-Benz knew this beforehand, deciding that the cost of engine development was worth one win at Indianapolis.

See also[edit]

References[edit]

  1. ^ a b Nice, Karim (2000-12-13). "HowStuffWorks "Camshaft Configurations"". Auto.howstuffworks.com. Retrieved 2011-09-07. 
  2. ^ "Automotive Engineering International Online: Powertrain Technology Newsletter". Sae.org. Retrieved 2011-09-07. 

External links[edit]


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19 news items

 
Bradenton Herald
Sun, 27 Jul 2014 20:56:15 -0700

On Friday, Richard Svec was doing an overhead valve job on a big six-cylinder Cummings engine that powers an Manatee County Area Transit bus. Someone else had a bus up on a massive lift, repairing the differential -- commonly known as the vehicle's ...
 
Car and Driver (blog)
Fri, 25 Jul 2014 10:08:04 -0700

By then work had commenced on a modern overhead-valve V-8 engine to take advantage of higher-octane gasoline and rising road speeds. The new design was introduced for the 1949 model year with a 3.81-inch bore and 3.63-inch stroke yielding 331 ...
 
Street Rodder Magazine
Fri, 18 Jul 2014 06:07:30 -0700

Most of the truly rare stuff comes in like this pair of Maxi overhead-valve conversion heads. They define the term exceedingly rare—the only other pair we've seen are on Isky's roadster (he cast his one-piece rocker covers in high school). So ...
 
Burlington County Times
Thu, 17 Jul 2014 10:06:20 -0700

When the car was shipped to Virginia from New York, the trailer also brought the original 948cc A-series overhead valve engine, as well as a pair of Skinners Union carburetors. Because it was a New York car, Smith expected there would be rust to deal ...
 
Belleville News Democrat
Tue, 15 Jul 2014 14:33:45 -0700

"The overhead-valve motors are usually the ones with the smaller cc number. They're a higher compression motor and much more efficient on their flow of intake and exhaust. So even though they have fewer cc's, they're still putting out the same gross ...
 
Autonet.ca
Sat, 28 Jun 2014 20:56:15 -0700

Only 59 were assembled with Willys' own 161 cubic inch overhead valve 6-cylinder that generated an anemic 90 hp. The bigger straight 6 is the same Continental engine that had powered Kaiser and Frazer cars since K-F's beginning in 1946. By 1953 its ...

Ultimate MotorCycling | News and Reviews

Ultimate MotorCycling | News and Reviews
Thu, 03 Jul 2014 11:22:30 -0700

Liquid now cools the engine and double overhead cams have replaced the pushrod overhead valve system. Multiport sequential electronic fuel injection feeds the big twin through two intake valves per cylinder, replacing the single Amal carburetor on the ...
 
The FINANCIAL
Thu, 03 Jul 2014 10:37:30 -0700

As elegant as any Bentley of the era, the '49 model was the first with Cadillac's modern overhead valve V-8. 2. 1961-63 Ford Thunderbird. The third generation T-Bird is arguably the prettiest. The epitome of jet-age, mid-century modern style, it was ...
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