Overhead valve (OHV)
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 producerproducing 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,and was first used in the 600 horsepower (447 kW) SRT-10 engine for the 2008 Dodge Viper.
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).
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.
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 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. Longditudinally 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.